Module 7: Sensory system
Introduction – changes to the sensory system
Body needs to maintain homeostasis (relatively stable conditions in the internal environment of the
body) and in order to do this, it is necessary for the body to detect changes in the external
environment so it can react appropriately
The human body has several sensory systems that allow it to detect external changes:
1. The somatosensory (touch) system
2. The visual system
3. The auditory and vestibular system
4. The olfactory (smell) system
5. The gustatory (taste) system
Examine how the events in the outside world are detected, converted to action potentials, travel to the
brain and become consciously perceived
Transduction of Environmental Information
Transduction of environmental information is how information from the external environment is
turned into language the brain understands—action potentials: language of the nervous system.
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 action potentials.
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.
o A mechanical 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
o A chemical stimulus, like a sour taste on the tongue or an odour in the nose, binds with a
receptor, causing a depolarization and then an
o Light energy is absorbed by photoreceptors of the
eye (rods and cones in the retina) and eventually
produces action potentials.
o 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
Some receptors can detect more than one type of stimulus
Adequate stimulus: the particular form of environmental stimulus to which the sensory receptor is
o Adequate stimulus for rods and cone cells (found in retina) is light
Sensory receptors do respond to other forms of energy but not in an optimal way
o The rod and cone cells also respond to pressure on the eyeball
Receptor (generator) potentials
Recall that at a chemical synapse an excitatory neurotransmitter first produces an EPSP that, if
strong enough, then generates an action potential at the axon hillock. This is similar to the events
that take place at a sensory receptor.
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
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), the depolarization has to spread to
the synapse to result in the release of a neurotransmitter.
Receptor potentials are similar to EPSPs and IPSPs and share some of the same characteristics. They
include: Receptor Potential and neural coding
In the nervous system module, neural coding informed the brain of the weight of an object in your
hand. 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 your hand. The Somatosensory System
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
Receptors in the skin are collectively known as cutaneous receptors. They include:
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
Our skin is covered with sensory receptors but each receptor will only respond to a stimulus within
a certain region on the skin
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 sensory neuron
o Any stimulus applied outside the receptive field will not generate an action potential
The action potentials that have been generated in the sensory nerve must propagate 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
Somatosensory pathways from the periphery to the brain: The spinothalamic (anterolateral) tract
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 smell). A second
synapse with a third order neuron occurs here and then travels to the somatosensory cortex. 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. (The interactive animation at right only shows information coming from
the right side of the body.)
Somatosensory pathway from the periphery to the brain: Dorsal column, Medial Lemniscal system
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
Again, sensory information from the right side of the body goes to the left side of the brain and vice
versa. (The interactive animation at right only shows information coming from the right side of the
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 post central gyrus behind the central sulcus 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
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 should be).
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 (although
babies prefer to use their mouths). 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 out external environment
The visual system consists of:
o The eye – which contains photoreceptors that convert light to action potentials
o The visual pathway – that transmits the action potentials
o The primary visual area in the occipital lobe of the brain – that processes the incoming
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 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 Rods are extremely sensitive to light and, therefore, function best under low light conditions. They
contain one type of photopigment (a chemical sensitive to light) and, consequently, do not detect
color. Rods are located mostly in the region of the retina outside and around the fovea.
Cones, on the other hand, 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 light
Other cells in the retina include:
o Bipolar cells
o Ganglion cells
o Horizontal cells
o Amacrine cells
The rod and cone cells do not generate action potential and therefore, these other cells are
responsible for the integration of information from the rods and cones and the production of
Transduction of Light to 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 neurotransmi