Biology 230| BIOL 230 - Human Physiology Midterm 2 COMPLETE NEW VERSION
UPDATE 2026 GUIDE EVERYTHING YOU NEED TO KNOW Athabasca University
Midterm 2
Chapter 9 - Sensory Systems
9.1 Overview of Sensation
● Sensation - the conscious or subconscious awareness of changes in the external or
internal environment
○ The process of sensation involves 4 events
■ Stimulation of the sensory receptor
● Stimulus - a change in the external or internal environment that
can activate a sensory neuron
■ Transduction of the stimulus - the sensory receptor converts the energy in
the stimulus into a graded potential
■ Generation of action potential - If the graded potential reaches threshold,
it triggers one or more action potentials which propagate into the CNS
■ Integration of sensory input - Processing of information. Is either modified,
allowed to continue or is filtered out. Integration occurs in the cerebral
cortex
○ Adequate stimulus - the type of stimulus to which a sensory receptor responds
best
○ Types of sensory receptors
■ Mechanoreceptors - sensitive to mechanical stimuli such as deformation,
stretching or bending of cells. Includes touch, pressure, vibration,
proprioception, hearing and equilibrium
■ Thermoreceptors - detect changes in temperature
■ Photoreceptors - detect light that strikes the retina of the eye
■ Chemoreceptors - detect chemicals in the mouth, nose and body fluids
■ Nociceptors - response to painful stimuli
● Perception - the conscious awareness and interpretations of sensations. Primarily a
function of the cerebral cortex
● Peripheral nerve endings - Can also serve as sensory receptors. Two types
○ Encapsulated nerve endings - include receptors for pressure, vibration, and
some touch. Encapsulation (with CT) enhances sensitivity or specificity of the
receptor
○ Free nerve endings
● Special cells - receptors for most of the special senses (ex: gustatory receptors,
photoreceptors, hair cells)
○ Note: olfactory receptors are not special cells
● Receptor potential - the graded potential that forms in a sensory receptor
● Receptive field - Is the stimulated physical area, specific group of chemicals or particular
set of sound frequencies that causes a response in that neuron
● Sensory coding - the use of organizational and functional features of the nervous system
to represent specific details about a stimulus. Sensory systems encode 4 attributes of a
stimulus
,○ Modality - each unique type of sensation
■ Labeled lines - the neural pathways that convey information about
modality from peripheral receptors to specific regions of the cerebral
cortex
■ Labeling line coding - the association of a modality with the activation of a
particular labeled line
○ Location - encoded by the location of the activated receptive field
■ Topographic pattern - orderly arrangement in which the relationship
between adjacent sensory receptors is maintained as information
processed in the CNS
■ Acuity - sharpness of perception (ability to precisely locate and distinguish
one stimulus from another). Two major factors affect acuity
● Size of the receptive field - varies inversely with the number of
sensory receptors that it contains (smaller field - greater number
of receptors = greater acuity)
○ Tactile acuity - sharpness of touch of perception. Has two
point discrimination
■ Ability to perceive two points applied to the skin as
two separate points
■ Two point discrimination threshold - the minimum
distance at which the two caliper points are
perceived as two separate points of touch ( a
reflection of the size of receptor fields)
● Lateral inhibition - input from the sensory receptors along the
border of a stimulus is substantially inhibited compared to input
from the sensory receptors at the center of the stimulus
○ Intensity - is encoded by two main factors
■ Frequency of action potentials generated in response to the stimulus
(more action potentials, greater intensity)
● Frequency coding - the use of action potential frequency to
determine stimulus intensity
■ The number of sensory receptors activated by that stimulus (more
receptors, greater intensity)
○ Duration - encoded by the duration of action potentials in the sensory neuron
(longer stimulus lasts = the longer sensory neurons produce action potentials)
■ Adaptation - a decrease in the response of a sensory receptor during a
maintained, constant, stimulus. (ie. receptor potential diminished while the
prolonged stimulus is present)
■ Slowly adapting receptors (aka tonic receptors) - adapt slowly and
continue to produce a significant response as long as the stimulus
persists (ex: baroreceptors)
■ Rapidly adapting receptors (aka phasic receptors) - Adapt very quickly.
Respond when stimulus is first applied, then cease to respond while the
stimulus is maintained
, ● A sensory pathway - a group of parallel chains of neurons that convey sensory
information from sensory receptors in the periphery to the cerebral cortex
○ First order neurons - sensory neurons, convey info into the CNS from sensory
receptors in the body. Synapse with axon of second order neuron.
○ Second order neurons - Usually located in the brainstem or spinal cord. Synapse
with both first and third order neurons.
○ Third order neurons - Located in thalamus (synapse with second and fourth)
○ Fourth order neurons - in a primary sensory area of the cerebral cortex, where
perception of the sensation occurs.
■ Higher order neurons - primary sensory area, where complex integration
takes place in association areas of the cerebral cortex
● Decussate - sensory pathways cross over to the opposite side as they course through
the spinal cord or brain stem
○ Sensory information from one side of the body is perceived by a specific region
of the cerebral cortex on the opposite side of the brain.
○ Some sensory pathways do not decussate, and their sensory info is perceived on
the same side of the brain.
9.2 The Somatic Sensory System
● Somatic sensation - arises from stimulation of sensory receptors embedded in the skin,
skeletal muscle, tendons and joints. Receptors are distributed unevenly throughout the
body. There are 4 modalities of somatic sensation
○ Tactile sensation - touch, pressure, vibration, itch and tickle. Mechanoreceptors.
■ Touch - stimulation of tactile receptors in the skin. Four types of touch
receptors
● Meissner corpuscles - located in the upper part of the dermis. Egg
shaped mass of nerve endings surrounded by CT. Rapidly
adapting receptors
● Hair root plexus - rapidly adapting touch receptors. Found in hairy
skin. Free nerve endings in hair follicles.
● Merkel discs (aka type 1 cutaneous mechanoreceptors) - saucer
shaped free nerve endings located in dermis and epidermis.
Respond to extended/continuous touch. Slowly adapting touch
receptor.
● Ruffini corpuscles (aka type 2 cutaneous mechanoreceptors) -
slowly adapting touch receptors. Elongated and encapsulated
receptors located in dermis and subcutaneous later. Highly
sensitive to stretching.
■ Pressure - a sustained sensation that occurs with deeper deformation of
the skin and subcutaneous layer.
● Receptors are Merkel discs and ruffini corpuscles
■ Vibration - sensation from rapid, repetitive sensory signals from tactile
receptors.
, ● Pacinian corpuscle (aka lamellated corpuscle) - nerve endings
surrounded by multilayered connective tissue. Looks like a sliced
onion. Rapidly adapting receptor. High frequency vibrations.
○ Transduction in tactile response research
○ The capsule of the pacinian corpuscle plays an important
role in how the corpuscle adapts (deformed when stimulus
is present, then rebounds when stimulus is gone)
● Meissner corpuscles also respond to low frequency vibrations
■ Itch - stimulation of free nerve endings in the skin by certain chemicals
(inflammatory response)
● Pruritogenic (aka itching) response
■ Tickle - stimulation of free nerve endings
○ Thermal sensation - coldness and warmth detected by thermoreceptors. Two
types of thermoreceptors (both rapidly adapting receptors)
■ Cold (temps between 10-35C)
■ Warm - less abundant than cold (30-45C)
■ Note: receptors outside these ranges are nociceptors
○ Nociceptors - receptors for pain.
■ Three types
● Mechanical receptors - respond to intense stimuli (ex: pinch or
puncture)
● Thermal nociceptors
● Polymodal nociceptors - respond to a variety of stimuli including
intense mechanical stimuli, extreme thermal and chemicals
released from damaged tissues
■ Hyperalgesia - an increased sensitivity to painful stimuli
■ Transduction in nociceptors
● Involves ions that are present in the membrane of the nociceptor
■ Types of pain
● Fast pain - sharp, prickling sensation. Signals are transmitted
along the A-delta fibers (large myelinated axons)
● Slow pain - dull, aching. Signals are transmitted along C fibers
(small, unmyelinated)
■ Pain pathways - two types
● Spinal reflex pathways - unconscious protective responses
● Ascending pathways to the brain - higher ordered
○ Involves glutamate and neuropeptide substance P as
neurotransmitters
■ Somatic pain - arises from stimulation of nociceptors in skin, skeletal
muscle and joints
■ Visceral pain - stimulation of nociceptors in internal organs
● Referred pain - pain felt at a site other than the place of origin
(converging of the second order neuron of the same ascending
pathway to the brain)
UPDATE 2026 GUIDE EVERYTHING YOU NEED TO KNOW Athabasca University
Midterm 2
Chapter 9 - Sensory Systems
9.1 Overview of Sensation
● Sensation - the conscious or subconscious awareness of changes in the external or
internal environment
○ The process of sensation involves 4 events
■ Stimulation of the sensory receptor
● Stimulus - a change in the external or internal environment that
can activate a sensory neuron
■ Transduction of the stimulus - the sensory receptor converts the energy in
the stimulus into a graded potential
■ Generation of action potential - If the graded potential reaches threshold,
it triggers one or more action potentials which propagate into the CNS
■ Integration of sensory input - Processing of information. Is either modified,
allowed to continue or is filtered out. Integration occurs in the cerebral
cortex
○ Adequate stimulus - the type of stimulus to which a sensory receptor responds
best
○ Types of sensory receptors
■ Mechanoreceptors - sensitive to mechanical stimuli such as deformation,
stretching or bending of cells. Includes touch, pressure, vibration,
proprioception, hearing and equilibrium
■ Thermoreceptors - detect changes in temperature
■ Photoreceptors - detect light that strikes the retina of the eye
■ Chemoreceptors - detect chemicals in the mouth, nose and body fluids
■ Nociceptors - response to painful stimuli
● Perception - the conscious awareness and interpretations of sensations. Primarily a
function of the cerebral cortex
● Peripheral nerve endings - Can also serve as sensory receptors. Two types
○ Encapsulated nerve endings - include receptors for pressure, vibration, and
some touch. Encapsulation (with CT) enhances sensitivity or specificity of the
receptor
○ Free nerve endings
● Special cells - receptors for most of the special senses (ex: gustatory receptors,
photoreceptors, hair cells)
○ Note: olfactory receptors are not special cells
● Receptor potential - the graded potential that forms in a sensory receptor
● Receptive field - Is the stimulated physical area, specific group of chemicals or particular
set of sound frequencies that causes a response in that neuron
● Sensory coding - the use of organizational and functional features of the nervous system
to represent specific details about a stimulus. Sensory systems encode 4 attributes of a
stimulus
,○ Modality - each unique type of sensation
■ Labeled lines - the neural pathways that convey information about
modality from peripheral receptors to specific regions of the cerebral
cortex
■ Labeling line coding - the association of a modality with the activation of a
particular labeled line
○ Location - encoded by the location of the activated receptive field
■ Topographic pattern - orderly arrangement in which the relationship
between adjacent sensory receptors is maintained as information
processed in the CNS
■ Acuity - sharpness of perception (ability to precisely locate and distinguish
one stimulus from another). Two major factors affect acuity
● Size of the receptive field - varies inversely with the number of
sensory receptors that it contains (smaller field - greater number
of receptors = greater acuity)
○ Tactile acuity - sharpness of touch of perception. Has two
point discrimination
■ Ability to perceive two points applied to the skin as
two separate points
■ Two point discrimination threshold - the minimum
distance at which the two caliper points are
perceived as two separate points of touch ( a
reflection of the size of receptor fields)
● Lateral inhibition - input from the sensory receptors along the
border of a stimulus is substantially inhibited compared to input
from the sensory receptors at the center of the stimulus
○ Intensity - is encoded by two main factors
■ Frequency of action potentials generated in response to the stimulus
(more action potentials, greater intensity)
● Frequency coding - the use of action potential frequency to
determine stimulus intensity
■ The number of sensory receptors activated by that stimulus (more
receptors, greater intensity)
○ Duration - encoded by the duration of action potentials in the sensory neuron
(longer stimulus lasts = the longer sensory neurons produce action potentials)
■ Adaptation - a decrease in the response of a sensory receptor during a
maintained, constant, stimulus. (ie. receptor potential diminished while the
prolonged stimulus is present)
■ Slowly adapting receptors (aka tonic receptors) - adapt slowly and
continue to produce a significant response as long as the stimulus
persists (ex: baroreceptors)
■ Rapidly adapting receptors (aka phasic receptors) - Adapt very quickly.
Respond when stimulus is first applied, then cease to respond while the
stimulus is maintained
, ● A sensory pathway - a group of parallel chains of neurons that convey sensory
information from sensory receptors in the periphery to the cerebral cortex
○ First order neurons - sensory neurons, convey info into the CNS from sensory
receptors in the body. Synapse with axon of second order neuron.
○ Second order neurons - Usually located in the brainstem or spinal cord. Synapse
with both first and third order neurons.
○ Third order neurons - Located in thalamus (synapse with second and fourth)
○ Fourth order neurons - in a primary sensory area of the cerebral cortex, where
perception of the sensation occurs.
■ Higher order neurons - primary sensory area, where complex integration
takes place in association areas of the cerebral cortex
● Decussate - sensory pathways cross over to the opposite side as they course through
the spinal cord or brain stem
○ Sensory information from one side of the body is perceived by a specific region
of the cerebral cortex on the opposite side of the brain.
○ Some sensory pathways do not decussate, and their sensory info is perceived on
the same side of the brain.
9.2 The Somatic Sensory System
● Somatic sensation - arises from stimulation of sensory receptors embedded in the skin,
skeletal muscle, tendons and joints. Receptors are distributed unevenly throughout the
body. There are 4 modalities of somatic sensation
○ Tactile sensation - touch, pressure, vibration, itch and tickle. Mechanoreceptors.
■ Touch - stimulation of tactile receptors in the skin. Four types of touch
receptors
● Meissner corpuscles - located in the upper part of the dermis. Egg
shaped mass of nerve endings surrounded by CT. Rapidly
adapting receptors
● Hair root plexus - rapidly adapting touch receptors. Found in hairy
skin. Free nerve endings in hair follicles.
● Merkel discs (aka type 1 cutaneous mechanoreceptors) - saucer
shaped free nerve endings located in dermis and epidermis.
Respond to extended/continuous touch. Slowly adapting touch
receptor.
● Ruffini corpuscles (aka type 2 cutaneous mechanoreceptors) -
slowly adapting touch receptors. Elongated and encapsulated
receptors located in dermis and subcutaneous later. Highly
sensitive to stretching.
■ Pressure - a sustained sensation that occurs with deeper deformation of
the skin and subcutaneous layer.
● Receptors are Merkel discs and ruffini corpuscles
■ Vibration - sensation from rapid, repetitive sensory signals from tactile
receptors.
, ● Pacinian corpuscle (aka lamellated corpuscle) - nerve endings
surrounded by multilayered connective tissue. Looks like a sliced
onion. Rapidly adapting receptor. High frequency vibrations.
○ Transduction in tactile response research
○ The capsule of the pacinian corpuscle plays an important
role in how the corpuscle adapts (deformed when stimulus
is present, then rebounds when stimulus is gone)
● Meissner corpuscles also respond to low frequency vibrations
■ Itch - stimulation of free nerve endings in the skin by certain chemicals
(inflammatory response)
● Pruritogenic (aka itching) response
■ Tickle - stimulation of free nerve endings
○ Thermal sensation - coldness and warmth detected by thermoreceptors. Two
types of thermoreceptors (both rapidly adapting receptors)
■ Cold (temps between 10-35C)
■ Warm - less abundant than cold (30-45C)
■ Note: receptors outside these ranges are nociceptors
○ Nociceptors - receptors for pain.
■ Three types
● Mechanical receptors - respond to intense stimuli (ex: pinch or
puncture)
● Thermal nociceptors
● Polymodal nociceptors - respond to a variety of stimuli including
intense mechanical stimuli, extreme thermal and chemicals
released from damaged tissues
■ Hyperalgesia - an increased sensitivity to painful stimuli
■ Transduction in nociceptors
● Involves ions that are present in the membrane of the nociceptor
■ Types of pain
● Fast pain - sharp, prickling sensation. Signals are transmitted
along the A-delta fibers (large myelinated axons)
● Slow pain - dull, aching. Signals are transmitted along C fibers
(small, unmyelinated)
■ Pain pathways - two types
● Spinal reflex pathways - unconscious protective responses
● Ascending pathways to the brain - higher ordered
○ Involves glutamate and neuropeptide substance P as
neurotransmitters
■ Somatic pain - arises from stimulation of nociceptors in skin, skeletal
muscle and joints
■ Visceral pain - stimulation of nociceptors in internal organs
● Referred pain - pain felt at a site other than the place of origin
(converging of the second order neuron of the same ascending
pathway to the brain)
