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Anatomy and Physiology
Special Senses Unit
Sensation

Conscious or subconscious awareness of
external stimuli.
Perception

Conscious awareness and interpretation of
sensations.
Modality

The distinct quality that makes one sensation
different from another.
Components of Sensation
Stimulation – Stimulus (change in the
environment that can activate certain
sensory neurons) occurs within the sensory
neuron’s receptive field (region that can
respond to stimuli).
2. Transduction – A sensory receptor or
sense organ must respond to the stimulus
and transduce (convert it to a generator
potential).
1.
Components of Sensation Cont’d
Impulse Generation and Conduction – When the
generator potential reaches threshold, nerve
impulses are sent.
First Order Neurons – Sensory neurons that send
messages to the CNS.
4. Integration – A region of the CNS must receive and
integrate the sensory nerve impulses into a
sensation. Part of the cerebral cortex is involved
with each type of sensation.
3.
Sensory Receptors

Respond vigorously to one type of stimulus,
but weakly or not at all to others, a quality
known as selectivity.
Classification of Receptors

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
Somatic Senses – Sensations of touch,
pressure, vibration, warmth, cold, pain and
detection of body positions and movements
Free Nerve Endings – The simplest type,
having no structural specializations;
examples include receptors for pain, temp.
itch and tickle
Special Senses – taste, smell, vision,
hearing and equilibrium
Classification by Location
Exteroceptors – Located at or near the surface of
the body; provide information about the external
environment.
2.
Interoceptors – Located in the blood vessels and
viscera to provide info about internal conditions
3.
Proprioceptors – Located in tendons, muscles,
joints and the internal ear
- Provide info about body position, muscle tension and
the position and activity of our joints.
1.
Classification by Stimulus
1.
2.
Mechanoreceptors – They detect
mechanical pressure or stretching; related
to touch, pressure, vibration, proprioception,
hearing, equilibrium and blood pressure.
Thermoceptors – They detect changes in
temperature.
Classification by Stimulus Cont’d
3.
4.
Nociceptors – They detect stimuli that
cause physical or chemical damage to
tissues.
Chemoreceptors – They detect chemicals
in the mouth (taste), nose (smell) and body
fluids.
Generator Potentials and Receptor
Potentials
-
-
-
The electrical response of a sensory receptor
will be one of these.
Both are graded potentials; vary in
amplitude.
They vary with the amplitude of the stimulus.
Generator Potentials

Generated by the senses of smell, touch,
pressure, stretching, vibration, temperature,
pain and proprioception.
Receptor Potential



* Generated by the senses of vision,
hearing, equilibrium and taste.
* Have short neurons that synapse with first
order sensory neurons.
* Causes an increase or decrease in
exocytosis from synaptic vesicles.
Adaptation


A change in sensitivity (usually a decrease)
over a long stimulation.
EX: A hot shower feels hot a first, then feels
comfortable.
Receptors and Adaptation


Rapidly Adapting Receptors – Associated
with pressure, smell and touch; adapt
quickly.
Slowly Adapting Receptors – Associated
with pain, body position and chemicals in the
blood; adapt slowly.
Somatic Senses

1.
Arise from receptors in the skin, muscles, joints,
tendons and inner ear.
Cutaneous Sensations – Touch, pressure,
vibration, thermal and pain.
- receptors are in skin, mucus membranes,
mouth and anus
- some areas have many receptors, others have
few
- receptors consist of dendrites of sensory neurons
Somatic – Cutaneous Sensations
Cont’d



Tactile Sensations – Touch, pressure,
vibration, itch and tickle.
Touch – Results from the stimulation of
tactile receptors immediately deep to the
skin.
Crude Touch – You know something has
touched you, but can’t determine its location,
shape, size or texture.
Somatic – Cutaneous Sensations
Cont’d


Discrimitive Touch – The ability to
determine the exact location and other
details without seeing them.
Pressure – These sensations result from
stimulation of deeper tissues.
–
–
Defined as a sustained sensation that is felt over
a larger area than touch
Receptors adapt quickly
Somatic – Cutaneous Sensations
Cont’d


Vibration – Sensations result from rapidly
repetitive sensory signals from tactile
receptors
Itch and Tickle – Sensations result from
stimulation of free nerve endings or by
certain chemicals
–
Tickle is the only sensation that most people can’t
elicit on themselves, though why is still a mystery
Somatic – Cutaneous Sensations
Cont’d

Thermal – Detects heat and cold; separate
receptors for each
Somatic – Cutaneous Sensations
Cont’d

Pain – Protects the body by letting you know
when you’ve come in contact with something
damaging.
–
–
Receptors are found in almost every body part
and respond to any type of stimulus that is strong
enough to cause tissue damage.
Receptors are slow to adapt, so pain is going to
linger for long periods of time
Types of Pain
1.
2.
Acute (fast) – Occurs fast (within 0.1 second of
stimulation) and is not felt in deeper tissues; aka
sharp, fast and pricking pain. EX: a needle
puncture or cut from a knife
Chronic (slow) – Begins after a second or more
and increases in intensity; aka burning, throbbing,
aching pain. It can be excruciating
- Can be in skin, deeper tissues viscera.
Pain



Superficial Somatic Pain – Pain that arises
in the skin
Deep Somatic Pain – Arises from receptors
in skeletal muscles, joints, tendons and
fascia
Visceral Pain – Comes from receptors in the
viscera (organs)
Referred Pain



When pain is felt in the organ as well as in
some surface area far from the stimulated
area.
The area to which the pain is referred and
the visceral organ involved are serviced by
the same area of the spinal cord.
The heart and skin on the medial portion of
the upper arm enter the spinal cord between
T1 and T5.
Phantom Pain




Pain experienced by patients that have had a limb
amputated.
They experience itching, pressure, tingling and pain
as though the limb were still there.
Possibly caused by stimulation from neurons that
used to receive signals from the appendage.
Possibly from the brain and its sense of body
awareness.
Proprioceptive Sensations



Awareness of body parts and movements.
Informs us of how far muscles are
contracted, tension on tendons, and change
in the position of a joint.
Allows us to do coordinated movements like
walking and typing.
Proprioceptive Sensations Cont’d


Also allows you to estimated the amount of
force needed to do something (lifting a bag of
feathers or a bag of rocks).
Receptors adapt very little if at all (you
constantly need that information).
Proprioceptive Receptors

Muscle spindles, tendon organs and joint
kinesthetic receptors as well as hair cells of
the internal ear, which provide information
about balance.
1. Muscle Spindles

Special groups of muscle fibers found in
regular fibers and oriented parallel to them.
–
–
–
–
They lack actin and myosin, so they can’t contract
They do contain two types of sensory receptors
They monitor changes in the length of a skeletal
muscle by responding to the rate and degree of
change in length
The information gets sent to the brain
2. Tendon Organs


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
Found in the junction of tendon and muscle
When tension increases in a tendon, a
message is sent to the brain.
Prevents damage to tendons and muscles
due to excessive tension
They also monitor the force of muscle
contractions.
3. Joint Kinesthetic Receptors


Associated with the articular capsule of
synovial joints
Responds to pressure and measures
acceleration and deceleration of joint
movement as well as joint strain.
Somatic Sensory Pathways

Pathways from receptors to the cerebrum
involve three-neuron sets
1. First-Order Neurons

Carry signals from the receptor to the brain
stem or spinal cord
–
–
Face, mouth, teeth and eyes; use cranial nerves
Back of the head, neck and body use spinal
nerves
2. Second Order Neurons


Carry signals from the spinal cord or brain
stem to the thalamus
Second order neurons cross over from right
to left and vice versa before ascending to the
thalamus
3. Third Order Neurons



Run from the thalamus to the primary
somatosensory area of the cerebral cortex where
conscious perception of sensations result
2nd order neurons synapse with 3rd order neurons in
the somatosensory area of the cerebral cortex
Impulses along this pathway give rise to several
highly evolved and refined sensations:
1. Discrimitive Touch

The ability to recognize the exact location of
a light touch and to make two-point
discriminations
2. Sterognosis

The ability to recognize by feel the size,
shape and texture of an object. Examples
are reading braille or identifying (with eyes
closed) a paperclip put in your hand
3. Proprioception

The awareness of the precise position of
body parts, and kinesthesia, the awareness
of direction of movement
4. Weight Discrimination

The ability to assess the weight of an object.
5. Vibratory Sensations

The ability to sense rapidly fluctuating touch.