13.5 Neuroanatomy: CNS Organization, Somatosensory & Motor Pathways

13.5 Neuroanatomy: CNS Organization, Somatosensory & Motor Pathways

The general anatomy outline lists neuroanatomy: central and peripheral nervous system structure and organization, the somatosensory system and the motor system. The physiology outline adds autonomic, motor and sensory neurophysiology. Every podiatric neurologic examination (monofilament, vibration, reflexes, Babinski sign, gait) tests these pathways. Peripheral nerves of the limb are covered in 4.1.

Organization of the Nervous System

Cerebral Cortex and Blood Supply

• The primary motor cortex (precentral gyrus) and primary somatosensory cortex (postcentral gyrus) map the body as a homunculus.
• The leg and foot sit on the medial surface in the paracentral lobule, supplied by the anterior cerebral artery (ACA). An ACA stroke causes contralateral leg-predominant weakness and sensory loss.
• A middle cerebral artery stroke affects the face and arm more than the leg (Circle of Willis in 13.1).
• The thalamus (ventral posterolateral nucleus) relays body sensation to the cortex. The posterior limb of the internal capsule carries corticospinal and thalamocortical fibers, so a small lacunar stroke there causes pure motor or pure sensory hemiparesis.

Spinal Cord

• The gray matter has dorsal horns (sensory processing, substantia gelatinosa for pain), ventral horns (alpha and gamma motor neurons) and lateral horns from T1–L2 (sympathetic preganglionic neurons).
• The lumbosacral enlargement (roughly cord segments L1–S3) supplies the legs. The cord ends at about the L1–L2 vertebral level as the conus medullaris, with the cauda equina below (13.4).
• White matter columns carry the ascending and descending tracts described below.

Somatosensory System

Receptors (Sensory Physiology)

Adaptation and coding. Rapidly adapting receptors signal change, such as vibration and movement. Slowly adapting receptors signal sustained stimuli. Stimulus intensity is coded by firing frequency and by recruiting more receptors. Two-point discrimination is finest where receptive fields are small (fingertips, toes).

Ascending Pathways

Clinical correlation. Diabetic large-fiber neuropathy and dorsal column disease (vitamin B12 deficiency, tabes dorsalis) impair vibration and joint position. Patients then have a positive Romberg sign: stable with eyes open, falling with eyes closed, because vision was compensating for lost proprioception.

Motor System

Corticospinal (Pyramidal) Tract

Upper motor neurons in the precentral gyrus descend through the posterior limb of the internal capsule, cerebral peduncle and basis pontis to the medullary pyramids. About 85–90% decussate at the pyramids to form the lateral corticospinal tract, which synapses on contralateral ventral horn lower motor neurons. The uncrossed fibers form the anterior corticospinal tract, which controls axial muscles.

Amyotrophic lateral sclerosis combines upper and lower motor neuron signs. A pes cavus foot with intrinsic wasting suggests a lower motor neuron or peripheral process such as Charcot-Marie-Tooth disease (14.4). A spastic equinovarus foot suggests an upper motor neuron lesion.

Basal Ganglia and Cerebellum

• Basal ganglia (striatum, globus pallidus, subthalamic nucleus, substantia nigra) modulate movement through the direct pathway (facilitates movement) and indirect pathway (inhibits movement). Parkinson disease (loss of nigral dopamine) causes bradykinesia, rigidity, resting tremor and a shuffling, festinating gait with high fall risk.
• Cerebellum coordinates movement on the same side of the body. Vermis lesions (for example from alcohol) cause truncal and wide-based gait ataxia; hemisphere lesions cause ipsilateral limb dysmetria and intention tremor.

Spinal Reflexes

• Stretch reflex: tapping a tendon stretches muscle spindles, and Ia afferents synapse monosynaptically on alpha motor neurons, with reciprocal inhibition of antagonists. The patellar reflex is L3–L4 (mainly L4) and the Achilles reflex is S1 (4.1).
• Withdrawal (flexor) reflex with a crossed extensor reflex in the opposite limb keeps a person upright after stepping on a sharp object.
• Plantar reflex: normal flexion of the toes; an extensor response indicates corticospinal disease, although it is normal in infants under about 1 year.

Spinal Cord Lesion Patterns

Autonomic Organization and the Lower Limb

• Sympathetic (thoracolumbar) outflow: preganglionic neurons in the T1–L2 lateral horns exit through white rami communicantes to the paravertebral chain. Fibers for the leg arise mainly from about T10–L2, descend to lumbar and sacral chain ganglia, and reach the lumbosacral plexus through gray rami. They travel with peripheral nerves and arteries to supply sweat glands, arteriolar smooth muscle and arrector pili.
• Parasympathetic (craniosacral) outflow: cranial nerves III, VII, IX and X plus S2–S4 (pelvic splanchnic nerves to the bladder, rectum and genitalia). The limbs receive essentially no parasympathetic supply.
• Neurotransmitters: all preganglionic fibers release acetylcholine (nicotinic receptors). Sympathetic postganglionic fibers release norepinephrine, except sweat glands, which receive sympathetic cholinergic (muscarinic M3) fibers.

Clinical relevance:

• Diabetic autonomic neuropathy causes anhidrosis, fissuring and arteriovenous shunting (8.3).
• Lumbar sympathectomy produces a warm, dry foot.
• Complex regional pain syndrome causes disproportionate pain with vasomotor, sudomotor, trophic and motor changes, often after foot or ankle trauma or surgery, and is diagnosed clinically with the Budapest criteria.