Sensation, Sensory Receptors, and Somatic Sensations

Sensation
Sensation is the conscious or subconscious
awareness of external or internal stimuli.
Impulses reaching the thalamus do what?
Provide a crude awareness of the location and type of sensation.
Impulses reaching the cerebral cortex do what?
Enable the precise location of the stimulus, as well as the type of stimulus.
Perception
Perception is the conscious awareness and
interpretation of the meaning of sensations.
What is specific about sensory impulses that do not reach the thalamus and cortex?
There is no perception of sensory impulses
that do not reach the thalamus and cerebral
cortex: blood pressure and carbon dioxide
concentration
What is a sensory modality and what types are included?
A sensory modality is a unique type of sensation:
touch
pain
vision
hearing
What are the two classes of sensory modality?
General senses
Special senses
General senses include …
Somatic senses
Visceral senses
What are the modalities of general senses?
– Tactile sensations: touch, pressure, vibration, itch, tickle
– Thermal sensations: warm, cold
– Pain sensations
– Proprioceptive sensations
– Dynamic sensations (movements of limbs and head)
What so visceral senses provide?
Visceral sensations provide information about
conditions within internal organs.
What are the special senses?
– Smell
– Taste
– Vision
– Hearing
– Equilibrium
What is the first step in the process of sensation?
Activation of a sensory receptor by a stimulus.
What is selectivity?
Each sensory receptor monitors one (and only one) type of stimulus to which it is sensitive, and responds weakly, if at all, to other kinds of stimuli.
Where must a stimulus occur?
The stimulus must occur within the receptive
field of the receptor, which is the body region
where stimulation elicits a response.
What is transduction of the stimulus?
Transduction is the conversion of stimulus
energy into electrical energy
The sensory receptor transduces energy in a
stimulus into a graded potential
– Graded potentials vary in amplitude depending
on the strength of the stimulus
– Graded potentials are not propagated
What is generation of impulses?
Graded potentials that sum to threshold in a
sensory neuron trigger one or more nerve
impulses that propagate toward the central
nervous system.
What is first-order neuron?
Sensory neurons that conduct impulses from the PNS into the CNS are termed first-order neurons.
Where are conscious sensations and perceptions integrated?
In the cerebral cortex.
What are sensory receptor classification (microscopic)?
– Free Nerve Endings
– Encapsulated Nerve Endings
– Separate Cells
What are free nerve endings?
– Bare dendrites
– No structural specialization microscopically
– Pain, thermal, tickle, itch, some touch
What are encapsulated nerve endings?
– Dendrites are enclosed in a connective tissue capsule
– Capsule enhances sensitivity or specificity of the receptor
– Pressure & vibration (lamellated), some touch (Meisner)
What are separate cells?
– Sensory receptors for certain special senses are
specialized, separate cells
– The specialized cell synapses with first-order sensory neuron
– Hearing and equilibrium hair cells (inner ear),
photoreceptors (retina), gustatory receptors (taste buds)
What are the 2 kinds of graded potentials produced by sensory receptors?
– Generator potentials
– Receptor potentials
Amplitude of both types of potentials varies
directly with the intensity of the stimulus
What are generator potentials produced by?
– Free nerve endings
– Encapsulated nerve endings
– Receptive part of olfactory receptors
What is the sensory receptor?
– It is the first-order neuron; they are one and the same.
– A sufficiently large generator potential will
generate an action potential.
Example of Free Nerve Ending
Example of Free Nerve Ending
Reiterate what free nerve ending is?
Free nerve endings are bare dendrites of first-order neurons.
Example of Encapsulated Nerve Ending
Example of Encapsulated Nerve Ending
Reiterate what an encapsulated nerve ending is?
Encapsulated nerve endings have dendrites of first-order neurons surrounded by a specialized capsule.
What are receptor potentials produced by?
– Hair cells of the inner ear (both equilibrium and hearing)
– Gustatory receptors (taste)
– Photoreceptors (sight)
What does the receptor potential trigger?
Triggers the release of a neurotransmitter, which diffuses across the synaptic cleft and produces a postsynaptic potential (PSP) in the first-order neuron
The sensory receptor is not the first-order neuron
The PSP may in turn trigger one or more nerve
impulses
Example of a Separate Receptor Cell
Example of a Separate Receptor Cell
What is specific about a specialized receptor cell?
Specialized receptor cells are separate and distinct
from the first-order neuron. There is a synapse
between the receptor and the first-order neuron.
What are the classifications by location and activating stimuli?
Exteroceptor
Interoceptor
Proprioceptor
What is an exteroceptor?
– Located at or near body surface
– Provide information about external environment
– Sight, odor, taste, touch, pressure, vibration, thermal, pain
What is an interoceptor AKA visceroceptors?
– Located in blood vessels, visceral organs, muscles, & nervous system
– Provide information about internal environment
– Impulses usually not consciously perceived
What is a propriceptor?
– Located in muscles, tendons, joints, inner ear
– Provide information about body position, muscle length and tension, position and motion of joints, and equilibrium
What are the classifications by type of stimulus detected?
Mechanoreceptors
Thermoreceptors
Nocireceptors
Photoreceptors
Chemoreceptors
Osmoreceptors
What are mechanoreceptors?
mechanical pressure
touch sensations
pressure
vibration
proprioception
hearing
equilibrium
stretching of blood vessels and internal organs
What are thermoreceptors?
Changes in temperature
What are nocireceptors?
Physical or chemical damage to tissue (pain)
What are photoreceptors?
Detect light that strikes the rtina
What are chemoreceptors?
Chemical in mouth (taste), nose (smell) and body fluids
What are osmoreceptors?
Osmotic pressure of body fluids
What is adaptation?
Most sensory receptors exhibit adaptation,
which is a decrease in the generator potential
or receptor potential amplitude when exposed
to a stimulus that is applied at a constant
level over a reasonably long term.
What are the two types of speed of adaption receptors?
– Rapidly adapting receptors
– Slowly adapting receptors
What rapidly adapting receptors?
– Adapt very quickly
– Are specialized for signaling changes in a stimulus
– Examples: touch, pressure, smell
What are slowly adapting receptors?
– Adapt slowly
– Continue to trigger nerve impulses as long as the stimulus persists
– Examples: pain receptors, proprioception, chemical composition of the blood
Examples of rapidly adapting receptors
– Meissner’s corpuscles
– Pacinian corpuscles
Examples of slowly adapting receptors
– Merkel’s disks
– Ruffini endings
Where do somatic sensations arise from?
Somatic sensations arise from stimulation of sensory receptors embedded in the skin or subcutaneous layer, in mucous membranes, muscles, tendons, joints, and inner ear
What are the 4 modalities of somatic sensations?
– Tactile
– Thermal
– Pain
– Proprioceptive
What are tactile sensations?
– Touch
– Pressure
– Vibration
– Itch
– Tickle
What are touch sensations usually do to?
Stimulation of tactile receptors in the skin or
subcutaneous layer.
What is crude touch?
the ability to perceive that something has contacted the skin, but its location, shape, size, and texture can not be ascertained
What is fine touch?
provides specific information about the location that is touched, and the shape, size, and texture of the source of stimulation
What are some examples of rapidly adapting touch receptors?
– Meissner corpuscles (encapsulated0
– Hair root plexuses (free dendritic endings)
What are some examples of slowly adapting touch receptors?
– Merkel discs (free dendritic endings)
– Ruffini corpuscles (encapsulated)
What is pressure?
Pressure is a sustained sensation that is felt
over a larger area than touch
It occurs with deformation of deeper tissues
than does touch, hence some of the receptors
for pressure are deeper
What are some examples of rapidly adapting pressure receptors?
– Meissner corpuscles
– Pacinian corpuscles
What are some examples of slowly adapting pressure receptors?
– Merkel discs
What do sensations of vibration result from?
Sensations of vibration result from rapid and
repetitive sensory signals from tactile receptors
What are examples of vibration receptors?
– Corpuscles of touch (Meissner’s corpuscles),
which detect lower-frequency vibrations
– Lamellated corpuscles (Pacinian corpuscles),
which detect higher-frequency vibrations
What do itch sensations result from?
Result from stimulation of free nerve endings by certain chemicals.
What do tickle sensations arise from?
Tickle sensations are thought to arise from free
nerve endings and lamellated corpuscles.
Thermal sensations comprise …
– Coldness
– Warmth
Thermoreceptors are unspecialized free nerve
endings that respond to absolute and relative
changes of temperature
Where are cold receptors located?
Cold receptors are located in the stratum
basale of the epidermis.
At what temperatures do cold receptors activate?
Between 10º C and 40º C (50º to 105º F)
What type of fiber and myelination are cold receptors?
Cold receptors apparently are thinly myelinated A fibers.
What is paradoxical cold?
Paradoxical cold is the sensation of coolness
that occurs when the receptive field of a cold
receptor is touched by a small probe having a
higher temperature than the skin.
Where are warm receptors located?
Warm receptors are located in the dermis.
At what temperatures do warm receptors activate?
Between 32º C and 48º C (90º
to 118º F).
What type of fiber and myelination are warm receptors?
Warm receptors are thought to be unmyelinated C fibers.
What are pain receptors and where are they found?
Pain receptors are free nerve endings found
in every tissue of the body except the brain.

Termed nociceptors, they exhibit very little
adaptation.

What chemicals stimulate nociceptors?
– Kinins
– Prostaglandins
– Potassium ions
What are kinins?
Kinins are polypeptides that induce vasodilation and increased blood vessel permeability, and
serve as chemotactic agents for phagocytes. They are formed in blood from inactive precursors
called kininogens.
What are prostaglandins?
Prostaglandins are lipids that intensify the effects of histamine and kinins, and stimulate the
emigration of phagocytes through capillary walls. They are released by damaged cells.
What is persistence of pain?
Persistence of pain after the stimulus is
removed occurs because the pain-mediating
chemicals linger.
What are the two types of pain and how are they differentiatied?
– Fast
– Slow

Differentiated based on
– Types of nerve fibers that propagate the impulses (hence the speed of propagation)
– Where in the body the particular type of pain can
occur

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What is fast pain?
Perception of pain occurs very rapidly
Propagated along medium-diameter,
myelinated A fibers
Also termed acute, sharp, or pricking pain
Not felt in deeper tissues of the body
Examples:
– Needle puncture
– Knife cut
What is slow pain?
Perception of pain occurs more slowly—a
second or more after the stimulus is applied
Propagated along small-diameter,
unmyelinated C fibers
Also termed chronic, burning, aching, or
throbbing pain, it may be excruciating
Felt in both the skin, deeper tissues of the
body, and internal organs
Example: toothache
Where is fast pain localized?
Where is slow pain localized?
Fast pain is very precisely localized to the
stimulated area
Slow somatic pain is reasonably well localized, but more diffuse
What is referred pain?
Referred pain occurs when nerve fibers from
regions of high sensory input (such as the
skin) and nerve fibers from regions of
normally low sensory input (such as the
internal organs) happen to converge on the
same levels of the spinal cord.
In what location is referred pain with regards to a heart attack.
Pain of a heart attack may be felt in the skin over the heart and along the left arm because sensory fibers from the heart, the skin over the heart, and the skin along the medial aspect of the left arm enter the same spinal cord segments.
Explain referred pain with regards to nerve signals.
Nerves from damaged heart tissue convey pain signals to spinal cord levels T1-T4 on the left side, which happen to be the same levels that receive sensation from the left side of the chest and part of the left arm. The brain isn’t used to receiving such strong signals from the heart, so it
interprets them as pain in the chest and left arm.
What is analgesia?
Analgesia is relief from pain
Some analgesic drugs block formation of
prostaglandins, which stimulate nociceptors
(aspirin, ibuprofen)
What is phantom limb sensation?
– Cerebral cortex interprets impulses arising in the proximal portions of sensory neurons that previously carried impulses from the limb as still coming from the nonexistent limb
– Neurons in the brain generate sensations of body awareness, including those neurons that previously received sensory impulses from the missing limb, thereby giving rise to false sensory perceptions
What is proprioception?
Proprioceptive sensations arise in receptors
termed proprioceptors, allowing us to know
where our head and limbs are located and
how they are moving even without looking at
them
Proprioceptors adapt slowly, and then only
slightly, so they provide continuous
information to the brain
What type of information do proprioceptors in muscle and tendons provide?
– degree of muscle contraction
– amount of tension on tendons
– position of joints
– relative rates of movement of body parts
– weights of objects
What are some type of proprioceptors?
Muscle spindles

Tendon organs

Joint kinesthetic receptors

What are muscle spindles?
Muscle spindles are proprioceptors in skeletal
muscles that participate in stretch reflexes

Their function is to measure muscle length—
how much a muscle is being stretched

Are they slow or fast adapting?
They are slowly adapting
What type of stretching will stimulate muscle spindles?
Either sudden or prolonged stretching of the
muscle will stimulate the muscle spindle
What comprises a muscle spindle?
Several sensory nerve endings wrapped
around 3 to 10 specialized muscle
fibers = intrafusal muscle fibers)

intrafusal muscle fibers are enclosed by
a connective tissue capsule = muscle spindle

What are gamma neurons?
They are motor neurons that terminate at intrafusal fibers and provide impulses from the brain that regulate sensitivity of muscle spindles by regulating their contraction.
Where are intrafusal muscle fibers found?
Within the muscle spindle.
What is the rest of the muscle called?
Extrafusal muscle fibers (outside).
What are extrafusal muscle fibers innervated by?
Extrafusal muscle fibers are innervated by
large-diameter A fibers called alpha motor
neurons.
What happens during the stretch reflex with regards to extrafusal fibers?
Impulses in muscle spindle sensory axons of the muscle propagate into the CNS and activate the
alpha motor neurons of that same muscle,
causing contraction of extrafusal fibers.
What are the concentration of muscle spindles in various muscles?
– Muscles requiring finely controlled movements
have numerous muscle spindles (greater density)

– Muscles involved in coarser movements have
fewer (lesser density)

– The small skeletal muscles of the middle ear have very few muscle spindles (at one time, it was
thought they had none)

Where are tendon organs?
Tendon organs are located at the junction of a
tendon and a muscle
What do tendon organs do and what is their rate of adaptability?
They provide information about changes in
muscle force

Tendon organs protect tendons and their
associated muscles from damage due to
excessive tension

They are slowly adapting

What is a tendon organ?
Sensory nerve endings wrapped around and among bundles of collagen fibers enclosed by a
capsule of connective tissue.
Example of Muscle Spindle and Tendon Organ
Example of Muscle Spindle and Tendon Organ
Where does a nerve impulse go from a tendon organ when tension is applied?
Propagate into the CNS.
How does a tendon organ prevent damage?
Tendon reflexes decrease muscle force by
causing muscle relaxation before the muscle
can be damaged.
What is special about joint kinesthetic receptors?
The term joint kinesthetic receptor does not refer to a particular kind of receptor, but rather to a particular function, which is providing information about parameters associated with the movement (kinesis) of synovial joints.
What parameters are associated with joint kinesthetic receptors?
– Pressure
– Changes in velocity
• Acceleration
• Deceleration
– Tension
Where are joint kinesthetic receptors located?
The receptors are located in and around the articular capsule of synovial joints.
Where is pressure detected?
Pressure is detected by receptors in the
capsule
– Free nerve endings
– Type II cutaneous mechanoreceptors (Ruffini
corpuscles)
Where is velocity changes detected?
Velocity changes are detected by lamellated
(Pacinian) corpuscles in the connective tissue
outside articular capsules
Where is tension sensed?
Tension is sensed by receptors in the articular
ligaments which are similar to tendon organs