Chapter 13
Each spinal nerve and cranial nerve consists of many individual axons and contains layers of protective connective tissue coverings. what are these layers?
Individual axons within a nerve, whether myelinated or unmyelinated, are wrapped in endoneurium, the innermost layer. The endoneurium consists of a mesh of collagen fibers, fibroblasts, and macrophages. Groups of axons with their endoneurium are held together in bundles called fascicles, each of which is wrapped in perineurium, the middle layer. The perineurium is a thicker layer of connective tissue. It consists of up to 15 layers of fibroblasts within a network of collagen fibers. The outermost covering over the entire nerve is the epineurium. It consists of fibroblasts and thick collagen fibers. Extensions of the epineurium also fill the spaces between fascicles. The dura mater of the spinal meninges fuses with the epineurium as the nerve passes through the intervertebral foramen. Note the presence of blood vessels, which nourish the spinal meninges.
what is knowing what each spinal cord segment supply to good for?
Knowing which spinal cord segments supply each dermatome makes it possible to locate damaged regions of the spinal cord. If the skin in a particular region is stimulated but the sensation is not perceived, the nerves supplying that dermatome are probably damaged. In regions where the overlap is considerable, little loss of sensation may result if only one of the nerves supplying the dermatome is damaged. Information about the innervation patterns of spinal nerves can also be used therapeutically. Cutting posterior roots or infusing local anesthetics can block pain either permanently or transiently. Because dermatomes overlap, deliberate production of a region of complete anesthesia may require that at least three adjacent spinal nerves be cut or blocked by an anesthetic drug.
Spinal cord injuries
Most spinal cord injuries are due to trauma as a result of factors such as automobile accidents, falls, contact sports, diving, and acts of violence. The effects of the injury depend on the extent of direct trauma to the spinal cord or compression of the cord by fractured or displaced vertebrae or blood clots. Although any segment of the spinal cord may be involved, the most common sites of injury are in the cervical, lower thoracic, and upper lumbar regions. Depending on the location and extent of spinal cord damage, paralysis may occur
direct motor pathways
Motor output to skeletal muscles travels down the spinal cord in two types of descending pathways: direct and indirect. The direct motor pathways include the lateral corticospinal, anterior corticospinal, and corticobulbar tracts. They convey nerve impulses that originate in the cerebral cortex and are destined to cause voluntary movements of skeletal muscles.
What are the two main routes for nerves impulses from spinal cord up to brain?
Nerve impulses from sensory receptors propagate up the spinal cord to the brain along two main routes on each side: the spinothalamic tract and the posterior column. The spinothalamic tract conveys nerve impulses for sensing pain, warmth, coolness, itching, tickling, deep pressure, and crude touch. The posterior column consists of two tracts: the gracile fasciculus and the cuneate fasciculus. The posterior column tracts convey nerve impulses for discriminative touch, light pressure, vibration, and conscious proprioception (the awareness of the positions and movements of muscles, tendons, and joints).
posterior root
The posterior (dorsal) root and rootlets contain only sensory axons, which conduct nerve impulses from sensory receptors in the skin, muscles, and internal organs into the central nervous system. Each posterior root has a swelling, the posterior (dorsal) root ganglion, which contains the cell bodies of sensory neurons.
Are there variation in cord segments?
The various spinal cord segments vary in size, shape, relative amounts of gray and white matter, and distribution and shape of gray matter. In addition, more sensory and motor tracts are present in the upper segments of the spinal cord than in the lower segments. Therefore, the amount of white matter decreases from cervical to sacral segments of the spinal cord. There are two major reasons for this variation in spinal cord white matter: (1) As the spinal cord ascends from sacral to cervical segments, more ascending axons are added to spinal cord white matter to form more sensory tracts. (2) As the spinal cord descends from cervical to sacral segments, the motor tracts decrease in thickness as more descending axons leave the motor tracts to synapse with neurons in the gray matter of the spinal cord.
white matter
The white matter of the spinal cord consists primarily of bundles of myelinated axons of neurons. Two grooves penetrate the white matter of the spinal cord and divide it into right and left sides (Posterior median sulcus and anterior median fissure)
columns
The white matter of the spinal cord is organized into regions. The anterior and posterior gray horns divide the white matter on each side into three broad areas called columns: (1) anterior (ventral) white columns, (2) posterior (dorsal) white columns, and (3) lateral white columns. Each column in turn contains distinct tracts.
Tracts
These bundles, which may extend long distances up or down the spinal cord that have a common origin or destination and carrying similar information are called tracts. -Recall that tracts are bundles of axons in the CNS, whereas nerves are bundles of axons in the PNS.
how does tension reflex protect muscle?
Thus, as tension on the tendon organ increases, the frequency of inhibitory impulses increases; inhibition of the motor neurons to the muscle developing excess tension (effector) causes relaxation of the muscle. In this way, the tendon reflex protects the tendon and muscle from damage due to excessive tension.
Tibial Medial plantar lateral plantar
Tibial Gastrocnemius, plantaris, soleus, popliteus, tibialis posterior, flexor digitorum longus, and flexor hallucis longus muscles. Branches of tibial nerve in foot are medial plantar nerve and lateral plantar nerve. Medial plantar (PLAN-tar) Abductor hallucis, flexor digitorum brevis, and flexor hallucis brevis muscles; skin over medial two-thirds of plantar surface of foot. Lateral plantar Remaining muscles of foot not supplied by medial plantar nerve; skin over lateral third of plantar surface of foot.
Reciprocal innervation
a polysynaptic reflex arc to the antagonistic muscles operates at the same time as a stretch reflex. This arc involves three neurons and two synapses. An axon collateral (branch) from the muscle spindle sensory neuron also synapses with an inhibitory interneuron in the integrating center. the interneuron synapses with and inhibits a motor neuron that normally excites the antagonistic muscles. So when the stretched muscle contracts during a stretch reflex, antagonistic muscles that oppose the contraction relax. This is reciprocal innervation.
Mixed nerve
a typical spinal nerve has two connections to the cord: a posterior root and an anterior root. The posterior and anterior roots unite to form a spinal nerve at the intervertebral foramen. Because the posterior root contains sensory axons and the anterior root contains motor axons, a spinal nerve is classified as a mixed nerve. The posterior root contains a posterior root ganglion in which cell bodies of sensory neurons are located.
crossed extensor reflex
the crossed extensor reflex involves a contralateral reflex arc: Sensory impulses enter one side of the spinal cord and motor impulses exit on the opposite side. Thus, a crossed extensor reflex synchronizes the extension of the contralateral limb with the withdrawal (flexion) of the stimulated limb. Reciprocal innervation also occurs in both the flexor reflex and the crossed extensor reflex. In the flexor reflex, when the flexor muscles of a painfully stimulated lower limb are contracting, the extensor muscles of the same limb are relaxing to some degree. If both sets of muscles contracted at the same time, the two sets of muscles would pull on the bones in opposite directions, which might immobilize the limb. Because of reciprocal innervation, one set of muscles contracts while the other relaxes.
reciprocal innervation of tension reflex
the sensory neuron from the tendon organ also synapses with an excitatory interneuron in the spinal cord. The excitatory interneuron in turn synapses with motor neurons controlling antagonistic muscles. Thus, while the tendon reflex brings about relaxation of the muscle attached to the tendon organ, it also triggers contraction of antagonists. This is reciprocal innervation. The sensory neuron also relays nerve impulses to the brain by way of sensory tracts, thus informing the brain about the state of muscle tension throughout the body.
reciprocal innervation
this type of arrangement, in which the components of a neural circuit simultaneously cause contraction of one muscle and relaxation of its antagonists, is termed reciprocal innervation. Reciprocal innervation prevents conflict between opposing muscles and is vital in coordinating body movements.
The meninges and other protection of spinal cord
three protective, connective tissue coverings that encircle the spinal cord and brain. they are the (1) dura mater, (2) arachnoid mater, and (3) pia mater. The spinal meninges are continuous with the cranial meninges. All three spinal meninges cover the spinal nerves up to the point where they exit the spinal column through the intervertebral foramina. The spinal cord is also protected by a cushion of fat and connective tissue located in the epidural space, a space between the dura mater and the wall of the vertebral canal
anterior median fissure posterior median sulcus
-The anterior median fissure is a wide groove on the anterior (ventral) side. -The posterior median sulcus is a narrow furrow on the posterior (dorsal) side.
What helps after a spinal injury?
In many cases of traumatic injury of the spinal cord, the patient may have an improved outcome if an anti-inflammatory corticosteroid drug called methylprednisolone is given within 8 hours of the injury. This is because the degree of neurologic deficit is greatest immediately following traumatic injury as a result of edema (collection of fluid within tissues) as the immune system responds to the injury.
This arrangement is called an ipsilateral reflex (ip-si-LAT-er-al = same side).
sensory nerve impulses enter the spinal cord on the same side from which motor nerve impulses leave it. All monosynaptic reflexes are ipsilateral.
conus medullaris filum terminale
Inferior to the lumbar enlargement, the spinal cord terminates as a tapering, conical structure called the conus medullaris, which ends at the level of the intervertebral disc between the first and second lumbar vertebrae (L1-L2) in adults. -Arising from the conus medullaris is the filum terminale, an extension of the pia mater that extends inferiorly, fuses with the arachnoid mater and dura mater, anchors the spinal cord to the coccyx.
Nerve to obturator internus perforating cutaneous posterior cutaneous nerve of thigh
Nerve to obturator internus (OB-too-rā′-tor in-TER-nus) and superior gemellus Obturator internus and superior gemellus muscles. Perforating cutaneous (kū′-TĀ-nē-us) Skin over inferior medial aspect of buttock. Posterior cutaneous nerve of thigh Skin over anal region, inferior lateral aspect of buttock, superior posterior aspect of thigh, superior part of calf, scrotum in male, and labia majora in female.
Spinal Nerves
Spinal nerves are the paths of communication between the spinal cord and specific regions of the body. -The spinal cord appears to be segmented because the 31 pairs of spinal nerves emerge at regular intervals from intervertebral foramina. Indeed, each pair of spinal nerves is said to arise from a spinal segment. Within the spinal cord there is no obvious segmentation but, the naming of spinal nerves is based on the segment in which they are located. There are 8 pairs of cervical nerves (C1-C8), 12 pairs of thoracic nerves (T1-T12), 5 pairs of lumbar nerves (L1-L5), 5 pairs of sacral nerves (S1-S5), and 1 pair of coccygeal nerves (Co1).
Superior gluteal Inferior gluteal nerve to piriformis Nerve to quadratus femoris
Superior gluteal (GLOO-tē-al) Gluteus minimus, gluteus medius, and tensor fasciae latae muscles. Inferior gluteal Gluteus maximus muscle. Nerve to piriformis (pir-i-FOR-mis) Piriformis muscle. Nerve to quadratus femoris (quod-RĀ-tus FEM-or-is) and inferior gemellus (jem-EL-us) Quadratus femoris and inferior gemellus muscles.
Sacral Plexus
Superior gluteal Inferior gluteal Nerve to piriformis Nerve to quadratus femoris and inferior gemellus Nerve to obturator internus and superior gemellus Perforating cutaneous Posterior cutaneous nerve of thigh Pudendal Sciatic Tibial Medial plantar (PLAN-tar) Lateral plantar Common fibular (FIB-ū-lar) Superficial fibular. Deep fibular -L4 to L5 and S1 to S4
anterior gray horns
The anterior (ventral) gray horns contain somatic motor nuclei, which are clusters of cell bodies of somatic motor neurons that provide nerve impulses for contraction of skeletal muscles.
Difference between intersegmental reflex arcs and stretch arcs
Through intersegmental reflex arcs, a single sensory neuron can activate several motor neurons, thereby stimulating more than one effector. The monosynaptic stretch reflex, in contrast, involves muscles receiving nerve impulses from one spinal cord segment only.
Roots
Two bundles of axons, called roots, connect each spinal nerve to a segment of the cord by even smaller bundles of axons called rootlets. Divided into anterior and posterior The anterior (ventral) root and rootlets contain axons of motor neurons, which conduct nerve impulses from the CNS to effectors (muscles and glands).
Meningitis Neuroalgia
-Inflammation of the meninges due to an infection, usually caused by a bacterium or virus. Symptoms include fever, headache, stiff neck, vomiting, confusion, lethargy, and drowsiness. Bacterial meningitis is much more serious and is treated with antibiotics. Viral meningitis has no specific treatment. Bacterial meningitis may be fatal if not treated promptly; viral meningitis usually resolves on its own in 1-2 weeks. A vaccine is available to help protect against some types of bacterial meningitis. -Attacks of pain along the entire course or a branch of a sensory nerve.
A stretch reflex operates as follows
1. Slight stretching of a muscle stimulates sensory receptors in the muscle called muscle spindles. The spindles monitor changes in the length of the muscle. 2. In response to being stretched, a muscle spindle generates one or more nerve impulses that propagate along a somatic sensory neuron through the posterior root of the spinal nerve and into the spinal cord.
what are the two spinal enlargements?
-The superior enlargement, the cervical enlargement, extends from the fourth cervical vertebra (C4) to the first thoracic vertebra (T1). Nerves to and from the upper limbs arise from the cervical enlargement. The inferior enlargement, called the lumbar enlargement, extends from the ninth to the twelfth thoracic vertebra. Nerves to and from the lower limbs arise from the lumbar enlargement.
Poliomyelitis
-simply polio, is caused by a virus called poliovirus. The onset of the disease is marked by fever, severe headache, a stiff neck and back, deep muscle pain and weakness, and loss of certain somatic reflexes. In its most serious form, the virus produces paralysis by destroying cell bodies of motor neurons, specifically those in the anterior horns of the spinal cord and in the nuclei of the cranial nerves. Polio can cause death from respiratory or heart failure if the virus invades neurons in vital centers that control breathing and heart functions in the brain stem. Even though polio vaccines have virtually eradicated polio in the United States, outbreaks of polio continue throughout the world. Due to international travel, polio could be easily reintroduced into North America if individuals are not vaccinated appropriately.
The pathway followed by nerve impulses that produce a reflex is a reflex arc (reflex circuit). A reflex arc includes the following five functional components:
1. Sensory receptor. The distal end of a sensory neuron (dendrite) or an associated sensory structure serves as a sensory receptor. It responds to a specific stimulus—a change in the internal or external environment—by producing a graded potential called a generator (or receptor) potential. If a generator potential reaches the threshold level of depolarization, it will trigger one or more nerve impulses in the sensory neuron.
The internal organization of the spinal cord allows sensory input and motor output to be processed by the spinal cord in the following way
1. Sensory receptors detect a sensory stimulus. 2. Sensory neurons convey this sensory input in the form of nerve impulses along their axons, which extend from sensory receptors into the spinal nerve and then into the posterior root. From the posterior root, axons of sensory neurons may proceed along three possible paths (see steps 3, 4 and 5).
DEEP (LARGELY MOTOR) BRANCHES
Ansa cervicalis (AN-sa ser-vi-KAL-is) Divides into superior and inferior roots. Superior root Infrahyoid and geniohyoid muscles of neck. Inferior root Infrahyoid muscles of neck. Phrenic (FREN-ik) Diaphragm. Segmental branches Prevertebral (deep) muscles of neck, levator scapulae, and middle scalene muscles.
How are tracts named?
As noted previously, one of the ways the spinal cord promotes homeostasis is by conducting nerve impulses along tracts. Often, the name of a tract indicates its position in the white matter and where it begins and ends. For example, the anterior corticospinal tract is located in the anterior white column; it begins in the cerebral cortex (superficial gray matter of the cerebrum of the brain) and ends in the spinal cord. Notice that the location of the axon terminals comes last in the name. This regularity in naming allows you to determine the direction of information flow along any tract named according to this convention.
Among the somatic reflexes of clinical significance are the following:
Babinski sign (ba-BIN-skē). This reflex results from gentle stroking of the lateral outer margin of the sole. The great toe extends, with or without a lateral fanning of the other toes. This phenomenon normally occurs in children under 1.5 years of age and is due to incomplete myelination of fibers in the corticospinal tract. A positive Babinski sign after age 18 months is abnormal and indicates an interruption of the corticospinal tract as the result of a lesion of the tract, usually in the upper portion. The normal response after age 18 months is the plantar flexion reflex, or negative Babinski—a curling under of all the toes.
what else does pain stimulate?
Besides initiating the flexor reflex that causes you to withdraw the limb, the pain impulses from stepping on the tack also initiate a crossed extensor reflex to help you maintain your balance.
These are spinal cord levels affected by a complete transection and not vertebral column levels. Recall that spinal cord levels differ from vertebral column levels because of the differential growth of the cord versus the column
C1-C3: no function maintained from the neck down; ventilator needed for breathing; electric wheelchair with breath, head, or shoulder-controlled device required C4-C5: diaphragm, which allows breathing C6-C7: some arm and chest muscles, which allows feeding, some dressing, and manual wheelchair required T1-T3: intact arm function T4-T9: control of trunk above the umbilicus T10-L1: most thigh muscles, which allows walking with long leg braces L1-L2: most leg muscles, which allows walking with short leg braces
common fibular superficial fibular deep fibular
Common fibular (FIB-ū-lar) Divides into superficial fibular and deep fibular branch. Superficial fibular Fibularis longus and fibularis brevis muscles; skin over distal third of anterior aspect of leg and dorsum of foot. Deep fibular Tibialis anterior, extensor hallucis longus, fibularis tertius, and extensor digitorum longus and extensor digitorum brevis muscles; skin on adjacent sides of great and second toes.
Dorsal scapular Long thoracic Nerve to subclavius suprascapular musculocutaneous
Dorsal scapular (SKAP-ū-lar) Levator scapulae, rhomboid major, and rhomboid minor muscles. Long thoracic (thō-RAS-ik) Serratus anterior muscle. Nerve to subclavius (sub-KLĀ-vē-us) Subclavius muscle. Suprascapular Supraspinatus and infraspinatus muscles. Musculocutaneous (mus′-kū-lō-kū-TĀN-ē-us) Coracobrachialis, biceps brachii, and brachialis muscles.
Brachial plexus
Dorsal scapular Long thoracic (thō-RAS-ik) Nerve to subclavius Suprascapular Musculocutaneous Lateral pectoral (PEK-tō-ral) Upper subscapular Thoracodorsal Lower subscapular Axillary (AK-si-lar-ē) Median Radial Medial pectoral Medial cutaneous nerve of arm Medial cutaneous nerve of forearm Ulnar -C5-C8 and T1
Elongation of spinal cord
During early childhood, both the spinal cord and the vertebral column grow longer as part of overall body growth. Elongation of the spinal cord stops around age 4 or 5, but growth of the vertebral column continues. Thus, the spinal cord does not extend the entire length of the adult vertebral column.
spinal shock areflexia
Following complete transection, and to varying degrees after hemisection, spinal shock occurs. Spinal shock is an immediate response to spinal cord injury characterized by temporary areflexia, loss of reflex function. The areflexia occurs in parts of the body served by spinal nerves below the level of the injury. Signs of acute spinal shock include slow heart rate, low blood pressure, flaccid paralysis of skeletal muscles, loss of somatic sensations, and urinary bladder dysfunction. Spinal shock may begin within 1 hour after injury and may last from several minutes to several months, after which reflex activity gradually returns.
Lumbar plexus
Iliohypogastric (il′-ē-ō-hī-pō-GAS-trik) Ilioinguinal (il′-ē-ō-ING-gwi-nal) Genitofemoral (jen′-i-tō-FEM-or-al) Lateral cutaneous nerve of thigh Femoral Obturator (OB-too-rā′-tor) -L1 to L4
Iliohypogastric Ilioinguinal genitofemoral
Iliohypogastric (il′-ē-ō-hī-pō-GAS-trik) Muscles of anterolateral abdominal wall; skin of inferior abdomen and buttock. Ilioinguinal (il′-ē-ō-ING-gwi-nal) Muscles of anterolateral abdominal wall; skin of superior and medial aspect of thigh, root of penis and scrotum in male, and labia majora and mons pubis in female. Genitofemoral (jen′-i-tō-FEM-or-al) Cremaster muscle; skin over middle anterior surface of thigh, scrotum in male, and labia majora in female.
Spinal Tap
In a spinal tap, a local anesthetic is given, and a long hollow needle is inserted into the subarachnoid space to withdraw CSF. During this procedure, the patient lies on his or her side with the vertebral column flexed. The spinal cord ends around the second lumbar vertebra (L2); however, the spinal meninges and circulating cerebrospinal fluid extend to the second sacral vertebra (S2). Between vertebrae L2 and S2 the spinal meninges are present, but the spinal cord is absent. Consequently, a spinal tap is normally performed in adults between the L3 and L4 or L4 and L5 lumbar vertebrae because this region provides safe access to the subarachnoid space without the risk of damaging the spinal cord.
indirect motor pathways
Indirect motor pathways include the rubrospinal, tectospinal, vestibulospinal, lateral reticulospinal, and medial reticulospinal tracts. These tracts convey nerve impulses from the brain stem to cause automatic movements and help coordinate body movements with visual stimuli. Indirect pathways also maintain skeletal muscle tone, sustain contraction of postural muscles, and play a major role in equilibrium by regulating muscle tone in response to movements of the head.
lateral cutaneous femoral obturator
Lateral cutaneous nerve of thigh Skin over lateral, anterior, and posterior aspects of thigh. Femoral Largest nerve arising from lumbar plexus; distributed to flexor muscles of hip joint and extensor muscles of knee joint, skin over anterior and medial aspect of thigh and medial side of leg and foot. Obturator (OB-too-rā′-tor) Adductor muscles of hip joint; skin over medial aspect of thigh.
Later pectoral upper subscapular thoracodorsal lower subscapular axillary
Lateral pectoral (PEK-tō-ral) Pectoralis major muscle. Upper subscapular Subscapularis muscle. Thoracodorsal (thō-RĀ-kō-dor-sal) Latissimus dorsi muscle. Lower subscapular Subscapularis and teres major muscles. Axillary (AK-si-lar-ē) Deltoid and teres minor muscles; skin over deltoid and superior posterior aspect of arm.
SUPERFICIAL (SENSORY) BRANCHES
Lesser occipital Skin of scalp posterior and superior to ear. Great auricular (aw-RIK-ū-lar) Skin anterior, inferior, and over ear, and over parotid glands. Transverse cervical Skin over anterior and lateral aspect of neck. Supraclavicular Skin over superior portion of chest and shoulder.
Medial pectoral medial cutaneous nerve of arm medial cutaneous nerve of forearm ulnar
Medial pectoral Pectoralis major and pectoralis minor muscles. Medial cutaneous nerve of arm (kū-TĀ-nē-us) Skin of medial and posterior aspects of distal third of arm. Medial cutaneous nerve of forearm Skin of medial and posterior aspects of forearm. Ulnar Flexor carpi ulnaris, ulnar half of flexor digitorum profundus, and most muscles of hand; skin of medial side of hand, little finger, and medial half of ring finger.
median radial
Median Flexors of forearm, except flexor carpi ulnaris; ulnar half of flexor digitorum profundus, and some muscles of hand (lateral palm); skin of lateral two-thirds of palm of hand and fingers. Radial Triceps brachii, anconeus, and extensor muscles of forearm; skin of posterior arm and forearm, lateral two-thirds of dorsum of hand, and fingers over proximal and middle phalanges.
What are the 5 types of paralysis?
Monoplegia is paralysis of one limb only. Diplegia is paralysis of both upper limbs or both lower limbs. Paraplegia is paralysis of both lower limbs. Hemiplegia is paralysis of the upper limb, trunk, and lower limb on one side of the body, and quadriplegia is paralysis of all four limbs.
Why are autonomic reflexes not practical diagnostic tools?
Most autonomic reflexes are not practical diagnostic tools because it is difficult to stimulate visceral effectors, which are deep inside the body. An exception is the pupillary light reflex, in which the pupils of both eyes decrease in diameter when either eye is exposed to light. Because the reflex arc includes synapses in lower parts of the brain, the absence of a normal pupillary light reflex may indicate brain damage or injury.
nerve impulses pathways
Nerve impulses propagating into, through, and out of the CNS follow specific pathways, depending on the kind of information, its origin, and its destination.
Among the somatic reflexes of clinical significance are the following:
Patellar reflex (knee jerk). This stretch reflex involves extension of the leg at the knee joint by contraction of the quadriceps femoris muscle in response to tapping the patellar ligament. This reflex is blocked by damage to the sensory or motor nerves supplying the muscle or to the integrating centers in the second, third, or fourth lumbar segments of the spinal cord. It is often absent in people with chronic diabetes mellitus or neurosyphilis, both of which cause degeneration of nerves. It is exaggerated in disease or injury involving certain motor tracts descending from the higher centers of the brain to the spinal cord.
Pudendal sciatic
Pudendal (pū-DEN-dal) Muscles of perineum; skin of penis and scrotum in male and clitoris, labia majora, labia minora, and vagina in female. Sciatic (sī-AT-ik) Actually two nerves—tibial and common fibular—bound together by common sheath of connective tissue; splits into its two divisions, usually at the knee. (See below for distributions.) As sciatic nerve descends through thigh, it sends branches to hamstring muscles and adductor magnus.
cauda equina
Recall that the spinal cord ends near the level of the superior border of the second lumbar vertebra (L2), and that the roots of the lumbar, sacral, and coccygeal nerves descend at an angle to reach their respective foramina before emerging from the vertebral column. This arrangement constitutes the cauda equina.
Cervical plexus
SUPERFICIAL (SENSORY) BRANCHES Lesser occipital Great auricular Tranverse cervical Supraclavicular DEEP (LARGELY MOTOR) BRANCHES Ansa cervicalis Superior Root Inferior root Phrenic Segmental branches -C1 to C4 with contributions of C5
sensory and motor tracts
Sensory (ascending) tracts consist of axons that conduct nerve impulses toward the brain. Tracts consisting of axons that carry nerve impulses from the brain are called motor (descending) tracts. Sensory and motor tracts of the spinal cord are continuous with sensory and motor tracts in the brain.
post-polio syndrome
Several decades after suffering a severe attack of polio and following their recovery from it, some individuals develop a condition called post-polio syndrome. This neurological disorder is characterized by progressive muscle weakness, extreme fatigue, loss of function, and pain, especially in muscles and joints. Post-polio syndrome seems to involve a slow degeneration of motor neurons that innervate muscle fibers. Triggering factors appear to be a fall, a minor accident, surgery, or prolonged bed rest. Possible causes include overuse of surviving motor neurons over time, smaller motor neurons because of the initial infection by the virus, reactivation of dormant polio viral particles, immune-mediated responses, hormone deficiencies, and environmental toxins.
intercostal nerves or thoracic nerves
The anterior rami of spinal nerves T2-T12 do not enter into the formation of plexuses and are known as intercostal nerves or thoracic nerves. These nerves directly connect to the structures they supply in the intercostal spaces. After leaving its intervertebral foramen, the anterior ramus of nerve T2 innervates the intercostal muscles of the second intercostal space and supplies the skin of the axilla and posteromedial aspect of the arm. Nerves T3-T6 extend along the costal grooves of the ribs and then to the intercostal muscles and skin of the anterior and lateral chest wall. Nerves T7-T12 supply the intercostal muscles and abdominal muscles, along with the overlying skin. The posterior rami of the intercostal nerves supply the deep back muscles and skin of the posterior aspect of the thorax.
Pia mater (PĒ-a MĀ-ter; pia = delicate)
This innermost meninx is a thin transparent connective tissue layer that adheres to the surface of the spinal cord and brain. It consists of thin squamous to cuboidal cells within interlacing bundles of collagen fibers and some fine elastic fibers. Within the pia mater are many blood vessels that supply oxygen and nutrients to the spinal cord. Between the arachnoid mater and pia mater is a space, the subarachnoid space, which contains shock-absorbing cerebrospinal fluid.
Flexor reflex is protective because
This reflex is protective because contraction of flexor muscles moves a limb away from the source of a possibly damaging stimulus.
spinal reflex cranial reflex somatic reflex
When integration takes place in the spinal cord gray matter, the reflex is a spinal reflex. An example is the familiar patellar reflex (knee jerk). If integration occurs in the brain stem rather than the spinal cord, the reflex is called a cranial reflex. An example is the tracking movements of your eyes as you read this sentence. You are probably most aware of somatic reflexes, which involve contraction of skeletal muscles. Equally important, however, are the autonomic (visceral) reflexes, which generally are not consciously perceived. They involve responses of smooth muscle, cardiac muscle, and glands.
Hemisection is ?
a partial transection of the cord on either the right or the left side. After hemisection, three main symptoms, known together as Brown-Séquard syndrome (sē-KAR), occur below the level of the injury: (1) Damage of the posterior column (sensory tracts) causes loss of proprioception and fine touch sensations on the ipsilateral (same) side as the injury. (2) Damage of the lateral corticospinal tract (motor tract) causes ipsilateral paralysis. (3) Damage of the spinothalamic tracts (sensory tracts) causes loss of pain and temperature sensations on the contralateral (opposite) side.
Shingles is ?
an acute infection of the peripheral nervous system caused by herpes zoster, the virus that also causes chickenpox. After a person recovers from chickenpox, the virus retreats to a posterior root ganglion. If the virus is reactivated, the immune system usually prevents it from spreading. From time to time, however, the reactivated virus overcomes a weakened immune system, leaves the ganglion, and travels down sensory neurons of the skin by fast axonal transport. The result is pain, discoloration of the skin, and a characteristic line of skin blisters. The line of blisters marks the distribution (dermatome) of the particular cutaneous sensory nerve belonging to the infected posterior root ganglion.
muscle spindles and tendon organs are sensitive to what?
muscle spindles are sensitive to changes in muscle length, tendon organs detect and respond to changes in muscle tension that are caused by passive stretch or muscular contraction.
Why do they do a spinal tap? Why is it done with a flexed back?
withdraw cerebrospinal fluid (CSF) for diagnostic purposes; to introduce antibiotics, contrast media for myelography, or anesthetics; to administer chemotherapy; to measure CSF pressure; and/or to evaluate the effects of treatment for diseases such as meningitis. -Flexion of the vertebral column increases the distance between the spinous processes of the vertebrae, which allows easy access to the subarachnoid space.
A tendon reflex operates as follows
1. As the tension applied to a tendon increases, the tendon organ (sensory receptor) is stimulated (depolarized to threshold). 2. Nerve impulses arise and propagate into the spinal cord along a sensory neuron. 3. Within the spinal cord (integrating center), the sensory neuron activates an inhibitory interneuron that synapses with a motor neuron. 4. The inhibitory neurotransmitter inhibits (hyperpolarizes) the motor neuron, which then generates fewer nerve impulses. 5. The muscle relaxes and relieves excess tension.
Steps of flexor reflex
1. Stepping on a tack stimulates the dendrites (sensory receptor) of a pain-sensitive neuron. 2. This sensory neuron generates nerve impulses, which propagate into the spinal cord. 3. Within the spinal cord (integrating center), the sensory neuron activates interneurons that extend to several spinal cord segments. 4.The interneurons activate motor neurons in several spinal cord segments. the motor neurons then generate nerve impulses, which propagate toward the axon terminals. 5. Acetylcholine released by the motor neurons causes the flexor muscles in the thigh (effectors) to contract, producing withdrawal of the leg.
steps of an extensor reflex
1. Stepping on a tack stimulates the sensory receptor of a pain-sensitive neuron in the right foot. 2. This sensory neuron then generates nerve impulses, which propagate into the spinal cord. 3. Within the spinal cord (integrating center), the sensory neuron activates several interneurons that synapse with motor neurons on the left side of the spinal cord in several spinal cord segments. Thus, incoming pain signals cross to the opposite side through interneurons at that level, and at several levels above and below the point of entry into the spinal cord.
What are the three functions of spinal cords and nerves?
1. The white matter of the spinal cord contains sensory and motor tracts, the "highways" for conduction of sensory nerve impulses toward the brain and motor nerve impulses from the brain toward effector tissues. 2. The spinal cord gray matter is a site for integration (summing) of excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs). 3. Spinal nerves and the nerves that branch from them connect the CNS to the sensory receptors, muscles, and glands in all parts of the body.
The pathway followed by nerve impulses that produce a reflex is a reflex arc (reflex circuit). A reflex arc includes the following five functional components:
2. Sensory neuron. The nerve impulses propagate from the sensory receptor along the axon of the sensory neuron to the axon terminals, which are located in the gray matter of the spinal cord or brain stem. From here, relay neurons send nerve impulses to the area of the brain that allows conscious awareness that the reflex has occurred.
The internal organization of the spinal cord allows sensory input and motor output to be processed by the spinal cord in the following way
3. Axons of sensory neurons may extend into the white matter of the spinal cord and ascend to the brain as part of a sensory tract. 4. Axons of sensory neurons may enter the posterior gray horn and synapse with interneurons whose axons extend into the white matter of the spinal cord and then ascend to the brain as part of a sensory tract. 5. Axons of sensory neurons may enter the posterior gray horn and synapse with interneurons that in turn synapse with somatic motor neurons that are involved in spinal reflex pathways.
A stretch reflex operates as follows
3. In the spinal cord (integrating center), the sensory neuron makes an excitatory synapse with, and thereby activates, a motor neuron in the anterior gray horn. 4. If the excitation is strong enough, one or more nerve impulses arises in the motor neuron and propagates, along its axon, which extends from the spinal cord into the anterior root and through peripheral nerves to the stimulated muscle. The axon terminals of the motor neuron form neuromuscular junctions (NMJs) with skeletal muscle fibers of the stretched muscle.
The pathway followed by nerve impulses that produce a reflex is a reflex arc (reflex circuit). A reflex arc includes the following five functional components:
3. Integrating center. One or more regions of gray matter within the CNS acts as an integrating center. In the simplest type of reflex, the integrating center is a single synapse between a sensory neuron and a motor neuron. A reflex pathway having only one synapse in the CNS is termed a monosynaptic reflex arc. More often, the integrating center consists of one or more interneurons, which may relay impulses to other interneurons as well as to a motor neuron. A polysynaptic reflex arc involves more than two types of neurons and more than one CNS synapse.
The pathway followed by nerve impulses that produce a reflex is a reflex arc (reflex circuit). A reflex arc includes the following five functional components:
4. Motor neuron. Impulses triggered by the integrating center propagate out of the CNS along a motor neuron to the part of the body that will respond. 5. Effector. The part of the body that responds to the motor nerve impulse, such as a muscle or gland, is the effector. Its action is called a reflex. If the effector is skeletal muscle, the reflex is a somatic reflex. If the effector is smooth muscle, cardiac muscle, or a gland, the reflex is an autonomic (visceral) reflex.
steps of an extensor reflex
4. The interneurons excite motor neurons in several spinal cord segments that innervate extensor muscles. The motor neurons in turn generate more nerve impulses, which propagate toward the axon terminals. 5. Acetylcholine released by the motor neurons causes extensor muscles in the thigh (effectors) of the unstimulated left limb to contract, producing extension of the left leg. In this way, weight can be placed on the foot that must now support the entire body. A comparable reflex occurs with painful stimulation of the left lower limb or either upper limb.
A stretch reflex operates as follows
5. Acetylcholine released by nerve impulses at the NMJs triggers one or more muscle action potentials in the stretched muscle (effector), and the muscle contracts. Thus, muscle stretch is followed by muscle contraction, which relieves the stretching.
The internal organization of the spinal cord allows sensory input and motor output to be processed by the spinal cord in the following way
6. Motor output from the spinal cord to skeletal muscles involves somatic motor neurons of the anterior gray horn. Many somatic motor neurons are regulated by the brain. Axons from higher brain centers form motor tracts that descend from the brain into the white matter of the spinal cord. There they synapse with somatic motor neurons either directly or indirectly by first synapsing with interneurons that in turn synapse with somatic motor neurons.
The internal organization of the spinal cord allows sensory input and motor output to be processed by the spinal cord in the following way
7 . When activated, somatic motor neurons convey motor output in the form of nerve impulses along their axons, which sequentially pass through the anterior gray horn and anterior root to enter the spinal nerve. From the spinal nerve, axons of somatic motor neurons extend to skeletal muscles of the body.
The internal organization of the spinal cord allows sensory input and motor output to be processed by the spinal cord in the following way
8. Motor output from the spinal cord to cardiac muscle, smooth muscle, and glands involves autonomic motor neurons of the lateral gray horn. When activated, autonomic motor neurons convey motor output in the form of nerve impulses along their axons, which sequentially pass through the lateral gray horn, anterior gray horn, and anterior root to enter the spinal nerve.
The internal organization of the spinal cord allows sensory input and motor output to be processed by the spinal cord in the following way
9. From the spinal nerve, axons of autonomic motor neurons from the spinal cord synapse with another group of autonomic motor neurons located in the peripheral nervous system (PNS). The axons of this second group of autonomic motor neurons in turn synapse with cardiac muscle, smooth muscle, and glands.
Rami
A short distance after passing through its intervertebral foramen, a spinal nerve divides into several branches. These branches are known as rami. The posterior (dorsal) ramus serves the deep muscles and skin of the posterior surface of the trunk. The anterior (ventral) ramus serves the muscles and structures of the upper and lower limbs and the skin of the lateral and anterior surfaces of the trunk. In addition to posterior and anterior rami, spinal nerves also give off a meningeal branch. This branch reenters the vertebral cavity through the intervertebral foramen and supplies the vertebrae, vertebral ligaments, blood vessels of the spinal cord, and meninges. Other branches of a spinal nerve are the rami communicantes, components of the autonomic nervous system.
stretch reflex
A stretch reflex causes contraction of a skeletal muscle (the effector) in response to stretching of the muscle. This type of reflex occurs via a monosynaptic reflex arc. The reflex can occur by activation of a single sensory neuron that forms one synapse in the CNS with a single motor neuron. Stretch reflexes can be elicited by tapping on tendons attached to muscles at the elbow, wrist, knee, and ankle joints. An example of a stretch reflex is the patellar reflex (knee jerk). This reflex is an ipsilateral reflex. Because the stimulus for the stretch reflex is stretching of muscle, this reflex helps avert injury by preventing overstretching of muscles.
Among the somatic reflexes of clinical significance are the following:
Abdominal reflex. This reflex involves contraction of the muscles that compress the abdominal wall in response to stroking the side of the abdomen. The response is an abdominal muscle contraction that causes the umbilicus to move in the direction of the stimulus. Absence of this reflex is associated with lesions of the corticospinal tracts. It may also be absent because of lesions of the peripheral nerves, lesions of integrating centers in the thoracic part of the cord, or multiple sclerosis.
Among the somatic reflexes of clinical significance are the following:
Achilles reflex (a-KIL-ēz) (ankle jerk). This stretch reflex involves plantar flexion of the foot by contraction of the gastrocnemius and soleus muscles in response to tapping the calcaneal (Achilles) tendon. Absence of the Achilles reflex indicates damage to the nerves supplying the posterior leg muscles or to neurons in the lumbosacral region of the spinal cord. This reflex may also disappear in people with chronic diabetes, neurosyphilis, alcoholism, and subarachnoid hemorrhages. An exaggerated Achilles reflex indicates cervical cord compression or a lesion of the motor tracts of the first or second sacral segments of the cord.
spinal cord compression
Although the spinal cord is normally protected by the vertebral column, certain disorders may put pressure on it and disrupt its normal functions. Spinal cord compression may result from fractured vertebrae, herniated intervertebral discs, tumors, osteoporosis, or infections. If the source of the compression is determined before neural tissue is destroyed, spinal cord function usually returns to normal. Depending on the location and degree of compression, symptoms include pain, weakness or paralysis, and either decreased or complete loss of sensation below the level of the injury.
cauda equina
As spinal nerves branch from the spinal cord, they pass laterally to exit the vertebral canal through the intervertebral foramina between adjacent vertebrae. because the spinal cord is shorter than the vertebral column, nerves that arise from the lumbar, sacral, and coccygeal regions of the spinal cord do not leave the vertebral column at the same level they exit the cord. The roots of these lower spinal nerves angle inferiorly alongside the filum terminale in the vertebral canal like wisps of hair. Accordingly, the roots of these nerves are collectively named the cauda equina, "horse's tail".
does brain know about reflex?
Axon collaterals of the muscle spindle sensory neuron also relay nerve impulses to the brain over specific ascending pathways. In this way, the brain receives input about the state of stretch or contraction of skeletal muscles, enabling it to coordinate muscular movements. The nerve impulses that pass to the brain also allow conscious awareness that the reflex has occurred.
Plexus
Axons from the anterior rami of spinal nerves, except for thoracic nerves T2-T12, do not go directly to the body structures they supply. Instead, they form networks on both the left and right sides of the body by joining with various numbers of axons from anterior rami of adjacent nerves. Such a network of axons is called a plexus. The principal plexuses are the cervical plexus, brachial plexus, lumbar plexus, and sacral plexus. A smaller coccygeal plexus is also present. Emerging from the plexuses are nerves bearing names that are often descriptive of the general regions they serve or the course they take. Each of the nerves in turn may have several branches named for the specific structures they innervate.
How are reflex arcs useful?
Because reflexes are normally so predictable, they provide useful information about the health of the nervous system and can greatly aid diagnosis of disease. Damage or disease anywhere along its reflex arc can cause a reflex to be absent or abnormal. For example, tapping the patellar ligament normally causes reflex extension of the knee joint. Absence of the patellar reflex could indicate damage of the sensory or motor neurons, or a spinal cord injury in the lumbar region. Somatic reflexes generally can be tested simply by tapping or stroking the body surface.
lateral gray horns
Between the posterior and anterior gray horns are the lateral gray horns, which are present only in thoracic and upper lumbar segments of the spinal cord. The lateral gray horns contain autonomic motor nuclei, which are clusters of cell bodies of autonomic motor neurons that regulate the activity of cardiac muscle, smooth muscle, and glands.
Epidural block Myelitis Nerve block
-injection of an anesthetic drug into the epidural space, the space between the dura mater and the vertebral column, in order to cause a temporary loss of sensation. Such injections in the lower lumbar region are used to control pain during childbirth. -Inflammation of the spinal cord -Loss of sensation in a region due to injection of a local anesthetic
The spinal cord has what two principal functions in maintaining homeostasis?
-nerve impulse propagation and integration of information. The white matter tracts in the spinal cord are highways for nerve impulse propagation. Sensory input travels along these tracts toward the brain, and motor output travels from the brain along these tracts toward skeletal muscles and other effector tissues. The gray matter of the spinal cord receives and integrates incoming and outgoing information.
Complete transection (tran-SEK-shun; trans- = across; -section = a cut) of the spinal cord means
the cord is severed from one side to the other, thus cutting all sensory and motor tracts. It results in a loss of all sensations and voluntary movement below the level of the transection. A person will have permanent loss of all sensations in dermatomes below the injury because ascending nerve impulses cannot propagate past the transection to reach the brain. At the same time, all voluntary muscle contractions will be lost below the transection because nerve impulses descending from the brain also cannot pass. The extent of paralysis of skeletal muscles depends on the level of injury. The closer the injury is to the head, the greater the area of the body that may be affected.
Horns posterior gray horns
The gray matter on each side of the spinal cord is subdivided into regions called horns (anterior and posterior gray horns). The posterior (dorsal) gray horns contain cell bodies and axons of interneurons as well as axons of incoming sensory neurons. Recall that cell bodies of sensory neurons are located in the posterior (dorsal) root ganglion of a spinal nerve.
nuclei
-In the gray matter of the spinal cord and brain, clusters of neuronal cell bodies form functional groups called nuclei. -Sensory nuclei receive input from receptors via sensory neurons -motor nuclei provide output to effector tissues via motor neurons.
Neuritis Paresthesia
-Inflammation of one or several nerves that may result from irritation to the nerve produced by direct blows, bone fractures, contusions, or penetrating injuries. Additional causes include infections, vitamin deficiency (usually thiamine), and poisons such as carbon monoxide, carbon tetrachloride, heavy metals, and some drugs. - An abnormal sensation such as burning, pricking, tickling, or tingling resulting from a disorder of a sensory nerve.
What types of diseases affect functions of spinal cord?
A number of degenerative diseases affect the functions of the spinal cord. One of these is multiple sclerosis. Another progressive degenerative disease is amyotrophic lateral sclerosis (Lou Gehrig's disease), which affects motor neurons of the brain and spinal cord and results in muscle weakness and atrophy
What is used as a landmark for a spinal tap?
A line drawn across the highest points of the iliac crests, called the supracristal line, passes through the spinous process of the fourth lumbar vertebra and is used as a landmark for administering a spinal tap.
Spinal nerves beginning
Spinal nerves T1-L5 exit the vertebral canal below their corresponding vertebrae. From the spinal cord, the roots of the sacral spinal nerves (S1-S5) and the coccygeal spinal nerves (Co1) enter the sacral canal, the part of the vertebral canal in the sacrum. Subsequently, spinal nerves S1-S4 exit the sacral canal via the four pairs of anterior and posterior sacral foramina, and spinal nerves S5 and Co1 exit the sacral canal via the sacral hiatus.
Spinal nerves
Spinal nerves are associated with the spinal cord and, like all nerves of the peripheral nervous system (PNS), are parallel bundles of axons and their associated neuroglial cells wrapped in several layers of connective tissue. Spinal nerves connect the CNS to sensory receptors, muscles, and glands in all parts of the body. The 31 pairs of spinal nerves are named and numbered according to the region and level of the vertebral column from which they emerge. Not all spinal cord segments are aligned with their corresponding vertebrae (ex cauda equina)
muscle tone
In addition to the large-diameter motor neurons that innervate typical skeletal muscle fibers, smaller-diameter motor neurons innervate smaller, specialized muscle fibers within the muscle spindles themselves. The brain regulates muscle spindle sensitivity through pathways to these smaller motor neurons. This regulation ensures proper muscle spindle signaling over a wide range of muscle lengths during voluntary and reflex contractions. By adjusting how vigorously a muscle spindle responds to stretching, the brain sets an overall level of muscle tone, which is the small degree of contraction present while the muscle is at rest.
Gray commissure anterior white commissure
The gray commissure forms the crossbar of the H. In the center of the gray commissure is a small space called the central canal; it extends the entire length of the spinal cord and is filled with cerebrospinal fluid. Anterior to the gray commissure is the anterior (ventral) white commissure, which connects the white matter of the right and left sides of the spinal cord.
Gray matter
The gray matter of the spinal cord is shaped like the letter H or a butterfly; it consists of dendrites and cell bodies of neurons, unmyelinated axons, and neuroglia. The gray commissure forms the crossbar of the H. At its superior end, the central canal is continuous with the fourth ventricle in the medulla oblongata of the brain. Anterior to the gray commissure is the anterior (ventral) white commissure, which connects the white matter of the right and left sides of the spinal cord.
spinal nerves beginning
The first cervical pair of spinal nerves emerges from the spinal cord between the occipital bone and the atlas (first cervical vertebra, or C1). Most of the remaining spinal nerves emerge from the spinal cord through the intervertebral foramina between adjoining vertebrae. Spinal nerves C1-C7 exit the vertebral canal above their corresponding vertebrae. Spinal nerve C8 exits the vertebral canal between vertebrae C7 and T1.
Protection of CNS
The first layer of protection for the central nervous system is the hard bony skull and vertebral column, providing strong protective defenses against damaging blows or bumps. The second protective layer is the meninges, three membranes that lie between the bony encasement and the nervous tissue in both the brain and spinal cord. Finally, a space between two of the meningeal membranes contains cerebrospinal fluid, a buoyant liquid that suspends the central nervous tissue in a weightless environment while surrounding it with a shock-absorbing, hydraulic cushion.
Flexor reflex
The flexor reflex causes withdrawal of a part of the body in response to a painful stimulus polysynaptic The flexor reflex is ipsilateral. The flexor reflex also illustrates another feature of polysynaptic reflex arcs. Moving your entire lower or upper limb away from a painful stimulus involves contraction of more than one muscle group. Hence, several motor neurons must simultaneously convey impulses to several limb muscles. Because nerve impulses from one sensory neuron ascend and descend in the spinal cord and activate interneurons in several segments of the spinal cord, this type of reflex is called an intersegmental reflex arc.
Dura mater (DOO-ra MĀ-ter = tough mother)
The most superficial of the tree spinal meninges is a thick strong layer composed of dense irregular connective tissue. The dura mater forms a sac from the level of the foramen magnum in the occipital bone, where it is continuous with the meningeal dura mater of the brain, to the second sacral vertebra. The dura mater is also continuous with the epineurium, the outer covering of spinal and cranial nerves.
reflexes
The second way the spinal cord promotes homeostasis is by serving as an integrating center for some reflexes. A reflex is a fast, involuntary, unplanned sequence of actions that occurs in response to a particular stimulus. Some reflexes are inborn, such as pulling your hand away from a hot surface before you even feel that it is hot. Other reflexes are learned or acquired. For instance, you learn many reflexes while acquiring driving expertise. Slamming on the brakes in an emergency is one example.
motor activities
The sensory systems keep the CNS informed of changes in the external and internal environments. The sensory information is integrated (processed) by interneurons in the spinal cord and brain. Responses to the integrative decisions are brought about by motor activities (muscular contractions and glandular secretions). The cerebral cortex, the outer part of the brain, plays a major role in controlling precise voluntary muscular movements. Other brain regions provide important integration for regulation of automatic movements.
dermatome
The skin over the entire body is supplied by somatic sensory neurons that carry nerve impulses from the skin into the spinal cord and brain. Each spinal nerve contains sensory neurons that serve a specific, predictable segment of the body. One of the cranial nerves, the trigeminal (V) nerve, serves most of the skin of the face and scalp. The area of the skin that provides sensory input to the CNS via one pair of spinal nerves or the trigeminal (V) nerve is called a dermatome. The nerve supply in adjacent dermatomes overlaps somewhat.
external anatomy of spinal cord
The spinal cord is roughly oval in shape, being flattened slightly anteriorly and posteriorly. In adults, it extends from the medulla oblongata, the inferior part of the brain, to the superior border of the second lumbar vertebra. In newborn infants, it extends to the third or fourth lumbar vertebra. The length of the adult spinal cord ranges from 16-18 in. Its maximum diameter is approximately 0.6 in. in the lower cervical region and is smaller in the thoracic region and at its inferior tip. A transverse section of the spinal cord reveals regions of white matter that surround an inner core of gray matter.
What can a stretch reflex help with?
The stretch reflex can also help maintain posture. For example, if a standing person begins to lean forward, the gastrocnemius and other calf muscles are stretched. Consequently, stretch reflexes are initiated in these muscles, which cause them to contract and reestablish the body's upright posture. Similar types of stretch reflexes occur in the muscles of the shin when a standing person begins to lean backward.
Tendon reflex
The stretch reflex operates as a feedback mechanism to control muscle length by causing muscle contraction. In contrast, the tendon reflex operates as a feedback mechanism to control muscle tension by causing muscle relaxation before muscle force becomes so great that tendons might be torn. Although the tendon reflex is less sensitive than the stretch reflex, it can override the stretch reflex when tension is great, making you drop a very heavy weight, for example. the tendon reflex is ipsilateral. The sensory receptors for this reflex are called tendon (Golgi tendon) organs, which lie within a tendon near its junction with a muscle. polysynaptic
Arachnoid mater (a-RAK-noyd MĀ-ter; arachn- = spider; -oid = similar to).
This layer, the middle of the meningeal membranes, is a thin, avascular covering comprised of cells and thin, loosely arranged collagen and elastic fibers. It is called the arachnoid mater because of its spider's web arrangement of delicate collagen fibers and some elastic fibers. It is deep to the dura mater and is continuous through the foramen magnum with the arachnoid mater of the brain. Between the dura mater and the arachnoid mater is a thin subdural space, which contains interstitial fluid.
Treatment of post-polio syndrome?
Treatment consists of muscle-strengthening exercises, administration of pyridostigmine to enhance the action of acetylcholine in stimulating muscle contraction, and administration of nerve growth factors to stimulate both nerve and muscle growth.
denticulate ligaments (den-TIK-ū-lāt = small tooth)
Triangular-shaped membranous extensions of the pia mater suspend the spinal cord in the middle of its dural sheath. These extensions, called denticulate ligaments, are thickenings of the pia mater. They project laterally and fuse with the arachnoid mater and inner surface of the dura mater between the anterior and posterior nerve roots of spinal nerves on either side. Extending along the entire length of the spinal cord, the denticulate ligaments protect the spinal cord against sudden displacement that could result in shock.
