7). Basal Ganglia, Thalamus, Somatosensory, and Motor Systems

Vocabulary flashcards covering neuroanatomy topics including the thalamus, basal ganglia, limbic system, somatosensory pathways, nerve fiber classifications, trigeminal system, motor pathways, and upper vs lower motor neuron deficits.

Thalamus

A small, egg-shaped structure in the brain that acts as a relay station for most sensory information (except smell), processing and transmitting it to the cerebral cortex for interpretation.

Lateral Geniculate Nucleus (LGN)

A thalamic visual relay nucleus where all visual information from the retina travels via optic tracts for processing before being sent to the primary visual cortex in the occipital lobe.

Superior Colliculi

Paired dorsal midbrain structures involved in processing:

• Optical stimuli

• Orienting attention

• Coordinating eye and head movements

Basal Ganglia

A subcortical collection of interacting clusters of cell bodies involved in:

• Reward, emotional, and motor circuits

• Comprising the caudate, putamen, nucleus accumbens, globus pallidus, substantia nigra, and subthalamic nucleus.

Nucleus Accumbens

A structure identified as the center of joy in the CNS that plays a crucial role in explaining how motivation and emotion influence visual attention, learning, and therapeutic engagement.

Limbic System

A neural system that influences:

• Emotions

• Visceral responses to emotion

• Motivation

• Mood

• Sensations of pain and pleasure

• Giving meaning to visual data by integrating it with memory and behavior.

Posterior Column–Medial Lemniscus System (DC/ML)

A somatosensory tract responsible for;

• Fine discriminative touch

• Pressure

• Proprioception

• Vibration perception

Anterolateral System (ALS / Spinothalamic Tract)

A somatosensory pathway responsible for transmitting pain, temperature, and crude touch information to the brain.

Trigeminal Sensory System

A sensory system responsible for the transmission of

• Tactile

• Proprioceptive

• Pain

• Temperature information from the head to the cerebral cortex, cerebellum, and reticular formation.

Proprioceptors

Sensory receptors found deep in muscles and joints that sense position and movement of body parts, providing information for body awareness.

First-Order Neurons

Sensory neurons that carry signals from sensory receptors to the central nervous system, with cell bodies located in the:

• Dorsal root ganglion = body

• Trigeminal ganglion = face

Second-Order Neurons

Sensory neurons that relay signals from first-order neurons to the thalamus, with cell bodies in the:

• Dorsal horn of the spinal cord or medullary nuclei for the body

• Trigeminal sensory nuclei for the face.

Third-Order Neurons

Sensory neurons that transmit signals from the thalamus to the cerebral cortex, projecting from the:

• Ventral posterolateral (VPL) nucleus for the body.

• Ventral posteromedial (VPM) nucleus for the face.

Primary Somatosensory Cortex

The cortical target in the postcentral gyrus receiving somatosensory signals, which then project to the secondary somatosensory cortex in the parietal operculum and posterior parietal lobule (areas 5 and 7).

Muscle Spindle

The major stretch receptor located within muscles.

A Alpha Nerve Fibers

Nerve fibers with a diameter of $12\text{--}20\,\mu\text{m}$ and conduction velocity of $70\text{--}120\,\text{m/s}$, responsible for proprioception and motor functions.

A Beta Nerve Fibers

Nerve fibers with a diameter of $5\text{--}12\,\mu\text{m}$ and conduction velocity of $30\text{--}70\,\text{m/s}$, responsible for pressure and touch.

A Gamma Nerve Fibers

Nerve fibers with a diameter of $3\text{--}6\,\mu\text{m}$ and conduction velocity of $15\text{--}30\,\text{m/s}$, responsible for muscle spindle innervation.

A Delta Nerve Fibers

Nerve fibers with a diameter of $2\text{--}5\,\mu\text{m}$ and conduction velocity of $12\text{--}30\,\text{m/s}$, responsible for touch, cold temperature, and fast pain.

B Nerve Fibers

Nerve fibers with a diameter of $< 3\,\mu\text{m}$ and conduction velocity of $3\text{--}15\,\text{m/s}$, serving autonomic functions.

C Nerve Fibers

Nerve fibers with a diameter of $0.3\text{--}1.3\,\mu\text{m}$ and conduction velocity of $0.7\text{--}2.3\,\text{m/s}$, serving autonomic functions and slow pain.

Ipsilateral

Located on or affecting the same side of the body.

Contralateral

Located on or affecting the opposite side of the body.

Trigeminal Ganglion

The cranial nerve equivalent of a dorsal root ganglion, containing cell bodies for the three main trigeminal branches:

1. Ophthalmic (V1)

2. Maxillary (V2)

3. Mandibular (V3)

Ruffini Ending

The primary mechanoreceptor located in the periodontal ligament (PDL).

Corneal Blink Reflex

A test of trigeminal nerve function mediated by polysynaptic connections in the brainstem between the trigeminal (CN V) and facial (CN VII) nerves.

Trigeminal Neuralgia (Tic Douloureux)

A condition producing excruciating, lightning strikes of facial pain lasting $< 1\,\text{minute}$ near the nose, lips, eyes, or ears without sensory loss

(Most often in the maxillary or mandibular branch distribution in patients over 40.)

Stretch Reflex

A monosynaptic reflex elicited when a muscle tendon is tapped with a soft rubber hammer, causing muscle fibers to contract.

Corticobulbar Tract

A pyramidal motor tract that innervates motor neurons of cranial nerve nuclei 5, 7, 12, and nucleus ambiguus (9+10);

(Its innervation is primarily bilateral, with the exception of CN VII.)

Corticospinal Tract

A collection of pyramidal motor axons controlling voluntary movement of the trunk and limbs;

• 85% decussating in the lower medulla to form the lateral corticospinal tract.

• 15% remaining uncrossed as the anterior corticospinal tract.

Upper Motor Neurons (UMN)

Motor neurons that:

• Originate in the motor cortex

• Terminate within the medulla or spinal cord

Damage results in spastic paralysis and exaggerated reflexes without muscle atrophy.

Lower Motor Neurons (LMN)

Motor neurons with cell bodies in the spinal cord or brainstem that innervate skeletal muscles.

Loss leads to:

• Weakness

• Muscle twitching

• Flaccid paralysis

• Muscle atrophy

Hemiparesis

Weakness affecting one side of the body.

Hemiplegia

Paralysis affecting only one side of the body, commonly caused by lesions of the corticospinal tract.

Babinski's Reflex

An abnormal reflex characterized by extension of the great toe toward the top of the foot and fanning of other toes when the sole is firmly stroked, indicating damage to the corticospinal tract.

Extrapyramidal Pathways

Involuntary motor pathways originating in the brainstem (including tectospinal, rubrospinal, reticulospinal, and vestibulospinal tracts) that regulate:

• Posture

• Muscle tone

• Balance

• Coordination

• Automatic visual reflexes

Decorticate Rigidity

A posture released by loss of cortical control characterized by elbow and wrist flexion with leg and foot extension;

(Caused by diffuse cerebral damage located above the red nuclei)

Decerebrate State

A condition characterized by exaggerated extensor reflexes leading to rigid extension of the limbs, hyperreflexia, and opisthotonus.

(Caused by brainstem lesions situated between the red nuclei and vestibular nuclei.)