• By the end of this section, you should be able to:
Define a sensory receptor and its adequate stimulus.
List four characteristics of generator potentials.
List the receptors responsible for touch, vibration, temperature, pain, and
proprioception (limb position and movement).
Define receptive field of a neuron. Name the two major ascending sensory
pathways and describe their anatomy and the information they carry.
List the somatotopic organization on the postcentral gyrus (somatosensory area),
going from medial to lateral on the cortex.
Draw and label a picture of the visual system and the eye.
List the cell types in the retina and draw a diagram of their anatomical
List the functional characteristics of the rod and cone systems.
Draw a flow diagram of the sequence of steps in the retina by which light is
transduced to action potentials.
List four types of eye movements, describe when they occur, and describe their
Draw a simple diagram of the auditory system.
List three ways in which the outer and middle ear act to transmit pressure waves
from air to fluid.
Describe how different frequencies of sound are transduced into action potentials.
Draw a simple diagram of a single semicircular canal with hair cells and cupula
and the utricle and saccule with otoliths.
List the major functions served by the vestibular system.
Name the movement detected by the semicircular canal receptors and the two
detected by the otolith organs.
Describe how angular motion of the head is transduced into action potentials.
Introduction – Changes to the Sensory System
• The human body has several sensory systems that allow it to detect these external
changes rapidly. These systems include:
The somatosensory (touch) system
The visual system
The auditory and vestibular system
The olfactory (smell) system
The gustatory (taste) system.
Section 7.3 Transduction of Environmental Information
• Transduction of environmental information:
It is how information from the external environment is turned into
language the brain understands – action potentials.
In order for the brain to know what is happening outside the body,
environmental stimuli (energy) like light, heat, touch, or sound must first
be detected by sensory receptors which then convert the information into
• In order for the brain to consciously perceive an environmental stimulus, that
stimulus must be detected by a sensory receptor.
• Environmental stimuli come in different forms and, therefore, will require
different receptors to detect the stimulus and then convert it to action potentials.
• For example:
Amechanical stimulus, like touching or vibrating the skin, will stretch
sensory receptors in the skin and open ion channels, causing a
depolarization of the sensory neuron producing an action potential.
Achemical stimulus, like a sour taste on the tongue or an odor in the
nose, binds with a receptor, causing a depolarization and then an action
Light energy is absorbed by photoreceptors of the eye (rods and cones in
the retina) and eventually produces action potentials.
Gravity and motion can also be detected by hair cells in the vestibular
system, which convert this form of external stimulus to action potentials.
Adequate Stimulus for the Receptor
• Although there are many types of stimuli and a corresponding receptor to detect,
some receptors can detect more than one type of stimulus. • Adequate stimulus:
The particular form of environmental stimulus to which the sensory
receptor is most sensitive.
The adequate stimulus for the rod and cone cells found in the retina of the
eye is light.
Sensory receptors do respond to other forms of energy but not in an
Receptor (Generator) Potentials
• Once the sensory receptor is stimulated by an environmental stimulus, it will
cause a change in ion permeability, leading to a local depolarization.
• This local depolarization is called a generator or receptor potential.
• Since the receptor does not have voltage-gated ion channels necessary to fire an
action potential, the receptor potential must spread to an area on the sensory
neuron that does contain these channels. This is usually at the first node of
Ranvier on the axon.
• The action potential will then be generated and propagated along the axon and
into the spinal cord.
• In receptors with no axons (like the hair cells in the inner ear that we will see
later), the depolarization has to spread to the synapse to result in the release of a
Receptor (Generator) Potentials (cont’d)
• Receptor potentials are similar to EPSPs
and IPSPs and share some of the same
• These characteristics
include those shown
at the left.
Receptor Potential and
• In the nervous system module, we talked
about how neural
the brain of the weight of an object in
• The weight of the object was "coded" into the action potentials (the heavier the
object, the more action potentials per second).
• How do you generate a large number of action potentials?
The heavier weight will trigger the receptor to produce a large receptor
potential. This large receptor potential will trigger many action potentials on the
sensory neuron's axon.
This burst of high-frequency action potentials will eventually reach the
brain where you will become consciously aware of the heavier weight in
The Somatosensory System
• Detects and processes the sensations of touch, vibration, temperature, and pain –
the majority of which originate in the skin.
• Detecting each sensation requires several different sensory receptors within the
skin, each developed to detect its adequate stimulus.
• The receptors in the skin are collectively referred to as cutaneous receptors. They
include the following:
1. Hair follicle receptors that are sensitive to fine touch and vibration
2. Free nerve endings that respond to pain and temperature (hot and cold)
3. Meissner's corpuscles that detect low-frequency vibrations (between 30
and 40 cycles/sec) and touch
4. Ruffini's corpuscles that detect touch
5. Pacinian corpuscles that detect high-frequency vibrations (250 to 300
cycles/sec) and touch
• The receptive field is the area on the surface of the skin where an adequate
stimulus will activate a particular receptor to fire an action potential in the neuron.
• In the animation at right, the receptive field is the third and fourth cells, which,
when touched, generate an action potential in the sensory neuron.
• Any stimulus applied outside the receptor field will not generate an action
• Now that action potentials have been generated in the sensory nerve, they must be
propagated to a specific area of the brain so that the individual becomes
consciously aware of the stimulus. These action potentials reach the brain via two
spinal tracts—let's look at these now.
Somatosensory Pathways from the Periphery to the Brain: The Spinothalamic
• The spinothalamic (anterolateral) tract:
Transmits information dealing with very basic sensations like pain,
temperature, and crude touch.
• The information from the sensory neuron (first order neuron) enters the spinal
cord where it synapses with a second order neuron.
• This neuron crosses to the opposite or contralateral side of the spinal cord and
ascends to a region of the brain called the thalamus. • The thalamus acts as a relay station for almost all sensory information (except
• Asecond synapse with a third order neuron occurs here and then travels to the
• It is important to realize that sensory information from the right side of the body
goes to the left side of the brain and vice versa.
Somatosensory Pathways from the Periphery to the Brain: Dorsal Column, Medial
• The dorsal column, medial lemniscal system:
Transmits information associated with the more advanced sensations of
fine detailed touch, proprioception (muscle sense), and vibration.
• The information from the sensory neuron (first order neuron) enters the spinal
cord and immediately travels up the spinal cord before crossing to the
contralateral side (unlike the spinothalamic system).
• In the upper spinal cord, the sensory neuron synapses with a second order
neuron which then crosses to the opposite side of the spinal cord.
• From here it continues to the thalamus where it synapses again onto a third
order neuron that then travels to the somatosensory cortex.
• Again, sensory information from the right side of the body goes to the left side of
the brain and vice versa
Primary Somatosensory Cortex
• Once the sensory information has reached the brain, it travels to the primary
somatosensory cortex, which is located in the parietal lobe on the postcentral
gyrus behind the central sulcus. Section 7.14
Primary Somatosensory Cortex—The Somatosensory Homunculus
• The primary somatosensory cortex is arranged in a very specific manner.
• The sensory information arriving at this cortex is not randomly scattered around
on the surface; rather, it is "geographically preserved."
• It is as if the entire body were projected onto the surface of the brain like a map.
• All the sensory information for the foot is located in one area – that of the leg just
next to it and the hip next to the leg, and so on – for the entire body.
• This topographical representation of the body on the surface of the cortex is called
the somatosensory homunculus
Primary Somatosensory Cortex—The Somatosensory Homunculus (cont’d)
• You should notice that the "picture" of the human body represented in the
homunculus is somewhat out of scale.
• Some of the representative areas are out of proportion (much larger than they
• This is because some areas on the cortex, like the areas dealing with the hand,
tongue, and lips, receive more sensory information and require more of the brain
to process that information.
• The hands, tongue, and lips are the most sensitive parts of the body; they contain
many more sensory receptors than any other part.
• After all, when you really want to experience how something feels, you use your
hands Section 7.16
The Visual System
• The visual system detects light, converts it into action potentials, and sends these
to the primary visual areas for processing.
• Once processed, we become aware of our visual world and are able to distinguish
and recognize features in our external environment.
• The visual system consists of the:
Eye (which contains photoreceptors that convert the light to action
Visual pathway (which transmits the action potentials)
Primary visual area in the occipital lobe of the brain (which processes
the incoming signals).
• After passing through the cornea, the amount of light is regulated by the iris,
which can constrict with bright light or dilate in low light.
• The lens flips the light (upside down and backwards) and focuses it onto the retina
at the back of the eye.
• The retina contains photoreceptors called rods and cones.
• The rods and cones actually point toward the back of the head.
• The center of your vision is focused onto a part of the retina called the fovea. This
area has the highest concentration of cone cells.
The Photoreceptors of the Eye—Rod Cells and Cone Cells
Extremely sensitive to light and function best under low light conditions.
They contain one type of photopigment (a chemical sensitive to light) and
do not detect color.
Rods are located mostly in the region of the retina outside and around the
Function best under bright light and are ideal for detecting detail.
There are three different types of cone cells, each with a different
photopigment and each sensitive to one primary color.
The cones are principally located in the region of the fovea where they are
found in large concentrations. • Notice that the rod and cone cells do not have axons and, therefore, do not
generate action potentials.
• However, they do generate receptor potentials that cause the release of an
inhibitory neurotransmitter (this is important) from their synaptic ending.
Other Cells of the Retina
• The retina contains a pigment layer at the very back of the eye that absorbs excess
• Other cells in the retina include bipolar cells, ganglion cells, horizontal cells, and
• As already mentioned, the rod and cone cells do not generate action potentials.
• These other cells are responsible for the integration of information from the rods
and cones and the production of action potentials.
• The visual system works "backwards."
• As you have seen, the light striking the retina has been flipped upside down and
backwards due to the lens.
• When depolarized, the rod and cone cells release an inhibitory
neurotransmitter, shutting off the bipolar cells.
• But, most importantly, when light strikes the retina, it does not excite and
depolarize the rod and cone cells.
• The light actually hyperpolarizes these cells and shuts them off. • Since these cells release an inhibitory neurotransmitter when depolarized in the
dark, they inhibit the bipolar cells.
• When light strikes the photoreceptors, they hyperpolarize, shut off, and stop
releasing inhibitory neurotransmitter.
• The bipolar cells, which can depolarize spontaneously (by themselves), now
• Eventually, the depolarization of the bipolar cells may lead to an action potential
in the ganglion cells (not shown).
How Light is Transformed into Action Potentials
• In the dark, Na are flowing into the photoreceptors, producing a depolarization.
• This leads to the release of the inhibitory neurotransmitter.
• However, when light strikes the rod and cone cells, it closes these Na channels
• With less Na coming in and K leaking out (as they always do in every cell), the
• With the rods and cones hyperpolarized, no inhibitory neurotransmitter is released
and the bipolar cell depolarizes.
Types of Eye Movements
• In order to be able to focus our attention on a particular object, we must be able to
direct our eyes to exactly the correct spot. In this way, the image of interest is
focused onto the fovea, which has the highest concentration of cone cells. In order
to do this, we must be able to move the eyes in a number of ways, depending on
whether the object is stationary or moving.
• There are four primary eye movements:
Rapid, jerky movements of the eye.
Used to rapidly move the eye to the object of interest
Ex: gazing around a room while holding your head still or reading
these words on your computer.
2. Smooth pursuit
Smooth movement of the eyes that is made to keep a moving
object of interest focused on the fovea
Ex: following the flight of a bird through the sky while keeping
your head still
3. The vestibular ocular reflex (VOR) An eye movement made when you focus your attention on an
object and then move your head back and forth or shake it up and
Ex: staring at someone with whom you are disagreeing or
Eye movements that are made when an object of interest is
approaching or moving away from you.
When the object is moving away, the eyes diverge; when the
object moves closer, the eyes converge
Ex: staring at a pencil while moving it away from and toward your
• The auditory system converts sound waves from the external environment into
action potentials that travel to the auditory system of the brain.
• Human ear can detect sound frequencies ranging from 20 waves per second (or
Hz) to as high as 20,000 Hz
• The large, basic structural features of the auditory system can be divided into
1. The external or outer ear contains the ear or auricle and the external
2. The middle ear consists of the eardrum (or tympanic membrane), the