In the last module we talked about how our brain causes muscles to move or actions to
occur in our bodies. This module will focus on how we sense our environment and relay
that information to our brain.
1) How are all the stimuli around us translated into action potentials, the language
our nervous system understands?
In order for us to sense and detect changes in the environment, we use receptors.
These act as sensors of our body and are responsible for initiating what we perceive as
one of our many senses (taste, smell, hearing, sight, touch).
We have numerous sensory receptors all over our body, each particularly good for
sensing one thing (adequate stimuli).
However, this doesn’t mean it ONLY senses one type of stimuli. A good example of this
is your photoreceptors in your eyes. They are particularly good at sensing Light
(adequate stimulus) but if you take your fingers and press on your eyelids, you will notice
that you begin to see spots! So while these receptors are most sensitive to light, they
can also be stimulated by a certain amount of pressure.
2) How are adequate stimuli translated into action potentials?
Surprise, it is all about ions moving! The movement of ions in a sensory receptor is
called a receptor potential. Receptor potentials ONLY occur in receptors. However, they
share many of the same properties you learned about in the last module for EPSP’s and
Hint-It would be helpful to make a chart of the similarities and differences between
these potentials so that you can understand them fully.
Receptor potentials behave very much like the post-synaptic potentials:
• Receptor potentials are graded (size matters), local, and deteriorate in strength
over time and distance.
• Receptor potentials can be depolarizing (mainly Na+ moving in) or
hyperpolarizing (mainly by K+ moving out).
• The most important detail to take away is that just like EPSP’s or IPSP’s,
receptor potentials may OR may not lead to an action potential…you need a
large enough depolarization
i) Sometimes, receptors are directly attached to their sensory neuron…and
then the receptor potential spreads to the “firing area”. A good example of
these types of receptors are your pain receptors.
ii) Some receptors are not attached to their sensory neuron, but there is a cleft
or space between them and the dendrites of the sensory neuron.
How can the receptor potential spread in this case?
These receptors have little vesicles containing neurotransmitters tha++are released when
a depolarizing receptor potential spreads, opening voltage-gated Ca channels and
neurotransmitter travels across the cleft and binds to their chemically-gated ion
channels. These channels open allowing ions to move and cause a graded potential just
as you learned for the generation of EPSP’s and IPSP’s. Examples of these types of
receptors (that have to release a neurotransmitter) are rods and cones in the eyes. 3) Summary of receptors
In this module you learned about many types of receptors, you should be familiar with
the different types and what their stimulus is;
A) The somatosensory system- detects and processes the sensations of
touch, vibration, temperature, and pain, mostly found in the skin. Collectively
called cutaneous receptors (Hair follicle, free nerve endings, Meissner’s
corpuscles, Ruffini’s corpuscles and Pacinian corpuscles)
These cutaneous receptors (as do all receptors) have a particular area that it is
sensitive for. For example, touching a spot on your arm isn’t sensed by Pacinian
receptors in your shoulder, rather there are localized Pacinian receptors in that
area in your arm that respond to what has just occurred. This is known as the
receptive field for that particular receptor type. Receptive fields can vary in size,
on your back they are very large but on your lips they are very small.
Where does the sensory information go?
Most of the sensory information including proprioception, vibration, touch, pain,
and temperature goes to our primary somatosensory cortex, located in the
postcentral gyrus in the parietal lobe (The sensation station). Of course we have
other sensory information which will go to “specialized cortex regions” such as
sight, which goes to the occipital lobe (visual cortex), and hearing which goes to
the temporal lobe (auditory cortex).
The somatosensory cortex is organized just as you saw for the motor cortex, in a
homunculus. It is somatotopically organized according to body parts, where sensations
that come from your legs or feet go to a particular area near the top center region of this
cortex and sensations from your face and lips go to the lateral or side areas of this
cortex. Realize that the area taken by each body part is out of proportion. This is because of
the size and number of receptive fields within each part. For example you need many
receptors have supersensitivity, like on your face and lips.
4) How does information get from the receptor all the way up to the
Just as you learned in the previous modules, we have “tracts”. These are like the 401
for sensory information. In module 6 you learned about the corticospinal tract … the 401
of MOTOR (voluntary movement) information. This tract only carries MOTOR
information. In this module, you learned about two tracts that carried sensory information
(Know what types of sensory information is carried in each tract system!)
i) Spinalthalamic tract- pain, temperature, crude touch
ii) Dorsal medial lemniscal system- proprioception, vibration, fine touch
- Each tract differs slightly in the area in which it travels in the spinal cord as well
as the area it crosses over.
- You need to have a clear understanding of the crossovers in the spinal column.
The dorsal column travels a bit up the spinal cord before it crosses to the opposite side of the spinal cord while the spinalthalamic tract neurons cross right
away once they enter the spinal cord. KEY POINT is it all crosses!
- So these stimuli from the right side of the body go to the left somatosensory
cortex (in the left hemisphere of the brain) while the stimuli from the left side of
the body go to the right somatosensory cortex (in the right hemisphere of the
Other sensory systems:
B) Proprioception- back to module 6. Don’t get confused, often students think
of muscle spindles as the motor system causing muscle contraction, but in fact
they are receptors. Proprioception is the “sensation” of what position your
skeletal muscles are in. It is the unconscious monitoring of your body position.
This is accomplished by different proprioception receptors in the muscles,
tendons and joints; the muscle spindles and golgi tendon organs. These
receptors each sense different things.
A) Golgi tendon organs: These are found in the tendons of your
muscles and detect muscle tension.
B) Muscle spindles: These are found in parallel (adjacent) to the
extrafusal muscle fibers (the part of the muscle that generates
movement). These muscle spindles consist of muscle fibers called
intrafusal fibers with a central sensory region, which senses muscle
stretch and how fast the length is changing.
- Alpha-gamma coactivaiton- The arrangement of muscles spindles
cannot tell you about muscle contraction. When the motor neuron sends
action potentials to contract your muscle (the extrafusal fibres); the
intrafusal fibers become limp (like wet spaghetti noodles). They have no
stretch left in them so they don’t have any receptor potential occurring
and no information is sent down the sensory neuron to the brain. The