Chapter 36 Sensory Reception

I. Sensory Systems: An Overview
A. Each sensory system has three component parts:
1 Sensory receptors are the branched endings of sensory neurons or specialized cells adjacent to them
that detect specific stimuli.
2 Nerve pathways lead to the brain.
3 Brain regions process the information into a sensation (and later, perhaps, a perception).
B. Types of Sensory Receptors
1 Chemoreceptors detect ions or molecules; they include olfactory and taste receptors.
2 Mechanoreceptors detect changes in pressure, position, or acceleration; they include receptors for
touch, stretch, hearing, and equilibrium.
3 Photoreceptors detect the energy of visible and ultraviolet light.
4 Thermoreceptors detect radiant energy, including infrared.
C. Sensory Pathways
1 Each sensory pathway starts at receptors of sensory neurons that are sensitive to the same type of
stimulus.
2 Sensory nerve pathways from different receptors lead to different parts of the cerebral cortex.
a. Signals from the skin receptors travel to the somatic sensory cortex.
b. The visual world is mapped onto the visual cortex.
D. Information Flow Along Sensory Pathways
1 All sensory receptors convert stimulus energy to local, graded potentials, which may result in an action
potential if the stimulus is intense or repeated fast enough.
2 Action potentials reach the brain via synapses with interneurons and provide the following information: a. Genetically determined networks of neurons in the brain can interpret incoming action potentials
only in specific ways; for example, receptors from eyes see only light.
b. Strong stimulation of a receptor causes a greater frequency of action potentials.
c. Strong stimulation causes a greater number of neurons to fire.

II. Somatic Sensations
A. Touch and Pressure
1 These sensations are dependent on mechanoreceptors, which are dendrites of sensory neurons, with
or without a capsule.
2 Receptors sensitive to touch produce action potentials only when the stimulus begins and ends. 3 Receptors sensitive to pressure respond to a constant stimulus.
B. Temperature Changes
1 When the body surface temperature remains constant, the neuron messages to the brain are a steady
stream.
2 Increases in temperature cause an increase in firing.
3 The mechanism for detection of cold has not been discovered yet.
C. Pain
1 Pain is the perception of injury to some region of the body.
a. The messages are directed through the thalamus and on to the parietal lobe for interpretation.
b. Responses are made to strong mechanical stimulation, intense heat and cold, and chemical irritation.
2 Much visceral pain is referred, that is, felt at some distance from the real stimulation point.
D. Muscle Sense
1 Receptors in skeletal muscle, joints, tendons, ligaments, and skin are responsible for awareness of the
body’s position in space and of limb movements.
2 Stretch receptors in the muscle tissue respond in accordance with the degree and speed of muscle
stretching.

III. The Special Senses
A. Taste and Smell
1 Taste receptors enable animals to distinguish nutritious from noxious substances.
a. Receptors of some animals are located on antennae, legs, tentacles, or fins.
b. In humans, taste receptors are often components of taste buds distributed mostly on the tongue.
2 Olfactory receptors detect odors.
a. Many times the receptors respond to molecules from food or predators.
b. Others respond to pheromones, which are molecules released outside the body to elicit a social
response in a member of the same species (example: bombykol in silkworm moths).
B. Hearing
1 Hearing requires mechanoreceptors that can detect vibrations, wavelike forms of mechanical energy
that show amplitude (loudness) and frequency (pitch).
a. In invertebrates, the vibrations directly stimulate mechanoreceptors attached to the membrane.
b. In vertebrates, the membrane vibrations cause a fluid inside the ear to be displaced, which in turn
causes mechanoreceptors to bend and result in the firing of action potentials.
2 The Mammalian Ear
a. The outer ear collects sound waves and channels them through a canal to an eardrum.
b. The eardrum vibrates and sets in motion a series of three small bones in the middle ear.
c. The vibrations set off pressure waves in the fluid of the coiled tube of the inner ear in which hair cells
are stimulated to produce the action potentials that are sent to the brain.
3 Echolocation
a. Bats, dolphins, and whales produce high-frequency sound waves which echo back from the objects
that they hit.
b. By perceiving variations in the echoes, these animals can pinpoint the distance and direction of
movement of predators, prey, and so on.
C. Balance
1 The sense of balance depends on the organs of equilibrium.
2 The vestibular apparatus is a closed system of fluid-filled sacs and canals inside the ear.
a. The otolith organs detect changes in the orientation of the head relative to gravity.
b. The semicircular canals detect changing movements because the canals are arranged in the three
planes of space.
c. Overstimulation of the hair cells of the vestibular apparatus can result in motion sickness.
D. Vision
1 A visual system includes structures that focus (lenses) light onto photoreceptors and a neural program
in the brain that can interpret the patterns.
2 The eyespots of some invertebrates function in photoreception, but form no images.
a. Some mollusks have image-forming eyes each with a lens, cornea, and retina; some also possess an
iris for adjusting light intensity and a pupil through which the light passes.
b. Insects and crustaceans have compound eyes with numerous photosensitive units, each capable of
sampling a portion of the visual field to assemble a visual mosaic.
E. The Vertebrate Eye
1 Eye Structure
a. The outer sclera (“white” of the eye) covers most of the eye; the cornea covers the front.
b. The middle consists of a dark-pigmented choroid, a lens with a pupil opening, and jellylike substances.
c. The inner layer is the retina.
1) Because of the bending of the light rays by the cornea, accommodation must be made by the
lens so that the image is in focus on the retina.
2) In fish and reptiles, the lens is moved forward and back (like a camera lens) to focus.
3) In birds and mammals, the ciliary muscle changes the shape of the lens to focus.
2 Photoreception
a. Photoreceptors, linked to neurons, are located in the retina.
1) Cones respond to high-intensity light, contribute to sharp daytime vision, and are packed at the
fovea; three types respond to red, green, or blue light.
2) Rods are sensitive to dim light and detect changes in light intensity.
3) Each rod contains molecules of rhodopsin that can be altered by light, resulting in voltage
changes in membranes.
b. Axons in the optic nerve conduct these signals to the brain for interpretation.
3 Processing Visual Information
a. The sense of vision is the result of processing the information through levels of synapsing neurons.
b. Stimulation begins in the rods and cones, then moves to adjacent neurons and bipolar cells, then to
ganglion cells whose axons form the optic nerves that lead to the brain.
c. The brain’s visual cortex has several visual fields sensitive to direction, movement, color, and so on,
which interpret the signals to produce an organized sense of sight.