Class Notes (838,386)
Canada (510,872)
Physiology (903)
PHGY 209 (410)
Erik Cook (23)

CNS sensory and motor (Lecture 3).pdf

11 Pages
Unlock Document

PHGY 209
Erik Cook

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 CNS. 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 system:  We will start with somatosensory soon, but first we need to understand how sensory information is encoded. 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 common:  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… -> stimulus goes off).  What are these adapting vs. non- adapting types of responses telling the CNS? o Non-adapting responses: encode stimulus 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 “off-response”. 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
More Less

Related notes for PHGY 209

Log In


Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.