Sensory Physiology
Sensations
Define sensation:
State of awareness of a stimulus.
Awareness
Stimulus is detected by the body; below the level of
consciousness. Change in blood pressure or blood pH, etc.
Perception
Stimulus reaches consciousness; pain, hunger, thirst, sight,
sound, etc.
Sensation requires
1) stimulus
Any change in the environment; internal or external,; can generate
a stimulus. Example: body temp, pH, BP, light, sound.
2) receptor
Structure which detects a stimulus; converts it into APs.
3) impulse conducted
Action potential is sent via dendrites to the CNS sensory somas.
4) translation
Particular regions of the CNS will receive the APs; convert them to
an awareness of the stimulus.
Receptor potential
Receptor potentials: Changes in the transmembrane potential
of a receptor caused by the stimulus.
Generator potential
Generator potentials: receptor potential that is strong enough
(reaches threshold) to generate an AP in a receptor.
Strong stimulus causes a large change in the receptor potential,
which is then more likely to cause an action potential to fire.
Remember that APs are all-or-none.
Stronger the stimulus (above threshold) the more APs over a given
time.
Characteristics of sensations
1)Projection
Perception of a stimulus is mapped to the surface of the body
where the stimulus is applied.
2)Adaptation
Frequency of action potentials declines over time even if the
stimulus is still present. Sensation declines.
Phasic
Rapidly adapting; pressure, smell, taste.
Tonic
Slowly adapting; pain.
Characteristics of sensations
3) Afterimages
Persistence of sensation after the stimulus is removed.
After image of bright lights, for example.
4) Modality
Feature by which one sensation is distinguished from an
another.
Sound is distinct from vision; sound vibrations in the air vs.
light.
Receptor
Q43
Define
Dendrite of a sensory neuron is modified to detect a
certain sensory modality; pressure receptors in the skin,
etc.
single or many (millions) of neurons
Single: touch, pain, pressure, etc.
Many: ear, eye, balance, etc.
contain dendrites
All contain modified dendrites.
Classification of receptors I. Location
1)externoceptors
Located on the body surface or specialized to detect external
stimuli.
Pressure, pain, temp, touch, etc.
2)visceroceptors
Within internal organs, detect internal stimuli.
Pressure, pain, fullness.
3)proprioceptors
Joints and muscles.
Vestibular structures and the semicircular canals of the inner
ear.
Limb and body position and movement.
Classification of receptors II. Modalities
1)mechanoceptive
Detects stimuli which mechanically deform the receptor;
pressure, vibration, touch, sound.
2)thermoceptive
Changes in temperature; hot/cold.
3)nociceptive (pain)
Damage to the structures.
4)photoreceptors
Light; vision, retina of the eye.
5)chemoceptive
Chemical stimuli: CO2 and O2 in the blood, glucose, smell,
taste.
Classification of receptors III. Complexity
1) simple receptors
Usually a single modified dendrite.
general sense
Touch, pressure, pain, vibration, temperature.
2)complex
Highly modified dendrites, organized into complex
structures; ear, eye.
special senses
Vision, hearing, smell, taste.
General sense I) Cutaneous A) tactile
1) touch
a) Hair root plexus
Base of hair shaft: detects
movement over the body surface.
b) Free nerve endings
Touch and pressure.
c) Merkel's disc or tactile discs
Fine touch and pressure.
Meissner's
corpuscles
d) Meissner's corpuscles
Fine touch, pressure and vibration.
e) Ruffini's end organs
Deep touch/pressure.
Meissner's
corpuscles
Pacinian
corpuscles
General sense I) Cutaneous A) tactile
2) pressure
f) Pacinian corpuscles (AKA, lamellated)
Deep pressure. Sensitive to highfrequency vibrations.
Pacinian
corpuscles
Free nerve endings and Meissner’s
Also detect pressure
Meissner's
corpuscles
General sense I) Cutaneous A) tactile
3) vibration Cyclic changes in pressure over time.
Meissner's and Pacinian
Detect vibration.
Meissner's
corpuscles
Pacinian
corpuscles
General sense I) Cutaneous B) thermoreceptive
Detect changes in temperature. Tend to adapt quickly.
free nerve endings?
Receptor types that detect
temperature, both hot and
cold.
General sense I) Cutaneous C) pain/nociceptors
Free nerve endings that detect trauma.
Tissue damage will tear open the membranes of pain receptors.
Na+ rapidly flows in, depolarizing the membrane and generating
action potentials.
speed/duration
Acute
Occurs at the time of trauma, while the tissue is being damaged.
Sensation lasts only as long the damage is happening
Chronic
Lasts as long as the damage is present.
Does not adapt well.
General sense I) Cutaneous C) pain/nociceptors
location Either within or on the surface of the body.
Somatic
Surface body pain; easy to localize.
Caused by some external trauma event.
Visceral
Within the body, associated with internal organs; hard to
localize
referred (visceral) pain
Pain sensations on visceral organs that are mapped or
referred to the surface of the body.
Reason: many organs and surface sites have common
embryological origins, so common innervation.
CNS sometimes cannot determine where the sensations
come from.
General sense II) proprioceptive sense
Include both simple and complex receptors
kinesthetic/proprioceptive
Gives you a sense of body position and limb movement.
sense of position
Vestibular and semicircular apparatus of the inner ear
determines how your body is positioned in 3D space.
General sense II) proprioceptive sense
1) muscle spindles
State of muscle contraction and stretch.
2) tendon organs
Amount of tension that tendons are under.
3) joint kinesthetic
Degree to which the joint is flexed or extended..
4) maculae & cristae
In the vestibule and semicircular canals of the inner ear; body
position, linear acceleration and rotation.