Sensory Systems


                With this chapter we will venture the way animals receive signals from the external and internal environments, and then decode the signals in ways that give rise to awareness of sound, sights, odors, pain, and other sensations.  Sensory neurons, nerve pathways, and brain regions are required for these tasks.  Together, they represent the portions of the nervous system called the sensory systems.


I.  Sensory Receptors and Pathways

A. Sensory Systems- the front doors of the nervous system, receive information    about specific changes inside and outside the body and notify the spiral cord and brain of what is going on.

1.Sensory Receptors

2.Nerve pathways leading from receptors to the brain.

3.Brain regions where sensory information is translated into sensations.

a. Sensation- conscious awareness of a stimulus.

b. Perception- (understanding what a sensation means)

4. Compound sensations arise when information about different stimuli is integrated at the same time.

EXAMPLE:  “Wetness” is not a single stimulus; our perception of it arises from simultaneous inputs concerning pressure, touch, and temperature.

B.      Types of Sensory receptors: receptionists at the front door of the nervous system. Six major categories of sensory receptors based on the type of stimuli energy that each type detects.

1.Mechanoreceptors- detect forms of mechanical energy (changes in pressure, position, or acceleration)

2.Thermoreceptors- detects infrared energy (heat).

3.Pain receptors- detects tissue damage.

4.Chemoreceptors- detects chemical energy of specific substances dissolved in the fluid surrounding them.

5.Osmoreceptors- detects changes in water volume (solute concentration) in the surrounding fluid.

6.Photoreceptors- detects visible and ultraviolet light.


 C.      Sensory Pathways

                1.Sensory axons carry signals from receptors to the brain.

a. Before this happens, the stimulus energy must be converted to  action potentials, the basis of neural messages.

2.When a stimulus disturbs the plasma membrane of a receptor’s ending, certain ions flow across a local patch of the membrane.

3.Intense stimulus or repeated fast enough for a summation of local signals, action potentials may be the result.

     a. Propagate themselves from the receptor to the axon endings of sensory neurons.

     b. Neurotransmitter is released from the presynaptic cell, and it influences the electrical activity of the cell adjacent to it (another interneuron or a motor neuron).

                       c. Disturbance may trigger action potentials in the postysynaptic cell, which is part of an information pathway leading to

                        the brain.

d. When traveling along a sensory neuron, do not vary in amplitude.

4.  How does the brain assess the nature of a given stimulus?

                       a. Which nerve pathways happen to be carrying action potentials?

                       b. Frequency of action potentials traveling along each axon.

                        c. The number of axons that the stimulus has recruited.

i. Decrease in frequency is sensory adaptation.

D.      Specific Sensory Receptors

1.Somatic Sensations- Sensory receptors that are present at more than one body location contribute to these types of sensations.

2.Special Senses- other types of sensory receptors that are restricted to particular locations, such as inside the eyes or ears, contribute to these types of sensations.


II. Somatic Sensations

A. Begin with receptors in the body’s surface tissues, skeletal muscles, and walls of internal organs.

1. Receptors are most highly developed in birds and mammals; amphibians have few, and apparently fishes have none.

2. Inputs from these receptors travel into the spinal cord and on to the somatosensory cortex, part of the surface layer of gray matter of the cerebral hemispheres.

B. Receptors near the Body Surface.

1.Sensations of touch, pressure, cold, warmth, and pain near the body surface.

2.Regions with the greatest number of sensory receptors, most sensitive to stimulation.

Example: fingertips and the tip of the tongue

3.Other regions do not have nearly as many receptors and are far less sensitive.

Example: back of the hand and neck.

4.Free nerve endings simplest receptors.

a.Thinly myelinated or unmyelinated (naked) branched endings of sensory neurons in the epidermis or in the underlying

dermis of skin.

5.Encapsulated receptors

a.A Meiscner’s corpuscle adapts slowly to vibrations of low frequencies.

b.Notably abundant in the lips, fingertips, eyelids, nipples, and genitals.

C.      Muscle Sense

1.Requires mechanoreceptors in skeletal muscle, joints, tendons, ligaments, and skin.

Examples: Stretch receptors of muscle spindles.

D.      Sensation of Pain

1.Pain is the perception of injury to some body region.

a.Most important pain receptors are subpopulations of free nerve endings, several million of which are distributed throughout the skin and internal tissues.

2.Somatic pain starts with pain receptors in skin, skeletal muscles, joints, and tendons.

3.Visceral pain is associated with the internal organs, are related to excessive chemical stimulation, muscle spasms, muscle fatigue,                inadequate blood flow to organs, and other abnormal conditions.

a. Responses to pain depend on the ability of the brain to identify                  the affected tissue and project the sensation back to it.

b. Referred pain is related to the way the nervous system is                            constructed.

4.Phantom pain-sensed by amputees.






III. Senses of Taste and Smell

A.      Both taste and smell are chemical sense; their sensory pathways start at chemo receptors, which are activated when they bind a chemical substance that is dissolved in the fluid bathing them.

                1.Receptors wear out and new ones replace them.

2.Sensory input travels from the receptors through the thalamus and on to the cerebral cortex.

3.Input also travels to the limbic system, which can integrate it with emotional states and stored memories.

B.      Taste Receptors

                1.Also called chemo receptors

                2.Depending on its location is where animals are allowed to taste.

Examples: Mouth, antennae, legs, tentacles, or fins.

C.      Olfactory Receptors

1.Detect water- soluble or volatile (easy vaporized) substances.

2.Vomeronsal organ, “Sexual nose” is common among animals,                                   including humans.

a. Receptors detect pheromones, a type of signaling molecule with roles in social aspects of reproduction.


IV. Sense of Balance

A. Inner Ears

            1. Present in a variety of fishes, amphibians, and reptiles, paired.

2. Evolved first as organs of equilibrium and had little, if anything, to do with hearing.

3. Systems of fluid-filled sacs and canals on both sides of the brain.

4. Some parts detect rotational, accelerated motions of the head.

                Example: riding a looping roller coaster.

5. Other parts detect linear motion of the head.

B. In humans

1. Vestibular apparatus is the part of the inner ear where organs of equilibrium are located.

2. Hair cells are a type of mechanoreceptor



Figure 3: External flaps of the outer ear collect sound waves, which move into on auditory canal and then arrive at the eardrum (the tympanic membrane).






VI. Vision

A. All organisms are sensitive to light.

B. Vision: an awareness of the position, shape, brightness, distance, and    movement of visual photoreceptors.

C.  Vision requires:

1.  A complex system of photoreceptors.

2.Complex brain centers that can receive and interpret the patterns of actions from different parts of the photoreceptor system.

3. Eyes: Photoreceptor organs that contribute to image formation.



VII. Invertebrate eyes:

A.  Eyespots: Clusters of photosensitive cells that are arranged in a cuplike depression in the epidermis.

1.Many invertebrates have eyespots not eyes.

B.  The eye of a mollusk.

1.Cornea: transparent lens with a transparent cover.

2.Retina: a light-sensitive tissue with densely packed photoreceptors.

3.Lens: a transparent cone or sphere that focuses incoming light onto a dense layer of photoreceptor cells behind it.

4.Iris: a ring of contractile tissue that can be adjusted to admit more or less light through a pupil

5.Pupil:  an opening at the ring’s center.

C.  Of all the invertebrates, octopuses and other cephalopods have the most complex eyes and refined sense of vision.

D. Compound eyes:  Photoreceptor organs that contain closely packed photosensitive units.

EXAMPLE: insects and crustaceans.




VIII. Vertebrate eyes:

A.  Eye structure

1.  Vertebrate eyes have three layers.

a. Outer layer: Sclera and transparent cornea.

b. Middle layer: Choroids and lens, as well as semi fluid and jellylike substances.

c. Inner layer:  Retina.

B. Focusing Mechanisms:

1. Light rays from sources at varying distances from the eye strike the cornea at different angles and will be focused at different distances behind it. Therefore, certain components of the eye must be adjusted, so that all of the incoming stimuli will be focused onto the retina.  Without it, rays being transmitted from very distant objects would be erroneously focused in front of the retina, and rays being transmitted from very close objects would be focused behind it.

2. In both fishes and reptiles, eye muscles move the entire lens forward or back, like the focusing apparatus in a camera.

3. In Birds and mammals, the shape if the lens can be adjusted itself.

4. In some cases, the lens cannot be adjusted enough to make

the focal point match up precisely with the retina.

EXAMPLE: nearsightedness and farsightedness


Figure 6: Structure f the human eye.


IX. Disorders of the eye

A. Two-thirds of all the sensory receptors in your body requires are in your eyes.

B.  Injuries, disease, inherited abnormalities, and advancing age can disrupt functions of the eyes.

1.  Colorblindness

2. Focusing problems: Astigmatism, nearsightedness, and farsightedness

3. Eye diseases: Histoplasmosis, herpes simplex, trachoma

4. Age-related problems: cataracts, glaucoma

5.     Eye-injuries: retinal detachments

C. Today a variety of tools are used to correct some eye disorders.

EXAMPLE: Corneal transplant surgery, radial keratotomy, Laser coagulation



X. From signaling to visual perception.

A. Organization of the Retina

1.The flow of information begins as light reaches the retina, at the back of the eyeball.

2.Covering the eyeball is the retina’s basement layer, a pigmented epithelium.

3.Resting on the epithelium are densely packed photoreceptors, called rod cells and cone cells.

a. Rod cells detect dim light.

b. Cone cells detect bright light.

4.Sensory neurons are organized in distinct layers above the rods and cones.

Figure 7: Mammalian photoreceptors: rods and cones.





5. Information flows linearly from these photoreceptors to the bipolar types of sensory neurons, then to the types called Ganglion cells, the axons of which form the optic nerves to the visual cortex.

6. A great deal of synaptic integration and processing goes on even before visual information is sent to the brain.


B.  Neuronal Responses to Light

1.Rod cells

a. A rod cell’s outer segment consists of several hundred membranous disks, each peppered with 108 molecules of rhodopsin.

b.With the rod cells neurons send signals about the visual stimulus.

2.Cone cells.

a. Red, green, and blue cone cells help with the vision during the day and for color.

b. The fovea, a funnel-shaped depression near the center of the retina, consists of slender cone cells beneath the layers of sensory neurons, where there is greatest acuity.

3.Ganglion cells: visual perception

4.Lateral Geniculate Nucleus

a. Each layer of this brain region has a distinctive bend to it. With this we have the visual stimulus of Form, movement, depth, color, texture and so on.