Chapter 13: The Spinal Cord, Spinal Nerves, and Somatic Reflexes

Comprehensive vocabulary flashcards covering the gross and cross-sectional anatomy, tracts, meninges, spinal nerves, plexuses, and somatic reflexes presented in Chapter 13.

Conduction (Spinal Cord)

The spinal cord function wherein nerve fibers conduct sensory and motor information up and down the spinal cord.

Neural Integration

The spinal cord function in which spinal neurons receive input from multiple sources, integrate the information, and execute an appropriate output (such as bladder control).

Central Pattern Generators

Groups of spinal cord neurons that coordinate repetitive sequences of muscle contractions for walking during locomotion.

Spinal Cord

A cylinder of nervous tissue averaging $1.8\,\text{cm}$ thick and $45\,\text{cm}$ long that arises from the brainstem at the foramen magnum, occupies the upper two-thirds of the vertebral canal, ends inferiorly at L1 or slightly beyond, and gives rise to 31 pairs of spinal nerves.

Segment (Spinal Cord)

The specific part of the spinal cord supplied by each pair of spinal nerves.

Cervical Enlargement

The thickened region of the spinal cord that gives rise to nerves supplying the upper limbs.

Lumbosacral Enlargement

The thickened region of the spinal cord that gives rise to nerves supplying the pelvic region and lower limbs.

Medullary Cone (Conus Medullaris)

The tapering, inferior point of the spinal cord located near vertebra L1.

Cauda Equina

A bundle of nerve roots occupying the vertebral canal from L2 to S5 that resembles a horse tail.

Meninges

Three fibrous membranes (dura mater, arachnoid mater, and pia mater) that enclose the brain and spinal cord, separating the soft CNS tissue from the bones of the cranium and vertebral canal.

Dura Mater

The outermost, thick, tough membrane composed of dense irregular connective tissue that forms a loose-fitting dural sheath around the spinal cord.

Epidural Space

The space between the dural sheath and the vertebral bones containing blood vessels, adipose tissue, and loose connective tissue; the site where epidural anesthetics are introduced.

Subarachnoid Space

The space between the arachnoid mater and pia mater, spanned by fibrous trabeculae and filled with cerebrospinal fluid (CSF).

Lumbar Cistern

The subarachnoid space located inferior to the medullary cone; the site where a lumbar puncture (spinal tap) is performed to sample cerebrospinal fluid.

Denticulate Ligaments

Extensions of the pia mater that pass through the arachnoid to attach to the dura mater, anchoring the spinal cord in place.

Terminal Filum

A fibrous strand extending inferiorly from the pia mater that fuses with the dura mater to form the coccygeal ligament.

Gray Matter (Spinal Cord)

The unmyelinated, butterfly-shaped central core of the spinal cord containing neuron cell bodies, dendrites, and proximal axon segments.

Posterior (Dorsal) Horns

Regions of gray matter in the spinal cord that receive sensory nerve fibers and synapse with interneurons.

Anterior (Ventral) Horns

Regions of gray matter in the spinal cord that contain the cell bodies of somatic motor neurons.

Lateral Horns

Lateral projections of spinal gray matter present only in segments T2 through L1, associated with the sympathetic nervous system.

Gray Commissure

A crossbridge of gray matter connecting the left and right sides of the spinal cord, with the central canal at its center.

White Matter (Spinal Cord)

The myelin-rich, pearly white outer region of the spinal cord consisting of axon bundles arranged into posterior, lateral, and anterior funiculi.

Decussation

The crossing of nerve tract fibers across the midline from one side of the central nervous system to the other.

Contralateral

A condition in which the origin and destination of a tract are on opposite sides of the body.

Ipsilateral

A condition in which the origin and destination of a tract are on the same side of the body without decussation.

First-Order Neuron

A sensory neuron that detects a stimulus and transmits the signal to the spinal cord or brainstem.

Second-Order Neuron

A sensory pathway neuron that receives input from a first-order neuron and conducts the signal to the thalamus at the upper end of the brainstem.

Third-Order Neuron

A sensory pathway neuron that carries sensory signals from the thalamus to the sensory region of the cerebral cortex.

Gracile Fasciculus

An ascending tract within the posterior funiculus carrying signals for vibration, visceral pain, deep and discriminative touch, and proprioception from midthoracic and lower body regions.

Cuneate Fasciculus

An ascending tract within the posterior funiculus carrying sensory signals from the chest and upper limbs to the medulla.

Proprioception

The nonvisual sense of the position and movements of the body.

Spinothalamic Tract

An ascending pathway within the anterolateral system carrying signals for pain, pressure, temperature, light touch, tickle, and itch to the contralateral cerebral cortex.

Spinoreticular Tract

An ascending tract in the anterolateral system carrying pain signals from tissue injury that end in the reticular formation of the medulla and pons.

Spinocerebellar Tracts

Posterior and anterior tracts in the lateral funiculus composed of second-order neurons that deliver proprioceptive feedback from the limbs and trunk to the cerebellum.

Upper Motor Neuron

A descending motor neuron originating in the cerebral cortex or brainstem that terminates on a lower motor neuron.

Lower Motor Neuron

A motor neuron whose cell body lies in the brainstem or spinal cord and whose axon projects directly to skeletal muscle or another effector.

Corticospinal Tracts

Descending tracts (lateral and anterior) originating from the primary motor cortex that control precise, finely coordinated skeletal muscle movements.

Tectospinal Tract

A descending motor tract originating in the tectum of the midbrain that coordinates reflex turning of the head in response to visual and auditory stimuli.

Reticulospinal Tracts

Lateral and medial descending tracts originating in the reticular formation of the brainstem that control limb muscles for balance and posture, and contain descending analgesic pathways regulating pain signals.

Vestibulospinal Tracts

Lateral and medial descending tracts originating in the brainstem vestibular nuclei that receive inner ear balance signals and control extensor limb muscles.

Poliomyelitis

A disease caused by poliovirus spreading via fecal contamination of water that destroys motor neurons in the brainstem and anterior horn of the spinal cord, leading to paralysis, muscle atrophy, and potential respiratory arrest.

Amyotrophic Lateral Sclerosis (ALS)

A disease (Lou Gehrig disease) marked by destruction of motor neurons, muscle atrophy, and lateral spinal cord sclerosis caused by astrocytes failing to reabsorb glutamate to prevent its accumulation to toxic levels.

Endoneurium

A thin layer of loose connective tissue surrounding an individual nerve fiber external to the neurilemma.

Perineurium

A connective tissue sheath composed of overlapping squamous cells that encases a fascicle (bundle of nerve fibers).

Epineurium

A tough sheath of dense irregular connective tissue that envelops the entire nerve trunk.

Ganglion

A knot-like cluster of neuron cell bodies (neurosomas) situated outside the central nervous system and wrapped in continuous epineurium.

Posterior (Dorsal) Root

The proximal branch of a spinal nerve that conveys sensory impulses into the posterior horn of the spinal cord via six to eight rootlets.

Anterior (Ventral) Root

The proximal branch formed by six to eight rootlets leaving the anterior aspect of the spinal cord that carries motor signals to effectors.

Anterior Ramus

A distal branch of a spinal nerve that gives rise to intercostal nerves in the thoracic region and converges to form nerve plexuses in other spinal regions.

Posterior Ramus

A distal branch of a spinal nerve that innervates the local muscles and joints of the spine and the skin of the back.

Meningeal Branch

A small distal branch of a spinal nerve that reenters the vertebral canal to innervate the meninges, vertebrae, and spinal ligaments.

Shingles (Herpes Zoster)

A localized reactivation of the varicella-zoster virus stored in posterior root ganglia, traveling down sensory nerves by fast axonal transport to create a painful unilateral vesicular eruption along a dermatome.

Cervical Plexus

A nerve plexus formed by anterior rami of C1 to C5 that innervates the neck and gives rise to the phrenic nerves to the diaphragm.

Brachial Plexus

A nerve network formed by anterior rami of C5 to T1 arranged into roots, trunks, divisions, and cords that supplies the upper limb and parts of the shoulder and neck.

Lumbar Plexus

A nerve network formed by anterior rami of L1 to L4 that innervates the abdominal wall, anterior thigh, and genitalia.

Sacral Plexus

A nerve network formed by anterior rami of L4, L5, and S1 to S4 that supplies the lower trunk and lower limb, giving rise to the sciatic nerve.

Dermatome

A specific area of skin that conveys sensory input to a single spinal nerve.

Somatic Reflexes

Quick, involuntary, stereotyped reactions of skeletal muscles to stimulation mediated by somatic nerve fibers.

Reflex Arc

The neural pathway mediating a reflex action, composed sequentially of a somatic receptor, afferent nerve fiber, integrating center, efferent nerve fiber, and effector.

Muscle Spindle

A stretch receptor proprioceptor embedded in skeletal muscles that monitors muscle length and the rate of length change to coordinate motor output.

Intrafusal Fibers

Modified muscle fibers located within a muscle spindle, innervated at their contractile ends by gamma motor neurons and monitored by sensory fibers.

Extrafusal Fibers

The standard force-producing skeletal muscle fibers situated outside the muscle spindle, innervated by alpha motor neurons.

Stretch (Myotatic) Reflex

A reflex contraction that occurs when a muscle is stretched, acting to maintain equilibrium, smooth movement, and stabilize joints.

Monosynaptic Reflex Arc

A reflex arc featuring a single direct synapse between an afferent neuron and an efferent neuron with minimal synaptic delay (such as the patellar tendon reflex).

Reciprocal Inhibition

A reflex mechanism that inhibits antagonist muscles from contracting against agonist muscles while the agonist is activated.

Flexor (Withdrawal) Reflex

A polysynaptic ipsilateral reflex arc that causes quick contraction of flexor muscles and relaxation of extensors to pull a limb away from a noxious or painful stimulus.

Crossed Extension Reflex

A contralateral reflex arc that causes contraction of extensor muscles in the opposite limb to sustain body balance when a stimulated limb is withdrawn.

Intersegmental Reflex

A reflex where sensory input entering at one spinal level triggers motor output that emerges from different levels or segments of the spinal cord.

Tendon Organs

Encapsulated proprioceptors measuring approximately $0.5\,\text{mm}$ in length situated in tendons near muscle junctions that monitor tension and protect tendons from excessive contraction.

Tendon Reflex

An inhibitory reflex activated by excessive tension on a tendon that moderates muscle contraction before damage or avulsion can occur.

Paraplegia

Paralysis involving both lower limbs, typically resulting from spinal cord damage below the cervical region.

Quadriplegia

Paralysis affecting all four limbs resulting from spinal cord transection or damage in the cervical region.