Bone - Mineralized Connective Tissue
Bone is a connective tissue with living cells (osteocytes) and collagen fibers distributed throughout a ground substance that is hardened by calcium salts. As bone develops, precursor cells called osteoblasts secrete collagen fibers and a ground substance of proteins and carbohydrates. Eventually, osteocytes reside within lacunae in the ground substance, which becomes mineralized by calcium deposits.
Bones are surrounded by a sturdy membrane called the periosteum. There are two kinds of bone tissue. Compact bone tissue forms the bone’s shaft and the outer portion of its two ends. Compact bone forms in thin, circular layers (osteons or Haversian systems) with small canals at their centers, which contain blood vessels and nerves. Osteocytes in the lacunae communicate by way of canaliculi (little canals). Spongy bone tissue is located inside the shaft of long bones.
Spongy and compact bone tissue in a femur.
Thin, dense layers of compact bone tissue form interconnected arrays around canals that contain blood vessels and nerves. Each array is an osteon (Haversian system). The blood vessel threading through it transports substances to and from osteocytes, living bone cells in small spaces (lacunae) in the bone tissue. Small tunnels called canaliculi connect neighboring spaces.
Click here for the Animation: Structure of the Human Thigh Bone. Please make sure that your sound is on and your volume is up.
|How a long bone forms. First, osteoblasts begin to function in a cartilage model in the embryo. The bone-forming cells are active first in the shaft, then at the knobby ends. In time, cartilage is left only in the epiphyses at the ends of the shaft.|
A bone develops on a cartilage model. Osteoblasts secrete material inside the shaft of the cartilage model of long bones. Calcium is deposited; cavities merge to form the marrow cavity. Eventually osteoblasts become trapped within their own secretions and become osteocytes (mature bone cells).
In growing children, the epiphyses (ends of bone) are separated from the shaft by an epiphyseal plate (cartilage), which continues to grow under the influence of growth hormone until late adolescence.
Click here for the Animation: How a Long Bone Forms. Please make sure that your sound is on and your volume is up.
Click here for the Video: Taller and Taller. Please make sure that your sound is on and your volume is up.
|Normal bone tissue.||After the onset of osteoporosis, replacements of mineral ions lag behind withdrawals. In time the tissue erodes, and the bone becomes hollow and brittle.|
Bone tissue is constantly “remodeled.” Bone is renewed constantly as minerals are deposited by osteoblasts and withdrawn by osteoclasts during the bone remodeling process. Before adulthood, bone turnover is especially important in increasing the diameter of certain bones. Bone turnover helps to maintain calcium levels for the entire body. A hormone called PTH causes bone cells to release enzymes that will dissolve bone tissue and release calcium to the interstitial fluid and blood; calcitonin stimulates the reverse. Osteoporosis (decreased bone density) is associated with decreases in osteoblast activity, sex hormone production, exercise, and calcium uptake.
The Skeleton: The Body’s Bony Framework
Bones are the main components of the human skeletal system. There are four types of bones: long (arms), short (ankle), flat (skull), and irregular (vertebrae). Bone marrow fills the cavities of bones. In long bones, red marrow is confined to the ends; yellow marrow fills the shaft portion. Irregular bones and flat bones are completely filled with the red bone marrow responsible for blood cell formation.
The skeleton: a preview. The 206 bones of a human are arranged in two major divisions: the axial skeleton and the appendicular skeleton. Bones are attached to other bones by ligaments; bones are connected to muscles by tendons.
Bone functions are vital in maintaining homeostasis. The bones are moved by muscles; thus the whole body is movable. The bones support and anchor muscles. Bones protect vital organs such as the brain and lungs. Bone tissue acts as a depository for calcium, phosphorus, and other ions. Parts of some bones are sites of blood cell production.
Functions of Bone
|Movement||Bones interact with skeletal muscles to maintain or change the position of body parts.|
|Support||Bones support and anchor muscles.|
|Protection||Many bones form hard compartments that enclose and protect soft internal organs.|
|Mineral Storage||Bones are a reservoir for calcium and phosphorus. Deposits and withdrawals of these mineral ions help to maintain their proper concentrations in body fluids.|
|Blood Cell Formation||Some bones contain marrow where blood cells are produced.|
The Axial Skeleton
Sinuses in bones in the skull and face.
The irregular junctions between different bones are called sutures.
An “inferior,” or bottom-up, view of the skull. The large foramen magnum is situated atop the uppermost cervical vertebra.
The Appendicular Skeleton
Bones of the pectoral girdle, the arm, and the hand.
The pectoral girdle and upper limbs provide flexibility. The pectoral girdle includes the bones of, and is attached to, the shoulder. The scapula is a large, flat shoulder blade with a socket for the upper arm bone. The clavicle (collarbone) connects the scapula to the sternum.
Each upper limb includes some 30 separate bones. The humerus is the bone of the upper arm. The radius and ulna extend from the hingelike joint of the elbow to the wrist. The carpals form the wrist; the metacarpals form the palm of the hand, and the phalanges the fingers.
|Structure of a femur (thighbone), a typical long bone.||Bones of the pelvic girdle, the leg, and the foot.|
The pelvic girdle and lower limbs support body weight. The pelvic girdle includes the pelvis and the legs. The pelvis is made up of coxal bones attaching to the sacrum in the back and forming the pelvic arch in the front. The pelvis is broader in females than males; this is necessary for childbearing.
The legs contain the body’s largest bones. The femur is the longest bone, extending from the pelvis to the knee. The tibia and fibula form the lower leg; the kneecap bone is the patella. Tarsal bones compose the ankle, metatarsals the foot, and phalanges the toes.
Joints—Connections Between Bones
The knee joint, an example of a synovial joint. The knee is the largest and most complex joint in the body. Part (a) shows the joint with muscles stripped away; in (b) you can see where muscles such as the quadriceps attach.
Click to enlarge
Synovial joints move freely. Synovial joints are the most common type of joint and move freely; they include the ball-and-socket joints of the hips and the hingelike joints such as the knee. These types of joints are stabilized by ligaments. A capsule of dense connective tissue surrounds the bones of the joint and produces synovial fluid that lubricates the joint.
Other joints move little or not at all. Cartilaginous joints (such as between the vertebrae) have no gap, but are held together by cartilage and can move only a little. Fibrous joints also have no gap between the bones and hardly move; flat cranial bones are an example.
Ways body parts move at synovial joints. The synovial joint at the shoulder permits the greatest range of movement.
Disorders of the Skeleton
Inflammation is a factor in some skeletal disorders. In rheumatoid arthritis, the synovial membrane becomes inflamed due to immune system dysfunction, the cartilage degenerates, and bone is deposited into the joint.
In osteoarthritis, the cartilage at the end of the bone degenerates. Tendinitis is the inflammation of tendons and synovial membranes around joints. Carpal tunnel syndrome is the result of the inflammation of the tendons in the space between a wrist ligament and the carpal bones, usually aggravated by chronic over use.
An x-ray of an arm bone deforemed by Osteogenesis Imperfecta (OI).
Little girl with OI, she had multiple fractures in her arms and legs at birth
A child with rickets
Joints also are vulnerable to strains, sprains, and dislocations. A strain results from stretching or twisting a joint suddenly or too far. A sprain is a tear of ligaments or tendons. A dislocation causes two bones to no longer be in contact.
In factures, bones break. A simple fracture is a crack in the bone; not very serious. A complete fracture separates the bone into two pieces, which must be quickly realigned for proper healing. A compound fracture is the most serious because it means there are multiple breaks with the possibility of bone fragments penetrating the surrounding tissues.
Other bone disorders include genetic diseases, infections, and cancer. Genetic diseases such as osteogenesis imperfecta can leave bones brittle and easily broken. Bacterial and other infections can spread from the blood stream to bone tissue or marrow. Osteosarcoma, bone cancer, usually occurs in long bones.
The Body’s Three Kinds of Muscle
The three kinds of muscle are built and function in different ways. Skeletal muscle, composed of long thin cells called muscle “fibers,” allows the body to move. Smooth muscle is found in the walls of hollow organs and tubes; the cells are smaller than those of skeletal muscle and are not striated. The heart is the only place where cardiac muscle is found.
Cardiac muscle and smooth muscle are considered involuntary muscles because we cannot consciously control their contraction; skeletal muscles are voluntary muscles. Skeletal muscle comprises the body’s muscular system.
Click here for the Animation: Major Skeletal Muscles. Please make sure that your sound is on and your volume is up.
The three kinds of muscle in the body and where each type is found
Some of the major muscles of the muscular system.
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The flexor digitorum superficialis, a forearem muscle that helps move the fingers.
The zygomaticus major, which helps you smile.
The Structure and Function of Skeletal Muscles
A tendon sheath. Notice the lubricating fluid inside each of the sheaths sketched here.
A skeletal muscle is built of bundled muscle cells. Inside each cell are threadlike myofibrils, which are critical to muscle contraction. The cells are bundled together with connective tissue that extends past the muscle to form tendons, which attach the muscle to bones.
Bones and skeletal muscles work like a system of levers. The human body’s skeletal muscles number more than 600. The origin end of each muscle is designated as being attached to the bone that moves relatively little; whereas the insertion is attached to the bone that moves the most. Because most muscle attachments are located close to joints, only a small contraction is needed to produce considerable movement of some body parts (leverage advantage).
Many muscles are arranged as pairs or in groups. Many muscles are arranged as pairs or grouped for related function. Some work antagonistically (in opposition) so that one reverses the action of the other. Others work synergistically, the contraction of one stabilizes the contraction of another. Reciprocal innervation dictates that only one muscle of an antagonistic pair (e.g. biceps and triceps) can be stimulated at a time.
Click here for the Animation: Two Opposing Muscle Groups in Human Arms. Please make sure that your sound is on and your volume is up.
“Fast” and “slow” muscle. Humans have two general types of skeletal muscles: “Slow” muscle is red in color due to myoglobin and blood capillaries; its contractions are slower but more sustained. “Fast” or “white” muscle cells contain fewer mitochondria and less myoglobin but can contract rapidly and powerfully for short periods.
When athletes train, one goal is to increase the relative size and contractile strength of fast (sprinters) and slow (distance swimmers) muscle fibers.
How Muscles Contract
|Zooming down through skeletalmuscle from a biceps to filaments of the proteins actin andmyosin. These proteins can contract.|
A muscle contracts when its cells shorten. Muscles are divided into contractile units called sarcomeres. Each muscle cell contains myofibrils composed of thin (actin) and thick (myosin) filaments. Each actin filament is actually two beaded strands of protein twisted together. Each myosin filament is a protein with a double head (projecting outward) and a long tail (which is bound together with others). The arrangement of actin and myosin filaments gives skeletal muscles their characteristic striped appearance.
Click here for the Video: Structure of a Sarcomere. Please make sure that your sound is on and your volume is up.
Click here for the Animation: Structure of Skeletal Muscle. Please make sure that your sound is on and your volume is up.
Click here for the Animation: Banding Patterns and Muscle Contraction. Please make sure that your sound is on and your volume is up.
Muscle cells shorten when actin filaments slide over myosin. Within each sarcomere there are two sets of actin filaments, which are attached on opposite sides of the sarcomere; myosin filaments lie suspended between the actin filaments.
During contraction, the myosin filaments physically slide along and pull the two sets of actin filaments toward each other at the center of the sarcomere; this is called the sliding-filament model of contraction. When a myosin head is energized, it forms cross-bridges with an adjacent actin filament and tilts in a power stroke toward the sarcomere’s center. Energy from ATP drives the power stroke as the heads pull the actin filaments along. After the power stroke the myosin heads detach and prepare for another attachment (power stroke).
Click here for the Animation: Muscle Contraction. Please make sure that your sound is on and your volume is up.
When a person dies, ATP production stops, myosin heads become stuck to actin, and rigor mortis sets in, making the body stiff.
Click here for the Video: Muscle Contraction Overview. Please make sure that your sound is on and your volume is up.
How the Nervous System Controls Muscle Contraction
Pathway for signals from thenervous system that stimulate contraction of skeletal muscle.
Calcium ions are the key to contraction. Skeletal muscles contract in response to signals from motor neurons of the nervous system. Signals arrive at the T tubules of the sarcoplasmic reticulum (SR), which wraps around the myofibrils. The SR responds by releasing stored calcium ions; calcium binds to the protein troponin, changing the conformation of actin and allowing myosin cross-bridges to form. Another protein, tropomyosin, is also associated with actin filaments.
When nervous stimulation stops, calcium ions are actively taken up by the sarcoplasmic reticulum and the changes in filament conformation are reversed; the muscle relaxes.
Animation: Pathway from Nerve Signal to Skeletal Muscle Contraction
Animation: Actin, Troponin, and Tropomyosin
Neurons act on muscle cells at neuromuscular junctions. At neuromuscular junctions, impulses from the branched endings (axons) of motor neurons pass to the muscle cell membranes. Between the axons and the muscle cell is a gap called a synapse. Signals are transmitted across the gap by a neurotransmitter called acetylcholine (ACh). When the neuron is stimulated, calcium channels open to allow calcium ions to flow inward, causing a release of acetylcholine into the synapse.
The interactions of actin, tropomyosin,and troponins in a skeletal muscle cell.
Click to enlarge
How a chemical messenger called a neurotransmitter carries a signal across a neuromuscular junction.
How Muscle Cells Get Energy
ATP supplies the energy for muscle contraction. Initiation of muscle contraction requires much ATP; this will initially be provided by creatine phosphate, which gives up a phosphate to ADP to make ATP. Cellular respiration provides most of the ATP needed for muscle contraction after this, even during the first 5-10 minutes of moderate exercise. During prolonged muscle action, glycolysis alone produces low amounts of ATP; lactic acid is also produced, which hinders further contraction.
Muscle fatigue is due to the oxygen debt that results when muscles use more ATP than cellular respiration can deliver.
Click here for the Animation: Energy Sources for Muscle Contraction. Please make sure that your sound is on and your volume is up.
Three metabolic pathways by whichATP forms in muscles in response to the demands of physical exercise.
Properties of Whole Muscles
(a) Example of motor units present in muscles.(b) The micrograph shows the axon endings of a motorneuron that acts on individual muscle cells in the muscle.
Several factors determine the characteristics of a muscle contraction. A motor neuron and the muscle cells under its control are a motor unit; the number of cells in a motor unit depends on the precision of the muscle control needed. A single, brief stimulus to a motor unit causes a brief contraction called a muscle twitch. Repeated stimulation makes the twitches run together in a sustained contraction called tetanus (tetany).
Click here for the Animation: Effects of Stimulation on Muscles. Please make sure that your sound is on and your volume is up.
Not all muscle cells in a muscle contract at the same time. The number of motor units that are activated determines the strength of the contraction: Small number of units = weak contraction; large number of units at greater frequency = stronger contraction. Muscle tone is the continued steady, low level of contraction that stabilizes joints and maintains general muscle health.
Muscle tension is the force a contracting muscle exerts on an object; to contract, a muscle’s tension must exceed the load opposing it. An isotonically contracting muscle shortens and moves a load. An isometrically contracting muscle develops tension but does not shorten.
Tired muscles can’t generate much force. Muscles fatigue when strong stimulation keeps a muscle in a state of tetanus too long. After resting, muscles will be able to contract again; muscles may need to rest for minutes up to a day to fully recover.
|Recordings of twitches in muscles artificially stimulated in different ways. (a) A single twitch. (b) Six persecond cause a summation of twitches, and (c) about 20 per second cause tetanic contraction. (d) Painting of a soldier dying of thedisease tetanus in a military hospital in the 1800s after the bacterium Clostridium tetani infected a battlefield wound.||(a) An isotonic contraction.The load is less than a muscle’s peakcapacity to contract, so the musclecan contract, shorten, and lift the load.(b) In an isometric contraction, the loadexceeds the muscle’s peak capacity. Itcontracts, but can’t shorten.|
Strains and tears are muscle injuries. Muscle strains come from movement that stretches or tears muscle fibers; ice, rest and anti-inflammatory drugs (ibuprofen) allow damage to repair. If the whole muscle is torn, scar tissue may develop, shortening the muscle and making it function less effectively.
Sometimes a skeletal muscle will contract abnormally. A muscle spasm is a sudden, involuntary contraction that rapidly releases, while cramps are spasms that don’t immediately release; cramps usually occur in calf and thigh muscles. Tics are minor, involuntary twitches of muscles in the face and eyelids.
Muscular dystrophies destroy muscle fibers. Muscular dystrophies are genetic diseases leading to breakdown of muscle fibers over time. Duchenne muscular dystrophy (DMD) is common in children; a single mutant gene interferes with sarcomere contraction. Myotonic muscular dystrophy is usually found in adults; muscles of the hands and feet contract strongly but fail to relax normally. In these diseases, muscles progressively weaken and shrivel.
Exercise makes the most of muscles. Muscles that are damaged or which go unused for prolonged periods of time will atrophy (waste away). Aerobic exercise improves the capacity of muscles to do work. Walking, biking, and jogging are examples of exercises that increase endurance. Regular aerobic exercise increases the number and size of mitochondria, the number of blood capillaries, and the amount of myoglobin in the muscle tissue. Strength training improves function of fast muscle but does not increase endurance. Even modest activity slows the loss of muscle strength that comes with aging.
- Skeletal and Muscular Pathophysiology
- Epiphyseal plate
- Bone remodeling
- Bone Metabolism
- exchangeable calcium
- loosely bound CaPO4
- quickly released in response to short term increases in PTH
- Parathyroid hormone (PTH)
- when blood levels elevated for longer peroids of time, PTH stimulates:
- promotes renal retention
- promotes intestinal calcium absorption
- Vitamin D (Cholcalciferol)
- produced in skin
- transformed into CACIDIOL
- calcidiol is then transported to the kidney where it is finally converted into CALCITRIOL, the most potent agent.
- at normal physiological concentrations calcitriol promotes bone deposition
- at higher than physiological levels calcitriol works with PTH to activate resorption and calcium release
- calcitriol also regulates the growth and development of other tissues
- secreted by the thyroid gland
- opposes the action of PTH
- Bone pathophysiology
- DiGeorge's syndrome
- multipleendocrine deficiency
- vitamin D deficiency
- genetic disorders
- osteogenesis imperfect
- achondroplastic dwarfism
- acquired bone disorders
- Paget's disease
- Bone tumors
- Skeletal muscle disorders
- Muscle cell degeneration
- muscular dystrophy
- Motor end plate disorders
- myasthenia gravis
- Motor neuron disorders
- amyotrophic lateral sclerosis
- extrapyramidal disorders
- Parkinson's disease
- Huntington's disease
- Demyelinating disorders
- multiple sclerosis
- Guillain-Barre syndrome
- Joint disorders
- Rheumatoid arthritis
- specific effects
- Other joint disorders
- Ankylosing spondylitis
The Nervous System
The role of the nervous system is to detect and integrate information about external and internal conditions and carry out responses. Neurons form the basis of the system’s communication network. There are three types of neurons:
- Sensory neurons are receptors for specific sensory stimuli (signals).
The functional zones of a motor neuron. The micrograph shows a motor neuron with its plump cell body and branching dendrites.
- Interneurons in the brain and spinal cord integrate input and output signals.
- Motor neurons send information from integrator to muscle or gland cells (effectors).
Neurons have several functional zones. Neurons form extended cells with several zones: The cell body contains the nucleus and organelles. The cell body has slender extensions called dendrites; the cell body and the dendrites form the input zone for receiving information. Next comes the trigger zone, called the axon hillock in motor neurons and interneurons; the trigger zone leads to the axon, which is the neuron’s conducting zone. The axon’s endings are output zones where messages are sent to other cells.
Only 10% of the nervous system consists of neurons; the rest of the 90% is composed of support cells called neuroglia, or glia. Neurons function well in communication because they are excitable (produce electrical signals in response to stimuli).
Properties of a neuron’s plasma membrane allow it to carry signals. The plasma membrane prevents charged substances (K+ and Na+ ions) from moving freely across, but both ions can move through channels. Some channel proteins are always open, others are gated. In a resting neuron, gated sodium channels are closed; sodium does not pass through the membrane, but potassium does. According to the gradients that form, sodium diffuses into the cell, potassium diffuses out of the cell. The difference across the membrane that forms because of the K+ and Na+ gradients results in a resting membrane potential of ‒70 millivolts (cytoplasmic side of the membrane is negative).
Ions and a neuron’s plasma membrane. (a) Gradients of sodium (Na+) and potassium (K+) ions across a neuron’s plasma membrane. (b) How ions cross the plasma membrane of a neuron. They are selectively allowed to cross at protein channels and pumps that span the membrane.
Ions and a neuron’s plasma membrane. (a) Gradients of sodium (Na+) and potassium (K+) ions across a neuron’s plasma membrane. (b) How ions cross the plasma membrane of a neuron. They are selectively allowed to cross at protein channels and pumps that span the membrane.
Action Potentials = Nerve Impulses
(1, 2) Steps leading to an action potential. (3, 4) How an action potential propagates.
Chemical Synapses: Communication Junctions
Action potentials can stimulate the release of neurotransmitters. Neurotransmitters diffuse across a chemical synapse, the junction between a neuron and an adjacent cell (between neurons and other neurons, or between neurons and muscle or gland cells). The neuron that releases the transmitter is called the presynaptic cell. In response to an action potential, gated calcium channels open and allow calcium ions to enter the neuron from the synapse. Calcium causes the synaptic vesicles to fuse with the membrane and release the transmitter substance into the synapse. The transmitter binds to receptors on the membrane of the postsynaptic cell.
Neurotransmitters can excite or inhibit a receiving cell. How a postsynaptic cell responds to a transmitter depends on the type and amount of transmitter, the receptors it has, and the types of channels in its input zone. Excitatory signals drive the membrane toward an action potential. Inhibitory signals prevent an action potential.
Examples of neurotransmitters:
Neuromodulators can magnify or reduce the effects of a neurotransmitter. One example includes the natural painkillers called endorphins. Release of endorphins prevents sensations of pain from being recognized. Endorphins may also play a role in memory, learning, and sexual behavior. Competing signals are “summed up.” Excitatory and inhibitory signals compete at the input zone. An excitatory postsynaptic potential (EPSP) depolarizes the membrane to bring it closer to threshold. An inhibitory postsynaptic potential (IPSP) either drives the membrane away from threshold by a hyperpolarizing effect or maintains the membrane potential at the resting level. In synaptic integration, competing signals that reach the input zone of a neuron at the same time are summed; summation of signals determines whether a signal is suppressed, reinforced, or sent onward to other body cells.
Neurotransmitter molecules must be removed from the synapse. Neurotransmitters must be removed from the synaptic cleft to discontinue stimulation. There are three methods of removal: Some neurotransmitter molecules simply diffuse out of the cleft. Enzymes, such as acetylcholinesterase, break down the transmitters. Membrane transport proteins actively pump neurotransmitter molecules back into the presynaptic cells.
Click here for the Video: Synapse Function. Please make sure that your sound is on and your volume is up.
Nerves are long-distance lines. Signals between the brain or spinal cord and body regions travel via nerves. Axons of sensory neurons, motor neurons, or both, are bundled together in a nerve. Within the brain and spinal cord, bundles of interneuron axons are called nerve tracts. Axons are covered by a myelin sheath derived from Schwann cells. Each section of the sheath is separated from adjacent ones by a region where the axon membrane, along with gated sodium channels, is exposed. Action potentials jump from node to node (saltatory conduction); such jumps are fast and efficient. There are no Schwann cells in the central nervous system; here processes from oligodendrocytes form the sheaths of myelinated axons.
Reflex arcs are the simplest nerve pathways. A reflex is a simple, stereotyped movement in response to a stimulus. In the simplest reflex arcs, sensory neurons synapse directly with motor neurons; an example is the stretch reflex, which contracts a muscle after that muscle has been stretched. In most reflex pathways, the sensory neurons also interact with several interneurons, which excite or inhibit motor neurons as needed for a coordinated response.
In the brain and spinal cord, neurons interact in circuits. The overall direction of flow in the nervous system: sensory neurons >>> spinal cord and brain >>> interneurons >>> motor neurons. Interneurons in the spinal cord and brain are grouped into blocks, which in turn form circuits; blocks receive signals, integrate them, and then generate new ones. Divergent circuits fan out from one block into another. Other circuits funnel down to just a few neurons. In reverberating circuits, neurons repeat signals among themselves.
Overview of the Nervous System
The central nervous system (CNS) is composed of the brain and spinal cord; all of the interneurons are contained in this system. Nerves that carry sensory input to the CNS are called the afferent nerves. Efferent nerves carry signals away from the CNS.
The peripheral nervous system (PNS) includes all the nerves that carry signals to and from the brain and spinal cord to the rest of the body. The PNS is further divided into the somatic and autonomic subdivisions. The PNS consists of 31 pairs of spinal nerves and 12 pairs of cranial nerves. At some sites, cell bodies from several neurons cluster together in ganglia.
Major Expressways: Peripheral Nerves and the Spinal Cord
The peripheral nervous system consists of somatic and autonomic nerves. Somatic nerves carry signals related to movement of the head, trunk, and limbs; signals move to and from skeletal muscles for voluntary control. Autonomic nerves carry signals between internal organs and other structures; signals move to and from smooth muscles, cardiac muscle, and glands (involuntary control). The cell bodies of preganglionic neurons lie within the CNS and extend their axons to ganglia outside the CNS. Postganglionic neurons receive the messages from the axons of the preganglionic cells and pass the impulses on to the effectors.
Autonomic nerves are divided into parasympathetic and sympathetic groups. They normally work antagonistically towards each other. Parasympathetic nerves slow down body activity when the body is not under stress. Sympathetic nerves increase overall body activity during times of stress, excitement, or danger; they also call on the hormone norepinephrine to increase the fight-flight response. When sympathetic activity drops, parasympathetic activity may rise in a rebound effect.
The spinal cord is the pathway between the PNS and the brain. The spinal cord lies within a closed channel formed by the bones of the vertebral column. Signals move up and down the spinal cord in nerve tracts. The myelin sheaths of these tracts are white; thus, they are called white matter. The central, butterfly-shaped area (in cross-section) consists of dendrites, cell bodies, interneurons, and neuroglia cells; it is called gray matter. The spinal cord and brain are covered with three tough membranes - the meninges. The spinal cord is a pathway for signal travel between the peripheral nervous system and the brain; it also is the center for controlling some reflex actions. Spinal reflexes result from neural connections made within the spinal cord and do not require input from the brain, even though the event is recorded there. Autonomic reflexes, such as bladder emptying, are also the responsibility of the spinal cord.
Click here for the Video: New Nerves. Please make sure that your sound is on and your volume is up.
The Brain - Command Central
The spinal cord merges with the body’s master control center, the brain. The brain is protected by bone and meninges. The tough outer membrane is the dura mater; it is folded double around the brain and divides the brain into its right and left halves. The thinner middle layer is the arachnoid; the delicate pia mater wraps the brain and spinal cord as the innermost layer. The meninges also enclose fluid-filled spaces that cushion and nourish the brain.
The brain is divided into a hindbrain, midbrain, and forebrain. The hindbrain and midbrain form the brain stem, responsible for many simple reflexes. Hindbrain. The medulla oblongata has influence over respiration, heart rate, swallowing, coughing, and sleep/wake responses. The cerebellum acts as a reflex center for maintaining posture and coordinating limbs. The pons (“bridge”) possesses nerve tracts that pass between brain centers. The midbrain coordinates reflex responses to sight and sound. It has a roof of gray matter, the tectum, where visual and sensory input converges before being sent to higher brain centers. The forebrain is the most developed portion of the brain in humans. The cerebrum integrates sensory input and selected motor responses; olfactory bulbs deal with the sense of smell. The thalamus relays and coordinates sensory signals through clusters of neuron cell bodies called nuclei; Parkinson’s disease occurs when the function of basal nuclei in the thalamus is disrupted. The hypothalamus monitors internal organs and influences responses to thirst, hunger, and sex, thus controlling homeostasis.
Cerebrospinal fluid fills cavities and canals in the brain. The brain and spinal cord are surrounded by the cerebrospinal fluid (CSF), which fills cavities (ventricles) and canals within the brain. A mechanism called the blood-brain barrier controls which substances will pass to the fluid and subsequently to the neurons. The capillaries of the brain are much less permeable than other capillaries, forcing materials to pass through the cells, not around them. Lipid-soluble substances, such as alcohol, nicotine, and drugs, diffuse quickly through the lipid bilayer of the plasma membrane.
A Closer Look at the Cerebrum
There are two cerebral hemispheres. The human cerebrum is divided into left and right cerebral hemispheres, which communicate with each other by means of the corpus callosum. Each hemisphere can function separately; the left hemisphere responds to signals from the right side of the body, and vice versa. The left hemisphere deals mainly with speech, analytical skills, and mathematics; nonverbal skills such as music and other creative activities reside in the right. The thin surface (cerebral cortex) is gray matter, divided into lobes by folds and fissures; white matter and basal nuclei (gray matter in the thalamus) underlie the surface. Each hemisphere is divided into frontal, occipital, temporal, and parietal lobes.
The cerebral cortex controls thought and other conscious behavior. Motor areas are found in the frontal lobe of each hemisphere. The motor cortex controls the coordinated movements of the skeletal muscles. The premotor cortex is associated with learned pattern or motor skills. Broca’s area is involved in speech. The frontal eye field controls voluntary eye movements.
Several sensory areasare found in the parietal lobe: The primary somatosensory cortex is the main receiving center for sensory input from the skin and joints, while the primary cortical area deals with taste. The primary visual cortex, which receives sensory input from the eyes, is found in the occipital lobe. Sound and odor perception arises in primary cortical areas in each temporal lobe. Association areasoccupy all parts of the cortex except the primary motor and sensory regions: Each area integrates, analyzes, and responds to many inputs. Neural activity is the most complex in the prefrontal cortex, the area of the brain that allows for complex learning, intellect, and personality.
The limbic system: Emotions and more. Our emotions and parts of our memory are governed by the limbic system, which consists of several brain regions. Parts of the thalamus, hypothalamus, amygdala, and the hippocampus form the limbic system and contribute to producing our “gut” reactions.
Click here for the Video: Limbic System. Please make sure that your sound is on and your volume is up.
Memory and Consciousness
Disorders of the Nervous System
Some diseases attack and damage neurons. Alzheimer’s disease involves the progressive degeneration of brain neurons, while at the same time there is an abnormal buildup of amyloid protein, leading to the loss of memory. Parkinson’s disease (PD) is characterized by the death of neurons in the thalamus that normally make dopamine and norepinephrine needed for normal muscle function. Meningitis is an often fatal inflammatory disease caused by a virus or bacterial infection of the meninges covering the brain and/or spinal cord. Encephalitis is very dangerous inflammation of the brain, often caused by a virus. Multiple sclerosis (MS) is an autoimmune disease that results in the destruction of the myelin sheath of neurons in the CNS.
The CNS can also be damaged by injury or seizure. A concussion can result from a severe blow to the head, resulting in blurred vision and brief loss of consciousness. Damage to the spinal cord can result in lost sensation, muscle weakness, or paralysis below the site of the injury. Epilepsy is a seizure disorder, often inherited but also caused by brain injury, birth trauma, or other assaults on the brain. Headaches occur when the brain registers tension in muscles or blood vessels of the face, neck, and scalp as pain; migraine headaches are extremely painful and can be triggered by hormonal changes, fluorescent lights, and certain foods, particularly in women.
The Brain on Drugs
Drugs can alter mind and body functions. Psychoactive drugs exert their influence on brain regions that govern states of consciousness and behavior. There are four categories of psychoactive drugs: Stimulants (caffeine, cocaine, nicotine, amphetamines) increase alertness or activity for a time, and then depress you.
Drug use can lead to addiction. As the body develops tolerance to a drug, larger and more frequent doses are needed to produce the same effect; this reflects physical drug dependence. Psychological drug dependence, or habituation, develops when a user begins to crave the feelings associated with using a particular drug and cannot function without it. Habituation and tolerance are evidence of addiction.
Click here for the Video: Brain Stem. Please make sure that your sound is on and your volume is up.
Hormones are signaling molecules that are carried in the bloodstream. Signaling molecules are hormones and secretions that can bind to target cells and elicit in them a response. Hormones are secreted by endocrine glands, endocrine cells, and some neurons. Local signaling molecules are released by some cells; these work only on nearby tissues. Pheromones are signaling molecules that have targets outside the body and which are used to integrate behaviors.
Hormone sources: The endocrine system. The sources of hormones (hormone producing glands, cells, and organs) may be collectively called the endocrine system. Endocrine sources and the nervous system function in highly interconnected ways.
Hormones often interact. In an opposing interaction the effect of one hormone opposes the effect of another. In a synergistic interaction the combined action of two or more hormones is necessary to produce the required effect on target cells. In a permissive interaction one hormone exerts its effect only when a target cell has been “primed” to respond by another hormone.
Click here for the Animation: Major Human Endocrine Glands. Please make sure that your sound is on and your volume is up.
Types of Hormones and Their Signals
Hormones come in several chemical forms. Steroid hormones are lipids made from cholesterol. Amine hormones are modified amino acids. Peptide hormones are peptides of only a few amino acids. Protein hormones are longer chains of amino acids. All hormones bind target cells; this signal is converted into a form that works in the cell to change activity. A target cell’s response to a hormone is dependent on two factors: Different hormones activate different cellular response mechanisms. Not all cells have receptors for all hormones; the cells that respond are selected by means of the type of receptor they possess.
Steroid hormones interact with cell DNA. Steroid hormones, such as estrogen and testosterone, are lipid-soluble and therefore cross plasma membranes readily. Once inside the cell, they penetrate the nuclear membrane and bind to receptors in the nucleus, either turning on or turning off genes. Switching genes on or off changes the proteins that are made by the cell, thus effecting a response. Some steroid hormones bind receptors in the cell membrane and change membrane properties to affect change to the target cell’s function.
Click here for the Video: Mechanism of a steroid hormone. Please make sure that your sound is on and your volume is up.
Nonsteroid hormones act indirectly, by way of second messengers. Nonsteroid hormones include the amine, peptide, and protein hormones. Nonsteroid hormones cannot cross the plasma membrane of target cells, so they must first bind to a receptor on the plasma membrane. Binding of the hormone to the receptor activates the receptor; it in turn stimulates the production of a second messenger, a small molecule that can relay signals in the cell. Cyclic AMP (cyclic adenosine monophosphate) is one example of a second messenger.
Click here for the Video: Mechanism of a peptide hormone. Please make sure that your sound is on and your volume is up.
The Hypothalamus and Pituitary Gland: Major Controllers
The hypothalamus and pituitary gland work jointly as the neural-endocrine control center. The hypothalamus is a portion of the brain that monitors internal organs and conditions. The pituitary is connected to the hypothalamus by a stalk. The posterior lobe consists of nervous tissue and releases two hormones made in the hypothalamus. The anterior lobe makes and secretes hormones that control the activity of other endocrine glands.
The posterior pituitary lobe produces ADH and oxytocin. Neurons in the hypothalamus produce antidiuretic hormone (ADH) and oxytocin, which are released from axon endings in the capillary bed of the posterior lobe. ADH (or vasopressin) acts on the walls of kidney tubules to control the body’s water and solute levels by stimulating reabsorption. Oxytocin triggers uterine muscle contractions to expel the fetus and acts on mammary glands to release milk.
Click here for the Animation: Posterior Pituitary Function. Please make sure that your sound is on and your volume is up.
The anterior pituitary lobe produces six other hormones. Corticotropin (ACTH) stimulates the adrenal cortex. Thyrotropin (TSH) stimulates the thyroid gland. Follicle-stimulating hormone (FSH) causes ovarian follicle development and egg production. Luteinizing hormone (LH) also acts on the ovary to release an egg. Prolactin (PRL) acts on the mammary glands to stimulate and sustain milk production. Somatotropin (STH), also known as growth hormone (GH), acts on body cells in general to promote growth. Most of these hormones are releasers that stimulate target cells to secrete other hormones; other hormones from the hypothalamus are inhibitors and block secretions.
Click here for the Animation: Anterior Pituitary Function. Please make sure that your sound is on and your volume is up.
Click here for the Video: Hypothalamus and Pituitary. Please make sure that your sound is on and your volume is up.
Factors That Influence Hormone Effects
Problems with control mechanisms can result in skewed hormone signals. Endocrine glands in general only release small quantities of hormones and control the frequency of release to make sure there isn’t too much or too little hormone.
Abnormal quantities of hormones can lead to growth problems. Gigantism results from an oversecretion of growth hormone in childhood. Pituitary dwarfism results from an undersecretion of GH. Acromegaly is a condition resulting from an oversecretion of GH in adulthood leading to abnormal thickening of tissues. Diabetes insipidus occurs when ADH secretions fall or stop, leading to dilute urine and the possibility of serious dehydration.
Hormone interactions, feedback, and other factors also influence a hormone’s effects. At least four factors influence the effects of any given hormone. Hormones often interact with one another. Negative feedback mechanisms control secretion of hormones. Target cells may react differently to hormones at different times. Environmental cues can affect release of hormones. Hormones throughout the body are affected in similar ways.
The Thymus, Thyroid, and Parathyroid Glands
Thymus gland hormones aid immunity. Thyroid hormones affect metabolism, growth, and development. The thyroid gland secretes thyroid hormone (TH), which has effects on metabolism, growth, and development; the thyroid gland also secretes calcitonin, which helps regulate calcium levels in the blood.
Click here for the Animation: Thyroid Hormone Action. Please make sure that your sound is on and your volume is up.
Iodine-deficient diets interfere with proper synthesis of thyroid hormones. Simple goiter is an enlargement of one or both lobes of the thyroid gland in the neck; enlargement follows low blood levels of thyroid hormones (hypothyroidism). Graves disease and other forms of hyperthyroidism result from too much thyroid hormone in the blood.
PTH from the parathyroids is the main calcium regulator. Humans have four parathyroid glands, which secrete parathyroid hormone (PTH), the main regulator of blood calcium levels. More PTH is secreted when blood calcium levels drop below a certain point; less is secreted when calcium rises. Calcitonin contributes to processes that pull calcium out of the blood. Rickets in children arises from a vitamin D deficient diet; vitamin D is needed to aid absorption of calcium from food. Hyperparathyroidism sees so much calcium being withdrawn from a person’s bones that the bone tissue is dangerously weakened.
Adrenal Glands and Stress Responses
The adrenal cortex produces glucocorticoids and mineralocorticoids. One adrenal gland is located on top of each kidney; the outer part of each gland is the adrenal cortex, the site of production for two major steroid hormones. Glucocorticoids raise the level of glucose in the blood. The main glucocorticoid, cortisol, is secreted when the body is stressed and blood sugar levels drop; cortisol promotes gluconeogenesis, a mechanism for making glucose from amino acids derived from protein breakdown. Cortisol also dampens the uptake of glucose from the blood, stimulates the breakdown of fats for energy, and suppresses inflammation. Hypoglycemia can result when the adrenal cortex makes too little cortisol; this results in chronically low glucose levels in the blood. Mineralocorticoids regulate the concentrations of minerals such as K+ and Na+ in the extracellular fluid; aldosterone is one example that works in the nephrons of the kidneys. The adrenal cortex also secretes sex hormones in the fetus and at puberty.
Click here for the Animation: Control of Cortisol Secretion. Please make sure that your sound is on and your volume is up.
Hormones from the adrenal medulla help regulate blood circulation. The inner part of the adrenal gland, the adrenal medulla, secretes epinephrine and norepinephrine. Secretion by the adrenal medulla influences these molecules to behave like hormones to regulate blood circulation and carbohydrate use during stress.
Long-term stress can damage health. Stress triggers the fight-flight response and the release of cortisol, epinephrine, and norepinephrine; constant release of these molecules can contribute to hypertension and cardiovascular disease. Excess cortisol suppresses the immune system, making individuals susceptible to disease. Social connections for support and exercise for health can reduce the effects of stress.
The Pancreas: Regulating Blood Sugar
The pancreas has both exocrine and endocrine functions; the endocrine cells are located in clusters called pancreatic islets.
Each pancreatic islet secretes three hormones:
Click here for the Animation: Hormones and Glucose Metabolism. Please make sure that your sound is on and your volume is up.
Disorders of Glucose Homeostasis
Diabetes mellitus is a disease resulting from the secretion of too little insulin. Without insulin, cells can’t remove glucose from the blood; the kidneys remove the excess in urine, creating imbalances in water-solute concentrations. Metabolic acidosis, a lower than optimal blood pH, can result because of this imbalance.
In type 1 diabetes (also known as “juvenile-onset diabetes”) the insulin is no longer produced because the beta cells have been destroyed by an autoimmune response. Only about 1 in 10 diabetics have this form of diabetes. Treatment is by insulin injection.
Type 2 diabetes is a global health crisis. In type 2 diabetes the insulin levels are near normal but the target cells cannot respond to the hormone. Beta cells eventually break down and produce less and less insulin. Excess glucose in the blood damages capillaries. Cardiovascular disease, stroke, heart attack, and other serious complications arise.
Metabolic syndrome is a warning sign. Prediabetes describes individuals with slightly elevated blood sugar levels that have an increased risk for developing type 2 diabetes; about 20 million Americans fall into this category and do not know it. A composite of features collectively called metabolic syndrome also describe risk for diabetes; these features include: “apple shaped” waistline, elevated blood pressure, low levels of HDL, and elevated glucose and triglycerides. Type 2 diabetes can be controlled with a combination of improved diet, exercise, and sometimes drugs.
Some Final Examples of Integration and Control
Light/dark cycles influence the pineal gland, which produces melatonin. Located in the brain, the pineal gland is a modification of a primitive “third eye” and is sensitive to light and seasonal influences; this gland secretes the hormone melatonin. Melatonin is secreted in the dark, and levels change with the seasons. The biological clock seems to tick in synchrony with day length and is apparently influenced by melatonin. Seasonal affective disorder (SAD) affects persons during the winter and may result from an out-of-sync biological clock; melatonin makes it worse; exposure to intense light helps. Melatonin levels may potentially be linked to the onset of puberty.
Hormones also are produced in the heart and GI tract. Atrial natriuretic peptide (ANP) produced by the heart atria regulates blood pressure. Gastrin and secretin from the GI tract stimulate release of stomach and intestinal secretions.
Prostaglandins have many effects. More than 16 prostaglandins have been identified in tissues throughout the body. When stimulated by epinephrine and norepinephrine, prostaglandins cause smooth muscles in blood vessels to constrict or dilate. Allergic responses to dust and pollen may be aggravated by the effects of prostaglandins on airways in the lungs. Prostaglandins have major effects on menstruation and childbirth.
Growth factors influence cell division. Hormonelike proteins called growth factors influence growth by regulating the rate of cellular division. Epidermal growth factor (EGF) influences the growth of many cell types, as does insulinlike growth factor (IGF). Nerve growth factor (NGF) promotes growth and survival of neurons in the developing embryo. The current list of growth factors is expanding rapidly; many of these factors may have applications in medicine.
Pheromones may be important communication molecules in humans. Pheromones are released outside of the body by several animals to serve as sex attractants, territory markers, and communication signals. Recent studies suggest that humans also may communicate using pheromones.
Are endocrine disrupters at work? Endocrine disrupters are proposed to be environmental substances that interfere with reproduction or development. Sperm counts in males in Western countries declined about 40% between the years 1938 and 1990, possibly due to exposure to estrogens in the environment.
Click here for the Video: Hormone-Induced Adjustments. Please make sure that your sound is on and your volume is up.
The Nervous system and the Endocrine System
I. Types of nervous tissue
A) Neurons – conduct nerve impulses
1. sensory neurons
B) Glia – do not conduct impulses, they provide support to neurons
C) Anatomy of a neuron
1. cell body
II. Properties of the Neuronal membranes
A) resting membrane potential
B) action potential
V. Organization of the Nervous System
A) Central Nervous system
B) Peripheral Nervous system – everything outside of the Central Nervous system
VII. Disorders of the Nervous system
A) Parkinson’s disease
VIII. The Brain and Drugs
IX. The Endocrine System