Lecture 2: Somatosensory System

Question 1

What is somatosensation?

Answer 1

sensation arising from the body; includes touch, pressure, vibration, temperature, limb position and pain.

Question 2

What detects somatosensation?

Answer 2

Detected by sensory cells whose somi are in the dorsal root ganglion (limbs & trunk) and trigeminal ganglion (face)
Single process from DRG cells extends to the periphery and to the CNS (spinal cord)

Note that mechanosensory fibers (touch, pressure, vibration) project up the ipsilateral side of the spinal cord.
Pain/temperature fibers cross immediately to the contralateral side of the spinal cord before projecting up towards the brain.

Question 3

What is sensory transduction?

Answer 3

Sensory transduction – the conversion of environmental stimuli into an neuro-electric signal

picture is ex of pacinian corpuscle

Question 4

What is sensory modality?

Answer 4

Refers to what environmental information a receptor responds to.

Question 5

Labeled Line reception

Answer 5

Labeled line - a receptor only responds to specific kind of stimulus
This is the primary method of encoding
For example, the body uses different receptors to detect mechanical stimuli (somatosensory receptors in the skin) as opposed to visual stimuli (rods & cones in the retina).

Question 6

Frequency encoding reception

Answer 6

Frequency coding - the pattern of activity communicates sensory information
Is used to detect differences between stimuli that are part of the same sensory modality.
For example the same thermal receptor can detect whether the temperature is 15° or 25°C; it distinguishes between the two by being more active for the warmer temperature.

Question 7

What are the 4 afferent axon types?

Answer 7

Question 8

Describe the receptive field. What is two point discrimination?

Answer 8

region of skin that is innervate by a given sensory neuron (or the region of skin that produces a response in a sensory neuron)

Response of neuron decreases as you move away from the center of the receptive field

Receptive field size varies in different regions of the body; smaller fields in more sensitive regions of the body (fingers & toes); smaller fields are also intrinsically more sensitive

Two-point discrimination – minimum distance between 2 stimuli that allows them to be distinguished as 2 stimuli

Question 9

What is adaptation? What are the two types we’re focusing on?

Answer 9

Adaptation – the capacity of a neuron to reduce its’ firing rate or even cease firing even though a stimulus is still being applied.

Rapidly-adapting afferents tend to encode temporal or dynamic properties of a stimulus (e.g. a vibration.)

Slowly-adapting afferents tend to encode information of a more static nature (e.g. the magnitude of the stimulus).

Question 10

What is the difference between primary and secondary afferents

Answer 10

Responses by primary & secondary afferents to (A) onset of a linear stretch, (B) a tap, (C) a vibration, and (D) release from a linear stretch.

The top traces show the type of stimulus applied. The middle traces show the response of the primary (type I) afferents. The bottom traces show the response of the secondary (type II) afferents.

Question 11

What are accessory structures and what do they do?

Answer 11

These are specializations at the endings of mechanosensory cells that help determine

[1] what kind of stimulus a sensory cell responds (temperature, pressure, vibrations, etc.) and

[2] how a sensory cell responds to a given stimulus. An example of this latter function is adaptation.

Question 12

Free nerve ending cutaneous mechanoreceptors

Answer 12

pain (nociception)

Question 13

Meissner corpuscle cutaneous mechanoreceptors

Answer 13

Rapidly adapting, detect low freq vibrations (< 40 Hz), good for detecting texture (feedback for grip control)

Consist of a capsule (Schwann cells) that contains multiple disc-shape nerve endings

Very sensitive, small receptive fields

Question 14

Merkel cells (discs)

Answer 14

Slow-adapting, very high spatial resolution, appear to be involved detecting curves, points and edges

Question 15

Ruffini endings

Answer 15

Slow adapting, appear to respond to cutaneous stretch (especially in regions where there is limb movement)

Question 16

pacinian corpuscles

Answer 16

Rapidly adapting; even more sensitive than Meissner corpuscles (10 nm displacement), but the corpuscle acts as a filter so that only high frequency stimuli reach the afferent fiber inside (≥ 250 Hz)

Likely provide critical sensory feedback for fine motor control, e.g. tool use.

Question 17

How is braille read?

Answer 17

all inputs fro merkel discs, meissner’s corpuscle, ruffini endings, and pacinian corpuscle at once

Question 18

Haptics

Answer 18

interpretation of complex spatiotemporal patterns that is the result of input from multiple types of cutaneous mechano-sensitive neurons.

Question 19

Stereognosis

Answer 19

the ability to identify an object by touch

Question 20

Describe the mechanosensory pathways from limb stimulation to brain.

Answer 20

1st order sensory neurons (mechanosensory reception from body)

Travel ipsolateral up dorsal column.

Synapse on dorsal column nuclei in the Medulla

crossing over

Travels up medial lemniscus on contralateral side

through pons, midbrain

Synapse in VPL of thalamus

Project to primary somatic sensory cortex (S1)

Question 21

Describe the mechanosensory pathway from the face to the brain

Answer 21

Mechanosensory receptors from face to trigeminal ganglion, to principle nucleus of trigeminal complex.

Contralateral projection to medial lemniscus

syanpses in VPM of thalamus

Projects to SI

Question 22

Where is S1 (somatic sensory cortex)

Answer 22

Parietal lobe; posterior to central sulcus

in postcentral gyrus

Question 23

Somatotopic order in the human primary somatic sensory cortex

Answer 23

Not all of the regions (represented by Brodmann’s areas) receive the same input

Part of this separation of function reflects different sets of inputs from the ventral posterior complex of the thalamus (VPL + VPM) that carry distinct somatosensory information.

However, not all regions of S1 receive the same level of thalamic input

Question 24

Neurons in the primary somatosensory cortex form functionally distinct columns

Answer 24

Neurons with similar response properties are clustered together to form functional columns (in this case similar response properties refers to position on the body and adaptation)

These columns extend through the depth of the cortex so that so that all of the neurons in the column (100,000s neurons in each) have the same functional properties, such as responding to slowly-adapting vs. rapidly-adapting sensory inputs.

The columns are organized so that they have a topographic organization (close proximity on the body translates to close proximity in the brain), but also that similar response properties are close to each other (i.e. adaptation).

Question 25

Descibe functional expansion of a cortical representation by a repetitive behavioral task

Answer 25

Following a task that required increase use if digits 2, 3 and 4, the corresponding receptive fields in S1 expanded.

Other examples of experience-dependent changes in S1 receptive field organization

Application of anesthetic to a region of skin causes the effected region of S1 to re-organize so that it receives input from the still-functioning skin around the effected area. When the anesthetic wears off “expanded” region of S1 returns to its original size, but there is a period when that region of skin feels larger than normal.

Loss of a digit will be reflected by a remapping of S1, so that neurons that use to receive input from lost digit now receive input from the remaining digits.