Chapter 18: Nervous System: General & Special Senses

Sensory data begins at specialized cells or processes known as receptors, which exhibit receptor specificity and operate within defined receptive fields. Sensory information is conveyed to the central nervous system (CNS) through patterns of action potentials, a process called sensory coding. Receptors are functionally categorized as tonic receptors, which are continuously active and slow-adapting (like those monitoring body position), and phasic receptors, which activate transiently in response to a change in condition and are considered fast-adapting. Adaptation—the reduction of sensitivity to constant stimuli—can occur at the receptor level (peripheral) or within the CNS (central). The general senses are mechanoreceptors, chemoreceptors, thermoreceptors, and nociceptors (pain receptors). Mechanoreceptors detect physical distortion, encompassing baroreceptors sensitive to pressure changes in major organs and vessels, proprioceptors monitoring joint position and muscle tension (e.g., muscle spindles), and various tactile receptors for touch, pressure, and vibration, ranging from simple free nerve endings to encapsulated structures like lamellated (pacinian) corpuscles and tactile (Meissner's) corpuscles. The special senses include olfaction (smell), utilizing bipolar olfactory receptor cells whose axons form the olfactory nerve and travel to the olfactory bulbs, notably providing the only sensory input that reaches the cerebral cortex without first synapsing in the thalamus, which explains emotional responses tied to scent. Gustation (taste) relies on gustatory cells clustered in taste buds found on papillae (fungiform, circumvallate) and detects six primary sensations: sweet, salty, sour, bitter, umami (sensitive to amino acids/glutamate), and water, with input monitored by cranial nerves VII, IX, and X. Equilibrium and hearing originate in the ear; sound waves collected by the external ear vibrate the tympanic membrane, which transfers energy via the three auditory ossicles (malleus, incus, stapes) to the oval window. Within the inner ear's bony and membranous labyrinths, equilibrium is detected in the vestibular complex; the utricle and saccule employ heavy otoliths (statoconia) within maculae to sense gravity and linear acceleration, while the semicircular ducts use moving endolymph to displace the cupula, detecting rotational movement. Hearing occurs in the cochlea, where pressure waves stimulate hair cells of the Organ of Corti by pushing them against the tectorial membrane, sending frequency and intensity information via the vestibulocochlear nerve (N VIII) to auditory processing centers. Vision involves accessory structures like the lacrimal apparatus and eyelids, and the highly structured eyeball wall, composed of the outer fibrous tunic (sclera and transparent cornea), the middle vascular tunic (iris, ciliary body, choroid), and the inner neural tunic (retina). The vascular tunic regulates light entry through the pupil and secretes aqueous humor, which circulates before draining into the canal of Schlemm. The retina contains two types of photoreceptors: light-sensitive rods (black-and-white vision) and color-sensitive cones (sharp vision, concentrated in the fovea centralis). Visual signals proceed from photoreceptors to bipolar cells to ganglion cells, whose axons converge at the optic disc (blind spot) to form the optic nerve (N II); visual pathways involve partial crossover at the optic chiasm before relaying through the lateral geniculate nuclei to the visual cortex for conscious perception and crucial depth perception.