The Senses Sensory receptors transduce different forms of energy in the ―real world‖ into nerve impulses. Different sensory perceptions (sound, light, pressure) arise from differences in neural pathways. If the optic nerve delivers an impulse, the brain interprets it as light.
Functional Categories of Sensory Receptors Receptors can be classified according to the type of signal they transduce: Chemoreceptors – sense chemicals in the environment Taste, smell, or blood Photoreceptors – sense light Thermoreceptors – respond to cold or heat Mechanoreceptors – stimulated by mechanical deformation of the receptor. Touch Hearing Nociceptors – sense pain; damaged tissue release chemicals that excite sensory endings
Nociceptors Pain receptors that depolarize when tissues are damaged. Stimuli can include heat, cold, pressure, or chemicals Glutamate and substance P are the main neurotransmitters. May be activated by chemicals released by damaged tissues, such as ATP. Perception of pain can be enchanced by emotions and expectations Pain reduction depends on endogenous opioids. Nociceptors can be myelinated or unmyelinated Sudden, sharp pain is transmitted by myelinated neurons. Dull, persistent pain in transmitted by unmyelinated neurons.
Tonic and Phasic Receptors Receptors can be categorized based on how they respond to a stimulus. Phasic: respond with a burst of activity when a stimulus is first applied but quickly decrease their firing rate—adapt to the stimulus—if the stimulus is maintained. (fast-adapting) Alerts us to changes in the environment Allow sensory adaptation Smell, touch, temperature Tonic: maintain a high firing rate as long as the stimulus is applied. (slow-adapting)
Cutaneous Receptors Pain, cold, and heat receptors are naked dendrites Cold receptors – located close to epidermis Warm receptors – located deeper in the dermis. Hot receptors – pain experienced by a hot stimulus is sensed by a special nociceptor called a capsaicin receptor. Touch and pressure receptors have special structures around their dendrites. Meissner’s corpuscles Encapsulated dendrites in connective tissue Changes in texture and slow vibration Pacinian corpuscles Encapsulated dendrites by concentric lamellae of connective tissue structures Deep pressure and fast vibrations Ruffini endings Sustained pressure Enlarged dendritic endings with open, elongated capsule Merkel’s discs Expanded dendritic endings Sustained touch and pressure Slow adapting
Two-Point Threshold Test Measures the density of touch receptors The minimum distance at which two points of contact can be felt.High density of receptive fields =shorter minimum distanceLow density of receptive fields =longer minimum distance
Vestibular Apparatus Provides a sense of equilibrium Located in the inner ear Consists of: Otolith organs Linear acceleration Utricle (horizontal) Saccule (vertical) Semicircular canals Rotational acceleration Both structures in the vestibular apparatus are: Filled with endolymph Contain sensory hair cells which are activated by bending.
Clinical ApplicationsNystagmus Vertigo Involuntary oscillations of the eyes Loss of equilibrium with the when spinning is suddenly illusion of spinning stopped. May be caused by anything that alters the firing rate of one of the Eyes continue to move in the vestibulocochlear nervers. direction of the spin, then jerk May be due to spinning or rapidly back to the midline. pathologically induced by by viral When a person begins spinning, the cupula infections. bends in the opposite direction. If the movement suddenly stops, inertia of Tx: Antivert® (meclizine) endolymph causes it to continue moving in Anticholinergic action the direction of the spin. Blocks conduction in the middle ear This is a normal phenomenon that helps vestibular-cerebellar pathways. maintain balance during spinning, however, nystagmus can also be a symptom of certain diseases, like Meniere’s disease.
Structures of the Middle Ear Cavity between the tympanic membrane and the cochlea Contains three bones called ossicles: Malleus Incus Stapes Vibrations are transmitted and amplified along the bones. The stapes is attached to the oval window, which transfers the vibrations into the inner ear.
Clinical ApplicationsConduction deafness Sensorineural deafness Sound waves are not conducted Nerve impulses are not conducted from the outer to inner ear. from the cochlea to the auditory May be due to a buildup of earwax, cortex. too much fluid in the middle ear, May be due to damaged hair cells. damage to eardrum, or May only impair hearing of a overgrowth of bone in the middle particular sound frequency. ear. May be helped by cochlear Impairs hearing of all sound implants. frequencies. Can be helped by hearing aids.
Functional Anatomy of the Eye Image is inverted on retina due to refraction of light. Degree of refraction depends on: Refractive index (RI) of media RI of air = 1.00 RI of cornea = 1.38 Curvature of the interface between the two media.
Photoreceptors Rods: Provide black and white vision under low light intensities Cones: Provide sharp color vision when light intensity is great Humans have trichromatic vision due to the presence of three different types of cones: Blue, Green, and Red.
Visual Acuity Sharpness of vision Depends upon resolving power Ability of the visual system to resolve two closely spaced dotsVisual Abnormalities Myopia (nearsightedness) Hyperopia (farsightedness) Astigmatism uneven cornea or lens Presbyopia hardening of the lens impedes accommodation