Sensory and Motor Pathways: A Detailed Look
Sensory Pathways
Nerve impulses from touch, pressure, heat, and cold receptors travel to the sensory cortex, triggering feelings. More sensitive body areas have a larger mapped cortex region. The sensory homunculus visually represents body regions relative to their allocated cerebral cortex, with larger areas like the face and extremities being the most sensitive.
Nerve impulses travel in bundles within the spinal cord, forming the spinothalamic pathway and the route of Goll and Burdach.
a. Spinothalamic Pathway: Transmits pain and temperature information from cutaneous receptors. It involves three neurons: the first synapses in the spinal ganglion, then with the second neuron. Fibers cross to the opposite side of the cord, ascending to the thalamus, where they synapse with the third neuron reaching the sensory cortex.
b. Route of Goll and Burdach: Transmits light touch and pressure impulses. It also comprises three neurons. The first enters the spinal cord, ascending through the posterior columns (gracilis and cuneatus). It synapses in the bulb with the second neuron, which crosses to the opposite side and reaches the thalamus, synapsing with the third neuron that carries information to the sensory cortex.
Motor Pathways
The motor homunculus represents body regions proportionally to their assigned motor cortex. Areas needing precise motor control, like hands, fingers, and tongue, have greater cortical representation.
Motor responses generate nerve impulses that travel from the motor cortex to the spinal cord via pyramidal and extrapyramidal pathways.
a) Pyramidal Pathways: Fibers originate in the frontal lobe and descend to cranial nerve nuclei or the anterior horn of the spinal cord. Fibers may cross to the opposite side at the pyramids. Three main tracts are: crossed pyramidal tract, direct pyramidal tract, and corticobulbar tract.
b) Extrapyramidal Pathways: Fibers start in the motor cortex and continue to the basal ganglia. Three main bundles are the rubrospinal, vestibulospinal, and tectospinal. The rubrospinal transmits impulses related to muscle tone and posture. The vestibulospinal controls head movements in response to visual stimuli. The tectospinal is involved in balance control, regulating muscle tone in response to head movements.
The simple act of grasping an object requires integrated information from receptors, motor response development, and specific muscle contractions and relaxations, highlighting the functional integration of the nervous system.
Spinal Nerves
There are 31 pairs of spinal nerves:
- 8 pairs of cervical spinal nerves (C1-C8)
- 12 pairs of thoracic spinal nerves (T1-T12)
- 5 pairs of lumbar spinal nerves (L1-L5)
- 5 pairs of sacral spinal nerves (S1-S5)
- 1 pair of coccygeal spinal nerves (Co)
Visual Pathway
Cornea, pupil, aqueous humor, vitreous (hyaloid canal), retina, optic nerve, visual cortex (occipital lobe)