Cutaneous Senses

Question 1

Describe the pathway of sensing.

Answer 1

Transduction of stimulus into action potentials
transmission/processing of signal below cortex
Initial processing of signal in cortex
interpretation of signal

Question 2

How do we start an action potential?

Answer 2

Sensory (Afferent) neurons have specialized channels that are opened or closed in response to a stimulus
ex: touch receptors have a Na+ channel that is opened with deformation of the cell membrane

Question 3

What is a generator potential?

Answer 3

first opening of these channels produces a local response (sub-threshold)

Question 4

How do move from a generator potential to an AP?

Answer 4

the stimulus most be strong enough or long enough to cause the afferent neuron to come to threshold and generate an AP

Question 5

What is pacinian corpuscle and what is the structure?

Answer 5

touch receptor

alternating layers of membrane w/ fluid between, surrounding nerve endings

Question 6

Describe how a pacinian corpuscle works?

Answer 6

1. when we touch something the layers of the membrane are deformed, opening mechosensitive Na+ channels on the membrane
2. generating an AP
3. if stimulus continues the Ap adaptes b/c the fluid is redistributed
4. removal of the stimulus generates another AP are the pacinian corpuscle reforms (afterdischarge)

Question 7

What is a sensory unit?

Answer 7

sensory nerve and all its branches

Question 8

What is a receptive field?

Answer 8

the area from which stimulation produces activation of the neuron

Question 9

How is the intensity of the stimulus transferred to the brain ?

Answer 9

the number of action potentials
when the stimulus is very very very intense the AP are pattered into doublets or triplets
also the number of receptors firing also increases with increased intensity

Question 10

What is just noticeable difference.

Answer 10

The smallest different that can be detected ( about 10%)

Question 11

How is the relationship between perceived strength and actual (measured) intensity>

Answer 11

Weber-Fechner Law: perceived intensity=log(measured intensity)

Perceived intensity= K(measured intensity)^A

K and A are constants
K and A depend on the type of the sensory receptor

Question 12

It is rare that a stimulus activates only one receptor due to the overlap of receptive fields, how is localized maintained?

Answer 12

Special case of inhibition (axo-axonal synapse)
the post-synaptic cell is a pre-synaptic terminal

Result: reduced neurotransmitter release from the inhibited pre-synaptic terminal

Question 13

How does axo-axonal synapse work?

Answer 13

Start: normal chemical synapse w/ neurotransmitter release from A to B, neuron A is also in contacted with neuron C

2. when activated neuron C release GABA, which then activated chloride entrance into neuron A
3. causing the pre-synaptic terminal of neuron A to hyper polarize and allowing less calcium to enter
4. reducing the probability of action potentials in neuron B

Question 14

Initial processing of signal in cortex.

Answer 14

the sensory cortex is arranged somatotopically
the somatosensory cortex is neocortex so it has six layers
neurons in the somatosensory cortex are arranged in columns ( sensory info. arrives at its respective column in layer IV (via the thalamus)

each column deals with one sensory modality in one part of the body
neighboring columns receives information from the same part of the body, but a different sensory modality

Question 15

Somatic Sensory Area (S1)

Answer 15

post-central gyrus
First stop for most cutaneous sense
somatotopic representation
Function: involved in the integration of the info. for position sense as well as size, shape discrimination

Question 16

Somatic Sensory Area 2 (S2)

Answer 16

wall of lateral (sylvian) fissure
received input from S1
somatotopic representation
Functions: sterognosis, comparisons b/w two different tactile sensations, determining whether something becomes a memory

Question 17

Higher level interpretation of signal

Answer 17

association areas (parieto-temoral-occipital assocation cortex) 
required for high-level interpretation of sensory inputs (S1 and S2)

Question 18

Function of PTO

Answer 18

analysis of the spatial coordinates of self/surrounding objects, naming of objects, there are many other functions related to other aspects of cognition

Question 19

How the cortex can be changed by sensation?

Answer 19

plasticity: refine the map that is genetically coded in the cortex
amputated: afferent input from remaining parts of the body will reinnervate the cortex

Question 20

What is the doctrine of specific nerve energies?

Answer 20

stimulation of a sensory pathway at any point leads to the perception of a sensation that is dictated by the nature of the receptor that started the pathway

Question 21

What is the doctrine of specific nerve energies?

Answer 21

means that if I stimulate the cortical column that receives input from the above pacinian corpuscle, you will perceive the sensation of light touch