12: Neural Pathways Involving the Brain

Flashcards covering the neural pathways involving the brain, including ascending somatosensory pathways and descending motor tracts (pyramidal and extrapyramidal).

Tract

A bundle of axons located within the Central Nervous System (CNS).

Nerve

A bundle of axons located within the Peripheral Nervous System (PNS).

Relay

A chain of 2-3 neurons that contribute to successive tracts along a neural pathway.

Decussation

The crossing over of a pathway from one side of the CNS to the other.

Somatotopy

A precise spatial relationship among tract fibers that reflects an orderly mapping of the body.

Symmetry

The condition where all pathways and tracts are paired (right and left) and present on each side of the spinal cord or brain.

Sensation

Awareness of a stimulus.

Perception

Interpretation of the meaning of a stimulus.

First-order neurons

Sensory neurons that conduct impulses from cutaneous receptors or proprioceptors to the spinal cord or brainstem.

Second-order neurons

Interneurons in the spinal cord or brainstem that extend their axons to the thalamus or cerebellum.

Third-order neurons

Interneurons in the thalamus that relay impulses to the ipsilateral primary somatosensory cortex.

Dorsal column-medial lemniscal pathway

An ascending pathway transmitting impulses for discriminative (fine) touch, vibration, and proprioception from the limbs, trunk, neck, and posterior head.

Fasciculus cuneatus

The lateral portion of the dorsal column that carries signals from the upper limb ($T6$ and above).

Fasciculus gracilis

The medial portion of the dorsal column that carries signals from the lower limb (below $T6$).

Spinothalamic pathway

An ascending pathway transmitting impulses for pain, temperature, crude touch, pressure, itch, and tickle from the limbs, trunk, neck, and posterior head.

Trigeminothalamic pathway

An ascending pathway transmitting all somatosensory impulses from the face, nasal cavity, and oral cavity.

Spinocerebellar pathway

A two-neuron ascending pathway that transmits proprioceptive impulses to the ipsilateral cerebellum for subconscious processing of posture and balance.

Upper motor neurons (UMNs)

Neurons with cell bodies in the motor cortex (precentral gyrus) that synapse with lower motor neurons in the brainstem or spinal cord.

Lower motor neurons (LMNs)

Neurons with cell bodies in brainstem nuclei or the ventral horn of the spinal cord that innervate skeletal muscles.

Pyramidal (direct) tracts

Motor pathways that control rapid, precise, and skilled voluntary movement and involve only one synapse between the UMN and LMN.

Extrapyramidal (indirect) tracts

Complex, multi-synaptic motor pathways involving subcortical nuclei that regulate subconscious (involuntary) motor functions such as balance and posture.

Corticospinal tracts

Direct motor pathways that control voluntary movements of the body excluding the head and neck.

Corticobulbar tracts

Direct motor pathways that control voluntary movements of the head and neck, such as chewing, facial expression, and speech.

Lateral corticospinal tract

A division of the corticospinal tract containing $80-90\%$ of axons that decussate in the medulla oblongata at the pyramidal decussation.

Ventral (anterior) corticospinal tract

A division of the corticospinal tract containing $10-20\%$ of axons that descend ipsilaterally before decussating at the appropriate spinal cord segment.

Reticulospinal tract

An indirect motor pathway originating in the reticular formation (brainstem) that maintains posture and balance.

Vestibulospinal tract

An indirect motor pathway originating in the vestibular nuclei (medulla and pons) that maintains posture and balance.

Rubrospinal tract

An indirect motor pathway originating in the red nucleus (midbrain) that maintains muscle tone in flexor muscles and precise movements of distal upper limbs.

Tectospinal tract

An indirect motor pathway originating in the tectum (posterior midbrain) that controls reflexive movement of the head, eyes, and upper body.