Chapter 13: The Spinal Cord and Spinal Nerves

endoneurium

Individual axons in a nerve, myleinated or unmyelinated, are wrapped in endoneurium (innermost layer, consists of a mesh of collagen fibers, fibroblasts + macrophages).

Polysynaptic reflex arc

Involves 2+ neuron types + 1+ CNS synapse.

The gray matter of the spinal cord

receives + integrates incoming + outgoing info.

Spinal reflex

reflex where integration takes place in the spinal cord gray matter. eg knee jerk/patellar reflex.

Patellar Reflex

stretch reflex involving extension of leg at knee joint by contraction of quadriceps femoris muscle in response to tapping of the patellar ligament. Reflex is blocked by damage to the sensory or motor nerves supplying the muscle or to the integrating centers in L2-4 of the spinal cord.

Autonomic (visceral) reflexes

Reflexes which generally aren't consciously perceived + involve responses of smooth muscle, cardiac muscle + glands. Body fxns (eg heart rate, digestion, urination) = controlled by the ANS through autonomic reflexes.

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.

Pia Mater

innermost meningeal membrane; thin transparent CT that adheres to the surface of the spinal cord + brain. Consists of thin squamous to cuboidal cells w interlacing bundles of collagen fibers + fine elastic fibers. Many blood vessels +triangular membranous extensions (Denticulate ligaments) suspend spinal cord. Btwn arachnoid + pia mater is the subarachnoid space, containing CSF.

spinothalamic tract

conveys nerve impulses for sensing pain, temp + pressure

4 Important somatic spinal reflexes

1. Stretch reflex

posterior column

consists of 2 tracts: gracile fasciculus + cuneate fasciculus. The posterior column tracts convey nerve impulses for discriminative touch, light pressure, vibration + conscious proprioception.

monosynaptic reflex arc

A reflex pathway w1 synapse in the CNS

All monosynaptic reflexes = ___lateral

All monosynaptic reflexes = ipsilateral

How many connections does a spinal nerve have with the spinal cord?

2: the posterior root + anterior root

reflex

fast, involuntary, unplanned sequence of actions responding to a stimulus. Some are inborn, others are learned

spinal nerve

Mixed nerve, connecting to spinal cord. Posterior root contains sensory axons + anterior root contains motor axons.

anterior (ventral) root and rootlets

contain axons of motor neurons, which conduct nerve impulses from the CNS to effectors (muscles and glands).

posterior (dorsal) root + rootlets

contain sensory axons, which conduct nerve impulses from sensory receptors in the skin, muscles+ internal organs into the CNS. Each posterior root has a swelling, the posterior (dorsal) root ganglion, which contains the cell bodies of sensory neurons

Meninges

3 protective CT coverings encircling spinal cord + brain. Superficial to deep: 1) dura mater 2) arachnoid mater +3) pia mater

Impt nerves from the brachial plexus

1) Axillary nerve supplies deltoid + teres minor muscles 2) Musculocutaneous nerve supplies anterior muscles of arm 3) Radial nerve supplies muscles on posterior aspect of arm +forearm 4) Median nerve supplies muscles of anterior forearm + hand 5) Ulnar nerve supplies anteromedial muscles of forearm + hand

Layers of CNS protection

1) skull + vertebral column. 2) meninges (3 membranes btwn bony encasement + nervous tissue in brain + spinal cord). 3) space btwn 2 of the meningeal membranes contains CSF (liquid suspending CNS tissue in weightless environment += shock-absorbing cushion)

Integrating center

1+ regions of gray matter w/in the CNS act as an integrating center. In the simplest type of reflex, the integrating center is a single synapse btwn a sensory neuron + a motor neuron. Usually, the integration center consists of 1+ interneurons which may relay impulses to other interneurons as well as to a motor neuron.

Pairs of spinal nerves

31 pairs emerging from intervertebral foramina. There are 8 pairs of cervical nerves: C1-8, 12 pairs of thoracic nerves T1-12, 5 pairs of lumbar nerves L1-5, 5 pairs of sacral nerves + 1 pair of coccygeal nerves Co1.

Rami

A short distance after passing through its intervertebral foramen, a spinal nerve divides into several branches (rami)

Lumbar Plexus

Anterior rami of spinal nerves L1-L4. Minimal intermingling of fibers. Supplies anterolateral abdominal wall, external genitals+part lower limbs

Sacral Plexus

Anterior rami of spinal nerves L4-L5 + S1-S4 form the sacral plexus. Anterior to the sacrum + supplies the buttocks, perineum + lower limbs. The largest nerve in the body (sciatic nerve) arises from the sacral plexus.

Coccygeal Plexus

Anterior rami of spinal nerves S4-5 + the coccygeal nerves form a small coccygeal plexus which supplies a small area of skin in the coccygeal region

Plexus

Axons from the anterior rami of spinal nerves form networks (plexus) by joining w various numbers of axons from anterior rami of adjacent nerves. Principal plexuses = cervical plexus, brachial plexus, lumbar plexus, and sacral plexus.

Crossed Extensor Reflex

Besides initiating the flexor reflex that causes limb withdrawal, pain impulses also initiate a crossed extensor reflex to help maintain balance. Contralateral reflex arc. Synchronizes the extension of the contralateral limb with withdrawal (flexion) of the stimulated limb. Reciprocal innervation.

Spinal Cord Enlargements

Cervical enlargement, from the C4 to the T1, and the lumbar enlargement, which extends from the T9-12. Visible below the lumbar enlargement is the conus medullaris, a tapering which marks where the spinal cord ends around L2. Further inferior is the filum terminale: anchors spinal cord to coccyx.

Flexor Reflex

E.g. stepping on a tack + immediately withdrawing leg. Polysynaptic reflex arc. Sensory receptors= dendrites. Ipsilateral. Affects multiple muscles/interneurons in multiple segments of the spinal cord, thus is an intersegmental reflex arc. Reciprocal innervation.

Stretch reflex

Contracts skeletal muscle (effector) in response to stretching of the muscle. Occurs via a monosynaptic reflex arc + can occur by activation of a single sensory neuron that forms 1 synapse in CNS w a single motor neuron. Can be elicited by tapping on tendons attached to muscles at elbow, wrist, knee + ankle joint. Sensory receptors= muscle spindles. Ipsilateral

Motor output: 2 descending pathways:

Direct + indirect. Direct pathways: lateral corticospinal, anterior corticospinal + corticobulbar tracts. Nerve impulses from cerebral cortex-> voluntary movements of skeletal muscles. Indirect pathways: rubrospinal, tectospinal, vestibulospinal, lateral reticulospinal +medial reticulospinal tracts. Nerve impulses from brain stem-> automatic mvmts+ coordinate mvmts w visual stimuli+ maintain skeletal muscle tone+ sustain contraction of postural muscles + regulate muscle tone in response to head mvmts

Sensory receptor

Distal end of a sensory neuron or an associated sensory structure serves as a sensory receptor. Responds to a specific stimulus by producing a graded potential. If a generator potential reaches the threshold of depolarization, it will trigger 1+ APs in the sensory neuron.

Tendon Reflex

Feedback mechanism to control muscle tension by causing muscle relaxation before muscle force is great enough to tear tendons. Less sensitive than the stretch reflex, but can override the stretch reflex when tension is great. Ipsilateral. The sensory receptors = tendon (Golgi tendon) organs

perineurium

Groups of axons w endoneurium are held together in bundles (fascicles), each wrapped in perineurium (middle layer, thickest layer of CT, consisting of up to 15 layers of fibroblasts in network of collagen fibers).

white matter tracts in the spinal cord

Highways for AP propagation. Sensory input travels along the tracts toward the brain+ motor output travels from the brain along these tracts toward skeletal muscles + other effector tissues

Meningitis

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 treatment. Bacterial meningitis may be fatal if not treated promptly; viral meningitis usually resolves on its own.

Arachnoid Mater

Middle of meningeal membranes, thin avascular covering comprised of cells + thin, loosely arranged collagen + elastic fibers. Continuous through the foramen magnum w the arachnoid mater of the brain. Btwn dura mater + arachnoid mater of the brain is a thin subdural space, containing ISF.

Dura Mater

Most superficial spinal meninges. Thick layer of dense, irregular CT. Forms a sac from foramen magnum in the occipital bone, where it is continuous w the meningeal dura mater of the brain, to the second sacral vertebra. Also continuous w the epineurium, the outer covering of spinal + cranial nerves.

Poliomyelitis (Polio)

Onset is marked by fever, sever headache, stiff neck + back, deep muscle pain, weakness + loss of certain somatic reflexes. In the most serious form, the virus produces paralysis. Post-polio syndrome: neurological disorder involving slow degenreration of motor neurons that innervate muscle fibers.

epineurium

Outer layer; consists of fibroblasts+ thick collagen fibers. Extensions of epineurium fill spaces btwn fascicles. Dura mater of spinal meninges fuses w epineurium as the nerve passes through the intervertebral foramen.

Spinal Nerves

Paths of communication btwn the spinal cord + body. Part of PNS. Appears segmented b/c the 31 pairs of spinal nerves emerge at regular intervals from intervertebral foramina. Parallel bundles of axons + associated neuroglial cells wrapped in several layers of connective tissue.

reciprocal innervation

Prevents conflict btwn opposing muscles += vital in coordinating body movements. When the stretched muscle contracts during a stretch reflex, antagonistic muscles that oppose the contraction relax.

brachial plexus

Provides nerve supply of the shoulders + upper limbs. Anterior rami of spinal nerves C5-C8 + T1 form the brachial plexus+ form (superior, middle+ inferior) trunks in the inferior part of the neck. Trunks divide into anterior+ posterior divisions that unite to form cords (lateral, medial+ posterior). Principal nerves branch from the cords.

cranial reflex

Reflex where integration occurs in the brain stem. eg tracking movements of the eye as you read

somatic reflexes

Reflex which involves contraction of skeletal muscles. You know about these ones.

cauda equina

Roots (bundles of axons) from the lower spinal nerves (because the spinal cord is shorter than vertebral column, these nerves don't leave the vertebral column at the same level they exit the cord, instead angling inferiorly alongside the filum terminale.

Sensory Neuron

The AP propagates from the sensory receptor along the axon of the sensory neuron to the axon terminals (in gray matter of spinal cord or brain stem). Then relay neurons send nerve impulses to A of the brain allowing awareness of reflex.

Intercostal/thoracic nerves

The anterior rami of spinal nerves T2-T12 do not enter into the formation of plexuses, instead= intercostal/thoracic nerves. Directly connect to structures they supply in the intercostal spaces.

Dermatomes

The area of the skin that provides sensory input to the CNS via 1 pair of spinal nerves or the trigeminal (V) nerve. The nerve supply in adjacent dermatomes overlaps.

Effector

The part of the body that responds to the motor nerve impulse (muscle/gland). Its action is called a reflex. If effector = skeletal muscle: somatic reflex. If effector is smooth muscle, cardiac muscle or a gland: autonomic (visceral) reflex

Reflex arc/circuit

The pathways followed by nerve impulses that produce a reflex. Includes 5 fxnal components: Sensory receptor, Sensory neuron, integrating center, motor neuron +effector.

Shingles

acute infection of the PNS caused by herpes zoster, which retreats to the posterior root ganglion after chicken pox. Causes pain, discoloration of skin + characteristic line of skin blisters

cervical plexus

formed by anterior rami of C1-4 w contributions from C5. There is 1 on each side of the neck alongside the first 4 cervical vertebrae. Supplies the spine + head muscles, neck +superior part of the shoulders + chest. The phrenic nerves arise from the cervical plexuses + supply motor fibers to the diaphrgm. Branches of the cervical plexus run parallel to accessory (XI) + hypoglossal (XII) nerve.

Spinal cord's 2 principal functions in maintaining homeostasis

nerve impulse propagation + integration of info

Babinski sign

results from gentle stroking of the lateral outer margin of the sole. Great toe extends w/ or w/o lateral fanning of other toes. Only in kids less than 1.5 yrs of age b/c incomplete myelination of fibers in corticospinal tract. After 1.5 yrs, the usual response is plantar flexion reflex or negative babinski: curling under of all the toes.

2 main routes on each side of spinal cord for info to brain:

the spinothalamic tract + posterior column. Nerve impulses from sensory receptors propagate up the spinal cord to the brain along these 2 routes.