Anatomy Chapter 13: Spinal Cord, spinal nerves, spinal reflexes

central nervous system

brain and spinal cord

peripheral nervous system

cranial nerves and spinal nerves


quick, automatic responses triggered by specific stimuli. spinal reflexes are controlled in the spinal cord. They can function without any input from the brain.
* a reflex controlled in your spinal cord makes you drop a sizzling hot frying pan, before the

spinal meninges

three membranous layers that surround the spinal cord. spinal cord ends between L1 and L2. The posterior surface of the spinal cord has a shallow longitudinal groove, the posterior median sulcus. The anterior median fissure is a deeper groove along the an


expanded segments of the spinal cord that have the most gray matter-dedicated to sensory and motor control of the limbs. the cervical enlargement supplies nerves to the shoulder and upper limbs. The lumbar enlargement provides innervation to structures of

conus medullaris

area inferior to lumbar enlargement where the spinal cord becomes tapered and conical

filum terminale

a slender strand of fibrous tissue, extends from inferior tip of conus medullaris

spinal cord division

the entire spinal cord can be divided into 31 segments on the basis of the origins of the spinal nerves. a letter and number designation is used.

dorsal root ganglia

associated with every spinal segment-located near spinal cord. contain the cell bodies of sensory neurons. The axons of the neurons form the dorsal roots, which bring sensory information into the spinal cord. A pair of ventrous roots contains the axons of

spinal nerve

sensory and motor roots. spinal nerves are classified as mixed nerves-they contain both afferent(sensory) and efferent (motor) fibers.31 pairs of spinal nerves.
thoracic spinal nerves named after thoracic vertebrae above them and cervical nerves named aft

spinal meninges 2

a series of specialized membranes surrounding the spinal cord, provide the necessary physical stability and shock absorption. blood vessels branching within these layers deliver oxygen and nutrients to the spinal cord.
the spinal meninges consist of three


caused by bacterial or viral infection. It is inflammation of the meningeal membranes. dangerous because it can disrupt the normal circulation of cerebrospinal fluid, damaging or killing neurons and neuroglia in the affected areas.

dura mater

tough and fibrous. outermost layer covering spinal cord. contains dense collagen fibers that are oriented along the longitudinal axis of the cord.

epidural space

between dura mater and walls of the vertebral canal. contains areolar tissue, blood vessels, and a protective padding of adipose tissue.

subdural space

in a healthy living person there is no subdural space-the inner surface of the dura mater and outer surface of the arachnoid mater are covered by a simple squamous epithelia.

arachnoid mater

middle meningeal layer
includes this epithelium, called the arachnoid membrane, and the arachnoid trabeculae, a delicate network of collagen and elastic fibers that extends between the arachnoid membrane and the outer space of the pia mater. the region be

lumbar puncture or spinal tap

withdrawal of cerebrospinal fluid involves insertion of a needle into the subarachnoid space in the inferior lumbar region

pia mater

innermost meningeal layer. consists of meshwork of elastic and collagen fibers that is firmly bound to the underlying neural tissue. the blood vessels servicing the spinal cord run along the surface of the spinal pia mater, within the subarachnoid space.

denticulate ligaments

extend from the pia mater through the arachnoid mater to the dura mater. they originate along either side of the spinal cord, prevent lateral(side to side) movement


often injected into the epidural space
epidural anesthetic has two advantages
1) it affects only the spinal nerves in the immediate area of the injection and 2) it provides mainly sensory anesthesia
-if a catheter is left in place, continued injection rbi


they are the areas of gray matter one each side of the spinal cord. The gray matter is dominated by the cell bodies of neurons, neuroglia, and unmyelinated axons. The superficial white matter contains large numbers of myelinated and unmyelinated axons.

nuclei(gray matter)

masses of gray matter within the central nervous system. sensory nuclei receive and relay sensory information from peripheral receptors.

motor nuclei(gray matter)

issue motor commands to peripheral effectors.

gray matter info

the posterior gray horns contain somatic and visceral sensory nuclei, and the anterior gray horns contain somatic motor nuclei. The lateral gray horns, located only in the thoracic and lumbar segments contain visceral motor nuclei. The gray commisures pos

white matter

the white matter on each side of the spinal cord can be divided into three regions called columns, or funiculi. the posterior white columns lie between the posterior gray horns and the posterior median sulcus. The anterior white columns lie between the an

white matter cont

each column contains tracts whose axons share functional and structural characteristics. A tract or fasciculus is a bundle of axons in the CNS that is somewhat uniform in diameter, myelination, and propagation speed. ascending tracts carry sensory informa

nerve plexuses

interwoven networks of spinal nerves

spinal nerve..

each is surrounded by a series of connective tissue layers continuous with those of the associated peripheral nerves. epineurium is the outermost layer, it consists of a dense network of collagen fibers. The fibers of the perineum, the middle layer, exten


the specific bilateral region of the skin surface monitored by a single pair of spinal nerves. dermatomes are clinically important because damage or infection of a spinal nerve or dorsal root ganglion produces a loss of sensation in the corresponding regi

peripheral neuropathies/nerve palsies

regional losses of sensory and motor function most often resulting from nerve trauma or compression.


-caused by virus varicella-zoster virus( VZV)
-herpes virus
-attacks the neurons in the dorsal roots of spinal nerves and sensory ganglia of cranial nerves
-produces a painful rash and blisters whose distribution corresponds to that of the affected sensor

four major plexuses

only the ventral rami(branches) form plexuses
1) cervical plexus
2) brachial plexus
3)lumbar plexus
4)sacral plexus

sensory information

a spinal nerve collects sensory information from peripheral structures and delivers it to sensory nuclei in the thoracic or superior lumbar segments of the spinal cord. The dorsal and ventral rami, and the white rami of the rami communicants, also contain

sensory info cont

1) the sympathetic nerve carries sensory information from the visceral organs
2) the ventral ramus carries sensory information from the ventrolateral body surface, structures in the body wall, and the limbs
3) the dorsal ramus carries sensory information

motor commands

a spinal nerve distributes motor commands that originate in motor nuclei of thoracic or superior lumbar segments of the spinal cord

motor commands cont

1) the ventral root of each spinal nerve forms just lateral to the intervertebral foramen, where the dorsal and ventral roots unite
2) the spinal nerve forms just lateral to the intervertebral foramen, where the dorsal and ventral roots unite
3) the dorsa

cervical plexus

consists of the ventral rami of spinal nerves C1-C5. the branches of the cervical plexus innervate the muscles of the neck and extend into the thoracic cavity where the control the diaphragmatic muscles. The phrenic nerve is the major nerve of the cervica

brachial plexus

innervates the pectoral girdle and upper limb, with contributions from the ventral rami of spinal nerves C5-T1. The brachial plexus can also have fibers from C4, T2, or both. the nerves that form this plexus originate from trunks and cords


large bundles of axons from several spinal nerves.


smaller branches that originate at trunks
*we name both trunks and cords according to their location relative to the axillary artery, a large artery supplying the upper limb


the lateral cord forms the musculocutaneous nerve exclusively, and together with the medial cord, contributes to the median nerve. The ulnar nerve is the other major nerve of the medial cord. The posterior cord gives rise to the axillary nerve and radial

the lumbar plexus

arise from the lumbar segments of the spinal cord. nerves arising at this plexus innervate the pelvic girdle and lower limbs. the lumbar plexus contains axons from the ventral rami of spinal nerves T12-L4. The major nerves of this plexus are the genitofem

the sacral plexus

arise from the sacral segments of the spinal cord. nerves arising at this plexus innervate the pelvic girdle and lower limbs.contains axons from the ventral rami of spinal nerves L4-S4. Two major nerves arise at this plexus: the sciatic nerve and the pude

sensory innervation in the hand

sensory function in the hand is serviced by three nerves of the brachial plexus. In carpal tunnel syndrome compression of one or more nerves at the wrist cause changes or losses of sensation in the areas innervated by the affected nerves.

musculocutaneous nerve

spinal segments: C5-T1
distribution:flexor muscles on the arm (biceps brachii, brachialis, and coracobrachialis); sensory from skin over the lateral surface of the forearm through the lateral antebrachial cutaneous nerve

radial nerve

spinal segments C5-T1
distribution: many extensor muscles on the arm and forearm (triceps brachii, anconeus, extensor carpi radialis, extensor carpi ulnaris, and brachioradialis) and abductor pollicis muscle by the deep branch; sensory from over the poste

median nerve

spinal segments: C6-T1
Distribution: flexor muscles on the forearm (flexor carpi radialis and palmaris longus muscles); pronator quadratus and pronator teres muscles; digital flexors(through anterior interosseous nerve); sensory from skin over the anterol

ulnar nerve

spinal segments: C8-T1
Distribution:flexor carpi ulnaris muscle, flexor digitorum profundus muscle, adductor pollicis muscle, and small digital muscles by the deep branch; sensory from skin over medial surface of the hand through the superficial branch

nerves of the brachial plexus

the nerves of the brachial plexus arise from one or more trunks or cords whose names indicate their positions relative to the axillary artery, a large artery supplying the upper limb

dorsal scapular nerve

spinal segment: C5
Distribution: rhomboid and levator scapulae muscles

nerve to subclavius

spinal segment: C5-C6
distribution: subclavius muscle

suprascapular nerve

spinal segment: C5-C6
distribution: supraspinatus and infraspinatus muscles; sensory fro shoulder joint and scapula

pectoral (medial and lateral) nerve

spinal segment: C5-T1
distribution: pectoralis muscles

subscapular nerve

spinal segments: C5-C6
distribution: subscapularis and teres major muscles

thoracodorsal nerve

spinal segments: C6-C8
distribution: latissimus dorsi muscle

axillary nerve

spinal segments: C5-C6
distribution: deltoid and teres minor muscles

medial antibrachial cutaneous

spinal segments: C8-T1
distribution: sensory from skin over anterior and medial surface of the arm and forearm

iliohypogastric nerve

spinal segments: T12-L1
distribution: external and internal oblique and transverse abdominis muscle; skin over the inferior abdomen and buttocks

ilioguinal nerve

spinal segments: L1
distribution: abdominal muscles (with iliohypogastric nerve); skin over superior, medial thigh and portions of external genitalia

genitofemoral nerve

spinal segments: L1-L2
distribution: skin over anteromedial thigh and portions of external genitalia

lateral femoral cutaneous nerve

spinal segments: L2-L3
distribution: skin over anterior, lateral, and posterior thigh

femoral nerve

spinal segments: L2-L4
distribution: quadriceps, Sartorius, pectineus, and iliopsoas muscles; skin of the anteromedial thigh, and medial surface of the leg and foot

obturator nerve

spinal segment: L2-L4
distribution: gracilis, and adductor magnus, brevis and longus muscles; skin from the medial surface of the thigh

superior gluteal nerve

spinal segments: L4-S2
distribution:gluteus minimus, gluteus medius, and tensor fasciae latae muscles

inferior gluteal nerve

spinal segments: L4-S2
distribution: gluteus maximus muscle

posterior femoral cutaneous nerve

spinal segments: S1-S3
distribution:skin over perineum and posterior thigh and leg

sciatic nerve

spinal segment: L4-S3
distribution: semimembranosus, semitendinosus, and adductor magnus muscles; branches into tibial and fibular nerves


spinal segment: S2-S4
distribution: muscles of perineum; skin of external genitalia, bulbospongiosus and ishciocavernosus muscles

sensory innervation in the ankle and foot

the normal distribution of sensory nerves innervating the ankle and foot. Mapping of touch and pain perception can be combined with observations of muscle function to detect nerve damage affecting specific peripheral nerves.

neuronal pools

functional groups of interconnected neurons. A neuronal pool may be scattered, involving neurons in several regions of the brain, or localized, with neurons restricted to one specific location in the brain or spinal cord. each pool has a limited number of

five neural circuit patterns

1. Divergence
2. Convergence
3. Serial Processing
4. Parallel processing
5. Reverberation


-spread of information from one neuron to several neurons, or from a pool to multiple pools.
-permits broad distribution of a specific input
-considerable divergence occurs when u bring info into the CNS
-example: visual info from eyes goes to your consci


several neurons synapse on a single post synaptic neuron. Several patterns of activity in the presynaptic neurons can thereof have the same effect on the post synaptic neuron.
-through convergence the same motor neurons can be subject to both conscious an

serial processing

information is relayed in a stepwise fashion, from one neuron to another or from one neuronal pool to the next
-this pattern occurs as sensory information is relayed from one part of the brain to another.
-for example, pain sensations on their way to your

parallel processing

occurs when several neurons or neuronal pools process the same information simultaneously. Divergence must take place before parallel processing can occur. as a result, many responses can occur simultaneously.
-example: withdraw foot, shift body weight, m


collateral branches of axons somewhere along the circuit extend back toward the source of an impulse and further stimulate the presynaptic neurons.
-once a reverberating circuit has been activated, it will continue to function until synaptic fatigue or in


-rapid, automatic responses to specific stimuli
-reflexes preserve homeostasis by making rapid adjustments in the function of organs or organ systems.

reflex arc

route followed by nerve impulses to produce a reflex.

Five steps in a simple neural reflex

1) The arrival of a stimulus and activation of a receptor: reflex arcs begin with activation of a receptor, which may be a specialized cell or the dendrites of a sensory neuron. receptors are sensitive to physical or chemical changes in the body or to cha


A reflex response generally removes or opposes the original stimulus, and this is an example of negative feedback. However, reflexes may be integrated to produce complex movements that support or enhance the primary response.
By opposing potentially harmf

events in a reflex arc

the "wiring" of a single reflex is called a reflex arc. It begins at a receptor and ends at a peripheral effector such as a muscle fiber or a gland cell.

monosynaptic reflex

in the simplest reflex arc, a sensory neuron synapses directly on a motor neuron. Because there is only one synapse, it is called a monosynaptic reflex. Transmission across a chemical synapse always involves a synaptic delay, but will only one synapse, th

stretch reflex

an example of a monosynaptic reflex Because there is only one synapse, there is little delay between sensory input and motor output. These reflexes control the most rapid motor responses of the nervous system. A stretch reflex provides automatic regulatio

the patellar reflex

an example of a stretch reflex. The stimulus is a tap on the patellar tendon that stretches receptors within the quadriceps muscles. The response is a brief contraction of those muscles, which produces a noticeable kick.

polysynaptic reflexes

they can produce far more complicated responses than monosynaptic reflexes, because the interneurons can control motor neurons that activate several muscle groups simultaneously.

withdrawal reflex

moves affected parts of the body away from a stimulus. A flexor reflex is an example of a withdrawal reflex that affects the muscles of a limb. In this example , the stimulus of a hot frying pan causes the contraction of the flexor muscles of the arm, yan

A crossed extensor reflex

involves a contralateral reflex arc. In other words, a motor response to the stimulus also occurs on a side opposite the stimulus. The crossed extensor reflex complements the flexor reflex and the two occur simultaneously.

classification of reflexes

based on 1) their development 2) the nature of the resulting motor response 3) the complexity of the neural circuit involved or 4) the site of information processing

innate reflexes

result from the connections that form between neurons during development. genetically programmed

acquired reflexes

more complex, learned motor patterns. repetition enhances such reflexes

somatic reflexes

-provide a mechanism for the involuntary control of the muscular system.
- superficial reflexes are triggered by stimuli a the skin or mucous membranes. stretch reflexes are triggered by sudden elongation of a tendon

visceral reflexes

also known as autonomic reflexes, control the activities of the other systems. somatic reflexes are absolutely vital, primarily because they are immediate. Making decisions and coordinating voluntary responses take time. Thus,somatic reflexes provide a ra


monosynaptic reflexes involve the simplest reflex arc. Most reflexes are complicated and have at least one interneuron between the sensory neuron and the motor neuron. These types are called postsynaptic reflexes. Postsynaptic reflexes have a longer delay

spinal reflexes vary in complexity

spinal reflexes range in complexity from single monosynaptic reflexes involving a single segment of the spinal cord to polysynaptic reflexes that involve many segments. In the most complicated spinal reflexes, called intersegmental reflex arcs, many segme

monosynaptic reflexes

there is little delay between sensory input and motor output. these reflexes control the most rapid, stereotyped (preexisting, mechanically repetitive) motor responses of the nervous system to specific stimuli.

the stretch reflex

best known monosynaptic reflex. it automatically regulates skeletal muscle length. The knee jerk or patellar reflex is an example.
When a physician taps on the patellar tendon with a reflex hammer, receptors in the quadriceps muscles are stretched. The di

stretch reflex cont

to summarize, the stimulus(increasing muscle length) activates sensory neuron, which triggers an immediate motor response(contraction of the stretched muscle) that counteracts the stimulus. The entire reflex is completed 20-40 msec because the action pote

muscle spindles

The sensory receptors involved in the stretch reflex are muscle spindles. Each consists of a small, specialized skeletal muscle fibers called intrafusal muscle fibers. The muscle spindle is surrounded by larger skeletal muscle fibers, called extrafusal mu

postural reflexes

reflexes that help us maintain a normal upright posture. Postural muscles generally maintain a firm muscle tone and have extremely sensitive stretch receptors.

gamma efferents

They let the CNS adjust the sensitivity of muscle spindles. Gamma efferents play a vital role whenever voluntary contractions change the length of a muscle.

polysynaptic reflexes cont

the interneurons involved can control several muscle groups. Moreover, these interneurons may produce either excitatory or inhibitory postsynaptic potentials (EPSPs or IPSPs) at CNS motor nuclei. As a result, the response can involve the stimulation of so

tendon reflex

the stretch reflex regulates the length of a skeletal muscle. The tendon reflex monitors the external tension produced during a muscular contraction and prevents tearing or breaking of the tendons. The receptors are stimulated when the collagen fibers are

withdrawal reflexes

withdrawal reflexes move affected parts of the body away from a stimulus. The flexor reflex, a representative withdrawal reflex, affects the muscles of a limb. when a specific muscle contracts, opposing muscles must relax to permit the movement. Interneur

crossed extensor reflexes

the stretch, tendon, and withdrawal reflexes involve ipsilateral(same side) reflex arcs. The sensory stimulus and the motor response occur on the same side of the body. The crossed extensor reflex involves a contralateral reflex arc(opposite) because the

general characteristics of polysynaptic reflexes

- They involve pools of interneurons:processing takes place in pools of interneurons before motor neurons are activated. The result may be excitation or inhibition. The tendon reflex inhibits motor neurons, and the flexor and crossed extensor reflexes dir

brain can affect spinal-cord based reflexes

Reflex motor behaviors happen automatically, without instructions from higher centers. However, higher centers can have a profound effect on the performance of a reflex. Processing centers in the brain can facilitate or inhibit reflex motor patterns based

voluntary movements and reflex motor patterns

The descending pathways from the brain provide appropriate facilitation, inhibition, or "fine-tuning" of the established patterns.
At the highest level, centers in the brain can modulate or build on reflexive motor patterns

reinforcement and inhibition

-A single EPSP may not depolarize the postsynaptic neuron sufficiently to generate an action potential, but it does make that neuron more sensitive to other excitatory stimuli.
-By stimulating excitatory or inhibitory interneurons within the brain stem or