Sensory Physiology

Sensory receptors provide the only channels of communication from the external (and internal) world to the nervous system.

Perception - Our conscious interpretation of the external world as created by the brain from a pattern of nerve impulses delivered to it from sensory receptors.  Sensory receptors transduce (change) different energy forms into AP's that are conducted into the CNS by sensory neurons.

Functional Categories of Sensory Receptors:

• Chemoreceptors - Monitor dissolved chemicals; include the taste buds and the olfactory epithelium

• Photoreceptors - Monitor light; include the rods and cones of retina

• Thermoreceptors - Monitor temperature

• Mechanoreceptors - Monitor mechanical distortion (force) or pressure; include the pressure receptors in skin and the hair cells in inner ear

• Nociceptors - Monitor tissue damage due to chemicals released from damaged tissues

• Proprioceptors - Monitor position of muscles and joints

• Osmoreceptors - Monitor solute concentration

Characteristics of Sensory Receptors:

• Each type of sensory receptor is highly selective and most sensitive to a particular modality (energy forms such as sound, light, pressure) to which it responds by ultimately causing the production of action potentials.

• Exhibit sensory adaptation which is the process by which a sensory system becomes less sensitive to stimuli during prolonged or repeated stimulation.  The result is a  change in membrane potential as a receptor is subjected to a maintained stimulus.

          Types of sensory adaptation:

• Phasic Adaptation

• Receptors adapt rapidly. There is an initial burst of action potentials when the stimulus is applied followed by a reduced rate of firing. Examples: Odor, touch, and temperature receptors

• Tonic Adaptation

• Receptors do not adapt or adapt slowly. Continue to fire at a relatively constant rate as long as the stimulus is applied. Example: Pain receptors

Law of Specific Nerve Energies - Stimulation of a sensory nerve produces only one sensation.  Specificity is due to the synaptic pathways between the receptors and the brain.  These pathways are often referred to as labeled lines.

Adequate stimulus - The normal stimulus for a particular sensory receptor.

Generator (receptor) potentials - Local, graded changes in the membrane potential produced by stimulation of sensory receptors.  They are usually depolarizations.

• Proportional to the strength of the stimulus.

• If these potentials summate to threshold, AP's will be generated.

Receptive Field - Area of the body that, when stimulated by a sensory stimulus, activates a particular sensory receptor and changes the firing rate of the sensory neuron.

• Back, arms, legs - Large area served by relatively few sensory endings; has less tactile acuity (sharpness of touch perception) and discrimination.

• Fingers - Many receptors serve a small area of skin; has greater acuity and discrimination

• Exploiting the acuity of the fingertips is the reason why Braille is possible.

Approximate size of the receptive field  can be mapped by using the two-point touch threshold (two-point discrimination) = minimum distance at which two points of touch can be perceived as separate.  It is a measure of tactile acuity.

Lateral inhibition - Sharpening of sensation that occurs in the neural processing of sensory input. Input from those receptors that are most greatly stimulated is enhanced, while input from other receptors is reduced.

Sound waves are traveling vibrations that propagate through air or water and are due to changes in pressure over time.

Sounds can be characterized by intensity or loudness (related to amplitude) and pitch (related to frequency).

• Human hearing is in the frequency range of 20-20,000 Hertz.

• Intensity is measured in decibels (dB).  Every 10 dB’s represents a 10-fold increase in loudness.

• Directs sound waves toward the middle ear by vibrating the tympanic membrane (eardrum).

• Contains ossicles (malleus, incus, and stapes) and serves to transmit sound waves to cochlea.

• Contains the cochlea = the organ of hearing where changes in membrane potential are generated in response to sound waves.

• Houses sensory systems for equilibrium (semicircular canals, utricle, saccule).

Relevant anatomy of inner ear:

• Oval window, scala vestibuli (SV), vestibular membrane, scala media (SM) = cochlear duct, scala tympani (ST), helicotrema, basilar membrane (BM), organ of Corti (contains hair cells), stereocilia, tectorial membrane, round window

• SV and ST contain perilymph; SM contains endolymph. These fluids differ in their ionic concentrations.

1) Sounds waves enter ear and cause the tympanic membrane to vibrate.

2) Vibration of tympanic membrane causes the ossicles to move and the last ossicle (stapes) to vibrate against the oval window.

3. Vibrations of the oval window set up traveling pressure waves of perilymph in the SV that 1) either pass around the helicotrema (when the frequency is low) to the ST or that 2) pass through the vestibular membrane (when the frequency is higher) to the SM. This causes displacement of the vestibular and basilar membranes.

4. Vibration of the basilar membrane causes a shearing force between the sensory hair cells ("hairs" on the cells = stereocilia) located on the basilar membrane and the overlying tectorial membrane. This causes the stereocilia on the hair cells to bend, which produces a generator potential (GP).

5. The greater the displacement of the basilar membrane and the bending of the stereocilia, the greater the amount of transmitter released by the hair cells, and therefore the greater the GP produced.  This causes a greater rate of AP's to be generated in the 8th cranial nerve (vestibulocochlear nerve).

• Sounds of high frequency cause maximum displacement of the basilar membrane closer to the oval window; sounds of lower frequency produce maximum displacement of the basilar membrane further from the oval window.  This allows for pitch discrimination.

Neural Pathways for Hearing

• 8th cranial nerve primary auditory cortex of temporal lobe = region of the brain responsible for interpreting the AP's.

• The auditory cortex is mapped according to tone = tonotopic map.  Each region of the basilar membrane is linked to a specific region of the auditory cortex in the temporal lobe.  In other words, there is a correlation between pitch location in the cochlea and in the auditory cortex.

• The eyes convert energy in the electromagnetic spectrum into generator potentials and, ultimately, AP's.

• Visible light is from 400 nanometers (blue) - 700  nanometers (red)

• Eye anatomy (review as needed to help you understand the physiology)

• Pathway of light - Cornea pupil  lens  retina

• Pupil regulation

• Contract radial muscles  dilation and is mediated by the sympathetic N.S.

• Contract circular muscles  constriction and is mediated by the parasympathetic N.S.

• Light rays passing through the cornea and lens are refracted, or bent.  The image projected on the retina is upside down and right to left.

• Accommodation - Adjustment of the curvature of the lens of the eye to bring images of objects from various distances into sharp focus on the retina.

• Can change the shape of the lens by relaxing or contracting the ciliary muscles and changing the tension on the suspensory ligaments.

• Myopia (nearsightedness)

• Eyeball too long

• Light rays focus in front of retina

• Concave lens corrects

• Hyperopia (farsightedness)

• Eyeball too short

• Light rays focus behind retina

• Convex lens corrects

Retina

• Contains photoreceptor cells - Rods (function in dim light) and cones (detect color and function best in brighter light).

• Also contains ganglion, amacrine, bipolar, and horizontal cells.

• Rods (~ 120 million/retina) contain a light-sensitive purple pigment (a chemical compound that absorbs light) called rhodopsin (R) which is activated by light.

• Rhodopsin = Retinaldehyde (also called retinene or retinal; from Vitamin A) + opsin (protein)

With light energy: 11-cis retinene all-trans retinene and it dissociates (separates) from the opsin.

The above reaction is called the bleaching reaction.

In the light: Photopigment activated  closure of sodium channels due to stoppage of the dark current   membrane hyperpolarizes decreased release of inhibitory transmitter to bipolar cell excitation of bipolar cell release of excitatory neurotransmitter by bipolar cell stimulates ganglion cell AP's generated in ganglion cell propagation of AP to primary visual cortex of occipital lobe for visual perception.

What happens in the dark?  ___________________________________________________________

• Cones (~ 6 million/retina)

• Are less sensitive than rods to light

• Provide color vision and greater visual acuity (sharpness of vision)

• 3 types of cones - Red, green, blue

• Each has a different opsin molecule (photopsin) that absorbs light maximally at a different wavelength

• Found almost exclusively within the fovea

• Little to no convergence in retinal pathways

• Perception of any color is determined by the relative degree to which each cone is stimulated by any given wavelength of light.

• Color blindness is due to a congenital lack of 1 or more cone types.

• Each cone in the fovea synapses with 1 bipolar cell, which synapses with 1 ganglion cell. This 1:1:1 relationship gives each ganglion cell a "private line" to the visual field, thereby increasing acuity.

• Outside the fovea, many rods synapse with a single bipolar cell, and many bipolar cells synapse with a single ganglion cell which is referred to as convergence. This results in impaired visual acuity, but increased sensitivity to low light levels.

Neural Pathways from the Retina

• Axons making up the optic nerve project, after making several synaptic connections, to the primary visual cortex of the occipital lobes. Determines "what" something is.

• Some of the neurons from the retina project to the superior colliculus (SC). Determines "where" something is.

• SC also coordinates movements of the eyes such as saccadic eye movements and the pupillary reflex.

• Some neurons from the retina project to the biological clock in the hypothalamus (suprachiasmatic nucleus) to synchronize biological rhythms to the light-dark cycle.