Spinal Cord, Spinal Nerves, and Autonomic Nervous System

Comprehensive vocabulary flashcards covering the gross anatomy, sensory and motor pathways, reflex mechanisms of the spinal cord, and the functional organization, divisions, and neurotransmitters of the autonomic nervous system.

Conus medullaris

The tapering inferior end of the spinal cord proper, typically located at the level of the $L_1$ vertebra.

Cauda equina

The bundle of spinal nerve roots extending inferiorly through the vertebral canal from the conus medullaris, resembling a horse's tail.

Filum terminale

A thin strand of pia mater extending from the conus medullaris that anchors the inferior end of the spinal cord to the coccyx.

Denticulate ligaments

Paired lateral, tooth-like extensions of the pia mater that attach to the dura mater to suspend and anchor the spinal cord laterally.

Posterior root ganglion

An enlargement along the posterior root of a spinal nerve that contains the cell bodies of somatic and visceral sensory neurons.

Posterior funiculus–medial lemniscal pathway

An ascending sensory pathway that conducts signals for discriminative touch, precise pressure, vibration, and proprioception to the primary somatosensory cortex, decussating in the medulla oblongata via the medial lemniscus.

Anterolateral pathway

An ascending sensory tract system (including anterior and lateral spinothalamic tracts) that relays sensations of pain, temperature, crude touch, and pressure to the thalamus and somatosensory cortex.

Spinocerebellar pathway

An ascending sensory pathway composed of anterior and posterior spinocerebellar tracts that relays proprioceptive information from joints, muscles, and tendons directly to the cerebellum.

Direct (pyramidal) pathway

A descending motor pathway originating from upper motor neurons in the primary motor cortex that directly synapses on lower motor neurons in the brainstem or spinal cord anterior horns to control conscious skeletal muscle movements.

Indirect pathway

A motor pathway originating in brainstem nuclei (including rubrospinal, reticulospinal, tectospinal, and vestibulospinal tracts) that regulates reflexive, unconscious control of muscle tone, posture, balance, and limb movements.

Dermatome

A specific segment of skin innervated by cutaneous sensory fibers of a single spinal nerve.

Nerve plexus

A network of interweaving anterior rami of spinal nerves, including the cervical, brachial, lumbar, and sacral plexuses, which redistributes fibers so that damage to a single spinal segment does not completely denervate a target region.

Phrenic nerve

A major branch of the cervical plexus (primarily receiving fibers from $C_3-C_5$) that provides motor innervation to the diaphragm.

Sciatic nerve

The largest and longest nerve in the body, originating from the sacral plexus and composed of two major divisions wrapped in a single connective tissue sheath: the tibial division and the common fibular division.

Reflex arc

The neural pathway responsible for generating an involuntary reflex response, typically consisting of a receptor, sensory neuron, integration center, motor neuron, and effector.

Monosynaptic reflex

A reflex arc characterized by direct synaptic communication between a sensory neuron and a motor neuron without intervening interneurons, exemplified by the stretch reflex.

Polysynaptic reflex

A reflex arc that includes one or more interneurons between the sensory neuron and the motor neuron, exemplified by the withdrawal reflex.

Reciprocal inhibition

A neural mechanism during a stretch reflex wherein collateral branches of sensory axons synapse with inhibitory interneurons that prevent contraction of the antagonist muscle.

Reciprocal activation

A neural mechanism in the Golgi tendon reflex where sensory axons stimulate interneurons that excite motor neurons to antagonist muscles while inhibiting the agonist muscle to relieve excess tendon tension.

Crossed-extensor reflex

A contralateral polysynaptic reflex accompanying a withdrawal reflex, wherein interneurons cross the spinal cord to excite extensor motor neurons on the opposite side of the body to support body weight.

Babinski sign

An abnormal plantar reflex response in adults (normal in infants) characterized by extension of the great toe and fanning of the other toes, indicative of spinal cord damage or upper motor neuron lesions.

Clonus

A sequence of rhythmic, oscillating involuntary muscle contractions and relaxations observed during reflex testing, typically indicating hyperactive reflexes from central nervous system damage.

Autonomic Nervous System (ANS)

The visceral motor division of the nervous system that unconsciously regulates the activity of cardiac muscle, smooth muscle, and glands using a two-neuron pathway.

Preganglionic neuron

The first neuron in an autonomic pathway whose cell body resides in the central nervous system (brainstem or spinal cord) and whose myelinated axon projects to an autonomic ganglion.

Ganglionic neuron

The second neuron in an autonomic pathway whose cell body is located in an autonomic ganglion and whose unmyelinated postganglionic axon extends to the effector organ.

Hypothalamus (in ANS regulation)

The master integration and command center for autonomic functions and emotional responses, directly influencing brainstem and spinal cord reflex centers.

Parasympathetic division

The craniosacral division of the ANS that maintains homeostasis under resting conditions ('rest-and-digest') by conserving energy and replenishing nutrient stores.

Sympathetic division

The thoracolumbar division of the ANS, originating from the lateral horns of the $T_1-L_2$ spinal cord segments, that prepares the body for physical activity, exercise, and emergencies ('fight-or-flight').

White rami communicantes

Short nerve pathways carrying myelinated preganglionic sympathetic axons from $T_1-L_2$ spinal nerves into the sympathetic trunk ('entrance ramps').

Gray rami communicantes

Short nerve pathways carrying unmyelinated postganglionic sympathetic axons from the sympathetic trunk out to all spinal nerves ('exit ramps').

Prevertebral ganglia

Sympathetic ganglia located anterior to the vertebral column on the surface of the abdominal aorta (including celiac, superior mesenteric, and inferior mesenteric ganglia) where splanchnic nerves synapse.

Pelvic splanchnic nerves

Parasympathetic preganglionic nerve bundles originating from the lateral gray regions of spinal cord segments $S_2-S_4$ that innervate distal abdominal and pelvic organs.

Enteric Nervous System (ENS)

An autonomic array of motor and visceral sensory neurons located entirely within the wall of the gastrointestinal tract that coordinates smooth muscle motility (peristalsis) and gland secretion.

Nicotinic receptors

Cholinergic receptors located on all autonomic ganglionic neurons, the adrenal medulla, and neuromuscular junctions; binding of acetylcholine opens cation channels ($Na^+$ influx) to produce excitation.

Muscarinic receptors

Cholinergic receptors found on all parasympathetic effector targets and select sympathetic targets (sweat glands); they utilize second-messenger systems to produce either excitatory or inhibitory responses depending on the tissue.

$\alpha_1$ adrenergic receptor

An adrenergic receptor subtype located on blood vessels of skin, viscera, and kidneys, whose stimulation causes smooth muscle contraction and vasoconstriction.

$\beta_1$ adrenergic receptor

An adrenergic receptor subtype located predominantly in cardiac muscle (SA node, myocardium) and kidneys; activation increases heart rate, force of contraction, and renin secretion.

$\beta_2$ adrenergic receptor

An adrenergic receptor subtype found on smooth muscle of bronchioles, coronary blood vessels, skeletal muscle blood vessels, and the GI tract; activation stimulates smooth muscle relaxation (bronchodilation and vasodilation).

Autonomic tone

The continuous, basal release of neurotransmitters from both sympathetic and parasympathetic divisions that maintains a baseline state of activity in effector organs.

Vasomotor tone

The ongoing partial constriction of systemic blood vessels maintained continuously by baseline sympathetic activity without parasympathetic innervation.

Dual innervation

The innervation of an organ or tissue by both sympathetic and parasympathetic divisions, resulting in either antagonistic actions (e.g., heart rate regulation) or cooperative effects.

Raynaud syndrome

A condition characterized by sudden, painful, exaggerated sympathetic vasoconstriction of small arteries in the digits, usually triggered by cold exposure or emotional stress.

Autonomic dysreflexia

A potentially life-threatening syndrome in individuals with spinal cord injuries at or above $T_6$, where a stimulus below the lesion triggers massive, unopposed sympathetic reflex activity and severe hypertension.