Cutaneous Senses

Question 1

sensory (afferent) neurons

Answer 1

• how we start an AP
• have specialized channels that are opened or closed in response to a stimulus
  
  • most open or close in response to a mechanical stimulus such as deformation of the cell membrane will cause the Na channel to open and depolarization of the membrane

Question 2

depolarization of sensory neurons

Answer 2

• produces a local response upon opening of the Na channels when the mechanical stimulus is initiated
• this depolarization in a sensory neuron is called “generator potential”
  
  • if stimulus is strong enough or lasts long enough, the generator potentials will cause the afferent neuron to come to threshold and generate an AP

Question 3

generator potential–touch/pressure

Answer 3

• Pacinian corpuscle is a receptor in the skin that responds to touch
• comprised of alternating layers of membrane with fluid b/w them surrounding the nerve ending
  
  • when we touch something, all the layers of the membrane are deformed and this mechanical stimulus leads to the opening of the mechxnosensitive Na channels on the membrane and the influx of Na and an AP sent

Question 4

adaptation

Answer 4

• if a stimulus is maintained, the AP gradually die as accommodation occurs
  
  • the number of AP dies away even though the stimulus is still there b/c the fluid rearranges in the Pacinian corpuscle and then there is no more transmission of force to the neuron
    
    • the force is still there and the outer layers still deformed by the inner layers rearrange so mechanical force no longer goes all the way to neuron and channels close
• *much but not all of the adaptation that occurs is the result of changes in the periphery

Question 5

afterdischarge

Answer 5

• sometimes when we remove the stimulus, this triggers APs as the ending reforms
  
  • when we remove the force, the fluid that reformed now has to move again so this triggers an AP, so this helps us know when the stimulus ends and goes away
• property of the R itself but adaptation can be due to brain and receptor

Question 6

sensory unit and receptive field

Answer 6

• sensory unit–sensory neuron and all its branches
  
  • all the branches within the area of the skin
• receptive field–area from which stimulation produces activation of the neuron
  
  • area on the skin, visual field, etc which will activate the same sensory neuron if activated
  • fine touch wold have a small receptive area where as crude touch would be in an area with a large receptive area

Question 7

coding sensory stimulus

Answer 7

• number of APs is one way of coding the intensity
• with greater intensity, we see more APs
• with further increases, we may see patterned discharges–doublets or triplets
• number of receptors firing also inc with increased intensity
  
  • stimulus may only produce activation in one neuron if small stimulus so neighboring neurons get no signal
  • if the stimulus is bigger, then we can get more than one neuron firing and at different rates b/c one neuron is impinging on the receptive field of another

Question 8

quantification of the coding of the sensory stimulus

Answer 8

-we need to see about a 10% change in for conscious recognition of the change

Question 9

perceived intensity vs. measured intensity

Answer 9

perceived intensity=K(measured intensity)^A

• K and A vary depending on the type of sensory R
  
  • muscle senses–both are close to 1 so our perceived intensity matches our actual (measured) intensity very closely
    
    • this is important so that we know exactly where our muscles are to make the motion that we want
  • cutaneous senses–more variability so what we perceive may diverge from the actual rather substantially
    
    • if inc actual intensity, then inc perceived, but don’t know how much or exact fine motions

Question 10

multiple pathways of central processing of the senses in the brain

Answer 10

1. dorsal columns–proprioceptive and discriminative (fine touch)
2. spinothalamic tract–thermal, nociceptive, and coarse touch
3. spinoreticulothalamic system–nociceptive
4. spinocerebellar tract

Question 11

pre-synaptic inhibition

Answer 11

• special case of inhibition that arises from an axo-axonal synapse and involves the post-synaptic cell which is a pre-synaptic terminal
• end result of pre-synaptic transmission: reduced NT release from the inhibited pre-synaptic terminal
• so, we start with a normal chemical synapse with NT release from neuron A to B
  
  • then we have another neuron that synapses on the pre-synaptic terminal (neuron C)
  • the third neuron when activated release GABA at the synapse b/w it and the first neuron which allows Cl to enter neuron A, so this hypoerpolarized neuron A, so when bring in Ca at the junction b/w A and B, it will not depolarize the membrane as much and won’t release as much NT and reducing the probability of APs from neuron B

Question 12

where does presynaptic inhibition occur?

Answer 12

b/w neighboring receptors at the first synapse in the their pathway
-increases the brain’s ability to localize the signal

Question 13

thalamus in transmitting central processing of senses

Answer 13

• often makes decision that the thalamus does not need certain sensory information
  
  • this is important during sleep where large chunks of info do not get past the thalamus b/c it is so hyper polarized
• regardless of which pathway is used, every synapse along the way represents a chance to modify or respond to the stimulus

Question 14

organization of the cortex

Answer 14

• sensory cortex is arranged somatotopically
  
  • homunculus is what represents the sensory going to the parts of the body from the cortex–hand, face, and mouth get huge areas of cortex devoted to them

Question 15

neocortex

Answer 15

• somatosensory cortex is neocortex so it has 6 layers and the neurons in the cortex are arranged in columns–each column deals with ONE sensory modality in ONE part of the body
• sensory information arrives at its respective column in layer IV–via the thalamus
  
  • neighboring columns receive information from the same part of the body but a different sensory modality

Question 16

somatic sensory area 1 (S1)

Answer 16

• post central gyrus
• Brodmann’s 1, 2, and 3
• first stop for most cutaneous senses
• somatotropic representaiton–toes medial, head lateral
• first time you are consciously aware of a the stimulus
  
  • involved in integration of the info for position sense as well as size, shape, discrimination–know characteristics of the object but not what the object is
• sends its information to S2

Question 17

somatic sensory area 2 (S2)

Answer 17

• wall of lateral (sylvian) fissure
• receives input from S1
• somatotopic representation–not as detailed as S1
• if S1 is damaged, then S2 doesn’t fcn
• required for cognitive touch
  
  • stereogenesis–ability to detect what something is based completely on touch rather than seeing it
  • comparisons b/w 2 different tactile sensations
  • detemine whether something becomes memory

Question 18

what differentiates human brains and brains of other species?

Answer 18

-association areas in the cortex–interconnections b/w layers 1-3 that is information traveling b/w different areas from different networks to derive associations

Question 19

parieto-temporo-occipial (PTO) association cortex

Answer 19

• required for high level interpretation of sensory inputs so it receives that inputs from the different sensor cortical areas including S1 and 2
  
  • gets info from parietal (auditory), temporal (olfactory), and occipital (vision) as well as somatosensory areas
• functions in:
  
  • analysis of spatial coordinates of self/surrounding objects
  • naming of objects

Question 20

modification of input for the central processing of senses

Answer 20

• neighboring sensory R can modify a signal

- activation of 2 things simultaneously can cause pre synaptic inhibition which will modify a signal

Question 21

plasticity

Answer 21

• early in life, many of our experiences enable us to refine the map that is genetically coded in the cortex which includes anatomically eliminating synapses as well as strengthening others
• if an areas of the body is amputated or denervated, afferent input from remaining parts of the body will reinnervate the cortex
  
  • if the neurons receiving no stimulus in a certain column, other parts of the body will spread out and take over those columns
• if an area of cortex is lost, those afferents will innervate remaining tradeoffs
  
  • so if we lose a column, the neuron from that column will remap into a neighboring column so we don’t lose the sense but we do lose localization so we may not know exactly where something happened b/c the column may not use the same sense modality as the original

Question 22

doctrine of specific N energies

Answer 22

• stimulation of a sensory pathway at any point leads to the perception of a sensation that is dictated by the nature of the R that started the pathway
• SO, if I stimulate the cortical column that receives input from the pacinian corpuscle, then you will perceive the sensation of touch
  
  • whatever sensory pathway you stimulate no matter where you are in the pathway, the sensation you perceive is dictated by the R that should normally be activated
• “what you feel”

Question 23

law of projections

Answer 23

• 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 R is located
• SO if the cortical column I stimulate receives input from a pacinian corpuscle in your left index finger, you perceive the touch as occurring on your left index finger