Lecture 13- The Somatosensory System

Question 1

What does the somatosensory system consist of?

Answer 1

• Somatosensory receptors & neurons
• Afferent axons and neurons that form the somatosensory pathways
• Neurons of somatosensory cortex

Question 2

What are the three types of sensory receptors according to how they are distributed throughout the body?

Answer 2

• Exteroceptive: external skin contact, temperature
• Proprioceptive: body, limb, joint position
• Enteroceptive: internal organ status (eg bladder fullness, blood gases), maintaining homeostasis

Question 3

What does the somatosensory system do?

Answer 3

• It provides the brain with information about the state of the body, and about some aspects of the external environment
• This information is used to help guide behaviour and to maintain homeostatic function

Question 4

What is the other way the brain receives important sensory information?

Answer 4

Special senses (vision, hearing, balance, smell, taste)

Question 5

What are the 4 main features of stimulus that is encoded in information from somatosensory receptors?

Answer 5

• Modality: receptor specificity (what ‘type’ of information), eg touch and temperature are reported by different receptors and signaled separately (the “labelled line”)
• Intensity: the frequency of action potential firing in a sensory axon, and the number of activated axons encodes the intensity (strength) of the stimulus
• Location: the somatotopic mapping of receptors in specific areas allows the location (site) of the stimulus to be known
• Duration: the beginning/end and pattern of action potential firing can encode the start and end of a stimulus

Question 6

How is stimulus intensity encoded?

Answer 6

The bigger the stimulus the higher the frequency of action potential firing (more likely to reach threshold for AP)

Question 7

What is signal transduction?

Answer 7

The process of taking a stimulus and converting it into action potentials. Action potentials in the CNS then result in sensation (brain)

Question 8

What is required for signal transduction to occur?

Answer 8

Need a receptor specific to the stimulus

Question 9

What are the three broad types of somatosensory receptors in the skin (Exteroceptive)?

Answer 9

• Mechanoreceptors: touch, pressure
• Thermoreceptors: temperature
• Nociceptors: noxious stimuli (triggering pain sensation)

Question 10

What are the two types of skin?

Answer 10

• Hairy= majority

- Hairless= found on palms of hands + soles of feet, lips and fingertips

Question 11

What is the general mechanism for which mechanoreceptors in the skin work?

Answer 11

• Have channels in membrane, when pressure is applied to membrane of receptor it distorts the protein, allowing for the entrance of positive ions into the cells (channel opens).
• This mechanical deformation results in a receptor potential and then if this great enough (threshold) an action potential will result in signals being sent to the brain

Question 12

What are the 5 types of skin mechanoreceptors + their general functions?

Answer 12

-Meissner’s corpuscles: Glabrous skin (mostly), pressure, dynamic,
rapidly adapting, low threshold. Very light touch will result in the perception of touch.

-Pacinian corpuscles : Subcutaneous (all skin), interosseous, viscera,
deep pressure, vibration, dynamic, rapidly adapting, low threshold. Light touch will not activate these.

• Merkel’s discs: All skin, static pressure, convey info about shape & texture of objects, slowly adapting, low threshold
• Ruffini endings: All skin, deeper pressure & stretch, slow adapting, low threshold
• Hair units: Hairy skin, hair displacement (due to breeze or touch= compute info about enviroment), low threshold, rapidly adapting

Question 13

In terms of the features of the skin’s mechanoreceptors what does rapidly adapting versus slow adapting mean?

Answer 13

• Rapidly adapting = Stop firing action potentials within a short period of stimulus onset
• Slowly adapting= Firing action potentials throughout the entire stimulus, only stop when the stimulus itself stops

Question 14

What are the skins thermoreceptors and nociceptors like? How are they typically classified?

Answer 14

-Free nerve endings in all skin types

-Typically classified by type of axon:
Myelinated axons= Temperature (usually change in temp)
Unmyelinated or thinly myelinated axons= Nociceptors (respond to mechanical, thermal, or chemical stimuli)

Question 15

What are Proprioceptors? What are they used for?

Answer 15

-Mechanoreceptors in muscles and tendons that provide information
relating to change in muscle length (muscle spindles) & force (Golgi tendon organ)
-Used by CNS to sense limb movement & limb/body position
-Crucial for stretch reflex

Question 16

What branch of the nervous system are enteroceptors usually associated with?

Answer 16

• Autonomic, sometimes we are not consciously aware of their outputs
• They are responsible for many important homeostatic reflexes like breathing and blood pressure control

Question 17

What are the three types of enteroceptors? + examples of what they do?

Answer 17

• Mechanoreceptors (eg bladder distension)
• Chemoreceptors (eg blood gases)
• Nociceptors – visceral pain (eg intestine/bladder distension)

Question 18

What are chemoreceptors? Where are two places they can be located/ examples?

Answer 18

-Sensory cells with receptors that respond to presence of a specific chemical

-Peripheral chemoreceptors:
• Aortic & carotid bodies detect pCO2, and [H+], (and O2) in blood
• Crucial in control of breathing

-Central chemoreceptors:
• On surface of medulla
• Detect pH of cerebrospinal fluid

Question 19

What are sensory neurons also called?

Answer 19

Primary afferent neurons

Question 20

Where are the different parts of sensory neurons usually located? What does this mean about their shape?

Answer 20

• Cell body typically located in dorsal root ganglia, or cranial nerve ganglia
• Peripheral process typically ends at receptive location (nerve endings in periphery are the cell’s dendrites)
• Central process enters CNS
• Shape= unipolar because 1 cell body with 2 processes coming off it (reaching in opposite directions)

Question 21

Complete the sentence:

Sensory neurons nerve endings may be….

Answer 21

Free or encapsulated

Question 22

What is the receptive field of a sensory neuron? What does the size of the receptive field relate to?

Answer 22

• The region/area whose stimulation affects the activity of the neuron
• Location & shape of RF is determined by location of the neuron’s sensory apparatus
• Size of receptive relates to two point discrimination: the ability to tell if a stimulus occurred in one place or another

Question 23

Where is the somatosensory cortex and explain the ‘mapping’ that occurs there?

Answer 23

-Somatosensory cortex= post-central gyrus
-Sensory axons from all body locations retain a strict spatial relationship to one
another along their projection pathways. Information thereby arrives in somatosensory cortex with a specific “map” of the body

Question 24

How is the somatosensory cortex map distorted?

Answer 24

Regions with high receptor density and smaller receptive field sizes occupy a
disproportionate area e.g. the fingertips and lips. The area set aside is in no way related to size in cms!

Question 25

List the three different ascending somatosensory pathways + what they are for….

Answer 25

1) Dorsal Columns (fine touch & vibration)
2) Spinothalamic (Anterolateral) tract (pain, temperature, crude touch: means its difficult to determine where it came from)
3) Spinocerebellar tracts (proprioception)

Question 26

What occurs in the Dorsal Columns (fine touch & vibration)?

Answer 26

-Axons enter dorsal roots and ascend in the spinal cord dorsal columns to
make synaptic contact on neurons in the medulla
-Neurons in the gracile & cuneate nuclei project across the midline and
form the medial lemniscus to ascend to the thalamus, where they synapse with cells of the ventral posterior lateral (vpl) nucleus
-VPL neurons project to somatosensory cortex
-Projections from periphery to cortex retain somatotopic organization

Note: due to midline crossing in medulla, body sensations are represented in
cortex on opposite side

Question 27

What occurs in the Spinothalamic (Anterolateral) tract (pain, temperature, crude touch)?

Answer 27

-Axons from pain and temp fibres enter via dorsal root and synapse on neurons in
spinal cord
-SC neurons project across the midline and ascend (spinothalamic tract) to
neurons in ventral posterior inferior nucleus (VPI) in thalamus (aka the anterolateral pathway)
-VPI neurons project to somatosensory cortex and other nuclei involved in pain response

Question 28

What happens in the Spinocerebellar tracts (proprioception)?

Answer 28

-Info from position-sense receptors goes to cortex and cerebellum via spinocerebellar tracts

1) Axons from sensory neurons in trunk & lower limbs synapse in
Nucleus dorsalis, whose axons ascend to form dorsal spinocerebellar tract to
cerebellum, others synapse in sc on neurons whose axons cross the midline and ascend to form the ventral spinocerebellar tract

2) Axons from upper limbs ascend to synapse in external cuneate nucleus, then to
cerebellum