Lecture 13: Somatic Sensation Flashcards
Is the dorsal root ganglion considered to be part of the central or peripheral process?
the central process
Describe the location of the different parts of the primary afferent neuron
the sensory endings of the axon are the sensory receptors and these are located in places like the skin
there is axon which leads to these sensory endings and this makes up the peripheral process
the cell body is located in the dorsal root ganglion
there is the central process and the dendrites in the CNS
What are the two different somatosensory pathways called?
anterolateral and dorsal column pathway
Describe the anterolateral pathway
the axons (afferent) cross over at the level of the spinal cord (across and forward) and then go up to the brainstem
What is the anterolateral pathway for?
pain and temperture
Describe the dorsal column pathway
this enters the dorsal root and stays on that side then crosses over higher (in the medulla oblongata)
What is the dorsal column pathway for?
sensory modalities of fine touch (tactile sensation), vibration and proprioception
Receptive fields vary in _______ and _______ on different parts of the body
size and density
What is the reticular activating system?
Network of _______ located in the _______ _______ that project ______ to the ________ to mediate _______ as well as both ________ to the _______ and directly to the ________ for activation of _______, desynchronised cortical _______ patterns. This is for __________ and _________ to the cerebellum
network of neurons located in the brain stem that project anteriorly to the hypothalamus to mediate behaviour, as well as both posteriorly to the thalamus and directly to the cortex for activation of awake, desynchronised cortical EEG patterns.
this is for proprioreception and touch to the cerebellum
What is the purpose of a sensory receptor in the skin?
to convert a physical stimulus to action potentials in the primary sensory neuron via the process of transduction
What is the process of transduction?
When one form of energy is converted to another
What are the 4 different “nodes”/types of receptors in the skin?
- temperature (thermoreceptors)
- position of limbs (proprioreceptors)
- touch receptors
- pain (nocireceptors)
Describe thermoreceptors
different types are more active at warm or cold temperature ranges, especially responsive to a changing temperature
Describe the proprioreceptors
these detect position in space and are muscle spindles, golgi tendon organs and joint capsules
In a proprioreceptor, what is wrapped around the muscle spindle and why is this?
nerve endings are wrapped around the muscle spindle to give sensory information about changes in muscle length
What does the golgi tendon organs detect?
muscle tension
What are touch receptors (mechanoreceptors) sensitive to?
mechanical deformation due to stretch, pressure (they detect rapid changes)
What do pain receptors respond to?
extreme mechanical, temperature, and/or chemical stimuli
What are 5 different touch receptors?
tactile (Meissner's corpuscle) tactile (merkel's) corpuscle free nerve endings Pacinian corpuscle Ruffini corpuscle
Describe what Meissner corpuscles detects and why their location is important?
this is to detect light touch
they are located in the folds of non-hairy skin which is important for the interaction of our body surface with something that we are touching
What do pain receptors look like and where are they located?
pain fibres are branching to have a wide receptive field everywhere in the body except in the brain
Where is the pacinian corpuscle located? Why is this important? What does it detect?
it is located deep in the skin which allows for mechanical transduction over quite a large area of skin surface
it detects deep pressure
Describe the Ruffini corpuscles
what are they?
what do they give feedback about?
they are stretch receptors that are sensitive to orientation and what direction the skin is being pulled
it gives feedback about the position of joints in fingers and tactile sensation
Describe the hair receptor
there are fine axons around the basis of the hair
this is the part of the somatosensory sensation because it lets us know about air flow
Is the Pacinian corpuscle associated with myelinated or unmyelinated fibres? What does this mean about the conduction speed of the central nervous system?
myelinated for rapid conduction of the central nervous system
Are pain receptors myelinated or unmyelinated? Why?
unmyelinated because this is a tonic, wide area sensation
How does a pacinian corpuscle sense?
In response to a mechanical stimulus, mechanosensitive channels are pulled open due to the membrane being stretched. This causes an influx of cations (mostly Na+ but also Ca2+) which depolarises the membrane which generates a receptive potential. This is a graded potential which falls away in amplitude and frequency over time but if it reached threshold, an action potential would be generated.
What is the advantage of the wrapping around of tissue around the Pacinian corpuscle?
increases the region of the body that this receptor can receive information
the layers allow a damp relaxation so if you disturb the sensory ending, there is a big burst of action potentials but then there is some slippage of the layers of membrane to relax again to allow the ion channels to close
What is intensity encoding?
the stronger the stimulus, higher frequency of action potentials and the more sensory endings are likely to be activated because more of the skin will be disturbed by that input
What two things are encoding intensity?
the frequency of action potentials and the number of axons activated
How do we know where that amount of input is occurring?
because it is mapped in the cerebral cortex (primary somatosensory cortex)
(mapping of receptive fields of individual primary afferents to specific cortical locations)
How do we know how long a stimulus is present?
The rate of adaptation depends on what type of
sensory ending it is: when the stimulus comes on, there is a high frequency of action potentials that falls off over time as the receptive potential falls away and as the neuron starts to run out of functional Na+ channels. The sensory receptor slowly adapting ones may fire throughout the duration of the stimulus and let us know when it starts and when it stops but the rapidly adapting ones fire rapid bursts and then fall away and nothing, and then as the stimulus comes off, the membrane is disturbed again and we get another burst of action potentials
Which sensory ending allow us to determine which direction the skin is being pulled?
Ruffini’s corpuscles
Which sensory ending is sensitive to a low-frequency sinusoidal stimulus?
Meissner’s corpuscles
Which sensory ending is sensitive to a high-frequency sinusoidal stimulus?
Pacinian corpuscle
What is meant by the sinusoidal stimulus?
they are vibration senses because if you put in a pressure wave that goes into the skin, then the Meissner’s corpuscles and Pacinian corpuscles are very sensitive to that but with steady pressure, they stop firing
What is the benefit of the vibration senses?
when we pull our fingers across a surface, the surface pulls and vibrates the skin and this is how we have the sensation of texture
What information is being brought up the anterolateral pathway?
pain, temperature and some touch but this is more of a wide, general area
Where does the primary afferent neuron go in the anterolateral pathway?
the primary afferent neuron (which is a pain or temperature receptor) is entering the spinal cord through the dorsal root and it is synapsing low in the spinal cord
Where does the secondary afferent neuron go in the anterolateral pathway?
from the synapse with the primary afferent neuron low in the spinal cord, it crosses over anteriorly and laterally and it travels up the spinal cord and branches into the reticular formation (part of the RAS to alert us to input) and then up into the thalamus where there is the second synapse
Where does the third afferent neuron go in the anterolateral pathway?
there is a synapse from the secondary to the third afferent neuron in the thalamus which then send the information to the right part of the somatosensory cortex
What is the role of the thalamus
it acts as a relay system to send information to the necessary parts of the somatosensory cortex so it can enter out conscious experience
What information is being brought up the dorsal column pathway?
information about fine touch and position from joints
Where does the primary afferent neuron go in the dorsal column pathway?
the primary afferent neuron (which is a touch or proprioceptive ending) is entering the spinal cord through the dorsal root and travelling up the spinal cord on the same side in the dorsal column and it synapses onto the second axon in the medulla oblongata
Where does the secondary afferent neuron go in the dorsal column pathway?
from the synapse with the primary neuron, it crosses over onto the other side in the brainstem and synapses onto the third axon in the thalamus
it also branches into the RAS
Information coming from separate axon coming from the skin can have what?
overlapping receptive fields for fine touch
Why do receptive fields in the skin overlap?
so they can interact such that we get additional precision from our information by comparing what is happening in 2 pathways
Describe lateral inhibition if a pencil in poked into the skin
The axon in the centre is going to be more strongly activated because it is getting more direct input and it activated inhibitory interneurons to shut down the ones either side of it. The ones either side of it are trying to do the same thing but they are less strongly activated so the one in the middle wins
What does lateral inhibition result in?
centre-surround inhibition
Where is afferent information mapped?
in the primary somatosensory cortex
Where is the primary somatosensory cortex?
just behind the central sulcus (infront is the motor cortex)
Within the primary somatosensory cortex, there is a homunculus. What does this do?
it is an ordered representation of how many receptors there are in each part of the body (how much reception we get from that part of the body)
