Lecture 12: Spinal Cord and PNS

Vocabulary flashcards covering spinal cord anatomy, meninges, sensory and motor pathways, peripheral spinal and cranial nerves, plexuses, reflex arcs, and the autonomic nervous system.

Denticulate ligaments

Specialized lateral extensions of the pia mater that adhere to and anchor the spinal cord to the dura mater, providing physical stability and support.

Cauda equina

A bundle of nerve roots that project inferiorly from the end of the spinal cord through the lower vertebral column, resembling a horse's tail.

Lumbar puncture (spinal tap)

A diagnostic procedure performed around vertebrae $L_4/L_5$ where the cauda equina is located, used to safely retrieve cerebrospinal fluid (CSF) from the subarachnoid space without damaging the spinal cord.

Anterior gray horn

The anterior region of gray matter in the spinal cord containing cell bodies (somas) and synapses of somatic motor neurons that innervate skeletal muscles.

Lateral gray horn

The gray matter region found exclusively in spinal cord segments $T_1\text{--}L_2$ that houses cell bodies and synapses of visceral motor (autonomic) neurons.

Posterior gray horn

The region of spinal gray matter where somatic and visceral sensory neurons enter and form synapses with interneurons.

Posterior (dorsal) root ganglion

An enlargement on the posterior root of a spinal nerve that contains the cell bodies (somas) of unipolar sensory neurons.

Posterior white column

The region of spinal cord white matter located posteriorly that exclusively contains ascending sensory axon tracts.

Epineurium

The tough, outermost layer of dense connective tissue that encloses and protects an entire peripheral nerve.

Perineurium

The connective tissue sheath that surrounds bundles of axons called fascicles within a nerve.

Endoneurium

The delicate layer of connective tissue that wraps around an individual axon and its myelin sheath within a peripheral nerve.

Phrenic nerve

A vital nerve originating from spinal cord levels $C_3\text{--}C_5$ that innervates the diaphragm to control breathing; remembered by the adage '$C_3\text{--}C_5$ keep the diaphragm alive.'

Mixed nerves

Nerves that contain both sensory (afferent) and motor (efferent) axons; represents the most common nerve type in the body and includes all $31$ pairs of spinal nerves.

Sensory cranial nerves

The three cranial nerves that contain exclusively sensory fibers: CN I (Olfactory), CN II (Optic), and CN VIII (Vestibulocochlear).

Motor cranial nerves

The five cranial nerves that carry exclusively motor fibers: CN III (Oculomotor), CN IV (Trochlear), CN VI (Abducens), CN XI (Accessory), and CN XII (Hypoglossal).

Mixed cranial nerves

The cranial nerves that contain both sensory and motor functional components: CN V (Trigeminal), CN VII (Facial), and CN IX (Glossopharyngeal).

Somatic sensory pathway

An ascending 3-neuron pathway (1st order unipolar neuron, 2nd order multipolar neuron, 3rd order multipolar neuron) transmitting signals from receptors through the posterior horn and white column, crossing over at the medulla oblongata, and synapsing in the thalamus before reaching the primary somatosensory cortex.

Somatic motor pathway

A descending 2-neuron pathway (upper/1st order multipolar and lower/2nd order multipolar) originating in the primary motor cortex, crossing over at the medulla oblongata, traveling down the lateral white column, and exiting through the anterior gray horn and anterior root to skeletal muscle.

Rami

Anterior and posterior branches that form immediately after the anterior and posterior roots merge into a mixed spinal nerve, carrying both sensory and motor neurons to distinct body areas.

Nerve plexus

A complex network of interwoven nerves formed exclusively by anterior rami of spinal nerves that supply the motor and sensory needs of the limbs.

Brachial plexus

A nerve network formed by anterior rami of $C_5\text{--}T_1$ that innervates the pectoral girdle and upper limb.

Axillary nerve

A terminal branch of the brachial plexus that innervates the deltoid muscle, responsible for shoulder movement and cutaneous shoulder sensation.

Radial nerve

A brachial plexus nerve innervating the triceps brachii and extensor digitorum, mediating elbow, wrist, and digit extension, forearm supination, and sensory detection across the posterior arm and forearm.

Musculocutaneous nerve

A nerve of the brachial plexus that innervates the biceps brachii to flex the elbow and supinate the forearm, and supplies sensation to the lateral forearm.

Median nerve

A nerve arising from the brachial plexus that innervates the pronator teres, mediates wrist flexion and forearm pronation, and provides cutaneous sensation to the lateral hand.

Ulnar nerve

A terminal branch of the brachial plexus innervating the flexor carpi ulnaris to flex the wrist, while providing sensory feedback from the medial hand.

Femoral nerve

A major lumbar plexus nerve that innervates the quadriceps (rectus femoris) to flex the hip and extend the knee, while supplying cutaneous sensory fibers to the anterior thigh and leg.

Obturator nerve

A branch of the lumbar plexus that traverses the obturator foramen to innervate the adductor muscles (adductor magnus) and supply sensation to the medial thigh.

Sciatic nerve

The primary nerve originating from the sacral plexus that innervates the hamstrings (biceps femoris) to extend the hip and flex the knee, and provides sensory innervation to the lower leg.

Dermatome

A specific patch of skin whose sensory receptors are monitored by a single spinal nerve, useful clinically for identifying nerve damage or posterior root ganglion lesions.

Reflex arc

A rapid, involuntary motor response pathway consisting of five ordered components: sensory receptor, sensory neuron, integration center (interneurons in the spinal cord), motor neuron, and effector organ.

Sympathetic division

The 'fight or flight' thoracolumbar autonomic division with preganglionic somas in spinal cord segments $T_1\text{--}L_2$, characterized by short preganglionic axons, ganglia near the spinal cord (sympathetic trunk), long postganglionic axons, and neurotransmitters like norepinephrine and acetylcholine.

Parasympathetic division

The 'rest and digest' craniosacral autonomic division with preganglionic somas in the brainstem or $S_2\text{--}S_4$ spinal cord segments, characterized by long preganglionic axons, ganglia located near or within effector organs, short postganglionic axons, and acetylcholine as its neurotransmitter.