Sensory Physiology (Dr. Karius)

Question 1

Describe the transduction of a stimulus into an Action Potential.

What is a Generator Potential?

Answer 1

Sensory (Afferent) Neurons have specialized channels that are opened or closed in response to a stimulus.

• Touch Receptors have a sodium channel that is OPENED with deformation of the cell membrane
• Generator Potential –> Like an EPSP; if the stimulus is strong enough, the Generator Potential is strong enough, we have get an action potential!

Question 2

Talk about the Pacinian Corpuscle.

How does Adaptation occur?

What is Afterdischarge?

Answer 2

When something touches the Pacinian Corpuscle, the layers are all going to be deformed and this opens the Na+ Channels!!!

• If you have a strong enough stimulus, you will have an Action Potential that will go back to the CNS.

Adaptation: If the stimulus is maintained, the deformed membrane is going to adapt to a new shape (because it is filled with water) and you will NO longer have deformation of the most INNER LAYER of the Pacinian Corpuscle which will lead to halting of Action Potentials

Afterdischarge: Action Potentials that are going to occur when the stimulus is removed! (i.e. you can still feel your sunglasses after you take them off your head)

Question 3

Differentiate between the advantages of Small and Large Motor Units.

Answer 3

Small Motor Unit –> Fine Motor Control

Large Motor Unit –> POWER (Postural Muscles)

Question 4

Define Sensory Unit.

Answer 4

Sensory Nerve and its branches

Larger Objects = Greater Intensity and MORE Action Potentials

If the stimulus increases past the Peak for SINGLE Action Potentials, it will use PATTERNED DISCHARGE (doublets or triplets)

Question 5

Define Receptive Field.

Answer 5

Area from which stimulation produces action of the neuron

Big Receptive Field –> Crude Touch

Small Receptive Field –> Fine Touch

Question 6

What is the “Just Noticable Difference”?

Answer 6

Smallest difference that can be detected.

Needs to change by about 10% for conscious recognition

Question 7

Describe the Weber-Fechner Law.

Answer 7

Perceived Intensity = log (measured intensity)

Action Potentials are going to change based on a Logarithmic Scale

NEW Formula –> Percieved Intensity = K(measured intensity)^A
K and A will vary based on the type of sensory receptor that we are using!
1. Muscle Senses: K and A are both close to 1 (Our percieved intensity matches the ACTUAL (measured) intensity very closely)
2. Cutaneous Senses: More variability in K an A (What we perceive may diverge from the actual rather substantially)

Question 8

Describe Pre-Synaptic Inhibition.

Answer 8

• Axo-axonal Synapse (Main receptor sends off another branch to get rid of “excess noise”)

Result: Reduced neurontransmitter release from the inhibited pre-synaptic terminal (Releases GABA which will activate a CHLORIDE channel –> GABAa channel; this will cause less calcium to enter the cell)

**** Going to help the Brain to LOCALIZE different sensations!

Question 9

Describe the organization of the Sensory Cortex.

Answer 9

Arranged SOMATOTOPICALLY

Toes –> Medial

Head –> Lateral

Question 10

Describe the Columnar organization of the Primary Somesthetic Area.

Answer 10

Each Column is going to get different modalities from the same area!

Sensory input is going to come into LAYER 4

Question 11

Describe Somatic Sensory Area 1 (S1).

Answer 11

Post-Central Gyrus

• Broadmann’s 1, 2, and 3

*** FIRST STOP for most CUTANEUOUS senses

*** Somatotopic Representation (Toes are medial; Head is lateral)

*** Physical characteristics of what you are HOLDING (size and shape discrimination)

*** DO NOT know that you are holding a TV remote

Question 12

Describe Somatic Sensory Area 2 (S2).

Answer 12

Located in Wall of Lateral (Sylvian) Fissure

• Receives input FROM S1
• Somatotopic Representation (Not as detailed as S1)

Function: “Cognitive Touch” ability to know an object just by touching it, can compare two objects.

*** Knows that the object in the right is heavier than the object in the left

*** STILL CANNOT NAME the object that you are holding!

Question 13

If you damage S2, will you impair the function of S1?

Answer 13

NOPE

Question 14

Which area in the brain is going to allow us to identify objects that we are holding?

Answer 14

ASSOCIATION CORTEX

Question 15

What is the function of the Pariteo-Temporal-Occipital (PTO) association Cortex?

Answer 15

• Analysis of the spatial coordinates of self/surrounding objects
• Naming of Objects

Question 16

Describe the Doctrine of Specific Nerve Energies.

Answer 16

Stimulation of a sensory pathway point leads to the perception of a sensation that is dictated by the nature of the receptor that STARTED THE PATHWAY

*** Stimulating Columns in the Brain

**** Type of SENSATION that you feel

Question 17

You can change your Cortex through experience. How do you do it?

Answer 17

Babies put fingers in mouth in order to map out the sensory pathways in their body

**** Includes Anatomically eliminating synapses as well as strengthening others

**** If an area of the cortex is lost, you are going to send afferent fibers to the neighboring columns!

Question 18

Describe the Law of Projections.

Answer 18

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

**** Part of the Body that you FEEL