Lecture 13 - Somatic sensation

Question 1

Note about somatovisceral

Answer 1

Note - sometimes see the term somatovisceral system to emphasise that we also get sensory information from the gut

Question 2

Primary afferent neuron

Answer 2

Primary afferents are sensory neurons (axons or nerve fibers) in the peripheral nervous system that transduce information about mechanical, thermal, and chemical states of the body and transmit it to sites in the central nervous system.
Line/barrier indicates entry of this axon into the level of the spinal cord
Area that this axon is responsive to is called the receptive field

Question 3

Receptive field

Answer 3

Area that an axon is responsive to is called the receptive field

Question 4

Pathways

Answer 4

Axons cross over low in spinal cord (anterolateral, or ventral spinothalamic pathway), for pain, temperature, or cross high, in medulla (dorsal column/medial lemniscal pathway), for fine touch, proprioception.
Branches also in brainstem, to ‘reticular activating system’ (RAS), and for proprioception and touch to cerebellum.
Receptive fields vary in size and density on different parts of the body

Question 5

Anterolateral pathway

Answer 5

Anterolateral pathway - directly after entering in through the dorsal root ganglion in the spinal cord, the anterolateral fibres pass to the front of the spinal cord and to the other side up to the brain stem and this transports general information such as brain and temperature (broad sensations)

Question 6

Dorsal column pathway

Answer 6

Dorsal column pathway - enters through the dorsal root and it stays at the back of the spinal cord on the dorsal side of it and goes up on the same side that it comes in and crosses over higher up

Question 7

Sensory receptors of the skin

Answer 7

Convert physical stimulus to action potentials in primary sensory neuron (transduction).
Sensation or stimuli are transduced into action potentials

Question 8

Temperature receptors are called

Answer 8

Temperature (Thermoreceptors)

Different types more active at warm or cold temperature ranges, especially responsive to changing temperature

Question 9

Proprioceptors

Answer 9

Position of limbs (proprioceptors)
Muscle spindles, golgi tendon organs, joint capsule) skin mechanoreceptors
Alerts to changes in the environment for safety

Question 10

Touch receptors

Answer 10

Touch receptors
Sensitive to mechanical deformation (mechanoreceptors)
Location corresponds to folds in the skin so they have a specialised location that is involved with that interaction of texture with that body surface with something we are touching

Various types, with different sensitivities (sub-modalities) and locations

Question 11

Nociceptors

Answer 11

Pain (nociceptors)

Respond to extreme mechanical, temperature, and/or chemical stimuli

Question 12

Skin touch receptors

Answer 12

Non hairy skin is called glabrous skin and includes the palms and the soles of the foot
Pain fibres have quite wide receptive fields and they are found everywhere in the body except the brain and the spinal cord
A = Tactile (Meissner’s) corpuscle - light touch
Close to the finger print i.e. on the surface of the skin

B = Tactile (Merkle’s) corpuscle - touch
C = Free nerve ending - pain
D = Lamellated (Pacinian) corpuscle - deep pressure
Tends to be deep in the skin and has a mechanical structure outside of the axon

E = Ruffini corpuscle - stretch, directional, it is stretch receptor and it is sensitive to the orientation at which the skin is being pulled
Hair receptor

Question 13

Tactile (Meissner’s) corpuscle

Answer 13

Tactile (Meissner’s) corpuscle - light touch

Close to the finger print i.e. on the surface of the skin

Question 14

Tactile (Merkle’s) corpuscle

Answer 14

touch

Question 15

Free nerve ending

Answer 15

pain

Question 16

Lamellated (Pacinian) corpuscle

Answer 16

deep pressure

Tends to be deep in the skin and has a mechanical structure outside of the axon

Question 17

Ruffini corpuscle

Answer 17

Ruffini corpuscle - stretch, directional, it is stretch receptor and it is sensitive to the orientation at which the skin is being pulled

Question 18

Hair receptor

Answer 18

associated with a hair follicle

Question 19

Pacinian corpuscles

Answer 19

Associated with myelinated fibres therefore there is precise real time sensation in terms of our interaction with the environment
If the receptor potential reaches threshold then it will generate an action potential
Influx of ions such as sodium or calcium which depolarises the membrane and causes a receptor potential to form
Adaption results from receptor mechanical properties, and axon ion channel properties

Question 20

Pacinian corpuscles steps

Answer 20

mechanosenstive channels
Mechanical stimulus - influx of ions (Na+, Ca2+) - receptor potential - action potential generated at first node - potential decays, and APs stop = adaptation. Adaptation results from receptor mechanical properties and axon ion channel properties

Question 21

Sensory coding list

Answer 21

Modality (e.g. touch vs temp)
Intensity
Location
Duration

Question 22

Sensory coding - modality

Answer 22

Modality (e.g. touch vs temperature)
Specificity of receptors, ‘labelled line’
The basic sensory modalities include: light, sound, taste, temperature, pressure, and smell.

Question 23

Sensory coding - intensity

Answer 23

Frequency of action potentials in each axon, umber of axons activated

Question 24

Sensory coding - location

Answer 24

Mapping of receptive fields of individual primary afferents to specific cortical locations. [Somatotopic representation in somatosensory cortex].

Question 25

Sensory coding - duration

Answer 25

“Rapidly adapting” receptors respond briefly, even if stimulus is sustained. Detect movement, changing pressure. “Slowly adapting” receptors can signal true duration of stimulus.

Question 26

Vibrations for humans

Answer 26

Vibration sensors are very useful for spiders - in humans they contribute to sensation of surface texture

Question 27

Sensory pathways in the CNS

Answer 27

Anterolateral pathway and dorsal column pathway

Question 28

Anterolateral pathway is for

Answer 28

Pain, temperature, some touch (broad surface area sensation)

Question 29

Dorsal column pathway is for

Answer 29

Fine touch, position (proprioception)

Question 30

Anterolateral pathway order

Answer 30

primary afferent neuron (pain or temperature receptor) - synapse in spinal cord - anterolateral position of axons - secondary axon through spinal cord and synapse in thalamus, secondary axon also has branches in reticular formation - 3rd axon projects from the thalamus to the somatosensory cortex

Question 31

Dorsal column pathway order

Answer 31

Primary afferent (touch and proprioceptive endings) - primary axon in dorsal column - synapse in dorsal column nucleus (spinal cord) - 2nd axon from spinal cord to thalamus where it synapses, 2nd axon has branches in brainstem - 3rd axon from the thalamus to the sensory cortex

Question 32

Lateral inhibition

Answer 32

Lateral inhibition is a fundamental mechanism to increase accuracy of sensory information.
Mediated by inhibitory interneurons
In somatic sensation lateral inhibition localizes sensation to a restricted area of skin.
Lateral inhibition is used wherever accurate location of a stimulus is required – or where a pattern of input needs to be discriminated [e.g. vision, smell…]
Lateral inhibition results in centre- surround inhibition
Information from separate icons coming from the skin can have overlapping receptive fields - the overlap allows them to interact such that we get extra precision on this information
In the image, pressure is activating all of these neurons because the receptors overlap. Red lines are the inhibitory interneurons and they cross connect these separate pathways that are all carrying information from the same region of the skin but the central neuron is going to be more strongly activated because it is getting more direct input which activates the inhibitory interneuron to shut down ones either side of it and without this happening the sensory cortex would see information coming from a bigger area of skin but with this system we get more contrast between centre of the field and the outside of the field
Yellow circle is the receptive field of the central neuron

Question 33

Primary somatosensory cortex

Answer 33

Post central gyrus
Information on sensation on the body is being mapped on the primary somatosensory cortex
Note the parietal-lobe association cortex is where different forms of information are bought together to produce a coherent picture of the environment

Question 34

Somatotopic representation in primary somatosensory cortex

Answer 34

Ordered representation of body
Area proportional density of receptors (fineness of discrimination) and use
Margins of representations are modifiable (“plastic”) - you are not stuck with the same mapping for life
Homunculus

Example - Claudia
Could stimulate sensory axons that used to carry information into her missing arm and then could get sensation as though her arm was still there