CNS sensory and motor (Lecture 3):
Blood-brain barrier: astrocytes (glia)
Astrocytes are part of the blood-brain barrier, because these non-neural cells in our brain
and CNS. The foot processes of these glial cells are thought to induce the tight junctions
in the capillaries that create the blood-brain barrier.
Astrocytes (glia) have other important roles in the CNS.
o They regulate ionic concentrations by taking up certain ions and neurotransmitters
floating around in the extracellular space having leaked out from the synapse.
o They also provide structure support: help to provide the physical structure of the
o They perform phagocytosis of debris floating in the extracellular space.
Now we will talk about the physiology of neural processing and perception.
Perception of the external world:
Sensation: Awareness of sensory stimulation.
Perception: The understanding of a sensation’s meaning.
o E.g. Feeling uncomfortable is more of a sensation.
o E.g. My tooth really hurts and I need to see a dentist is more of a perception.
We do not perceive the “energy” of a sensory stimulus directly.
We only perceive the neural activity that is produced by sensory stimulation.
We can extend this argument/hypothesis to a few laws.
o Law of specific nerve energies: Regardless of how a sensory receptor is
activated, the sensation felt corresponds to that of which the receptor is
specialized/to what the receptor encodes. E.g. Rub your eyes hard and you will see light.
Although the photoreceptors in your eyes respond to photons, if
you press on your eye orbit, which pushes on your retina a little
bit, a release of neurotransmitters and the production of action
potentials result, so you might see some flashes of light.
Although there is no light and your receptors are stimulated in
some other manner, this is what you perceive – the light!
o Law of projection: Regardless of where in the brain you stimulate a sensory
pathway (at the photoreceptor, optic chiasm, or optic nerve), the sensation is
always felt at the sensory receptors location.
E.g. Neurosurgeon Penfield electrically stimulated somatic sensory cortex
and patients perceived somatic sensation in the body.
Because he was stimulating a place on the somatic sensory cortex
that maps to the foot.
No matter where along this pathway from toe to brain that you
activate the sensory input, what you perceive is at the location of
where the receptor is.
E.g. Phantom limb pain after amputation.
People who are amputated commonly perceive and feel sensation
in the limb, even though it is not there.
Why? Because when you amputate the limb and cut the nerves,
peripheral axons still try to grow back but there is no place to go,
so they connect to wrong places.
o For e.g., if my arm is amputated, some of my
somatosensory nerves might innervate some of the muscle
tissues in my shoulders and get activated when I contract
one of these muscles. This gives me perception of
somatosensation in my hand and my brain thinks it’s in my
hand, but obviously my hand is no longer there.
Sensory modalities of the sensory
We will start with
somatosensory soon, but first
we need to understand how
sensory information is
Modality is encoded by a labeled-
line code: Modality: general class of a stimulus (somatosensory, touching, sound, smell)
Labeled-line: The brain “knows” the modality and location of every sensory afferent.
o For every axon entering in the CNS, your brain knows what it encodes and where
its sensory receptor is situated in the body. We don’t know how this comes about.
o See law of specific nerve energies.
Sensory receptors and electrical physiology:
Different sensory receptors for different
sensory systems, but they all have in
Stimulus energy -> Receptor
membrane -> Transduction -> Ion
channel activation -> Afferent
o The stimulus needs to be
specific, which means that
there has to be a match
between the receptor and
the stimulus energy.
o When stimulus energy comes along, it interacts with a special receptor membrane
and transduction happens, that is the opening of closing of ion channels in the
receptor membrane. Once these ion channels are activated (opened or closed), we
have depolarization and activation of afference, which then signals to CNS.
o In some cases, we have specialized receptor cells, such as photoreceptors in the
retina and hair cells in the ear that become depolarized, release neurotransmitters
onto the afference and then that information is sent to CNS.
Stimulus energy is converted into afferent activity:
The strength of the stimulus is encoded at various stages in this process.
o Stimulus energy comes on.
o Receptor potential: Change in membrane potential. What happens is that the
membrane potential is depolarized around the receptor cell.
Could also be hyperpolarized
E.g. Photoreceptors: channels close when light activates them.
o Action potentials: If the receptor potential is big enough, it crosses the threshold
(dashed line) for action potential production.
o Propagation of action potentials: APs is sent down the axon to the CNS.
o Where are the cell bodies located? Outside the CNS, so these afferences send their
axons into the CNS and then release neurotransmitters. Stimulus intensity and afferent response:
How does this change as you vary the strength of stimulus energy.
o Subthreshold activation: you’re opening or closing ion channels in the receptor
cell, yet this produces a receptor potential that is not big enough to trigger APs, so
nothing goes to CNS. You do not perceive a subthreshold stimulus.
o Weak stimulus: might produce a small, but superthreshold(?) receptor potential,
which produces APs.
o Strong stimulus: bigger receptor potential, lots of APs, and these turn into the
magnitude of neurotransmitter released in the CNS (none -> a little -> a lot).
In short, the strength of the stimulus is reflected in the receptor potential, the frequency of
APs and the magnitude of neurotransmitter release, but is NOT reflected in the APs’
height (only the frequency is modulated).
Adaptation of afferent response:
What happens when you apply a stimulus, like touching your finger and recording from
an efferent nerve in the arm, using a tiny electrode?
The minority of afferent responses are non-adapting. You encounter what you might
expect. When the stimulus goes on, APs go off: pa-pa-pa-pah! And when the stimulus
goes off, there are no more APs.
Most afferent response requires slow adaptation.
o When the stimulus first goes on, they first fire a lot of APs (at high frequency),
but then adapt by slowing down, slowly (over a few 100s msec or sec) or rapidly
(right away: stimulus goes on -> pa-pa-pah…...pa-pa-pah -> stimulus goes off).
What are these adapting vs. non-
adapting types of responses
telling the CNS?
o Non-adapting responses:
magnitude and not very
sensitive to fast changes
in stimulus, like vibration
on the skin. o Adapting responses: tell you about the magnitude, but they also tell you a lot
about when the stimulus is turning on and off.
In fact, for rapidly adapting afference, we have an “on-response” and an
o Stimulus green line: Encodes the changes in stimulus for rapidly adapting
afference. It tells you when the derivative is non-zero(?).
o Generally, we are very sensitive to changes in our sensory environment, and
not so sensitive to constant stimulus inputs.
E.g(1). The amount of light constantly striking your retina is not important
to you, rather we care about the difference/contrast in brightness. So we
don’t really care about the absolute stimulus.
E.g(2). When we put on clothes, we feel them, but after a while we don’t
anymore. Yet, if you lightly touch the back of the neck of the person in
front of you, they will notice it, because we’re very sensitive to changes.
o Slowly-adapting afference: may encode some of the stimulus intensity, but they
encode moderate changes in stimulus intensity over time.
o Rapidly adapting afference: fast stimulus changes over time.
o This adaptation allows us to be sensitive to changes in the sensory input, which
are more meaningful than constant inputs.
Receptive field (RF):
Another general property of afference. It is the region in space that activates a sensory
receptor or neuron.
o E.g. Afference has a receptor field in the part of the skin that activates it.
See image: If I record from the afferent with an electrode, notice that if we
stimulate in the
middle of the receptor
field (A), we get an
AP response. As we
start moving away
from the center, the
response starts to go
down. If we