Sensory Function I - Lecture 32

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Sensory function

General Properties of Receptors

Senses: 5?

Somatic Sensations

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Sensory Function

We have discussed homeostasis - control of fluids and solutes; respiratory system - control of oxygen, CO₂, and circulatory system - controls blood flow and oxygen delivery

We have also discussed muscle physiology and a bit about nerve physiology - we have not covered neuro-endocrine physiology or digestive system and nutrition

Last topic before we start to wrap-up this presentation of Physiology --- Sensory systems

Your awareness of the world around you and your body's awareness of the inside environment is determined by the physiological mechanisms involved in processing information - Sensory Perception

This should sound familiar because this is where we began our discussions

Initial step is the conversion of stimulus energy into action potentials that are carried in nerve fibers

How?

1. Receptors

A. General properties

1. Receptors act as transducers:

change 1 form of energy into another i.e. mechanical, light, pressure -- electrical

2. Law of specific nerve energies:

each receptor type is sensitive to one form of energy, though it may respond to another i.e. eye responds to very low light level, but will also respond to much greater energy forms of pressure i.e. hit on the head ----- see stars - also hear bells.

push on eye and you will see light even if the eye is closed

3. receptors respond to stimulus by producing graded potentials

and action potentials result from adequate stimuli.

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4. Dynamic range - the range of stimulus intensities to which a receptor can respond without saturation

there are limits on system - why saturation point  - i.e. - above a certain temperature - your senses can no longer indicate if an object continues to get hotter - this a  problem because you are not aware that additional tissue damage will occur

finite numbers of ionic channels which can respond : when all respond --- increased stimulus but no increased response

1) AP frequency is limited by refractory period during saturation - body cannot sense increased stimulus - this may lead to damage of tissue

5. Range fractionation : each receptor of a group (i.e. photoreceptors) responds to a given dynamic range - photo cells, taste buds, pressure cells

(salt, blue, green, red, high pitch, low pitch, dog whistle)

some receptors respond to rapid changes in pressure, others to slow changes in pressure -i.e. if you increase the temperature very-very slowly a frog can be boiled with out sensing the temperature - the receptors only respond to rapid changes in temp.

6. Receptor adaptation : continued stimulation -- receptor may not continue to respond - i.e. bright light is not as bright after awhile

a. depletion of receptor molecule i.e. pigment in eye retinal cells bright light -- less pigment

b. accessory structure adapt

1. Pacinian corpuscle - sensory receptor in skin, muscle, mesentery, joints - (test in lab)

adaptation preserves ability to respond to a new stimulus - when remove stimulus - again noticed

7. Techniques to enhance receptor sensitivity - sum input from several different receptors

i.e. retina of eye - one photon on a single receptor (rod), no perception - a single photon on several receptors - get perception - due to summation of graded potentials - if threshold potential is reached - get action potential and thus perception of stimulus

taught about five senses - actually many more

all of the above are CONSCIOUS SENSATIONS

Somatic (Body) Sensations - respond to mechanical stimulation of the skin or hairs (inner ear), rotation or bending of joints, temp.

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How do we determine the type of stimulus? ie. touch or sound

remember all stimuli produce action potentials

pathways for somatic sensations --- cross in spinal cord or brain stem

simple sense detected by thalamus - i.e. -  if something is hot or cold

-intensity, localization, identity, comes from somatosensory cortex

neurons from different parts of the body go to discrete locations in the somatosensory cortex; the parts with the greatest sensitivity - fingers, thumb, and lips, face, genitalia -

higher centers for further elaboration such as shape, texture, and temperature

a) Touch - pressure

-rapidly adapting - onset and removal of pressure -- Phasic - i.e. - Pacinian Corpusle

-slowly adapting - sustained discharge -- Tonic - i.e. - muscle stretch receptor

within each category - test in lab

-small self defined receptive fields; precise information - finger tips

-large receptive fields- palms of hand

b) Proprioception - sense of position in space

Kinesthesia - joint position and movement

receptors in joints, tendons, and muscles - muscle spindles also eye and vestibular apparatus - later.

c) Temperature -

-skin - nerve endings

Ruffini's organ - warm - increase firing from 30-43^oC

Krause's bulb - cold - increase firing from 35-20^oC

how? - not known exactly

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d) Pain - 3 types

1) mechanical - cut, crush --- mechanoreceptors

2) thermal - temperature extremes --- thermoreceptors

3) polymodal - all damage (cut or temp.) and chemicals released by injured tissue

- nociceptors

a stimulus about to cause tissue damage will elicit a sensation of pain

-location, intensity, duration

mechanical and thermal - A delta Fibers - myelinated - large and fast - sharp pain

polymodal - unmyelinated C fibers - slow - dull, more painful

i.e. - in burn -- first sharp pain and then a dull aching poorly localized pain (the latter is due to chemical release of bradykinin from the damaged tissue)

along with perceived sensation - get reflex escape or withdrawal response and physiological changes mediated via sympathetic nerves

increase HR , increase C.O. , increase pressure, dilate pupils, emotional response - fear

therefore, it varies from one person to another

-athletes can hurt themselves - continue to play

Mechanisms

Receptors - naked nerve endings

-nociceptors

chemicals released from damaged tissue - increase number of action potentials in polymodal receptors only

-histamine increases permeability, but primarily bradykinin increases permeability, also potassium ions

brain has pathways to decrease firing of pain receptors

release of enkephalin

also endocrine system release endorphins (pituitary and adrenals)

these endogenous opiates decrease release of Sub P from pain fibers at synapse in spinal cord

similar to drugs opium, morphine --

heroin is often used for its CNS effects because it crosses blood brain barrier very fast

these are narcotics - used as analgesics (remove pain)

acupuncture - stimulates release of endogenous opiates, also exercise -runners high

prostaglandins produced by tissues - increase pain - increase receptor sensitivity - i.e. - pain receptor in foot -- aspirin decreases PG and, therefore, reduces pain

type IV afferents - unmyelinated - type c - pain only

4 groups of fibers

Ia - large diameter - Golgi tendon organs

IIa - large - muscle spindle

IIIa - small quickly adapting - some slow

IVc - small slowly adapting

REVIEW QUESTIONS

1. The Law of specific nerve energies means that:
   1. receptors change one form of energy into another form of energy.
   2. receptors can respond to 520 nm light but not to 430 nm light.
   3. the intensity of a stimulus is converted to an amplitude modulated graded potential.
   4. some receptors respond to rapid changes in pressure but not to slow changes in pressure.
   5. None of the above are correct.
2. Which of the following is not a somatic sensation?
   1. temperature of the blood
   2. touch
   3. pain
   4. rotational acceleration.
   5. All of the above are somatic sensations.
3. The electrical gradient of a receptor depends on the fact that the cytoplasm has _____ than the exterior of the cell.
   1. more potassium and less sodium
   2. more potassium and sodium
   3. more sodium and less potassium
   4. less potassium and sodium
   5. none of the above
4. Which of the following is true?
   1. all action potentials for the same nerve are alike
   2. receptors respond to stimuli by producing graded potentials
   3. different nerves convey information by going to different areas of the brain
   4. differences in intensities may be due to the frequency of action potential fired by a neuron
   5. all of the above are true
5. A person that is extremely sensitive to pain
   1. has few nerves
   2. has a low stimulus threshold
   3. has a high stimulus threshold
   4. has no myelinated nerves
   5. has consumed opium
6. Pain relievers such as endorphins and enkephalins are similar to
   1. depressants
   2. narcotic analgesics
   3. stimulants
   4. hallucinogens and psychedelics
   5. antipsychotics
7. Which of the following is incorrect?
   1. The neurotransmitter associated with pain is substance P.
   2. A Pacinian corpuscle is a rapidly adapting receptor.
   3. A Ruffini organ is a cold receptor.
   4. Aspirin decreases pain by blocking prostaglandin production.
   5. A polymodal receptor is a chemical receptor.

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