Comprehensive Nervous System: Sensory Receptors, Vision, Hearing, and Taste

Somatosensory System

Encompasses various receptors that relay information from the body surface and internal structures to the cerebral cortex.

Somatosensory Receptors

Main types include Proprioceptors, Mechanoreceptors, Thermoreceptors, and Nociceptors.

Mechanoreceptors

Receptors that respond to mechanical stimuli; can be rapidly or slowly adapting.

Rapidly adapting Mechanoreceptors

Examples include Pacinian corpuscle, Meissner's corpuscle, Hair follicle receptor.

Slowly adapting Mechanoreceptors

Examples include Free nerve endings, Merkel's disk, Ruffini's ending.

Thermoreceptors

Receptors that respond to temperature changes; utilize Transient Receptor Potential ion channels (TRP).

Nociceptors

Receptors that transmit pain, utilizing free nerve endings and A-delta or C fibers.

Warm receptors

Free nerve endings that respond to temperatures in the range of 30-43°C; frequency increases as temperature increases.

Cold receptors

Possible free nerve endings that respond to temperatures in the range of 20-35°C; frequency increases as temperature decreases.

Paradoxical cold

Occurs when cold receptors fire at 45°C, perceiving the temperature as painfully hot.

Mechanical nociceptors

Nociceptors that respond to mechanical stimuli.

Thermal nociceptors

Nociceptors that fire when temperatures are over 44°C.

Polymodal nociceptors

Nociceptors that respond to mechanical stimuli, extreme cold/heat, and chemicals.

Activators of polymodal nociceptors

Include Histamine, bradykinin, prostaglandins, Potassium, and Serotonin.

Dorsal column-medial lemniscal pathway

Pathway that carries touch and pressure sensations; decussation occurs in the Medulla.

Spinothalamic tract

Pathway that carries pain and temperature sensations; decussation occurs in the spinal cord.

Primary Somatosensory Cortex

Organized topographically by body region; area devoted to a specific region is proportional to its sensitivity.

Fast Pain

Sharp, pricking sensation easily localized, transmitted by Aδ fibers.

Slow Pain

Dull, aching sensation poorly localized, transmitted by C fibers.

1st order pain neurons

Terminate in the dorsal horn; neurotransmitters involved include substance P, glutamate, and CGRP.

Visceral pain

Pain originating in internal organs, often experienced as referred pain.

Gate-control theory

Suggests pain can be modulated by Aβ fibers, such as when rubbing a sore spot.

Endogenous opioids

Like Enkephalin, can block the release of substance P or act as competitive inhibitors at opioid receptors.

Hyperalgesia

Enhanced pain, sometimes associated with cytokines, fever, and fatigue.

Phantom limb pain

Involves activation of 2nd and 3rd order neurons and convergent stimulation, activating the pain pathway for the missing limb.

Choroid

Middle layer of the eye containing blood vessels.

Ciliary body

Structure in the eye that includes the ciliary muscle and zonular fibers.

Iris

Part of the eye that regulates the amount of light entering through the pupil.

Pupil

Opening in the iris that allows light to enter the eye.

Retina

Inner layer of the eye containing photoreceptors.

Fovea

Area in the retina with a high concentration of cones for sharp vision.

Optic disk

Blind spot in the retina where the optic nerve exits.

Inner circular muscle (constrictor)

Muscle controlled by parasympathetic input that constricts the pupil.

Outer radial muscle (dilator)

Muscle controlled by sympathetic input that dilates the pupil.

Refraction

Bending of light waves when passing from one medium density to another.

Accommodation

Mechanism for shifting focus from far to near objects by increasing lens curvature.

Emmetropia

Normal vision.

Myopia

Near-sightedness.

Hyperopia

Far-sightedness.

Presbyopia

Hardening of the lens.

Cataract

Opacity of the lens.

Glaucoma

Increased volume of aqueous humor leading to optic nerve degeneration.

Astigmatism

Irregularities of the lens or cornea.

Phototransduction

Conversion of light energy to nerve signals.

Rhodopsin

Photopigment in rods responsible for black-and-white vision.

Cones

Photoreceptors responsible for color vision, containing three different opsins.

Optic nerve

Cranial nerve II formed by ganglion cell axons.

Optic chiasm

Point where axons from the nasal retina decussate.

Lateral geniculate body

Thalamic structure where optic tract synapses occur.

Decussate

Crossing over of axons from the nasal retina to the opposite side.

Cochlea

Inner ear structure responsible for sound transduction.

Decibels (dB)

Unit of measurement for loudness.

Hertz (Hz)

Unit of measurement for frequency.

Organ of Corti

Structure in the cochlea containing hair cells.

Bending towards tall stereocilia

Opens tip link proteins, allowing K+ and Ca2+ to enter, leading to depolarization.

Bending towards short stereocilia

Closes channels, leading to hyperpolarization.

Coding of intensity (loudness)

Coded by the degree of deflection/channel opening.

Coding of frequency (pitch)

Coded by the location of deflection on the basilar membrane.

Cochlear nerve (Cranial Nerve VIII)

Hair cells synapse with afferent neurons of this nerve.

Neural pathway for sound

Signal travels from the cochlear nuclei in the medulla to the thalamus and then to the auditory cortex.

Tonotopic arrangement

Arrangement of the auditory cortex based on frequency.

Conductive deafness

Inadequate conduction of sound waves through the external and/or middle ear.

Sensorineural deafness

Inadequate transduction in the inner ear (cochlea) or damage to Cranial Nerve VIII.

Central deafness

Damage to the CNS neural pathway for sound.

Vestibular apparatus

Mediates equilibrium.

Semicircular canals

Three canals that detect rotational movement of the head.

Anterior canal

Detects head movement up or down on axis.

Posterior canal

Detects head tilt to the shoulder or movement left to right on axis.

Lateral or horizontal canal

Detects turning of the head from side to side on axis.

Utricle

Detects forward/backward motion.

Saccule

Detects up/down motion.

Taste buds

Found on the tongue, roof of the mouth, and pharynx, containing 50-150 modified epithelial receptor cells.

Primary tastes

Sour (due to H+), Salty (due to Na+), Sweet (due to small carbohydrates), Bitter (due to nitrogen containing compounds), Umami (due to glutamate and other amino acids).

Pattern theory of taste coding

Involves the activation of multiple receptors and neurons.

Olfaction

Depends on airborne chemicals binding to chemoreceptors in the olfactory epithelium.

Olfactory epithelium

Contains receptor cells, basal cells, and sustentacular cells.

Transduction in olfaction

Chemical binding activates a G protein, leading to depolarization.

Olfactory pathway

Olfactory receptor axons form Cranial Nerve I and synapse in the olfactory bulb without relay in the thalamus.