Neuroscience 1 - Nerve Fibers and Peripheral Nerves PDF

1A NUEROANATOMY Chapter 3:NERVE FIBERS & PERIPHERAL NERVES DR. A. VIADO, M.D. NERVE FIBERS...

1A NUEROANATOMY Chapter 3:NERVE FIBERS & PERIPHERAL NERVES DR. A. VIADO, M.D. NERVE FIBERS The connective tissue sheaths serve to support the nerve fibers and Nerve fiber is the name given to an axon (or a dendrite) of a nerve their associated blood vessels and lymph vessels. cell. Nerve tracts – bundles of fibers found in CNS Spinal Nerves and Spinal Nerve Roots Peripheral nerves –bundles of fibers found in PNS 31 pairs of spinal nerves 2 types of nerve fibers found in nervous system:  Leave the spinal cord and pass through intervertebral 1. Myelinated foramina in the vertebral column. 2. Nonmyelinated  Each connected to the spinal cord by TWO ROOTS: o Anterior root (EFFERENT FIBERS) Myelinated Nerve Fibers  Carries nerve impulses away from the CNS surrounded by a myelin sheath. o Posterior root (AFFERENT FIBERS) myelin sheath –formed by a supporting cell  Carries nerve impulses towards CNS  Oligodendrocyte –supporting cell in the CNS  Sensory fibers  Schwann cell –supporting cell in the PNS  Cell bodies situated at the posterior root ganglion Myelin Sheath  Begin to form before birth and during the first year postnatally Cranial Nerves  segmented, discontinuous layer interrupted at regular 12 pairs of cranial nerves intervals by the nodes of Ranvier  Leave the brain and pass through the foramina of the skull  CNS - each oligodendrocyte may form and maintain myelin  Entirely of sensory afferent nerve fibers: sheaths for as many as 60 nerve fibers (axons). o olfactory  PNS - only one Schwann cell for each segment of one nerve o optic fiber. o vestibulocochlear Nonmyelinated Nerve Fibers  Entirely of efferent fibers:  smaller axons of the CNS o oculomotor  postganglionic axons of ANS o trochlear  some fine sensory axons associated with reception of pain o abducent Peripheral Nervous System o accessory  each axon indents the surface of the Schwann cell so that o hypoglossal it lies within a trough  As many as 15 or more axons may share a single Schwann  Both afferent and efferent: cell, each lying within its own trough or sometimes sharing o Trigeminal a trough o Facial  In some situations, the troughs are deep and the axons are o Glossopharyngeal embedded deep in the Schwann cells, forming a mesaxon o vagus from the Schwann cell plasma membrane  Schwann cells lie close to one another along the length of Sensory Ganglia the axons The sensory ganglia of the posterior spinal nerve roots and of the  NO nodes of Ranvier trunks of the trigeminal, facial, glossopharyngeal,and vagal cranial  In synapses, axon emerges from the trough of the nerves have the same structure. Schwann cell for a short distance exposing the active  Each ganglion is surrounded by a layer of connective tissue region of the axon that is continuous with the epineurium and perineurium of the peripheral nerve Central Nervous System,  Neurons are UNIPOLAR, possessing cell bodies that are oval  Nonmyelinated neve fibers run in small groups in shape  Not particularly related to oligodendrocytes  Cell bodies aggregated and separated by bundles of nerve fibers PERIPHERAL NERVES  A single nonmyelinated process leaves each cell body and, Peripheral nerve after a convoluted course, bifurcates at a T junction into  Collective term for the cranial and spinal nerves. peripheral and central branches  Consist of afferent or efferent bundles  The former axon terminates in a series of dendrites in a  Axons may be myelinated or nonmyelinated peripheral sensory ending, and latter axon enters the central  nerve trunk surrounded by epineurium nervous system.  bundles of nerve fibers surrounded by perineurium  The nerve impulse, on reaching the T junction, passes directly  Between individual nerve fibers is a loose, delicate from the peripheral axon to the central axon, thus bypassing connective tissue referred to as the endoneurium the nerve cell body. Trans 3 | Andam, Allida, Andoy, Brul 1 of 7  Neuroanatomy Chapter 3:NERVE FIBERS & PERIPHERAL NERVES  Each nerve cell is surrounded by CAPSULAR CELLS or 2. Initiated by an adequate stimulus being applied to the surface SATELLITE CELLS of the neuron o Capsular cells –similar structure to Schwann cells and 3. Initiation usually occurs at the initial segment of the axon which are continuous with these cells as they envelop the is the most sensitive part of the neuron peripheral and central process of each neuron 4. Stimulus alters the permeability of the membrane to Na ions at the point of stimulation; Na ions rapidly enter the axon; positive Autonomic Ganglia ions outside the axolemma quickly decrease to zero; the  Sympathetic and Parasympathetic ganglia membrane potential is reduced to zero and is said to be  Found in the sympathetic trunks, prevertebral autonomic DEPOLARIZED. plexuses and as ganglia in or close to viscera Resting potential is -80 mV, outside of the membrane  Each ganglion is surrounded by a layer of connective tissue positive to the inside continuous with the epineurium and perineurium of the Action potential is about 40 mV, with outside of the peripheral nerve. membrane negative to the inside *in small diameter axons,  Neurons are MULTIPOLAR, possess cell bodies that are action potential may not reach 40 mV irregular in shape. 5. Negatively charged point on the outside of the axolemma acts  Dendrites make synaptic connections with the myelinated as a stimulus to the adjacent positively charged axolemma, and axons of preganglionic neurons in less than 1 msec, the polarity of the adjacent resting  Axons are of small diameter (C fibers) and unmyelinated; potential is reversed. they pass to viscera, blood vessels and sweat glands. 6. Action potential moves along the axolemma from the point  Each nerve cell body is surrounded by CAPSULAR CELLS or originally stimulated to the adjacent point on the membrane SATELLITE CELLS travelling along the full length of a nerve fiber to its end. 7. As the action potential moves along the nerve fiber, the entry of Peripheral Nerve Plexuses the Na ions into the axon ceases, and the permeability of the  Composed of bundles of nerve fibers. axolemma to K ions increases. 8. K ions rapidly diffuse outside the axon (since the concentration  Sometimes divide into branches that join neighboring is much higher within the axon than outside so that the original peripheral nerves forming a NERVE PLEXUS. resting membrane potential is restored)  Formation of nerve plexus allows individual nerve fibers to 9. The permeability of the axolemma decreases and the status quo pass from one peripheral nerve to another is restored by the active transport of the Na ions out of the  In MOST instances, branching of nerve fibers does not take axon and the K ions into the axon. place 10. The outer surface of the axolemma is again electrically positive  A plexus permits a redistribution of the nerve fibers within compared with that of the inner surface. the different peripheral nerves Absolute Refractory Period  Anterior rami of the spinal nerves form complicated  for a short time after the passage of a nerve impulse along a structures at the root of the limbs nerve fiber, while the axolemma is still depolarized, a second  Cervical and Brachial plexuses –root of upper limbs stimulus however strong, is unable to excite the nerve.  Lumbar and Sacral plexuses – root of lower limbs  Na channels become inactivated and no stimulation however  Cutaneous nerves commonly form fine plexuses that again strong will open the Na gates permit a rearrangement of nerve fibers before they reach  This period is followed by a further short interval during which their terminal sensory endings. the excitability of the nerve gradually returns to normal.This latter period is called the relative refractory period. The autonomic nervous system also possesses numerous nerve  Refractory period makes a continuous excitatory state of the plexuses that consist of preganglionic and postganglionic nerve nerve impossible and limits the frequency of the impulses. fibers and ganglia. Conduction velocity of a nerve is proportional to the cross-sectional CONDUCTION IN PERIPHERAL NERVES area of the axon.  Resting unstimulated state:  Thicker fibers conducts more rapidly A nerve fiber is polarized (interior is negative to exterior)  In nonmyelinated fibers - action potential passes Resting membrane potential: –80 mV continuously along the axolemma produced by the diffusion of sodium and potassium ions  In myelinated fibers, maintained by the sodium-potassium pump  presence of a myelin sheath serves as an insulator 3 Na pumped outside, 2 K ions pumped inside through  a myelinated nerve fiber can be stimulated only at active transport the nodes of Ranvier  the action potential jumps from one node to the next NERVE IMPULSE (ACTION POTENTIAL) (SALTATORY CONDUCTION)  starts at the initial segment of the axon  SALTATORY CONDUCTION is a more rapid 1. self propagating wave of electrical negativity that passes rapidly mechanism than is found in nonmyelinated fibers along the surface of the plasma membrane (axolemma) S1T3 2 of 7  Neuroanatomy Chapter 3:NERVE FIBERS & PERIPHERAL NERVES RECEPTOR ENDINGS  Enclosed by a connective tissue continuous with the Five basic Functional Types: endoneurium of the nerves that enter it. 1. Mechanoreceptors -respond to mechanical deformation  Considerable reduction between birth and old age 2. Thermoreceptors -respond to changes in temperature; some  Very sensitive to touch and are rapidly adapting respond to cold and others to heat mechanoreceptors 3. Nociceptors -respond to any stimuli that bring about damage  TWO-POINT TACTILE DISCRIMINATION - able to distinguish to the tissue between two pointed structures when they are placed close 4. Electromagnetic receptors -the rods and cones of the eyes are together on the skin sensitive to light intensity and wavelength 5. Chemoreceptors - respond to chemical changes associated Pacinian Corpuscles with taste and smell and oxygen and carbon dioxide  Widely distributed throughout the body concentrations in the blood  Abundant in the dermis, subcutaneous tissue, ligaments, peritoneum, nipples, and external genitalia Anatomical Types of Receptors  Ovoid in shape  Consists of a capsule and a central core conaining the nerve Nonencapsulated Receptors ending Free Nerve Endings  Rapidly adapting mechanoreceptor  Widely distributed throughout the body  Particularly sensitive to VIBRATION  Found between epithelial cells of the skin, cornea and the  Can respond up to 600 stimuli per second alimentary tract  In connective tissues including the dermis, fascia, ligaments, Ruffini Corpuscles joint capsules, tendons, periosteum, perichondrium, haversian  Located in the dermis of hairy skin systems of bone, tympanic membrane and dental pulp  Consists of several large unmyelinated nerve fibers ending  Also present in muscles within a bundle of collagen fibers and surrounded by a cellular  Afferent nerve fibers from free nerve endings are either capsule myelinated or nonmyelinated  Slowly adapting mechanoreceptors  Terminal endings devoid of a myelin sheath  Stretch receptors; respond when the skin is stretched  No schwann cells covering the tips  Most detect pain, while others detect crude touch, pressure, Function of Cutaneous Receptors tickle sensations, and possibly cold and heat  there are areas of the body that have only one or two histologic types of receptors and yet they are sensitive to a Merkel Discs variety of different stimuli  Found in hairless skin (e.g. fingertips) and in hair follicles  although the body has these different receptors, all nerves  Passes into the epidermis and terminates as a disc shaped only transmit nerve impulses expansion that is applied closely to a dark-staining epithelial  type of sensation felt is determined by the specific area of the cell in the deeper part of the epidermis called Merkel cell. central nervous system to which the afferent nerve fiber  Tactile domes -clusters of Merkel discs found in the epidermis passes. between the hair follicles of the hairy skin.  Slowly adapting touch receptors that transmit information Transduction of Sensory Stimuli Into Nerve Impulses about the degree of pressure exerted on the skin, such as  Transduction - one form of energy is changed into another when one is holding a pen. form of energy.  The Stimulus, when applied to the receptor, brings about a Hair Follicle Receptors change in potential of the plasma membrane of the nerve  Nerve fibers wind around the follicle in its outer connectiv ending. tissue sheath below the sebaceous gland. receptor potential- stimimulus takes place in a  Stimulated by bending of the hair receptor  Belongs to the rapidly adapting group of mechanoreceptors  The amplitude of the receptor potential is proportional to the  While hair remains bent, receptor is silent but when released, intensity of the stimulus. a further burst of nerve impulses is initiated.  With chemoreceptors and photoreceptors, the receptor potential is produced by second messengers activated when Encapsulated Receptors the stimulus agent binds to the membrane receptors coupled to G proteins. Meissner’s Corpuscles  Located in the dermal papillae of the skin especially on the Joint Receptors palm of the hand and sole of the foot; nipple and external  Four types of sensory endings genitalia  Three of these endings are encapsulated and resemble  Ovoid in shape pacinian, Ruffini, and tendon stretch receptors.  Consists of stack of modified flattened Schwann cells arranged  provide the CNS with information regarding the position and transversely across the long axis of the corpuscle movements of the joint. S1T3 3 of 7  Neuroanatomy Chapter 3:NERVE FIBERS & PERIPHERAL NERVES  A fourth type of ending is non encapsulated, sensitive to  activated by being squeezed by the adjacent tendon fibers excessive movements, transmit pain sensations. within the spindle when tension develops in the tendon  Detects changes in muscle tension Neuromuscular Spindles Function of the Neurotendinous Spindle  found in skeletal muscle and are most numerous toward the  Increased muscle tension stimulates the neurotendinous tendinous attachment of the muscle. spindles  provide the CNS with sensory information regarding the  Inhibitory reflex; inhibits muscle contraction. muscle length and the rate of change in the muscle length.  the tendon reflex prevents the development of too much  This information is used by the central nervous system in the tension in the muscle. control of muscle activity.  protective mechanism  its main function is to provide the CNS with information that Function of the Neuromuscular Spindle can influence voluntary muscle activity.  The neuromuscular spindle plays a very important role in keeping the central nervous system informed about the length EFFECTOR ENDINGS of a muscle and the rate of change of its length, thereby Innervation of Skeletal Muscle indirectly influencing the control of voluntary muscle.  innervated by one or more nerves.  limbs and head and neck, the innervation is usually single Stretch Reflex  large muscles of the abdominal wall, the innervation is  Stretching a muscle results in elongation of the intrafusal fibers multiple of the muscle spindle and stimulation of the annulospiral and  The nerve supply and blood supply to a muscle enter it at a flower-spray endings. more or less constant position called the neurovascular hilus.  The nerve impulses reach the spinal cord in the afferent  contains motor and sensory fibers. neurons and synapse with the large alpha motor neurons  The motor fibers are of three types: situated in the anterior gray horns of the spinal cord. (1) large alpha myelinated fibers,  Nerve impulses now pass via the efferent motor nerves and (2) small gamma myelinated fibers, and stimulate the extrafusal muscle fibers, and the muscle (3) fine unmyelinated C fibers. contracts.  The large myelinated axons of the alpha anterior horn cells  This simple stretch reflex depends on a two-neuron arc supply the extrafusal fibers that form the main mass of the consisting of an afferent neuron and an efferent neuron. muscle.  Reciprocal inhibition - muscle spindlle afferent impulses  The small gamma myelinated fibers supply the intrafusal fibers inhibit the alpha motor neuros supplying the antagonist of the neuromuscular spindles. muscles  The fine unmyelinated fibers are postganglionic autonomic efferents that supply the smooth muscle in the walls of blood Control of the Intrafusal Fibers of the vessels. Neuromuscular Spindle  The sensory fibers are of three main types:  In the brain and spinal cord, there are centers that give rise to (1) the myelinated fibers,which originate in the tracts that synapse with gamma motor neurons in the spinal annulospiral and flowerspray endings of the cord neuromuscular spindles reticular formation (2) the myelinated fibers, which originate in the basal ganglia neurotendinous spindles cerebellum (3) the myelinated and nonmyelinated fibers, which  The gamma efferent motor fibers cause shortening of the originate from a variety of sensory endings in the intrafusal fibers, stretching the equatorial regions and connective tissue of the muscle. stimulating the annulospiral and flower-spray endings, initiating the reflex contraction of the extrafusal fibers Motor Unit  one-third of all the motor fibers passing to a muscle are  the single alpha motor neuron and the muscle fibers that it gamma efferents; the remaining two thirds are the large alpha innervates. motor fibers.  The muscle fibers of a single motor unit are widely scattered  nuclear bag fibers - dynamic responses; position and velocity throughout the muscle. of contraction  Where fine,precise muscle control is required, such as in the  nuclear chain fibers - slow static contractions of voluntary extraocular muscles or the small muscles of the hand,the muscle. motor units possess only a few muscle fibers.  Where precise control is not necessary,however,as in a large Neurotendinous Spindles limb muscle such as the gluteus maximus,a single motor nerve  Golgi tendon organs may innervate many hundreds of muscle fibers.  located near the junctions of tendons with muscles  Provide the CNS with sensory information regarding the Neuromuscular Junctions in Skeletal Muscle tension of muscles S1T3 4 of 7  Neuroanatomy Chapter 3:NERVE FIBERS & PERIPHERAL NERVES  Skeletal muscle fibers are innervated by large, alpha immediately undergoes hydrolysis by the enzyme myelinated nerve fibers acetylcholinesterase (AChE).  As each myelinated fiber enters a skeletal muscle, it branches  The acetylcholine remains for about 1 msec in contact with the many times. postsynaptic membrane, and it is rapidly destroyed to prevent  A single branch then terminates on a muscle fiber at a site reexcitation of the muscle fiber. referred to as a neuromuscular junction or motor endplate.  After the fall in concentration of ACh in the cleft, the ionic  The great majority of muscle fibers are innervated by just one channels close and remain closed until the arrival of more ACh. motor end-plate. Skeletal muscle fiber contraction is thus controlled by the  On reaching the muscle fiber,the nerve loses its myelin sheath frequency of the nerve impulses that arrive at the motor and breaks up into a number of fine branches. nerve terminal.  Each branch ends as a naked axon and forms the neural  Nicotine and carbamylcholine mimics acetylcholine element of the motor end-plate.  Competitive blocking agents such as d-tubocurarine causes  The axon is expanded slightly and contains many mitochondria muscle to relax and not to contract by preventing the action of and vesicles. acetylcholine  At the site of the motor end-plate, the surface of the muscle fiber is elevated slightly to form the muscular element of the plate, often referred to as the sole plate.  The elevation is due to sarcoplasm and the presence of numerous nuclei and mitochondria  The expanded naked axon lies in a groove on the surface of the muscle fiber outside the plasma membrane (sarcolemma).  axons are truly naked; the Schwann cells merely serve as a cap or roof to the groove and never project into it.  The floor of the groove is formed of the plasma membrane, Neuromuscular Junctions in Smooth Muscle which is thrown into numerous small folds, called junctional  action is slow and widespread folds; these serve to increase the surface area of the plasma  autonomic nerve fibers branch extensively membrane that lies close to the naked axon.  a single neuron exerts control over a large number of muscle  The plasma membrane of the axon is separated,by a space fibers. about 30 to 50 nm wide, from the plasma membrane of the  In some areas (e.g., the longitudinal layer of smooth muscle in muscle fiber. This space constitutes the synaptic cleft. the intestine), only a few muscle fibers are associated with  The synaptic cleft is filled with the basement membranes of autonomic endings, the wave of contraction passing from one the axon and the muscle fiber. muscle cell to another by means of gap junctions  The motor end-plate is strengthened by endoneurium, which  In smooth muscle, in which the action is fast and precision is becomes continuous with the endomysium. required, such as in the iris, the branching of the nerve fibers is  A nerve impulse causes the opening of voltage-gated Ca less extensive; thus, a single neuron exerts control over only a channels that allow Ca ions to enter the axon. few muscle fibers.  This stimulates the fusion of some of the synaptic vesicles with  The autonomic nerve fibers, which are postganglionic, are the presynaptic membrane and causes the release of nonmyelinated and terminate as a series of varicosed acetylcholine into the synaptic cleft. branches. An interval of 10 to 100 nm may exist between the  The acetylcholine is discharged into the cleft by exocytosis axon and the muscle fiber. and diffuses rapidly across the cleft to reach the nicotinic type  At the site where transmission is to occur, the Schwann cell is of ACh receptors on the postsynaptic membrane of the retracted junctional folds.  part of the axon is naked, permitting free diffusion of the  Once the ACh-gated channels are opened, the postsynaptic transmitter substance from the axon to the muscle cell. membrane becomes more permeable to Na ions, which flow  the axoplasm contains numerous vesicles into the muscle cell,and a local potential called the end-plate  Smooth muscle is innervated by sympathetic and potential is created. (The ACh-gated channels are also parasympathetic parts of the autonomic system. permeable to K ions, which flow out of the cell but to a lesser  Cholinergic nerves liberate acetylcholine at their endings by extent.) exocytosis  The wave of depolarization is carried into the muscle fiber to  Noradrenergic nerves liberate norepinephrine at their endings the contractile myofibrils through the system of T tubules. by exocytosis  This leads to the release of Ca2 ions from the sarcoplasmic  Both acetylcholine and norepinephrine bring about reticulum,which,in turn,causes the muscle to contract. depolarization of the muscle fibers innervated  The amount of acetylcholine released at the motor end-plate  The acetylcholine is hydrolyzed in the presence of will depend on the number of nerve impulses arriving at the acetylcholinesterase in the nerve terminal.  the norepinephrine is taken up by the nerve endings.  Once the acetylcholine crosses the synaptic cleft and triggers  in some areas of the body (e.g., bronchial muscle), the ionic channels on the postsynaptic membrane, it norepinephrine causes smooth muscle to relax and not contract. S1T3 5 of 7  Neuroanatomy Chapter 3:NERVE FIBERS & PERIPHERAL NERVES motor neuron in the anterior gray column (horn) of the Neuromuscular Junctions spinal cord and all the muscle fibers it supplies in Cardiac Muscle  In a large buttock muscle, such as the gluteus maximus, fine  Nonmyelinated post ganglionic sympathetic and control is unnecessary, a given motor neuron may supply as parasympathetic autonomic nerves many as 200 muscle fibers.  At the site where transmission takes place, the axon becomes  in the small muscles of the hand or the extrinsic muscles of the naked because of the retraction of the Schwann cell permitting eyeball,fine control is required, one nerve fiber supplies only a free diffusion of neurotransmitter from the axon to the muscle few muscle fibers. fiber  Every skeletal muscle while resting is in a partial state of  intermittent desmosomes and gap junctions makes excitation contraction, a condition referred to as muscle tone. and contraction of one muscle fiber rapidly spread from fiber  muscle fibers are either fully contracted or relaxed to fiber.  different groups of motor units and different groups of muscle fibers are brought into action at different times to avoid fatigue; accomplished by the asynchronous discharge of Nerve Endings on Secretory Cells of Glands nervous impulses in the motor neurons in the anterior gray  Nonmyelinated postganglionic autonomic nerves extend into horn of the spinal cord the connective tissue of glands and branch close to the  muscle tone is dependent on the integrity of a simple secretory cells. monosynaptic reflex arc.  In many glands, the nerve fibers have been found to innervate  The lengthening and shortening in a muscle are detected by only the blood vessels. sensitive sensory endings called muscle spindles; tension is detected by tendon spindles. SEGMENTAL INNERVATION OF SKIN  The nervous impulses travel in the large afferent fibers to the spinal cord; they synapse with the motor neurons situated in  Dermatome - the single spinal nerve segment that supplies an the anterior gray column, which send impulses down their area of the skin axons to the muscle fibers.  On the trunk, the dermatomes extend round the body from  The muscle spindles themselves are innervated by small the posterior to the anterior median plane. gamma efferent fibers that regulate the response of the  Adjacent dermatomes overlap considerably, so to produce a muscle spindles, acting synergically with external stretch. region of complete anesthesia, at least three contiguous spinal Maintaining the muscle tone reflexly and adjusted to the nerves have to be sectioned. needs of posture and movement.  the area of tactile loss is always greater than the area of loss of  If afferent or efferent pathways of the reflex arc are cut, the painful and thermal sensations. muscle would lose its tone immediately and become flaccid.  the degree of overlap of fibers carrying pain and thermal  A flaccid muscle loses resilience, quickly atrophies and sensations is much more extensive than the overlap of fibers becomes reduced in volume. carrying tactile sensations.  the degree of activity of the motor anterior column cells and  Dermatomal charts for the anterior and posterior surfaces of the degree of muscle tone depend on the summation of the the body are shown in nerve impulses received  In the limbs,the arrangement of the dermatomes is more  Muscle movement is accomplished by bringing into action complicated because of the embryologic rotation of the limbs increasing numbers of motor units and reducing the activity of as they grow out from the trunk. the motor units of muscles that will oppose or antagonize the  In the face, the divisions of the trigeminal nerve supply a movement. precise area of skin, and there is little or no overlap SUMMATION OF MOTOR UNITS SEGMENTAL INNERVATION OF MUSCLES When a muscle begins to contract, the smaller motor units are  innervated by more than one spinal nerve and by the same stimulated first. number of segments of the spinal cord. The reason for this is that the smaller motor units are innervated by  to paralyze a muscle completely, it is necessary to section smaller neurons in the spinal cord and brainstem, and they have a several spinal nerves or destroy several segments of the spinal lower threshold of motor units are brought into action. This cord. phenomenon causes a gradual increase in muscle strength as the  Biceps brachii tendon reflex C5-6 muscle contracts.  Triceps tendon reflex C6-7 and C8  Brachioradialis tendon reflex C5-6 and C7 MUSCLE FATIGUE  Abdominal superficial reflexes. Upper abdominal skin T6-7;  The progressive loss of strength of a muscle with prolonged middle abdominal skin T8-9; lower abdominal skin T10-12. strong contraction is due to the reduction in the amounts of  Patellar tendon reflex (knee jerk) L2, L3, and L4 ATP within the muscle fibers..  Achilles tendon reflex (ankle jerk) S1 and 2 POSTURE MUSCLE TONE AND MUSCLE ACTION  position adopted by the individual within his or her  motor unit environment. S1T3 6 of 7  Neuroanatomy Chapter 3:NERVE FIBERS & PERIPHERAL NERVES  In the standing position, the line of gravity passes through the odontoid process of the axis, behind the centers of the hip joints, and in front of the knee and ankle joints.  antigravity muscles stabilize the body and prevent it from collapsing  posture depends on the degree and distribution of muscle tone  In order to maintain posture,the simple muscle reflex,on which muscle tone is dependent, must receive adequate nervous input from higher levels of the nervous system.  When an individual assumes a given posture, the tone of the muscles controlling that posture is constantly undergoing fine adjustments so that the posture is maintained.  Normal posture depends not only on the integrity of the reflex arc but also on the summation of the nervous impulses received by the motor anterior gray column cells from other neurons of the nervous system. S1T3 7 of 7