Sensory Physiology

Front 1

Describe the structure of the pacinian corpuscle.

Back 1

Comprised of alternating layers of membrane with fluid between them, surrounding the nerve ending

Front 2

What happens to the pacinian corpuscle when we touch something?

Back 2

All the layers of the membrane are deformed
This leads to opening of mechanosensitive Na+ channels on the membrane and influx of sodium and the generation of action potentials back to the CNS

Front 3

What happens when the stimulus on the pacinian corpuscle is maintained?

Back 3

The action potentials gradually die away as adaptation occurs
Adaptation is due to redistribution of the fluid in the corpuscle so that the force is no longer transmitted to the nerve ending
Much of the adaptation that occurs is the result of changes in the periphery (directly altering the afferent)

Front 4

What is an afterdischarge?

Back 4

The removal of a stimulus triggers action potentials as the ending "reforms"

Front 5

What is the sensory unit?

Back 5

The sensory nerve and all its branches
The smaller the sensory unit is, the finer its perception

Front 6

What is the receptive field?

Back 6

The area from which stimulation produces activation of the neuron

Front 7

How is stimulus intensity coded in regards to action potentials?

Back 7

The number of action potentials is one way of coding the intensity
With greater intensity, we see more action potentials
With further increases, may see patterned discharges (doublets, triplets, etc)

Front 8

How is stimulus intensity coded in regards to the receptors?

Back 8

The number of receptors firing increases with increased intensity

Front 9

What is the just noticeable difference?

Back 9

The smallest difference that can be detected
A change of about 10% is usually required for conscious recognition of the change

Front 10

What is the Weber-Fechner Law?

Back 10

The relationship between perceived strength and actual (measured) intensity
Perceived intensity = log (measured intensity)

New formula:
perceived intensity = K(measured intensity)^A
where K and A are constants

Front 11

What is significant about K and A from the perceived intensity equation?

Back 11

K and A vary depending on the type of sensory receptor
Muscle senses: both are close to 1 (want perceived intensity to match measured intensity)
Cutaneous senses: more variability
The CNS does not alter this much, if at all

Front 12

What are the pathways to the brain for central processing of the senses?

Back 12

*Dorsal columns - proprioceptive and discriminative (fine touch)
*Spinothalamic tract - thermal, nociceptive, and 'coarse' touch
Spinoreticulothalamic system - nociceptive
Spinocerebellar tract

Front 13

What is pre-synaptic inhibition?

Back 13

Special case of inhibition
Axo-axonal synapse
Post-synaptic cell is a pre-synaptic terminal
End result of pre-synaptic transmission: reduced NT release from the inhibited pre-synaptic terminal

Front 14

Describe the process of pre-synaptic inhibition?

Back 14

Start with "normal" chemical synapse with NT release from neuron A to neuron B
Now add another neuron (C) that synapses on the pre-synaptic terminal
When activated neuron C releases GABA which then activates Cl- entrance into neuron A
This causes the pre-synaptic terminal of neuron A to hyperpolarize, allowing less Ca2+ to enter, leading to less NT release and reducing the probability of action potentials in neuron B

Front 15

Where does pre-synaptic inhibition occur?

Back 15

Between neighboring receptors at the first synapse in their pathway
*This increases the brain's ability to localize the signal
Regardless of which pathway is used, every synapse along the way represents a chance to 'ignore' or modify the stimulus

Front 16

How is the sensory cortex arranged?

Back 16

Somatotopically -- homunculus
Somatosensory cortex is neocortex and has 6 cell layers

Front 17

How are neurons in the somatosensory cortex arranged?

Back 17

In columns that extend from the surface into the cortex
Each column deals with one sensory modality in one part of the body
Sensory cortex is arranged somatotopicaly because the columns are arranged this way

Front 18

Where does sensory information arrive in the somatosensory cortex?

Back 18

Arrives at its respective column in layer IV (via the thalamus)

Front 19

T/F: Neighboring columns in the somatosensory cortex receive information from the same part of the body, but a different sensory modality.

Back 19

True

Front 20

What is somatic sensory area 1 (S1)?

Back 20

Post-central gyrus
Brodmann's 1, 2, and 3
First stop for most of the cutaneous senses
Somatotopic representation (toes medial, head lateral)

Front 21

What is somatic sensory area 2 (S2)?

Back 21

Wall of lateral (sylvian) fissure
Receives input from S1
Somatotopic representation - not as detailed as S1

Front 22

What is S1 responsible for?

Back 22

Involved in the integration of the information for position sense as well as size, shape discrimination
Lesions here also disrupt function of S2

Front 23

What is S2 responsible for?

Back 23

S2 is required for 'cognitive touch'
Stereogenesis - ability to recognize from touch only the identity of an object
Comparisons between two different tactile sensations
Determining whether something becomes a memory
Lesions here do not later function of S1

Front 24

What is the parieto-temporal-occipital (PTO) association cortex required for and what are its functions?

Back 24

Required for high-level interpretation of sensory inputs Receives the input from the different sensory cortical areas
With these inputs, PTO functions in: analysis of spatial coordinates of self/surrounding objects, language comprehension, initial processing of visual language, naming of objects

Front 25

What does the PTO function in on the non-dominant side?

Back 25

Functions in:
Understanding and interpreting music
Body language
Interpretation of non-verbal visual experiences

Front 26

Describe the concept of plasticity.

Back 26

Early in life, many of our experiences enable us to refine the map that is genetically coded in the cortex including anatomically eliminating synapses as well as strengthening others
If an area of the body is denervated, afferent input from remaining parts of the body will reinnervate the cortex
If an area of cortex is lost, those afferents will innervate neighboring (remaining) columns

Front 27

What does the doctrine of specific nerve energies state?

Back 27

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

Front 28

What does the Law of Projections state?

Back 28

No matter where along the path we stimulate it, the perceived sensation is always referred back to the area of the body in which the receptor is located