Muscle Tissue Overview Outline PDF

MUSCLE TISSUE OVERVIEW OUTLINE I. OVERVIEW OF MUSCLE TISSUE A. Types of Muscular Tissue 1. Skeletal Muscle: a. Location: skeleton b. Function: body movement, heat production, posture c. Appearance: long, slender, striated, multi...

MUSCLE TISSUE OVERVIEW OUTLINE I. OVERVIEW OF MUSCLE TISSUE A. Types of Muscular Tissue 1. Skeletal Muscle: a. Location: skeleton b. Function: body movement, heat production, posture c. Appearance: long, slender, striated, multinucleated fibers d. Control: voluntary 2. Cardiac Muscle a. Location: only in the heart wall b. Function: pump blood c. Appearance: branched, striated fibers with central nucleus d. Control: involuntary e. Cardiac muscle fibers contract when stimulated by their own autorhythmic fibers. f. Several hormones and neurotransmitters can adjust heart rate by speeding or slowing the pacemaker. 3. Smooth Muscle a. Location: walls of hollow internal structures b. Function: movement of substances within body c. Appearance: spindle-shaped fibers, no striations, central nucleus d. Control: involuntary B. Functions of Muscular Tissue 1. Producing body movements 2. Stabilizing body positions 3. Storing and moving substances within the body 4. Generating heat C. Properties of Muscular Tissue 1. Electrical excitability = ability to respond to certain stimuli by production of action potentials 2. Contractility = ability to contract when stimulated by an action potential 3. Extensibility = ability to stretch without being damaged 4. Elasticity = ability to return to its original length after contraction or extension II. SKELETAL MUSCLE TISSUE A. Connective Tissue Components B. 1. Endomysium, perimysium, epimysium extend from fascia to encase individual muscles and their components 2. Epimysium surrounds entire muscle 3. Perimysium surrounds bundles of muscle fibers called fascicles 4. Endomysium separates individual muscle fibers 5. Fascia is a sheet or band of dense irregular connective tissue that is deep to the skin and surrounds muscles and other organs of the body. a. These connective tissue layers often extend beyond the muscle and connect to bones by forming rope-like structures called tendons. b. An aponeurosis is a flat sheet of connective tissue that connects one muscle to another muscle or to bone. D. C. Nerve and Blood Supply 1. The neurons that stimulate skeletal muscles to contract are called somatic motor neurons. 2. Capillaries are plentiful in muscular tissue (Figure 10.9d) D. Microscopic Anatomy of a Skeletal Muscle Fiber E. 1. Sarcolemma = plasma membrane of muscle cell. 2. Transverse (T) tubules are tiny invaginations of the sarcolemma that quickly spread the muscle action potential to all parts of the muscle fiber. 3. Sarcoplasm is the muscle cell cytoplasm and contains a large amount of glycogen for energy production and myoglobin for oxygen storage. 4. Myofibrils = threadlike contractile organelles within muscle fibers. 5. The sarcoplasmic reticulum encircles each myofibril, is similar to smooth endoplasmic reticulum in non-muscle cells, and stores calcium ions that are released to trigger muscle contraction. 6. Terminal cisterns = dilated end sacs of the sarcoplasmic reticulum. 7. Triad = one transverse tubule + the two terminal cisterns on either side.  8. Muscular hypertrophy after birth occurs by enlargement of existing muscle fibers, due to increased production of myofibrils, sarcoplasmic reticulum, mitochondria, and other organelles. 9. Fibrosis = replacement of muscle fibers by scar tissue that may occur after skeletal muscle damage or degeneration. 10. Atrophy = decrease in size of muscle fibers due to progressive loss of myofibrils. 11. Filaments (myofilaments) are the contractile protein structures within myofibrils, which are arranged in compartments called sarcomeres (Table 10.1). a. The darker middle portion of a sarcomere is the A band, which extends the entire length of the thick filaments. b. The lighter ends of the sarcomeres are the I bands, which consist of thin filaments only. c. The Z disc passes through the center of the I band, and separates one sarcomere from the next. d. The narrow H zone in the center of each A band contains thick but not thin filaments. e. Supporting proteins that hold the thick filaments together at the center of the H zone form the M line. D. Muscle Proteins (Table 10.2) 1. Contractile proteins (Figure 10.4) a. Myosin = main component of thick filaments b. Actin = main component of thin filaments 2. Regulatory proteins a. Include tropomyosin and troponin, both are part of the thin filament.  b. In a relaxed muscle, tropomyosin (which is attached to troponin) blocks the myosin-binding sites on actin preventing myosin from binding to actin. c. In a contracting muscle, calcium binds to troponin, causing tropomyosin to move away from the myosin-binding sites on actin, allowing myosin to bind to actin. 3. Structural proteins a. Titin connects Z discs with M lines, helping sarcomeres return to their resting length after a muscle has contracted or been stretched. b. Nebulin anchors thin filaments to the Z discs. c. Alpha-actinin: binds to actin and titin in Z discs. d. Myomesin: forms the M-line, connecting thick filaments together. e. Dystrophin links thin filaments to the sarcolemma (Figure 10.2d). III. CONTRACTION AND RELAXATION OF SKELETAL MUSCLE FIBERS A. The sliding filament mechanism: During muscle contraction, myosin cross bridges pull on actin filaments, causing them to slide inward toward the H zone; Z discs come toward each other and the sarcomere shortens, but the myosin and actin filaments do not change in length. The sliding of filaments and shortening of sarcomeres causes the shortening of the whole muscle fiber and ultimately the entire muscle. This is called the sliding filament mechanism. B. Steps in the Contraction Cycle 1. ATP hydrolysis: myosin heads hydrolyze ATP and become reoriented and energized.  2. Attachment of myosin to actin to form cross-bridges. 3. Power stroke: the cross-bridge rotates, generating force and releasing ADP. 4. Detachment of myosin from actin: as ATP binds to the ATP-binding site on the myosin head, the myosin head detaches from actin. C. Excitation-Contraction Coupling 1. Ca2+ is released from the sarcoplasmic reticulum (figure 10.7a,b). 2. Ca2+ binds to troponin, causing tropomyosin to move away from the myosin-binding sites on actin, allowing myosin to bind to actin. 3. Ca2+active transport pumps move Ca2+ back into the sarcoplasmic reticulum. 4. Calesequestrin binds to Ca2+ inside the sarcoplasmic reticulum. 5. Rigor Mortis: After death, Ca2+ leaks out of the sarcoplasmic reticulum, allowing myosin heads to bind to actin. ATP is not synthesized after breathing stops, so myosin cannot detach from actin. D. Length-Tension Relationship 1. The amount of tension (force) developed by a muscle depends on the length of the sarcomeres within a muscle before contraction begins. 2. Maximal tension is developed when there is an optimal zone of overlap between the thick and thin filaments. E. The Neuromuscular Junction (NMJ) 1. The neuromuscular junction is the synapse between a somatic motor neuron and a skeletal  muscle fiber. 2. A synapse is a region where communication occurs between a neuron and a target cell. 3. At the NMJ, neurotransmitter molecules called acetylcholine (ACh) are released by exocytosis from synaptic vesicles inside the synaptic end bulbs of the somatic motor neuron, after an action potential traveling down the neuron causes calcium ions to enter the synaptic end bulbs. 4. The ACh diffuses across the synaptic cleft and binds to ACh receptors on the motor end plate of the muscle fiber, causing the opening of ion channels that allow sodium ions to enter the muscle fiber, creating a muscle action potential. 5. The muscle action potential releases calcium ions from the sarcoplasmic reticulum that combine with troponin, causing it to pull on tropomyosin to change its orientation, thus exposing myosin-binding sites on actin (Figure 10.9a) and allowing actin and myosin to bind together. 6. When action potentials in the motor neuron cease, ACh is no longer released, and an enzyme called acetylcholinesterase (AChE) breaks down the ACh present in the synaptic cleft, ending the production of muscle action potentials. 7. Without muscle action potentials causing release of calcium ions from the sarcoplasmic reticulum, active transport pumps return calcium ions to the sarcoplasmic reticulum, ending muscle contraction. 8. Electromyography – measures the electrical activity of muscle cells. F. Summary of events of contraction and relaxation in a skeletal muscle fiber (Figure 10.10) 1. Nerve impulse arrives at synaptic end bulbs of motor neuron, causing channels to open that allow calcium ions to enter, causing synaptic vesicles to release ACh into the synaptic cleft. 2. (Please make sure to go over the remaining steps in the figure.) IV. MUSCLE METABOLISM A. Sources of ATP in muscle fibers 1. Stored ATP a. The ATP present inside muscle fibers is enough to power contraction for only a few seconds. Muscle fibers have three ways to produce additional ATP: 2. Creatine phosphate a. Since the formation of ATP from creatine phosphate occurs very rapidly, creatine phosphate is the first source of energy when muscle contraction begins.  b. Stores of creatine phosphate and ATP can power maximal muscle contraction for about 15 seconds (Figure 10.11). 3. Anaerobic glycolysis a. The partial catabolism of glucose to generate ATP occurs in the cytosol, and can occur whether oxygen is present (aerobic conditions) or absent (anaerobic conditions). b. Produces two molecules of ATP per molecule of glucose. c. This system can provide enough energy for about 2 minutes of maximal muscle activity. d. Under anaerobic conditions, the pyruvic acid generated from glycolysis is converted into lactic acid. The accumulation of lactic acid is thought to be responsible for the muscle soreness that is felt during strenuous exercise. 4. Aerobic respiration a. If sufficient oxygen is present, the pyruvic acid formed by glycolysis enters the mitochondria, where it undergoes aerobic respiration, a series of oxygen-requiring reactions (the Krebs cycle and electron transport chain) that produce ATP, carbon dioxide, water, and heat. b. Produces 30-32 molecules of ATP per molecule of glucose. c. Muscular activity lasting more than several minutes depends increasingly on aerobic respiration. d. Muscle tissue has two sources of oxygen: diffusion from blood and release by myoglobin inside muscle fibers. e. The aerobic system will provide enough ATP for prolonged activity (rest or light to moderate exercise) so long as sufficient oxygen and nutrients are available. B. Muscle Fatigue 1. Tiredness before actual muscle fatigue occurs is caused by caused by changes in the central nervous system, called central fatigue. 2. Factors that are thought to contribute to muscle fatigue include buildup of lactic acid and ADP, decreased release of calcium ions from the SR, depletion of creatine phosphate and glycogen, and decreased ACh release from the motor neuron. C. Oxygen Consumption after Exercise 1. Oxygen debt (recovery oxygen uptake) refers to elevated oxygen consumption after exercise, which is due to the following factors: a. Replenishment of creatine phosphate and ATP stores. e. Conversion of lactic acid into pyruvic acid.  f. Replacement of the oxygen removed from myoglobin. g. Elevation in body temperature after strenuous exercise. h. Tissue repair processes occurring at a faster pace. V. CONTROL OF MUSCLE TENSION A. Motor units = a somatic motor neuron plus all the muscle cells it innervates (Figure 10.12) 1. A single motor unit may innervate as few as 10 or as many as 3,000 muscle fibers, with an average of 150 fibers being innervated by each motor neuron. B. Twitch contraction 1. The brief contraction of all muscle fibers in a motor unit in response to a single action potential in its motor neuron. 2. Muscle contractions are recorded by a myogram (Figure 10.13) a. Latent period: the delay following an action potential and the onset of contraction. b. Contraction period: Ca2+ binds to troponin and peak contraction is developed. c. Relaxation period: Ca2+ is transported back into the sarcoplasmic reticulum. d. Refractory period: the period of lost excitability. C. Frequency of stimulation 1. Wave summation: When a second stimulus occurs before the muscle has relaxed, the second contraction will be stronger than the first (Figure 10.14b). 2. Tetanus: unfused or incomplete - sustained but wavering contraction. 3. Fused or complete tetanus – individual twitches cannot be detected (Figure 10.14d). D. Motor Unit Recruitment – the process in which the number of active motor units increases. 1. Clinical connection: Anaerobic training vs Aerobic training  E. Muscle Tone 1. Even at rest, a skeletal muscle exhibits muscle tone, or a small amount of tension 2. Hypotonia = decreased or lost muscle tone 3. Hypertonia = increased muscle tone F. Isotonic and Isometric Contractions G. 1. Isotonic contraction – tension is almost constant while the muscle changes length a. Concentric isotonic contraction – when the length of the muscle shortens and the object is moved (Figure 10.15a). b. Eccentric isotonic contraction – when the length of the muscle increases during contraction (Figure 10.15b). 2. Isometric contraction – muscle contracts but does not change length (Figure 10.15c). VI. SKELETAL MUSCLE FIBER TYPES A. Types of skeletal muscle fibers (Table 10.4) 1. Color varies according to the content of myoglobin, an oxygen-storing reddish pigment. Red muscle fibers have a high myoglobin content while the myoglobin content of white muscle fibers is low. 2. Contraction velocity, fiber diameter, method of ATP production, number of mitochondria and blood capillaries, and resistance to fatigue also differs between fibers.  3. 4. Slow oxidative (SO) fibers a. Small, dark red (due to large amounts of myoglobin and many blood capillaries), fatigue resistant. b. Contain low amounts of glycogen. 4. Fast oxidative-glycolytic fibers a. Typically largest in size, dark red (due to large amounts of myoglobin and many blood capillaries), moderately resistant to fatigue. b. Contain intermediate amounts of glycogen. 5. Fast glycolytic (FG) fibers a. White (due to low amounts of myoglobin and few blood capillaries), fatigue quickly. c. Contain large amounts of glycogen. VII. EXERCISE AND SKELETAL MUSCLE TISSUE A. Relative ratio of fast glycolytic and slow oxidative fibers in each muscle is genetically determined. B. Total number of skeletal muscle fibers does not usually increase with exercise C. Endurance (aerobic) exercises can transform some FG fibers into FOG fibers. D. Exercises that require great strength for short periods produce an increase in the size and strength of FG fibers. E. Stretching is most effective when the muscles are warm. F. Strength training increases the deposition of bone minerals in young adults, and raises resting metabolic rate by increasing muscle mass. VIII. CARDIAC MUSCLE TISSUE (Figure 20.9) IX. A. Contains sarcomeres similar to skeletal muscle. B. Has intercalated discs – contain desmosomes and gap junctions that allow muscle action potentials to spread from one muscle fiber to another (Figure 4.2e) C. Contractions last 10-15 times longer than skeletal muscle due to prolonged delivery of calcium into the sarcoplasm. D. Mitochondria are larger and more numerous than in skeletal muscle fibers. X. SMOOTH MUSCLE TISSUE XI. A. Usually activated involuntarily, by the autonomic nervous system, stretch, hormones, or local factors such as changes in pH, temperature, etc. B. Visceral (single unit) smooth muscle tissue – contracts as one single unit, similar to cardiac muscle (Figure 10.16a). C. Multiunit smooth muscle tissue – consists of individual fibers that contract independently D. E. Fibers are thick in the middle and tapered at the end (Figure 10.16c). F. Contain intermediate filaments in addition to thick and thin filaments. G. Filaments are connected to structures called dense bodies, which are functionally similar to Z discs (Figure 10.16c). H. Contraction and relaxation are regulated by calmodulin and myosin light chain kinase.  I. Smooth muscle tone is maintained due to the prolonged presence of calcium in the cytosol. J. When smooth muscle fibers are stretched, they first contract and develop increased tension, but then relax. This is called the stress-relaxation response. XII. REGENERATION OF MUSCLE TISSUE A. Growth of skeletal muscle after birth is due mainly to hypertrophy, the enlargement of existing cells, rather than hyperplasia, an increase in the number of fibers. B. Cardiac muscle fibers can undergo hypertrophy in response to increased workload, and can regenerate under certain circumstances. C. Smooth muscle fibers can undergo hypertrophy and have a limited capacity for regeneration XIII. AGING AND MUSCULAR TISSUE K. Between the ages of 30 and 50, humans undergo a slow, progressive loss of skeletal muscle mass that is replaced largely by fibrous connective tissue and adipose tissue. L. Lower limb muscles atrophy before the upper limbs. SKELETAL MUSCLE OUTLINE I. HOW SKELETAL MUSCLES PRODUCE MOVEMENT A. Muscle Attachment Sites: Origin and Insertion 1. Skeletal muscles produce movements by exerting force on tendons, which in turn pull on bones or other structures, such as skin. 2. Most muscles cross at least one joint and are attached to the articulating bones that form the joint (Figure 11.1a).  3. 4. When a muscle contracts, one articulating bone remains stationary or near its original position, either because other contracting muscles stabilize that bone or because its structure makes it less movable. a. The attachment to the stationary bone is the origin. b. The attachment to the movable bone is the insertion. c. The fleshy part of the muscle between the tendons is the belly (body). d. The actions of a muscle are the main movements that occur when the muscle contracts. e. Some muscles are capable of reverse muscle action (RMA), in which the positions of the origin and insertion are switched. B. Lever Systems and Leverage 1. Bones serve as levers and joints serve as fulcrums. 2. The lever is acted on by two different forces: resistance (load) and effort (Figure 11.1b). 3. Levers are categorized into three types, according to the position of the fulcrum, effort, and load a. first-class (EFL) (Figure 11.2a) - the fulcrum is between the effort and the load. An example is pair of scissors. b. second-class (ELF) (Figure 11.2b) - the load is between the fulcrum and effort. An example is a wheelbarrow.  c. third-class (FEL) (Figure 11.2c) - the effort is between the fulcrum and the load. An example is a pair of forceps. 4. Mechanical advantage: a relatively small effort is required to move a large load (over a small distance). 5. Mechanical disadvantage: a relatively large effort is required to move a small load (over a large distance; that is, at a greater speed). C. Effects of Fascicle Arrangement 1. Skeletal muscle fibers (cells) are arranged within the muscle in bundles called fascicles. 2. The muscle fibers are arranged in a parallel fashion within each bundle, but the arrangement of the fascicles with respect to the tendons may take one of five characteristic patterns: parallel, fusiform, circular, triangular, and pennate (Table 11.1). 3. Fascicular arrangement is correlated with the power of a muscle and the range of motion. 4. Clinical Connection: intramuscular injections D. Coordination Within Muscle Groups 1. Most movements are coordinated by several skeletal muscles acting in groups rather than individually, and most skeletal muscles are arranged in opposing (antagonistic) pairs at joints. 2. A muscle that causes a desired action is referred to as the prime mover (agonist); the antagonist produces an opposite action. 3. Most movements also involve muscles called synergists, which serve to steady a movement, thus preventing unwanted movements and helping the prime mover function more efficiently.  4. Some synergist muscles in a group also act as fixators, which stabilize the origin of the prime mover so that it can act more efficiently. 5. Under different conditions and depending on the movement and which point is fixed, many muscles act, at various times, as prime movers, antagonists, synergists, or fixators. 6. In the limbs, a compartment contains a group of skeletal muscles with a common function. 7. Clinical Connection: benefits of stretching II. HOW SKELETAL MUSCLES ARE NAMED A. Muscle naming involves many categories such as (table 11.2): 1. Location 2. Size 3. Number of origins 4. Shape 5. Direction of fibers 6. Origin and insertion 7. Muscle action III. PRINCIPAL SKELETAL MUSCLES AND MAJOR ACTIONS (You only need to know the following muscles and actions for the exam; you do not need to know the origins, insertions, and innervation) A. Muscles of the head that produce facial expressions (figure 11.4) 1. Orbicularis oris: closes and protrudes lips 2. Orbicularis oculi: closes eye 3. Bell’s palsy: unilateral paralysis of the muscles of facial expression B. Muscles that move the mandible and assist in mastication 1. Masseter: elevates mandible 2. Temporalis: elevates and retracts mandible 3. Clinical Connection: gravity and the mandible C. Muscles of the neck that move the head 1. Sternocleidomastoid: acting together, flexes neck; acting singly, laterally flexes neck and rotates head D. Muscles of the abdomen that protect abdominal viscera and move the vertebral column 1. Rectus abdominis: flexes vertebral column and compresses abdomen 2. External oblique: acting together, flexes vertebral column and compresses abdomen; acting singly, laterally flexes and rotates vertebral column  3. Internal oblique: acting together, flexes vertebral column and compresses abdomen; acting singly, laterally flexes and rotates vertebral column 4. Transversus abdominis: compresses abdomen 5. Inguinal hernia: rupture or separation of a portion of the inguinal area of the abdominal wall resulting in the protrusion of a part of the small intestine E. Muscles of the thorax that assist in breathing 1. Diaphragm: increases vertical dimension of thoracic cavity, causing inhalation F. Muscle of the thorax that move the pectoral girdle 1. Anterior thoracic muscles a. Serratus anterior: abducts and upwardly rotates scapula (RMA: elevates ribs when scapula is stabilized) 2. Posterior thoracic muscles a. Trapezius: superior fibers rotate scapula upwards; middle fibers adduct scapula; inferior fibers depress and rotate scapula upwards (RMA: superior fibers can help extend head) G. Muscles of the thorax and shoulder that move the humerus 1. Axial muscles that move the humerus a. Pectoralis major: as a whole, adducts and medially rotates arm; clavicular head flexes arm, and sternocostal head extends flexed arm b. Latissimus dorsi: extends, adducts, and medially rotates arm (RMA: elevates vertebral column and ribs 2. Scapular muscles that move the humerus (subscapularis, supraspinatus, infraspinatus, and teres minor are the rotator cuff muscles, which join the scapula to the humerus) a. Deltoid: lateral fibers abducts arm; anterior fibers flex and medially rotate arm; posterior fibers extend and laterally rotate arm b. Subscapularis: medially rotates arm c. Supraspinatus: abducts arm d. Infraspinatus: laterally rotates arm e. Teres major: extends arm; assists in adduction and medial rotation of arm f. Teres minor: laterally rotates and extends arm 3. Impingement syndrome: impingement of rotator cuff tendons where they pass through the subacromial space 4. Rotator cuff injury: strain or tear in the rotator cuff muscles H. Muscles of the arm that move the radius and ulna 1. Forearm flexors a. Biceps brachii: flexes and supinates forearm; flexes arm b. Brachialis: flexes forearm  c. Brachioradialis: flexes forearm; supinates and pronates forearm to neutral position 2. Forearm extensors a. Triceps brachii: extends forearm; extends arm 3. Forearm pronators a. Pronator teres: pronates forearm I. Muscles of the forearm that move the wrist, hand, thumb and digits 1. Superficial anterior (flexor) compartment of forearm a. Flexor carpi radialis: flexes and abducts hand b. Flexor carpi ulnaris: flexes and adducts hand c. Flexor digitorum superficialis: flexes fingers and hand d. Golfer’s elbow: strain of the flexor muscles in the forearm, causing pain near the medial epicondyle 2. Superficial posterior (extensor) compartment of forearm a. Extensor carpi radialis longus: extends and abducts hand b. Extensor digitorum: extends fingers and hand c. Extensor carpi ulnaris: extends and adducts hand 3. Carpal tunnel syndrome: condition due to compression of the median nerve within the carpal tunnel J. Muscles of the neck and back that move the vertebral column 1. Erector spinae muscle group consists of the iliocostalis, longissimus, and spinalis muscles: extend vertebral column and head 2. Clinical connection: Back injuries and heavy lifting K. Muscles of the gluteal region that move the femur 1. Psoas major: flexes and laterally rotates thigh; flexes trunk on hip 2. Iliacus: flexes and laterally rotates thigh; flexes trunk on hip 3. Gluteus maximus: extends and laterally rotates thigh (RMA: extends torso) 4. Gluteus medius: abducts and medially rotates thigh 5. Tensor fasciae latae: flexes and abducts thigh 6. Piriformis: laterally rotates and abducts thigh 7. Groin pull: rupture or tear of one or more of the five major muscles of the inner thigh L. Muscle of the thigh that move the femur, tibia and fibula 1. Medial (adductor) compartment of the thigh a. Adductor longus: adducts, flexes, and rotates thigh (RMA: extends thigh) b. Adductor magnus: adducts and rotates thigh; anterior part flexes thigh; posterior part extends thigh c. Gracilis: adducts and medially rotates thigh; flexes leg 2. Anterior (extensor) compartment of the thigh  a. Quadriceps femoris group (= rectus femoris + vastus lateralis + vastus medialis + vastus intermedius): all four heads extend leg; rectus femoris also flexes thigh 3. Posterior (flexor) compartment of the thigh a. Hamstrings group (= biceps femoris + semitendinosus + semimembranosus): flexes leg and extends thigh M. Muscles of the leg that move the foot and toes 1. Anterior compartment of the leg a. Tibialis anterior: dorsiflexes and inverts foot b. Extensor digitorum longus: dorsiflexes foot and extends toes c. Clinical connection: Shin splint syndrome 2. Lateral (fibular) compartment of the leg a. Fibularis (peroneus) longus: plantar flexes and everts foot 3. Superficial posterior compartment of the leg a. Gastrocnemius: plantar flexes foot and flexes leg 4. Deep posterior compartment of the leg a. Flexor digitorum longus: plantar flexes foot and flexes toes II-V NERVOUS SYSTEM OUTLINE I. OVERVIEW OF THE NERVOUS SYSTEM 1. The nervous system, along with the endocrine system, helps to keep controlled conditions within limits that maintain homeostasis. 2. The nervous system is responsible for our perceptions, behaviors, memories, and movements. 3. The branch of medical science that deals with the normal functioning and disorders of the nervous system is called neurology. A. Central Nervous System  1. The central nervous system (CNS) consists of the brain and spinal cord (Figure 12.1a). B. Peripheral Nervous System 1. The peripheral nervous system (PNS) consists of cranial and spinal nerves with sensory (afferent) and motor (efferent) components, ganglia, enteric plexuses, and sensory receptors. a. SAD DAVE: Sensory Afferent Dorsal, Dorsal=Afferent; Ventral=Efferent b. The sensory system consists of a variety of different receptors as well as sensory neurons. c. The motor system conducts nerve impulses from the CNS to muscles and glands. d. A nerve contains axons plus associated connective tissue and blood vessels that lies outside the central nervous system. e. Ganglia are small masses consisting primarily of neuron cell bodies. f. Enteric plexuses are networks of neurons located in the walls of organs of the gastrointestinal tract. g. A sensory receptor is a structure in the nervous system that monitors changes in the external or internal environment.  2. The PNS is also subdivided into somatic (voluntary), autonomic (involuntary), and enteric nervous systems (Figure 12.1b) 3. The somatic nervous system (SNS) consists of neurons that conduct impulses from somatic and special sense receptors to the CNS, and motor neurons that conduct impulses from the CNS to skeletal muscle tissue. 4. The autonomic nervous system (ANS) contains sensory neurons from visceral organs and motor neurons that convey impulses from the CNS to smooth muscle tissue, cardiac muscle tissue, and glands. a. The motor part of the ANS consists of the sympathetic division (which supports “fight-or-flight” responses) and the parasympathetic division (which supports “rest-and-digest” responses). 5. The enteric nervous system (ENS) consists of neurons in enteric plexuses that extend most of the length of the GI tract. a. Many neurons of the enteric plexuses function independently of the ANS and CNS to some extent. b. Sensory neurons of the ENS monitor chemical changes within the GI tract and stretching of its walls, whereas enteric motor neurons govern contraction and secretion of GI tract organs, and activity of the GI tract endocrine cells. C. Functions of the Nervous System 1. The sensory function of the nervous system is to sense changes in the internal and external environment through sensory receptors. Sensory neurons serve this function. 2. The integrative function is to analyze the sensory information, store some aspects, and make decisions regarding appropriate behaviors. Association or interneurons serve this function. 3. The motor function is to respond to stimuli by activating effectors (muscles and glands). Motor neurons serve this function. II. HISTOLOGY OF THE NERVOUS SYSTEM A. Parts of a neuron 1. Neurons have the property of electrical excitability. 2. Most neurons, or nerve cells, consist of a cell body (soma), many dendrites, and usually a single axon (Figure 12.2). 3. The cell body contains a nucleus, lysosomes, mitochondria, a Golgi complex, cytoplasmic inclusions such as lipofuscin, clusters of rough endoplasmic reticulum called Nissl bodies, microtubules, and neurofibrils. 4. The dendrites are the receiving or input portions of a neuron. 5. The axon propagates nerve impulses toward another neuron or to an effector organ of the body (muscle or gland). a. The axon often joins to the cell body at a cone-shaped elevation called the axon hillock. b. The part of the axon closest to the axon hillock is the initial segment. c. In most neurons, nerve impulses arise at the junction of the axon hillock and the initial segment, called the trigger zone.  d. The cytoplasm of the axon is called the axoplasm. e. The plasma membrane of the axon is called the axolemma. f. The axons end at fine processes called axon terminals. 6. The site of functional contact between two neurons or between a neuron and an effector cell is called a synapse. a. The tips of axon terminals may swell into synaptic end bulbs or varicosities, which contains tiny membrane-enclosed sacs containing neurotransmitters. 7. Slow axonal transport coveys axoplasm from the cell body toward the axon terminals. 8. Fast axonal transport uses proteins that function as motors to move materials along the surfaces of microtubules in both directions in the axon, either in an anterograde (forward) direction or in a retrograde (backward) direction. B. Structural diversity in neurons 1. Both structural and functional features are used to classify the various neurons in the body. 2. On the basis of structure, neurons are classified as multipolar, bipolar, and unipolar based on the number of processes extending from the cell body (Figure 12.3a,b,c). 3. Some neurons are named for the histologist who first described them (Purkinje cells) or named for their shape or appearance (pyramidal cells) (Figure 12.5) 4. On the basis of function, neurons are classified as sensory or afferent neurons, interneurons or association neurons, and motor or efferent neurons. C. Neuroglia 1. Neuroglia (or glia) are specialized tissue cells that support neurons, attach neurons to blood vessels, produce the myelin sheath around axons, and carry out phagocytosis. 2. Neuroglia of the CNS a. Astrocytes are the largest and most numerous neuroglia, whose functions include: 1) Microfilaments for strength; 2) Processes form the blood brain barrier; 3) Regulate growth, migration, and interconnection of neurons; 4) Maintain chemical environment for the generation of action potentials; 5) Learning and memory. b. Oligodendrocytes produce the myelin sheath around CNS axons. c. Microglial cells functions as phagocytes. d. Ependymal cells produce and assist the circulation of cerebrospinal fluid. 3. Neuroglia of the PNS a. Schwann cells produce the myelin sheath around PNS axons.  b. Satellite cells regulate the exchange of material between neuronal cell bodies and interstitial fluid. D. Myelination 1. The myelin sheath is a multilayered lipid and protein covering produced by Schwann cells and oligodendrocytes, which electrically insulates the axon and increases the speed of nerve impulse conduction (Figure 12.8a,b). 2. Schwann cells produce the myelin sheath in the PNS. a. The outer nucleated cytoplasmic layer of the Schwann cell, which encloses the myelin sheath, is called the neurolemma (sheath of Schwann) and is found only around axons in the PNS. b. The neurolemma aids in regeneration in an injured axon by forming a regeneration tube that guides and stimulates regrowth of the axon. c. The myelin sheath has gaps called nodes of Ranvier along the axon. 3. Oligodendrocytes form myelin sheaths for CNS axons. a. No neurolemma is formed. b. Little regrowth after injury occurs. E. Collections of nervous tissue 1. Clusters of neuronal cell bodies in the PNS are called ganglia. 2. Clusters of neuronal cell bodies in the CNS are called nuclei. 3. Nerves and tracts are groups of axons in the PNS and CNS, respectively.  4. Gray and white matter in brain and spinal cord a. White matter is composed primarily of myelinated axons, whereas gray matter contains neuron cell bodies, dendrites, axon terminals, unmyelinated axons, and neuroglia (Figure 12.9). b. In the spinal cord, gray matter forms an H-shaped inner core, surrounded by white matter; in the brain a thin outer shell of gray matter covers the surfaces of the largest portions of the brain. III. ELECTRICAL SIGNALS IN NEURONS A. Excitable cells communicate with each other by action potentials or graded potentials. B. Action potentials allow communication over long distances whereas graded potentials are used for short distances only. 1. Production of both types of potentials depend upon the existence of a resting membrane potential and the presence of certain types of ion channels. 2. The membrane potential is an electrical voltage across the membrane. 3. Graded and action potentials occur because of ion channels in the membrane that allow ion movement across the membrane that can change the membrane potential. a. Figure 12.10 depicts nervous system functions provided by the different signals acting through and on various cells in the pathway. C. Ion Channels 1. Leak channels are randomly alternate between open and closed positions (Figure 12.11a). 2. Voltage-gated channels respond to a change in the membrane potential (Figure 12.11d). 3. Ligand-gated channels respond to the binding of a specific chemical stimulus (Figure 12.11b).  4. Mechanically-gated ion channels respond to mechanical stimuli such as vibration, touch, or pressure (Figure 12.11c). D. Resting membrane potential 1. A typical value for the resting membrane potential is -70mV, wih the inside of the cell negative relative to the outside, and the membrane is said to be polarized (Figure 12.12b) 2. The resting membrane potential is determined by several factors (Figure 12.13). a. The unequal distribution of ions across the plasma membrane: Extracellular fluid has more Na+ than the cytosol, and the cytosol has more K+ than the extracellular fluid, due to the sodium-potassium pumps (Na-K ATPases) that pump sodium ions out of the cell and pump potassium ions into the cell. b. Because the plasma membrane typically has more K+ leak channels than Na+ leak channels, more potassium ions diffuse out of the neuron down their concentration gradient compared to the number of sodium ions that diffuse into the neuron down their concentration gradient, making the inside of the neuron more negative. c. Most anions cannot leave the interior of the cell to reduce the negative resting membrane potential because they are attached to large nondiffusible molecules such as ATP and proteins. E. Graded potentials 1. A graded potential is a small deviation from the resting membrane potential that makes the membrane either more polarized (hyperpolarization) or less polarized (depolarization) (Figure 12.14). 2. Graded potentials occur most often in the dendrites and cell body of a neuron.  3. Graded potentials occur as the result of opening of ligand-gated or mechanically gated channels (Figure 12.15) 4. The signals are graded, meaning they vary in amplitude (size) depending on the strength of the stimulus (Figure 12.16), and are localized, meaning that they die out as they spread from the stimulated area. 5. Graded potentials can be summated to create larger membrane voltage values (Figure 12.17) F. Generation of action potentials 1. An action potential (AP) or impulse is a sequence of rapidly occurring events that decrease and eventually reverse the membrane potential (depolarization) and then restore  it to the resting state (repolarization). 2. During an action potential, voltage-gated Na+ and K+ channels open in sequence (Figure 12.18). 3. Rapid opening of voltage-gated Na+ channels causes depolarization. If the depolarization is to threshold, an action potential is generated, and enough sodium rushes into the neuron so that the membrane potential reverses and becomes positive. Depolarization occurs due to the opening of the voltage-gated Na+ channel activation gates; the later closing of the Na+ channel inactivation gates contributes to  repolarization (Figures 12.18 and 12.20). 4. The slower opening of voltage-gated K+ channels and closing of the Na+ channel inactivation gates leads to repolarization, the recovery of the resting membrane potential. 5. Different stimulus strength will affect action potential generation (Figure 12.19) a. A subthreshold stimulus does not generate an action potential b. A threshold stimulus generates an action potential c. A suprathreshold stimulus generates an action potential and often more than one action potential. d. The amplitude (size) of an action potential is always the same, regardless of the strength of the stimulus. e. An action potential is generated in response to a threshold stimulus, but does not form when there is a subthreshold stimulus. This is called the all-or-none principle. 6. During the refractory period (Figure 12.20), another impulse cannot be generated at all (absolute refractory period) or can be triggered only by a suprathreshold stimulus (relative refractory period). G. Propagation of action potentials 1. An action potential conducts or propagates (travels) from point to point along the membrane; the traveling action potential is a nerve impulse. 2. Clinical Connection: Local anesthetics and certain neurotoxins prevent opening of voltage-gated Na+ channels so nerve impulses cannot pass the obstructed region. 3. The step-by-step depolarization of each adjacent area of the plasma membrane is called continuous conduction (Figure 12.21a). Nerve impulse conduction in which the impulse jumps from node to node in a myelinated axon is called saltatory conduction (Figure 12.21b). 4. The propagation speed of a nerve impulse is not related to stimulus strength. 5. 6. Myelinated fibers conduct impulses faster than unmyelinated fibers. 7. Larger-diameter fibers conduct impulses faster than those with smaller diameters. 8. Nerve fibers conduct impulses faster when warmed and slower when cooled. 9. Neurons are classified by diameter and myelination, and therefore conduction speed. a. A fibers are the largest-diameter axons and are myelinated. b. B fibers are medium-diameter axons and are myelinated. c. C fibers are the smallest-diameter axons and all are unmyelinated. 10. Encoding of stimulus intensity: The intensity of a stimulus is coded in the frequency of action potentials. H. Comparison of electrical signals produced by excitable cells. 1. Nerve and muscle cells differ in the resting membrane potential, duration of the action potentials, and propagation speed of the action potentials.  2. Graded potentials are not propagated, so can function only in short-distance communication, whereas action potentials are propagated, and are therefore useful in long-distance communication. 3. The various differences between graded potentials and action potentials are summarized in Table 12.2. IV. SIGNAL TRANSMISSION AT SYNAPSES A. A synapse is the functional junction between one neuron (presynaptic neuron) and another (postsynaptic neuron) or between a neuron and an effector such as a muscle or gland. B. Electrical Synapses 1. At an electrical synapse, ionic current spreads directly from one cell to another through gap junctions. 2. Electrical synapses allow faster communication and can synchronize the activity of a group of neurons or muscle fibers. C. Chemical Synapses 1. At a chemical synapse, there is only one-way information transfer from a presynaptic neuron to a postsynaptic neuron (Figure12.23). 2. In response to a nerve impulse, the presynaptic neuron releases neurotransmitters that diffuse across the synaptic cleft and bind to receptors on the plasma membrane of the postsynaptic neuron, producing a graded potential called a postsynaptic potential. The time required for these processes is called the synaptic delay. D. Excitatory and inhibitory postsynaptic potentials 1. Neurotransmitters at chemical synapses cause either an excitatory or inhibitory graded potential.  2. A neurotransmitter that causes depolarization of the postsynaptic membrane is excitatory because it brings the membrane closer to the threshold for an action potential. A depolarizing postsynaptic potential is called an excitatory postsynaptic potential (EPSP). 3. A neurotransmitter that causes hyperpolarization of the postsynaptic membrane is inhibitory because it moves the membrane potential further from the threshold for an action potential. A hyperpolarizing postsynaptic potential is called an inhibitory postsynaptic potential (IPSP). E. Structure of Neurotransmitter Receptors 1. Neurotransmitter receptors have two types (Figure 12.24) a. Ionotropic receptors are ion channels that directly bind to the neurotransmitter. b. Metabotropic receptors contain a neurotransmitter binding site but not an ion channel. Neurotransmitter binding activates a G protein, which in turn opens or closes an ion channel or activates second messenger molecules in the cytosol. F. Removal of neurotransmitter 1. Neurotransmitter is removed from the synaptic cleft in three ways: diffusion, enzymatic degradation, and uptake into cells (neurons and glia). G. Spatial and temporal summation of postsynaptic potentials 1. Spatial summation results from the buildup of neurotransmitter released simultaneously by several or more different presynaptic end bulbs (Figure 12.25a). 2.  3. Temporal summation can result from the buildup of neurotransmitter released by the same presynaptic end bulb in rapid succession (Figure 12.25b). 4. 5. Clinical Connection: Strychnine poisoning demonstrates the importance of inhibitory neurons. V. NEUROTRANSMITTERS A. Both excitatory and inhibitory neurotransmitters are present in the CNS and PNS; the same neurotransmitter may be excitatory in some locations and inhibitory in others. 1. Many neurotransmitters are also hormones released by endocrine glands. 2. Within the brain, certain neurons, called neurosecretory cells, also secrete hormones. B. Small-molecule neurotransmitters 1. Acetycholine (ACh) = excitatory neurotransmitter at neuromuscular junctions 2. Amino Acids a. Glutamate = neurotransmitter released by most excitatory neurons in the CNS b. Aspartate = excitatory c. Gamma aminobutyric acid (GABA) = most common inhibitory neurotransmitter in CNS; opens Cl- channels; antianxiety drugs such as Valium enhance the action of GABA. d. Glycine = inhibitory; opens Cl- channels  3. Biogenic amines a. Norepinephrine (NE): involved in arousal, dreaming. b. Epinephrine c. Dopamine (DA): involved in emotion, addictive behaviors, and movement. d. Serotonin (5-HT): involved in mood, appetite, temperature regulation, and induction of sleep. Selective serotonin reuptake inhibitors are drugs that may provide relief from some forms of depression. e. Norepinephrine, dopamine, and epinephrine are classified chemically as catecholeamines, which are broken down by the enzymes called catechol-O- methyltranferase (COMT) and monoamine oxidase (MAO). f. Amphetamines promote release of dopamine and norepinephrine. 4. ATP and other Purines 5. Nitric Oxide 6. Carbon Monoxide C. Neuropeptides 1. Enkephalins 2. Endorphins 3. Dynorphins 4. Substance P: enhances perception of pain. 5. Enkephalins, endorphins, and dynorphins are called opioid peptides, and are thought to be the body’s natural painkillers. VI. NEURAL CIRCUITS A. Neurons in the CNS are organized into patterns called neural circuits. B. Simple series circuit: A single presynaptic neuron stimulates a single postsynaptic neuron.  C. Diverging circuit: A single presynaptic neuron stimulates several postsynaptic neurons. D. Converging circuit: A single postsynaptic neuron receives input from several presynaptic neurons. E. Reverberating circuit: Branches from later neurons synapse with earlier neurons, sending impulses back through the circuit again and again. VII. A. Parallel after-discharge circuit: A single presynaptic neuron stimulates multiple groups of neurons, each of which synapses with a common postsynaptic neuron. VIII. REGENERATION AND REPAIR OF NERVOUS TISSUE A. Throughout life, the nervous system exhibits plasticity, the capability for change based on experience. B. Despite plasticity, neurons have a limited capacity to repair or replicate themselves.  C. In the PNS, damage to dendrites and myelinated axons may be repaired if the cell body remains intact, if Schwann cells are active, and if scar tissue formation does not occur too rapidly. D. In the CNS, there is little or no repair of damage to neurons. Although, epidermal growth factor was shown to stimulate neurogenesis in the brains of adult mice. E. Damage and repair in the peripheral nervous system (figure 12.29) 1. When there is damage to an axon, the Nissl bodies in the cytosol break down in a process called chromatolysis. 2. By the third to fifth day, degeneration of the distal portion of the axon and myelin sheath occurs (Wallerian degeneration); afterward, macrophages phagocytize the remains. 3. The Schwann cells may form a regeneration tube through which the new axon may grow. SPINAL NERVES AND REFLEXES OUTLINE I. INTRODUCTION A. The spinal cord is the pathway for sensory input to the brain and motor output from the brain. B. The spinal cord and spinal nerves mediate reactions to environmental changes through quick, automatic responses called reflexes. II. SPINAL CORD ANATOMY A. The spinal cord is protected by the vertebral column, three connective tissue membranes called meninges (including the dura mater, arachnoid mater, and pia mater), and cushions of fat in the epidural space and cerebrospinal fluid in the subarachnoid space. 1. The vertebral column provides a bony covering of the spinal cord (Figure 13.1). 2. Meninges a. The protective meninges are three coverings that run continuously around the spinal cord and brain (Figures 13.1, 14.2, respectively). b. The tough outermost layer is the dura mater, which consists of dense irregular connective tissue. c. The middle layer is the arachnoid mater, which contains loosely arranged collagen and elastic fibers. d. The innermost layer is the pia mater, which adheres to the surface of the spinal cord and brain, and attaches to the arachnoid mater and dura mater via extensions called denticulate ligaments that anchor the spinal cord within its dural sheath. 3. The subarachnoid space carries cerebrospinal fluid (CSF). 4. Clinical Connection: A spinal tap is done to withdraw CSF for diagnostic purposes. B. External Anatomy of the Spinal Cord 1. The spinal cord begins as a continuation of the medulla oblongata and terminates at about the second lumbar vertebra in an adult (Figure 13.2).  2. It contains cervical and lumbar enlargements that serve as points of origin for nerves to the extremities. 3. The tapered lower end of the spinal cord is the conus medullaris, from which arise the filum terminale (anchors spinal cord to coccyx) and cauda equina (“horse’s tail” formed by roots of lumbar, sacral, and coccygeal spinal nerves). C. Internal Anatomy of the Spinal Cord 1. The anterior median fissure and the posterior median sulcus penetrate the white matter of the spinal cord and divide it into right and left sides (Figure 13.3). 2. The gray matter of the spinal cord is shaped like the letter H or a butterfly and is surrounded by white matter. a. The gray matter consists primarily of neuron cell bodies, neuroglia, unmyelinated axons, and dendrites. 3. The white matter consists of bundles of myelinated axons. 4. The gray commissure forms the cross bar of the H-shaped gray matter. a. In the center of the gray commissure is the central canal, which runs the length of the spinal cord and contains cerebrospinal fluid. b. Anterior to the gray commissure is the anterior white commissure, which connects the white matter of the right and left sides of the spinal cord. 5. The gray matter is divided into horns, which contain cell bodies of neurons. Clusters of neuron cell bodies form functional groups called nuclei. a. Posterior gray horns contain axons of incoming sensory neurons, and cell bodies and axons of interneurons. b. Anterior gray horns contain cell bodies of somatic motor neurons, which control skeletal muscles.  c. Lateral gray horns are present only in the thoracic and upper lumbar segments of the spinal cord, and contain cell bodies of autonomic motor neurons, that control cardiac and smooth muscle and glands. 6. The white matter is divided into anterior (ventral), posterior (dorsal), and lateral columns. Each column contains distinct bundles of nerve axons called tracts, which have a common origin or destination and carry similar information. a. Sensory (ascending) tracts conduct nerve impulses toward the brain. b. Motor (descending) tracts conduct impulses away from the brain. III. SPINAL NERVES A. Spinal nerves are parallel bundles of axons and neuroglial cells wrapped in several layers of connective tissue, and connect the CNS to sensory receptors, muscles, and glands as part of the peripheral nervous system. 1. The 31 pairs of spinal nerves are named and numbered according to the region and level of the vertebral column from which they emerge (Figure 13.2). 2. There are 8 pairs of cervical nerves, 12 pairs of thoracic nerves, 5 pairs of lumbar nerves, 5 pairs of sacral nerves, and 1 pair of coccygeal nerves. 3. Roots are the two points of attachment that connect each spinal nerve to a segment of the spinal cord (Figure 13.3). a. The posterior or dorsal (sensory) root contains sensory nerve fibers and conducts nerve impulses from the periphery into the spinal cord; the posterior root ganglion contains the cell bodies of the sensory neurons. b. The anterior or ventral (motor) root contains motor neuron axons and conducts impulses from the spinal cord to the periphery; the cell bodies of motor neurons are located in the gray matter of the spinal cord. B. Connective Tissue Coverings of Spinal Nerves C. 1. Individual axons are wrapped in endoneurium. 2. Bundles of axons called fascicles are wrapped in perineurium. 3. The outermost covering of the entire nerve is called the epineurium. D. Distribution of Spinal Nerves 1. Shortly after passing through its intervertebral foramen, a spinal nerve divides into several branches; these branches are known as rami (Figure 13.6). a. The posterior (dorsal) rami serve the posterior aspect of the trunk. b. The anterior (ventral) rami serve the anterior and lateral aspects of the trunk, and the upper and lower limbs. c. The meningeal branches supply the vertebrae, vertebral ligaments, blood vessels of the spinal cord, and meninges. d. The rami communicantes are part of the autonomic nervous system. 2. The anterior rami of spinal nerves, except for T2-T12, form networks of nerves called plexuses (Figure 13.2 and Exhibits 13.A-13.D). Emerging from the plexuses are nerves bearing names that are often descriptive of the general regions they supply or the course they take. E. The cervical plexus supplies the skin and muscles of the head, neck, and upper part of the shoulders and chest; and supplies the diaphragm via the phrenic nerve (Figure 13.7, Exhibit 13.A). Complete severing of the spinal cord above the origin of the phrenic nerves (C3-C5) causes respiratory arrest. F. G. The anterior (ventral) rami of spinal nerves T2-T12 do not enter into the formation of plexuses and are known as intercostal or thoracic nerves. These nerves directly innervate structures they supply in the intercostal spaces. H. The brachial plexus constitutes the nerve supply for the upper extremities and a number of neck and shoulder muscles (Figures 13.8, Exhibit 13.B). I. J. 1. The axillary nerve supplies the deltoid and teres minor muscles. 2. The musculocutaneous nerve supplies the anterior muscles of the arm. 3. The radial nerve supplies the posterior muscles of the arm and forearm. a. Radial nerve injury causes wrist drop, the inability to extend the wrist and fingers. 4. The median nerve supplies most of the anterior muscles of the forearm and some of the hand muscles. a. Injury to the median nerve may cause numbness, tingling, and pain in the palm and fingers. 5. The ulnar nerve supplies the anteromedial muscles of the forearm and most of the hand muscles. a. Injury to the ulnar nerve may cause a condition called clawhand, which includes the inability to abduct or adduct the fingers. K. The lumbar plexus supplies the anterolateral abdominal wall, external genitals, and part of the lower limbs (Figure 13.10, Exhibit 13.C). L. M. 1. The largest nerve arising from the lumbar plexus is the femoral nerve, which serves the flexor muscles of the hip joint and extensor muscles of the knee joint (anterior thigh muscles), and the skin over the anterior and medial aspects of the thigh and medial side of the leg and foot. a. Injury to the femoral nerve is indicated by an inability to extend the leg and by loss of sensation in the skin over the anteromedial aspect of the thigh.  2. The obturator nerve supplies the adductor muscles of the hip joint and the skin over the medial aspect of the thigh. a. Obturator nerve injury is a common complication of childbirth and results in paralysis of the adductor muscles of the hip joint and loss of sensation over the medial aspect of the thigh. N. The sacral plexus supplies the buttocks, perineum, and part of the lower limbs (Figure 13.11, Exhibit 13.D). O. 1. The largest nerve arising from the sacral plexus (and the largest nerve in the body) is the sciatic nerve, which innervates the hamstrings, leg, and foot muscles.  2. a. Injury to the sciatic nerve results in sciatica, pain that extends from the buttock down the posterior and lateral aspect of the leg. P. Dermatomes  Q. 1. Spinal nerves innervate specific segments of the skin called dermatomes (Figure 13.11). 2. Knowledge of dermatomes helps a physician to determine which segment of the spinal cord or which spinal nerve is malfunctioning. IV. SPINAL CORD PHYSIOLOGY A. The spinal cord has two principal functions. 1. The white matter tracts are highways for nerve impulse conduction to and from the brain. 2. The gray matter receives and integrates incoming and outgoing information. B. Sensory and Motor Tracts 1. Figure 13.12 shows the principal sensory and motor tracts in the spinal cord. 2. 3. Sensory information from receptors travels up the spinal cord to the brain along two main routes on each side of the cord: the spinothalamic tracts and the posterior columns. a. The spinothalamic tracts carry nerve impulses for pain, temperature, deep pressure, itching, and crude touch. Crude touch refers to the ability to perceive that something has simply touched the skin. b. The posterior columns carry nerve impulses for discriminative touch, light pressure, vibration, and conscious proprioception (awareness of muscle, joint, and tendon positions and movements). Discriminative touch provides specific information about a touch sensation such as location, shape, size, and texture of the source of stimulation. 4. Motor information travels from the brain down the spinal cord to effectors (muscles and glands) along two types of descending tracts: direct pathways and indirect pathways. a. Direct pathways include the lateral and anterior corticospinal tracts, and the corticobulbar tracts. These pathways carry nerve impulses that cause voluntary movements of skeletal muscles. b. Indirect pathways include the rubrospinal, tectospinal, vestibulospinal, and lateral and medial reticulospinal tracts. These pathways carry nerve impulses from the brainstem to cause automatic movements, and also maintain skeletal muscle tone and body posture. C. Reflexes and Reflex Arcs 1. The spinal cord serves as an integrating center for spinal reflexes, which are involuntary actions occurring in response to a particular stimulus. This integration occurs in the gray matter (Figure 13.13). 2. Reflexes may be classified as spinal or cranial in regards to location of the integration center, and somatic or autonomic in regards to the effector organs (skeletal muscle, or smooth muscle, cardiac muscle, and glands, respectively). 3. A reflex arc is the pathway followed by nerve impulses that produce a reflex. 4. The five functional components of a reflex arc are the sensory receptor, sensory neuron, integrating center, motor neuron, and effector (Figure 13.13). 5. Reflexes help to maintain homeostasis by permitting the body to make exceedingly rapid adjustments to homeostatic imbalances. 6. Some important somatic spinal reflexes include the stretch reflex, tendon reflex, and flexor and crossed extensor reflexes. 7. The stretch reflex causes contraction of a skeletal muscle in response to stretching of the muscle. a. Muscle stretch is sensed by sensory receptors called muscle spindles located within the muscle. b. This reflex is useful in maintaining posture, for example, by helping to keep muscle length relatively constant. c. At the same time that the stretched muscle is contracting, antagonist muscles are inhibited from contracting, in a process called reciprocal innervation. d. In this reflex, sensory nerve impulses enter the spinal cord on the same side from which nerve impulses leave it. This is called an ipsilateral (meaning same side) reflex. 8. The tendon reflex causes relaxation of a strongly contracting muscle, and thus prevents damage to muscles and tendons as a result of stretching (Figure 13.15). This is an ipsilateral reflex. 9. a. The sensory receptors for this reflex are called tendon organs (or Golgi tendon organs), which are located within tendons and sense muscle tension. 10. The flexor (withdrawal) reflex is a protective reflex that moves a limb to avoid pain (Figure 13.16). This is an ipsilateral reflex.  a. This reflex results in contraction of flexor muscles to move a limb to avoid injury or pain. b. The flexor reflex works with the crossed extensor reflex to maintain balance. 11. The crossed extensor reflex is a balance-maintaining reflex that causes a synchronized extension of the joints of one limb in conjunction with a flexor reflex occurring in the opposite limb (Figure 13.17). a. The crossed extensor reflex, which is contralateral, helps to maintain balance during the flexor reflex. b.