7.2 Somatic Sensations, Motor/Cerebral Circuits & the Autonomic Nervous System

Somatic Sensations

Somatic sensation includes touch, pressure, vibration, temperature, pain, itch, and proprioception (position and movement sense). Receptors include Merkel discs (sustained pressure, high resolution), Meissner corpuscles (dynamic touch, low-frequency vibration), Pacinian corpuscles (high-frequency vibration, deep pressure), Ruffini endings (sustained stretch), free nerve endings (pain and temperature), muscle spindles (proprioception; intrafusal fibers), and Golgi tendon organs (tension). Adaptation is classified as rapidly adapting (Meissner, Pacinian) versus slowly adapting (Merkel, Ruffini).

Ascending Pathways

Two main pathways carry somatic information from the body to the thalamus and cortex:

• Dorsal column–medial lemniscus pathway: fine (discriminative) touch, vibration, and proprioception. Primary afferents enter the dorsal column and ascend ipsilaterally to the dorsal column nuclei (gracilis and cuneatus) in the medulla, where they synapse and decussate as the internal arcuate fibers, then ascend as the medial lemniscus to the contralateral VPL (ventral posterolateral) nucleus of the thalamus, and finally project to the postcentral gyrus (primary somatosensory cortex, S1).
• Anterolateral (spinothalamic) pathway: pain, temperature, and crude touch. Primary afferents synapse in the dorsal horn within one or two segments, second-order neurons decussate via the anterior white commissure and ascend in the contralateral anterolateral tract to the VPL and posterior thalamus, then to S1.

Sensation from the face travels via the trigeminal system: light touch through the principal sensory nucleus of V, pain/temperature through the spinal trigeminal nucleus, both projecting via the trigeminothalamic tract to the VPM (ventral posteromedial) thalamus. The somatosensory homunculus in S1 maps body parts somatotopically, with the lips and fingertips greatly magnified relative to the trunk.

Motor and Cerebral Circuits

Voluntary movement is generated by a hierarchical network:

1. Association cortex and supplementary motor area (SMA) plan the movement.
2. Primary motor cortex (precentral gyrus, Brodmann area 4) issues the command via upper motor neurons. The motor homunculus is somatotopically organized, with the hands, face, and tongue disproportionately represented.
3. The corticospinal tract descends through the internal capsule, cerebral peduncles, and medullary pyramids; about 85-90% of fibers decussate at the pyramidal decussation to form the lateral corticospinal tract (distal limb muscles, fine control), while the remaining uncrossed fibers form the ventral (anterior) corticospinal tract (proximal and axial muscles).
4. The basal ganglia (caudate, putamen, globus pallidus, subthalamic nucleus, substantia nigra) form a parallel loop that selects and scales movements. Dopamine from the substantia nigra pars compacta facilitates movement; its loss produces Parkinson disease (bradykinesia, rigidity, resting tremor). Striatal degeneration produces Huntington disease (chorea).
5. The cerebellum compares intended with actual movement and corrects errors in real time. The lateral hemispheres plan coordinated limb movements, the vermis regulates axial and proximal muscles, and the flocculonodular lobe regulates balance and eye movements. Cerebellar lesions produce dysmetria, intention tremor, ataxia, and dysdiadochokinesia without weakness.
6. Lower motor neurons (alpha motor neurons in the ventral horn or cranial nerve nuclei) are the final common pathway; they innervate extrafusal muscle fibers through the neuromuscular junction.

Upper motor neuron lesions produce spastic paralysis, hyperreflexia, Babinski sign, and increased tone. Lower motor neuron lesions produce flaccid paralysis, hyporeflexia, fasciculations, and muscle atrophy.

The Autonomic Nervous System (ANS)

The autonomic nervous system regulates involuntary visceral functions including heart rate, digestion, bronchial tone, pupil size, and bladder emptying. It has two divisions:

Sympathetic Division

Preganglionic neurons lie in the intermediolateral cell column of the spinal cord at segments T1-L2. Preganglionic fibers are short, synapsing in the paravertebral sympathetic chain or prevertebral (collateral) ganglia (celiac, superior mesenteric, inferior mesenteric). Postganglionic fibers are long and travel to effectors. All preganglionic sympathetic neurons release acetylcholine onto nicotinic receptors in ganglia; postganglionic sympathetic neurons release norepinephrine onto adrenergic receptors (exception: sweat glands receive cholinergic sympathetic input onto muscarinic receptors). The adrenal medulla is a modified sympathetic ganglion: preganglionic fibers synapse on chromaffin cells that release epinephrine (80%) and norepinephrine (20%) into the blood.

Parasympathetic Division

Preganglionic neurons lie in brainstem nuclei of cranial nerves III (oculomotor), VII (facial), IX (glossopharyngeal), and X (vagus), plus S2-S4 sacral segments. Preganglionic fibers are long and synapse in terminal ganglia near or within the target organ; postganglionic fibers are short. Both pre- and postganglionic parasympathetic neurons release acetylcholine, onto nicotinic receptors in ganglia and muscarinic receptors at effectors.

Receptor Pharmacology

Sympathetic vs Parasympathetic Comparison

Visceral Reflexes

Visceral afferent fibers travel with autonomic nerves to the spinal cord or brainstem, where they integrate with preganglionic neurons. The baroreceptor reflex is the best-studied example: increased blood pressure stretches carotid sinus and aortic arch baroreceptors, afferents via CN IX and X excite the nucleus tractus solitarius, which increases parasympathetic outflow (CN X) to the heart and decreases sympathetic outflow, lowering heart rate and pressure. The micturition reflex is a parasympathetic sacral reflex: bladder stretch activates S2-S4 preganglionic parasympathetic neurons, causing detrusor contraction and internal sphincter relaxation.

Per the PA-CAT Bulletin of Information, rev. 20240815, the Nervous System group of the Physiology content area explicitly includes somatic sensations, motor and cerebral circuits, and autonomic organization; expect questions testing receptor identity, ganglion location, and the dual-transmitter rule (cholinergic preganglionic for both divisions).