XV. The Sensory System
Meaningful motor activity is only possible if appropriate sensory input is present. It is sensory input that defines the conditions which motor activity must meet, and this sensory input begins with the sensory receptors.
A. Sensory unit - These are the components necessary for sensation. They include the following.
1. Receptor - This converts environmental changes into action potentials.
2. Neural pathway - This conducts the action potentials from the sensory receptors to the interpreting centers of the brain.
3. Interpreting center - This is where perception i.e., light, sound, pain, temperature, occurs.
B. Classification of receptors - There are three broad categories of receptors.
1. Exteroceptors - Respond to changes in the external environment.
2. Visceroceptors (enteroceptors) - Respond to changes inside of the body.
3. Proprioreceptors - Provide information about body positions and movement.
C. Characteristics of receptors
1. Transducers - A transducer converts on form of energy to another. Receptors convert various forms of energy into action potentials which the nervous system can interpret.
2. Adaptation - This is where a constant level stimulus results in a reduction in sensitivity. There are two types.
a. Peripheral adaptation - This is where the receptor or its sensory neuron reduces its level of activity.
b. Central adaptation - This is another type of sensory adaptation, involving the CNS. Here the receptors may continue to respond, but the CNS circuits will become unresponsive.
3. Specificity (Law of specific energies) - Each receptor has a particular type of energy to which it responds best. Mechanical receptors respond to mechanical energy but not light. Light receptors respond to light, but not sound.
D. Functioning of receptors - Receptors respond to a stimulus by initiating a graded, local potential, known as a generator potential. If the stimulus is intense enough then the generator potential will reach sufficient magnitude and an action potential will be triggered on the sensory neuron.
E. Types of senses - The senses are usually grouped into three major categories, the general senses, the propriosenses, and the special senses which are associated with the head region.
1. General (cutaneous) senses - These sense are largely associated with the skin although pain and temperature receptors are located in other areas as well. They are embedded in the skin and detect changes in the external environment. They are divided into two major groups based upon their structure.
a. Encapsulated nerve endings - These are nerve endings with one or more layers of membranes surrounding them. The major ones are as follows.
(1) Meissner's corpuscle, Merkel's disk, and the end organ of Ruffini are all associated with touch. The end organ of Ruffini was previously thought to be a heat detector.
(2) Pacinian corpuscle - This detects pressure.
b. Free nerve endings - These are essentially dendrites. They function in several different areas.
(1) Pain reception - These are sometimes called nociceptors and they respond to any type of stimulus which is of sufficient intensity to cause tissue damage.
(2) Touch - Certain free nerve endings wrap around the base of hair shafts thereby forming a sensitive lever system for touch.
(3) Temperature - Although they have not been specifically identified, it is thought that certain populations of free nerve endings are responsible for temperature detection.
c. Neural pathways of the cutaneous receptors - These are the spinothalamic tracts and they conduct impulses from the cutaneous receptors to the primary somesthetic area of the brain which is located on the post central gyrus of the parietal lobes. They consists of a chain of three neurons which for the most part cross over from one side of the nervous system to the other. Consequently, cutaneous sensation from the right side of the body is interpreted by the left side of the brain. All of the spinothalamic fibers synapse in the thalamus, and it is from here that the third neuron leaves for the cortex.
2. Proprioreceptors - These are the receptors which provide information as to the status of the muscles in terms of contraction and the position of the body. There are three types.
a. Tendon receptors - These detect the amount of contraction which a muscle is undergoing by measuring the stretch of the tendon.
b. Muscle spindle - These lie inside of the muscle parallel to the fibers. They detects the degree of relaxation.
c. Joint receptors - This receptor type measures the angle of the joint and therefore permit knowledge of limbs positions. This is the only proprioreceptor which works at the conscious level.
The proprioreceptors work in conjunction with the eyes and the inner ear in maintaining balance, equilibrium, and coordination.
3. Taste and Smell - These are the first of the special sense which are located in the head area. Both taste and smell are chemical senses, the receptors respond to chemicals from the environment.
a. Taste - The receptors are located in mounds of tissue on the tongue known as taste buds. Each bud has a gustatory pore which permits saliva containing dissolved chemicals to enter the bud and come in contact with the sensory cells which are then activated.
(1) The receptors respond to five classes of chemicals: sour, sweet, bitter, salty, and Umami. Umami is the most recently discovered taste and it detects amino acids, especially glutamate.
(2) The complex perception which we consider taste results from a blending of the inputs from these receptors in the taste center along with those coming from the olfactory receptors and touch receptors.
b. Smell (olfaction) - Unlike the sense of taste, the sense of smell responds to literally thousands of different odors. The cells are also very sensitive, being able to detect very low concentrations of molecules.
(1) The sensory cells of smell are embedded in a stratified epithelium about one inch square. There is one epithelium in each nostril, with part of each located on the septum and part on the lateral wall.
(2) The olfactory receptors adapt more readily than do any other type of receptor. This is why people can live next door to pulp mills with no discomfort. It is also why those with strong body odor are usually not aware of it (although everybody else is).
4. Eye - This is the most complex of the sense organs. It functions to detect light, but the photoreceptors are located deep inside of a complex optical organ.
a. Anatomy - The eye is organized into three layers. There are several structures associated with each layer. The layers and their structures are as follows.
(1) Sclera - This is the tough outer layer made up largely of white connective tissue. At the anterior side there is a clear curved portion known as the cornea. The cornea admits light within the eyeball and is primarily responsible for the bending the light so that it comes to a proper focus on the receptor cells. The anterior part of the sclera (the part exposed to the air) is covered by a thin membrane known as the conjunctiva. It contains cutaneous receptors as well as blood vessels (except for the portion covering the cornea) which nourish the anterior portion of the sclera.
(a) Anterior chamber - This is the cavity located behind the cornea but in front of the iris.
(b) Posterior chamber - This is a second cavity, located behind the iris but in front of the lens. Both chambers contain a fluid termed the aqueous humor. This fluid nourishes the lens and cornea and also provides proper intraocular pressure.
(2) Choroid layer - This is the middle layer of the eye. It is heavily pigmented and contains the blood vessels for the eyeball.
(a) Ciliary body - A muscular ring formed by the choroid coat. It is somewhat like a doughnut with a hole in the center. The lens of the eye is suspended from the edges of this hole, and it is the contraction of the ciliary body which changes the shape of the lens and therefore the focus of the eye.
(b) Iris - This is a muscular structure with a hole in the center known as the pupil. Contraction of the muscles of the iris changes the size of the pupil and therefore the amount of light that enters into the interior of the eye. The iris also contains the pigment which is responsible for eye color.
(c) Lens - This is a transparent structure enclosed in an elastic membrane. It along with the cornea is responsible for proper eye focus. It can change shape which results in a change of focus.
(d) Vitreous body - A jelly-like fluid which fills the large vitreous cavity behind the lens.
(3) Retina - This is the third layer of the eye. It consists of three layers. The innermost layer is the photoreceptors proper, known as the rods and cones. The photorecptors synapses with a middle layer of bipolar neurons. These in turn synapse with the outer layer of neurons known as the ganglion cells. The axons of these neurons form the optic nerves. Where these neurons converge and exit the eye (optic nerve) there is a spot where there are no receptor cells and therefore a blind spot. This spot is referred to as the optic disk.
b. Mechanism of vision - focusing on the retina
(1) Light enters through the cornea and pupil. The iris adjusts the diameter of the pupil and thereby controls the amount of light that enters the eye. This is accomplished by a reflex.
(2) The light is bent (focused) by the cornea, lens, and eye fluids. The lens is the only structure which can vary its diameter and therefore adjust the focus of the eyes from close to distant.
(3) The change in lens shape is brought about by the ciliary body which can increase or decrease tension on the edge of the lens, changing its diameter.
(4) When the eyes are focused on a near object the ciliary body is contracted, reducing tension on the lens, which then assumes a round shape and bends light more.
(5) When viewing objects at a distance the ciliary body relaxes and the hole in the center increases its diameter. This pulls on the edges of the lens causing it to flatten it out.
(6) Once light is focused on the retina it initiates a series of photochemical reactions in the receptors cells that in turn initiate action potentials.
c. Receptor cells - There are two types of receptor cells, the rods and the cones.
(1) Rods - These function in the following ways.
(a) They are used in black and white vision only.
(b) The function in low light only (night).
(c) They serve in low acuity vision only.
(2) Cones - These function in the following ways.
(a) They are responsible for color perception. There are three types of cones: those that respond to red light, others that respond to blue, and a third population that responds to green. The spectrum of colors is perceived by the interaction of inputs from these three types.
(b) They are active in bright light only (daylight).
(c) They are responsible for high acuity (detailed) vision.
/1/ There is in the retina a small pit known as the fovea centralis. It contains only cones and they are packed very tightly together. This is the spot of greatest visual acuity and the spot which we normally focus bright light on.
d. Transduction of light into action potentials in rods - While the focusing of the eye and the formation of an image on the retina is well understood, the conversion of this image into a series of action potentials is more complex. The process is more well understood in the rods than in the cones, although both receptor types do appear similar. The following events occur.
(1) There exists in the rods a light sensitive pigment referred to as rhodopsin or visual purple. This chemical consists of the protein opsin and the pigment retinal which is derived from vitamin A. Rhodopsin is also found in cones, but in a different form, in fact, a different form for each of the three different populations of cones. The retinal is the same for all, but the opsins differ.
(2) There are two three dimensional forms for retinal, the 11 cis form and the 11 trans form. In the dark all retinal is in the cis form but when it absorbs light it converts to the trans form. Following conversion to the trans form, a series of reactions occur which result in a complete separation of the retinal from the opsin. This results in colorless products and consequently the process is referred to as the bleaching of rhodopsin.
(3) Once light has been absorbed and bleaching has occurred, further activity is not possible until the rhodopsin is regenerated. This occurs in the dark. A series of reactions result in the trans form of retinal converting back to the cis form which then rejoins with opsin and the process is ready to begin again. In very strong light (day light or bright artificial light) all of the rhodopsin in all of the rods is bleached and therefore the rods do not function. It takes about 30 minutes of darkness for all of the rhodopsin to regenerate and for the eyes to become dark adapted.
(4) Once light has struck the rhodopsin molecule and the cis retinal has been converted to trans retinal, the opsin acts as an enzyme. It activates a second enzyme, transducin, or G-protein.
(5) G-protein then activates another enzyme that ultimately results in a closing of the sodium channels in the rod membrane. In a dark adapted rod, sodium channels are open and sodium is constantly moving inward (and being actively pumped outward) creating the so called "dark current." This results in a hypopolarized cell which constantly releases the neurotransmitter glutamate onto the bipolar neurons. Glutamate hyperpolarizes the bipolar neurons and consequently no activation of the ganglion cells can take place.
(6) The activation of rhodopsin by light results in a closing of the sodium channels. This results in a hyperpolarization of the rods which in turn shuts off the flow of glutamate to the bipolar cells. The bipolar cells will now hypopolarize. This releases an exciatory neurotrnasmitter on to the ganglion cells initiating an action potential which will be transmitted to the visual interpreting center in the brain.
(7) As strange as it may be seen, light actually turns off the rods, thereby permitting the ganglion cells to become active. This is another example of the fact that inhibition in the nervous system is every bit as important as is excitation.
5. Ear - This organ is the site of both equilibrium and sound detection. The structures for these function lie within a bony canal found in each of the temporal bones.
a. Anatomy - The ear is divided into three regions: outer, middle, and inner.
(1) Outer ear - This begins with the ear flap (pinna or auricle) and extends to the tympanic membrane (ear drum. The pinna has little significance in humans. The canal to the tympanic membrane is largely protective. It contains wax and hairs which keep foreign material out.
(2) Middle ear - Here is found the tympanic membrane and the air filled cavity behind it. The cavity contains the ear ossicles, three small bones which function to transmit sound vibrations from the tympanic membrane to the inner ear. The three bones are the malleus, incus, and stapes. The middle ear is connected to the pharynx (throat) by the auditory (eustachian) tubes which permit equalization of atmospheric pressure on both sides of the tympanic membrane.
(3) Included here are the vestibule, semicircular canals, and cochlea. These structures collectively form the bony or osseous labyrinth.
(a) The cochlea is the auditory portion and resembles a snail shell. It is divided into three chambers by a membranous partition known as the scala media or cochlear duct.
(b) The upper chamber is the scala vestibuli and the lower chamber is the scala tympani. The upper and lower chambers communicate at the tip of the cochlea.
(c) The upper and lower chambers are filled with a fluid termed perilymph. The middle chamber (scala media) is filled with a slightly different fluid known as endolymph.
(d) The upper membrane of the scala media is the vestibular membrane and the lower one is the basilar membrane. It is on the basilar membrane that the actual organ of hearing, the organ of Corti, is found.
b. Mechanism of hearing - Sound is a mechanical vibration that can be transmitted through solid, liquid, or gas. Sound vibrations are usually communicated to the ear by means of air molecules which vibrate at the same frequency as the sound being heard.
(1) The vibrating air molecules hit the tympanic membrane causing it to vibrate at the same frequency as the air molecules.
(2) The tympanic membrane in turn causes the ear ossicles to vibrate. These serve as a lever system to transfer the sound vibration across the middle ear to the inner ear.
(3) The last ossicle, the stapes, is connected to a membranous oval window, located in the wall of the vestibule.
(4) Movement of the ossicles causes the oval window to move back and forth. This in turn sets up waves in the perilymph of the scala vestibuli. These waves move to the tip of the cochlea and into the scala tympani. They now proceed back toward the vestibule where the collide with a second membrane window, the round window. The round window bulges back into the middle ear and thereby absorbs the energy from the moving waves, preventing back reflection.
(5) Movement of the waves through the scala tympani cause the basilar membrane to move up and down, much like a rubber raft following waves in the ocean. This is turn causes the organ of Corti, which sits on the basilar membrane, to move up and down in the endolymph.
(6) Located in the organ of Corti are hair cells. The movement of the organ through the endolymph causes the hair cells to bend and when they bend they generate action potentials. These are carried by the acoustic nerve to the hearing interpreting centers where they are perceived as sound.
c. Equilibrium - The inner ear is a major contributor to balance. The two major parts that contribute are the semicircular canals and the vestibule.
(1) Semicircular canals - There are three of these oriented at right angles to one another. They are U-shaped and communicate with the vestibule. Within each canal is endolymph and hair cells. Movement of endolymph when the body is in motion causes the hair cells to bend and generate action potentials. These are transmitted to the appropriate regions of the brain, especially the cerebellum. Note that the semicircular canals are only active when the body is in motion. They detect acceleration in a given direction and therefore play a key role in dynamic or moving equilibrium.
(2) Vestibule - Within the vestibule are found two membrane sacs, the utricle and the saccule. The utricle is the larger and is associated with the semicircular canals that open into it. Both sacs have special regions termed the macula. The macula contains hair cells which are covered with a gelatinous suspension containing otoliths (ear stones). Gravity pulls the otoliths on to the hair cells generating action potentials. When the head moves the otoliths move also, activating new populations of hair cells. In this way we always know where gravity is and therefore which way is up and down, static equilibrium.
F. Pathologies of the senses
1. Myopia - This is nearsightedness. With this condition one can see well close up, but not at a distance. It is usually caused by an eyeball that is too long which results in the image coming to focus in front of the retina. It can be corrected with concave lenses.
2. Hyperopia - The opposite of myopia, this is farsightedness. Persons with this condition can see well at a distance but not up close. Usually the eyeball is too short which results in the image coming to focus behind the retina. It can be corrected with convex lenses.
3. Presbyopia - This is the farsightedness of old age. With age the lens loses its elasticity and therefore its ability to assume highly spherical shapes. As a result the near point of vision moves further and further away. Although the process begins early in life, it is usually not noticeable until the forties when the near point moves so far that reading becomes difficult. It is then that most people have to get reading glasses.
4. Astigmatism - This defect is caused by unequal thickness in the cornea or lens. It is corrected with special cylindrical lenses.
5. Glaucoma - Results from abnormally high pressure inside of the eyeball due to a failure of aqueous humor to drain properly. It can result in blindness as the increasing pressure will collapse the blood vessels supplying the retina, resulting in retinal death.
6. Cataracts - A clouding over of the lens to the point that it becomes opaque. It is due to a breakdown in the integrity of the lens capsule. Ultraviolet light can also play a significant role in initiating this process.
7. Deafness - This is a lack of hearing or at least a significant loss of hearing. There are two general types of deafness.
a. Sensorineural - Impairment of the cochlea or some component in the neural pathway and/or interpreting center is responsible for this form.
b. Conduction - Here there is impairment of the middle ear components. This type can sometimes be surgically corrected or corrected by use of hearing aids.
8. Vertigo - This is a sensation of motion. The world seems to be moving around the affected individual, or the individual seems to be revolving in space. In may be due to inner ear disturbances, CNS disorders, or psychogenic.
9. Motion sickness - This is somewhat difficult to explain, but appears to be due to a mismatch of equilibrium information. The eyes fix on a position and provide information of position in space, but movement causes the inner ear to provide different information. The conflict results in the symptoms that are known as motion sickness. Various drugs can aid by diminishing signals from the vestibular apparatus.