CNS Lab 3: Pons, Midbrain, Medulla, Spinal Cord, and Histology

Vocabulary practice flashcards covering brainstem levels (pons, midbrain, medulla), spinal cord white/gray matter, peripheral sensory receptors, and cerebellar histology.

Pons at Level of Facial Colliculus (Inferior Level)

Transverse section through the lower pons characterized by the facial colliculus, abducent nucleus, motor nucleus of the facial nerve, trapezoid body, and middle cerebellar peduncles.

Facial Colliculus

An elevation in the floor of the fourth ventricle formed by the motor fibers of the facial nerve (internal genu) looping around the abducent nucleus at the inferior level of the pons.

Abducens Nucleus (VI)

A motor nucleus located in the dorsomedial tegmentum of the lower pons beneath the facial colliculus, giving rise to fibers that emerge ventrally at the pontomedullary junction.

Trapezoid Body

A transverse bundle of auditory fibers located at the ventral boundary of the pontine tegmentum at the inferior level of the pons, formed by decussating acoustic fibers from the cochlear nuclei.

Basis Pontis

The large ventral division of the pons containing longitudinal bundles of corticospinal and corticonuclear fibers, pontine nuclei, and transverse pontocerebellar fibers.

Pons at Level of Trigeminal Nerve (Middle Level)

Transverse section through the middle pons identified by the entry of the trigeminal nerve root (CN V), the motor and main sensory nuclei of CN V, and the superior cerebellar peduncles.

Superior Medullary Velum

A thin layer of white matter stretched between the two superior cerebellar peduncles that forms the roof of the upper part of the fourth ventricle.

Midbrain at Inferior Level

Transverse section through the caudal midbrain characterized by the inferior colliculi in the tectum, the trochlear nucleus (CN IV), and the decussation of the superior cerebellar peduncles in the tegmentum.

Inferior Colliculus

Paired rounded eminences located on the dorsal aspect (tectum) of the caudal midbrain that serve as principal auditory relay nuclei.

Trochlear Nucleus (CN IV)

The motor nucleus located in the ventral periaqueductal gray matter at the level of the inferior colliculus whose emerging fibers decussate dorsally.

Decussation of Superior Cerebellar Peduncles

The crossing of the main efferent fiber bundles of the cerebellum located in the central tegmentum of the caudal midbrain at the level of the inferior colliculus.

Substantia Nigra

A large, dark-pigmented motor nucleus located between the tegmentum and the crus cerebri of the midbrain that produces dopamine.

Midbrain at Superior Level

Transverse section through the rostral midbrain distinguished by the superior colliculi, prominent bilateral red nuclei, the oculomotor nucleus (CN III), and the crus cerebri.

Superior Colliculus

Paired rounded elevations on the dorsal tectum of the upper midbrain involved in visual reflexes and tracking movements.

Red Nucleus

A large, vascular, pinkish-red mass of gray matter situated in the tegmentum of the rostral midbrain that receives cerebellar efferents and gives rise to the rubrospinal tract.

Oculomotor Nucleus (CN III)

A somatic motor and parasympathetic complex located in the periaqueductal gray matter ventral to the cerebral aqueduct at the superior colliculus level.

Crus Cerebri

The massive ventral portion of the cerebral peduncle carrying descending corticospinal, corticonuclear, frontopontine, and temporopontine fibers.

Ventral Tegmental Decussation

The crossing of rubrospinal fibers arising from the red nucleus at the superior level of the midbrain.

Dorsal Tegmental Decussation

The crossing of tectospinal fibers arising from the superior colliculus at the superior level of the midbrain.

Meissner's Corpuscle

An encapsulated sensory receptor composed of modified Schwann cells arranged horizontally around a spiraling nerve terminal, found within the dermal papillae of fingertips to detect light touch.

Pacinian Corpuscle

A large, encapsulated mechanoreceptor with an onion-shaped profile composed of concentric lamellae of modified Schwann cells surrounding a central axon, located in deep connective tissue to sense vibration and pressure.

Muscle Spindle

An encapsulated stretch receptor nestled between extrafusal muscle fibers, containing specialized intrafusal muscle fibers with both afferent sensory and efferent motor innervation.

Intrafusal Muscle Fibers

Modified skeletal muscle fibers found inside the connective tissue capsule of a muscle spindle that monitor changes in muscle length and stretch velocity.

Spinal Cord Regional Levels

Distinct segments exhibiting morphological differences: cervical (broad ventral horn, thin dorsal horn), thoracic (lateral horn present, thin horns), lumbar (broad ventral and dorsal horns, gracile tract only), and sacral (reduced white matter).

Cervical Spinal Cord Segment

A spinal cord section marked by a broad ventral horn, thin dorsal horn, high white matter volume, both fasciculus gracilis and cuneatus, the anterior corticospinal tract, and the medial longitudinal bundle.

Thoracic Spinal Cord Segment

A spinal cord level identifiable by relatively thin dorsal and ventral gray horns alongside a prominent lateral horn (intermediolateral cell column) containing sympathetic preganglionic neurons.

Lumbar Spinal Cord Segment

A spinal cord cross-section distinguished by large, broad ventral and dorsal gray horns, the absence of the cuneate tract (gracile tract only), and the absence of the anterior corticospinal tract.

Fasciculus Gracilis

The medially positioned ascending dorsal column tract carrying conscious proprioception, vibration, and fine touch from the lower limbs and lower trunk (below $T6$).

Fasciculus Cuneatus

The laterally positioned ascending dorsal column tract carrying proprioception, vibration, and fine touch from the upper trunk and upper limbs, present from cervical down through upper thoracic levels ($T6$ and above).

Posterior Spinocerebellar Tract

An ascending tract located on the posterolateral border of the spinal cord that enters the cerebellum via the inferior cerebellar peduncle to relay unconscious proprioception from the trunk and lower limbs.

Anterior Spinocerebellar Tract

An ascending tract situated on the anterolateral margin of the spinal cord that ascends to the midbrain and enters the cerebellum via the superior cerebellar peduncle.

Motor Decussation (Closed Medulla)

The level of the lower closed medulla where approximately $85\%$ of descending corticospinal fibers cross the midline in the pyramids to form the lateral corticospinal tracts.

Internal Arcuate Fibers

Axons of second-order sensory neurons emerging from the nucleus gracilis and nucleus cuneatus that curve anteromedially across the midline to form the medial lemniscus.

Sensory Decussation (Closed Medulla)

The level of the closed medulla where internal arcuate fibers from the gracile and cuneate nuclei decussate in the midline to form the contralateral medial lemniscus.

Open Medulla

Transverse section of the rostral medulla characterized by the open fourth ventricle floor, large convoluted inferior olivary nuclei, pyramids, medial lemnisci, and the hypoglossal and vagal nuclei.

Inferior Olivary Nucleus (Olive)

A large, highly folded, purse-like gray matter lamina in the anterolateral open medulla that receives sensorimotor inputs and projects climbing fibers to the contralateral cerebellum.

Hypoglossal Nucleus (CN XII)

A somatic motor nucleus located in the dorsomedial floor of the fourth ventricle beneath the hypoglossal trigone in the open medulla, innervating intrinsic and extrinsic tongue muscles.

Dorsal Motor Nucleus of Vagus (CN X)

A general visceral efferent (parasympathetic) nucleus located lateral to the hypoglossal nucleus beneath the vagal trigone in the open medulla.

Cerebellar Cortex Layers

The tripartite gray matter layering of the cerebellar cortex consisting of an outer molecular layer, a middle Purkinje cell monolayer, and an inner granular layer.

Purkinje Cell

Large flask-shaped inhibitory projection neurons situated between the molecular and granular layers; their dendritic trees arborize in the molecular layer, and their axons provide the sole output from the cerebellar cortex.

Granule Cells (Cerebellum)

Small, numerous, tightly packed excitatory neurons in the granular layer of the cerebellum whose axons ascend to the molecular layer and bifurcate to form parallel fibers.

Climbing Fibers

Afferent cerebellar axons originating exclusively from the contralateral inferior olivary nucleus that wind directly around the primary dendrites of Purkinje cells.

Mossy Fibers

Afferent cerebellar axons originating from the spinal cord, pontine nuclei, and vestibular system that terminate on granule cell dendrites within cerebellar glomeruli.