Lecture 26 - Michael Hedrick PDF

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Michael Hedrick...

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Somatic Sensory system

● Somatic senses -

○ Touch

○ Temperature

○ Pain

○ Itch

○ Proprioception

○ Pathway

● Reflex arc -

○ Doesn’t necessarily have to involve the brain

○ CNS - Integration of afferent -> efferent response

● Sensory Receptors -

○ The nervous system can only interpret electrical signals in the form of

action potentials

○ Sensory signals -

■ Light

■ Hearing

■ Touch

■ Pain

■ Taste

■ Smell

○ Sensory transduction -

■ Is the process of converting a sensory signal to an electrical

signal in the sensory neuron

■ The process of transduction is converting all of these sensory

signals into an electrical signal. Which the central nervous

system, the brain, can interpret

■ The process of reception is dependent on the stimuli itself. The

type of receptor, receptor specificity, and the receptor field,

which can vary depending on the receptor type

■ 2 ways the Transduction process takes place:

● 1. Epithelial cells that are induced to specialize in

performing some type of sensory transduction

○ A specialized cell in a particular sensory pathway

that has a sensory cell that produces a

neurotransmitter in response to the stimulus and

that in turn leads to deep polarization of the

afferent nerve fibers and ultimately action

potentials that are being carried towards the axon

and down the axon

○ A sensory epithelial cell producing a graded

response in proportion to the stimulus and

producing an increase in X potential frequency

○ The epithelial cell is separated from the nerve

endings

○ True for hearing, taste, or vision

(chemoreceptors)

■ When that cell is activated, there is usually

an influx of calcium

■ It releases a chemical messenger, a

neurotransmitter on to a afferent nerve fiber

(typical Synapse)

○ Have ligand gated channels

■ It usually is opening sodium channels as a

result of ligand gated sodium channels and

this is going to result in a deep polarization

● OR

● 2. Neurons that grown into the area where the stimuli can

be detected

○ Touch receptors or pain receptors found in the

skin

○ These are receptors that are sometimes modified

nerve endings. Sometimes they are just free nerve

endings

○ What they do is that the stimulus results in what’s

called a generator potential

● In both cases, there is a generated potential. A local

potential that once if the cell is brought to a threshold,

then action potential will be produced and the frequency

of action potential are proportional to the amount of

stimulus because the amount of stimulus generates or the

generate potential is proportional to the stimulus

○ The stimulus would produce more release of

neurotransmitter and a greater D polarization on

the afferent side and ultimately increased

frequency of action potentials

○

○ (This can only be used to categorize muscle sensory axons)

■ For the velocity or the speed at which action potentials can be

propagated down the axon, it is a function of the size or the

diameter of those axons whether those axons are myelinated or

unmyelinated

● Decreasing order of velocity. A alpha being the highest

● C-fibers are unmyelinated and they are the slowest

○ Have a small diameter

○ Don’t contain myelin

○ Transfers action potentials at pretty slow

frequencies (2m/sec)

■ For Muscle (sensory axons), it only applies to muscle sensory

axons.

● Categorized as roman numerals

● One being the fastest group

○

○ (General layout of a sensory receptor)

○ Receiving stimulus producing action potentials and this cell bodies

that are usually collected together in a ganglion that lies outside of the

spinal cord

○ Sensory receptor -> PNS -> CNS

○ The receptors is producing an action potential and that potential goes

from the peripheral nervous system, which is where these receptors

are located to the central nervous system (brain and spinal cord)

○ Bipolar neurons -

■ Dendreons

● Dendrons and axons are myelinated

■ Where the action potentials originate and travel down the axon

○ In the cell bodies, carcinomas, it contains glial cells

○ The cell bodies are usually collected together just outside the spinal

cord and whenever you have a collection of cell bodies, they might

have traveled together in the same nerve trunk

○ The cell bodies are collected together in a group called a ganglion

■ Ganglion -

● Is a group of cell bodies outside of the central nervous

system

○ If we have a group of cell bodies that is within the central nervous

system, it is called a nucleus (different from the nucleus of a cell)

○ Mechanoreceptors -

■ Stimulated by mechanical force (touch, vibrations, pressure; ex.

skin, muscles, joints, hearing, and balance)

■ Responds to mechanical deformation

● Movement of proteins and skeletal elements when

pressure is applied (pulls the channel apart)

○ This allows influx of usually sodium and that is

going to result in a D polarization and ultimately

an action potential

● The transduction of a mechanical stimulus producing an

action potential depends simply on pulling ion channels

apart and allow the influx of ions(sodium) across the cell

membrane causing a D polarization

■ 4 different types of receptors in the skin -

● Pacinian corpuscle -

○ If you put pressure and deform the dermis layer,

then that deforms the pacinian corpuscle and it

results in sending action potentials down down that

axon

○ Fairly large

○ Onion-like

● Meissner corpuscle -

○ More in the dermal capelli

○ More superficial than the pacinian corpuscle

○ Much smaller than pacinian corpuscle

● Ruffini’s corpuscles -

○ In the dermis

● Merkel’s disks -

○ In the dermis

● Free nerve endings -

○ In the dermis

● Technically all of these are free nerve endings with a

specialized capsule that surrounds the free nerve endings

● All are low threshold fibers (high sensitivity)

○ It doesn’t take much stimulus to get them to

generate action potentials, though they have high

sensitivity

■ Thermoreceptors -

● Respond to changes in temperature of skin

● Don't have any specialized receptor cells

● They are free nerve endings that respond to relative

changes in temperature

● These receptors are generally comparing one temperature

versus another temperature between different neurons

and the firing of action potentials depends on thermal

history

○ Cold fibers might fire over a pretty broad range of

temperatures but the peak is about 20 - 22 celsius

so the onset of firing(activity) is 5 or 6 degrees

which is pretty cold and increases fire in frequency

and then starts to decrease

○ Right around when the cold fibers starts to

decrease, warm fibers that respond to increase

temperature start firing and they increase up to a

peak about 45 celsius (Normal body temp is 37

celsius)

■ Photoreceptors -

● Respond to light energy (eye)

● Biochemical transduction

■ Chemoreceptors -

● Respond to chemical in solution

○ Ex. taste, smell(odorants), blood gases(within

cardiovascular system, partial pressure of oxygen

and carbon dioxide), pH of blood

■ Nociceptors -

● Respond to pain

● Unspecialized nerve endings that initiate the sensation of

pain in the skin or wherever you have free nerve endings

● Mechanosensitive nociceptors -

○ Excessive pressure that can be perceived as a

painful stimulus

○ Among the slower of the slowest of the myelinated

sensory fibers that you find in the somatic nervous

system

● Mechanothermal nociceptors -

○ Excessive temperatures that can be perceived as a

painful stimulus

● Polymodal nociceptors -

○ Means that different kinds of stimuli can activate

these types of C fibers nociceptors

● Thermoreceptors vs. nociceptors -

○ Nociceptors doesn’t start firing until the

temperature gets pretty hot

■ As temperature goes higher, they fire at an

even higher rate

■ Because of this, it allows you to percieve

painful hot temperatures

○ Thermoceptors is responding to warm temperature

and doesn’t change its firing rate throughout the

stimulus

■ Touch receptors -

● Rapid adapting -

○ Fire action potentials when there is an onset and

they stop firing. Then they fire again when the

stimulus goes off

■ Starts firing based on the onset and offset of

the stimulus

○ When there is a change in the stimulus

○ If this stimulus was bigger, they might fire more

action potentials

■ So if we made this stimulus 50% bigger, you

might have a higher frequency of action

potentials at the beginning and the offset,

but they would not fire when that stimulus is

being sustained

● So the stimulus goes on, it stays on,

and then it goes off

○ The adaptation part is the fact that action potentials

frequency usually is highest at the beginning of a

stimulus and then slows down or stops

■ And those that stop after the beginning, the

stimulus is usually characterized as rapidly

adapting

○ Meissner’s corpuscle and pacinian corpuscle

● Slowly adapting -

○ They fire when the stimulus goes on, but that

stimulus is sustained, they will fire action

potentials all throughout the stimulus and they stop

firing at the offset

○ Mostly are firing during the length of the stimulus

and they might fire at an increased frequency or

burst if activity at the beginning

○ Merkel cells and ruffini endings

● Difference between rapidly adapting and slowly

adapting -

○ The slow adapting receptors continue firing

throughout the length of the stimulus

■ The slowly adapting receptors would allow

you to perceive a stimulus and the stimulus

intensity throughout the duration of the

stimulus

○ The rapidly adapting receptors allows receptors to

respond to the different types of stimuli

■ Ex. if there was something touching you,

over time you no longer feel that stimulus

■ More sensitive than slowly adapting

receptors

● Sensitive to the onset and offset

stimulus, but do not convey any

information during the duration of the

stimulus

○ If the stimulus was a greater intensity, you would

see greater frequency of action potentials

■ Receptive fields -

● Area over which a stimulus excites a sensory receptor

● Meissner’s corpuscles and merkel cells have a small

pin-point receptive field

● Pacinian corpuscle and ruffini endings have a wider

receptive field

● Two-point touch discrimination -

○ Is minimum distance at which two points of touch

can be perceived as separate

○ Measure of tactile acuity or distance between

receptive fields

○ Smaller receptive fields would be like your

fingertips and larger receptive fields would be like

your back or the back of your arm where your

sensory reception is not finely tuned...