Lecture 13: Somatic Sensation

Question 1

Is the dorsal root ganglion considered to be part of the central or peripheral process?

Answer 1

the central process

Question 2

Describe the location of the different parts of the primary afferent neuron

Answer 2

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

Question 3

What are the two different somatosensory pathways called?

Answer 3

anterolateral and dorsal column pathway

Question 4

Describe the anterolateral pathway

Answer 4

the axons (afferent) cross over at the level of the spinal cord (across and forward) and then go up to the brainstem

Question 5

What is the anterolateral pathway for?

Answer 5

pain and temperture

Question 6

Describe the dorsal column pathway

Answer 6

this enters the dorsal root and stays on that side then crosses over higher (in the medulla oblongata)

Question 7

What is the dorsal column pathway for?

Answer 7

sensory modalities of fine touch (tactile sensation), vibration and proprioception

Question 8

Receptive fields vary in _______ and _______ on different parts of the body

Answer 8

size and density

Question 9

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

Answer 9

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

Question 10

What is the purpose of a sensory receptor in the skin?

Answer 10

to convert a physical stimulus to action potentials in the primary sensory neuron via the process of transduction

Question 11

What is the process of transduction?

Answer 11

When one form of energy is converted to another

Question 12

What are the 4 different “nodes”/types of receptors in the skin?

Answer 12

• temperature (thermoreceptors)
• position of limbs (proprioreceptors)
• touch receptors
• pain (nocireceptors)

Question 13

Describe thermoreceptors

Answer 13

different types are more active at warm or cold temperature ranges, especially responsive to a changing temperature

Question 14

Describe the proprioreceptors

Answer 14

these detect position in space and are muscle spindles, golgi tendon organs and joint capsules

Question 15

In a proprioreceptor, what is wrapped around the muscle spindle and why is this?

Answer 15

nerve endings are wrapped around the muscle spindle to give sensory information about changes in muscle length

Question 16

What does the golgi tendon organs detect?

Answer 16

muscle tension

Question 17

What are touch receptors (mechanoreceptors) sensitive to?

Answer 17

mechanical deformation due to stretch, pressure (they detect rapid changes)

Question 18

What do pain receptors respond to?

Answer 18

extreme mechanical, temperature, and/or chemical stimuli

Question 19

What are 5 different touch receptors?

Answer 19

tactile (Meissner's corpuscle)
tactile (merkel's) corpuscle
free nerve endings
Pacinian corpuscle
Ruffini corpuscle

Question 20

Describe what Meissner corpuscles detects and why their location is important?

Answer 20

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

Question 21

What do pain receptors look like and where are they located?

Answer 21

pain fibres are branching to have a wide receptive field everywhere in the body except in the brain

Question 22

Where is the pacinian corpuscle located? Why is this important? What does it detect?

Answer 22

it is located deep in the skin which allows for mechanical transduction over quite a large area of skin surface
it detects deep pressure

Question 23

Describe the Ruffini corpuscles
what are they?
what do they give feedback about?

Answer 23

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

Question 24

Describe the hair receptor

Answer 24

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

Question 25

Is the Pacinian corpuscle associated with myelinated or unmyelinated fibres? What does this mean about the conduction speed of the central nervous system?

Answer 25

myelinated for rapid conduction of the central nervous system

Question 26

Are pain receptors myelinated or unmyelinated? Why?

Answer 26

unmyelinated because this is a tonic, wide area sensation

Question 27

How does a pacinian corpuscle sense?

Answer 27

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.

Question 28

What is the advantage of the wrapping around of tissue around the Pacinian corpuscle?

Answer 28

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

Question 29

What is intensity encoding?

Answer 29

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

Question 30

What two things are encoding intensity?

Answer 30

the frequency of action potentials and the number of axons activated

Question 31

How do we know where that amount of input is occurring?

Answer 31

because it is mapped in the cerebral cortex (primary somatosensory cortex)
(mapping of receptive fields of individual primary afferents to specific cortical locations)

Question 32

How do we know how long a stimulus is present?

Answer 32

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

Question 33

Which sensory ending allow us to determine which direction the skin is being pulled?

Answer 33

Ruffini’s corpuscles

Question 34

Which sensory ending is sensitive to a low-frequency sinusoidal stimulus?

Answer 34

Meissner’s corpuscles

Question 35

Which sensory ending is sensitive to a high-frequency sinusoidal stimulus?

Answer 35

Pacinian corpuscle

Question 36

What is meant by the sinusoidal stimulus?

Answer 36

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

Question 37

What is the benefit of the vibration senses?

Answer 37

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

Question 38

What information is being brought up the anterolateral pathway?

Answer 38

pain, temperature and some touch but this is more of a wide, general area

Question 39

Where does the primary afferent neuron go in the anterolateral pathway?

Answer 39

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

Question 40

Where does the secondary afferent neuron go in the anterolateral pathway?

Answer 40

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

Question 41

Where does the third afferent neuron go in the anterolateral pathway?

Answer 41

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

Question 42

What is the role of the thalamus

Answer 42

it acts as a relay system to send information to the necessary parts of the somatosensory cortex so it can enter out conscious experience

Question 43

What information is being brought up the dorsal column pathway?

Answer 43

information about fine touch and position from joints

Question 44

Where does the primary afferent neuron go in the dorsal column pathway?

Answer 44

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

Question 45

Where does the secondary afferent neuron go in the dorsal column pathway?

Answer 45

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

Question 46

Information coming from separate axon coming from the skin can have what?

Answer 46

overlapping receptive fields for fine touch

Question 47

Why do receptive fields in the skin overlap?

Answer 47

so they can interact such that we get additional precision from our information by comparing what is happening in 2 pathways

Question 48

Describe lateral inhibition if a pencil in poked into the skin

Answer 48

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

Question 49

What does lateral inhibition result in?

Answer 49

centre-surround inhibition

Question 50

Where is afferent information mapped?

Answer 50

in the primary somatosensory cortex

Question 51

Where is the primary somatosensory cortex?

Answer 51

just behind the central sulcus (infront is the motor cortex)

Question 52

Within the primary somatosensory cortex, there is a homunculus. What does this do?

Answer 52

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)