Sensory, Motor & Integration
Systems
Chapter 15
Sensation & Perception
• Sensation is the detection of stimulus of
internal or external receptors. It can be
either conscious or subconcious
• Perception is the awareness and conscious
interpretation of sensations. It is how the
brain makes sense of or assigns meaning to
the sensation.
Modalities
• A modality is a unique type or classification of
stimulus.
• We often divide them between general and special
senses.
• General Senses include:
– Somatic
•
•
•
•
•
Touch
Temperature
Pain
Pressure
Proprioception
Modalities (cont’d)
• Visceral Senses
– Sense internal conditions, e.g.:
• pH
• Osmolarity
• O2 and CO2 levels
Special Senses:
•
•
•
•
•
Vision
Hearing
Equilibrium
Taste
Olfaction
Sensation
• Sensory receptors are “tuned” or selective
to specific types of stimulus
• They are specific for a particular region of
the body or receptive field
• For a stimulus to be detected it must be
transduced
Transduction
• Transduction is the conversion of a stimulus into
an electrical event or potential
• A potential is a change in the membrane’s
electrical condition
• There are graded potentials which are localized,
variable in amplitude and fade with distance
• They can “sum” (or result in summation)
• If there is sufficient stimulus (reaching threshold,
then an action potential may be generated
• Sensory neurons carrying impulses to the PNS
are called first order neurons
Sensory Receptors
• Sensory receptors may be classified by
– Anatomical type
– Modality
– Location
Fig.15.01
Anatomical classes
Based on microscopic features
Receptive Field
• Area is monitored by a single receptor cell
• The larger the receptive field, the more difficult it
is to localize a stimulus
Figure 15–2
Classification by location
• Exteroceptors: sense stimuli from outside
the body (includes cutaneous receptors and
most special senses except equilibrium)
• Interoceptors: sense stimuli from within
(chemoreceptors, visceral stretch and
pressure and pain)
• Proprioceptors: deal with muscle & joint
position and equilibrium sense
Tonic Receptors
• Are always active
Phasic Receptors
• Are normally inactive
• Become active for a short time whenever a
change occurs
• Provide information about the intensity and
rate of change of a stimulus
Sensory adaptation
• Generator or receptor potential amplitudes
will decline over time if a stimulus remains
constant or below threshold for a given
length of time
• Some adapt rapidly, some slowly
Rapidly vs. Slowly
• Rapid
– Pressure
– Touch
– Smell
• Slow
– Proprioceptors
– pH & osmoreceptors
– Pain (really doesn’t
adapt much)
Any receptor can act as a pain receptor if the
stimulus is of adequate amplitude!
Phasic adaptation
• Response characteristic of phasic receptors
Tonic Adaptation
• Show little peripheral adaptation
• Called slow-adapting receptors
• Remind you of an injury long after the initial
damage has occurred
General Senses
by location
• Exteroceptors - Provide information about
the external environment
• Proprioceptors - Report the positions of
skeletal muscles and joints
• Interoceptors - Monitor visceral organs and
functions
Classification System
by stimulus
• Divides the general sensory receptors into
4 types by the nature of the stimulus that
excites them:
–
–
–
–
nociceptors (pain)
thermoreceptors (temperature)
mechanoreceptors (physical distortion)
chemoreceptors (chemical concentration)
Pain Receptors
• Also called nociceptors
• Are common in the:
– superficial portions of the
skin
– joint capsules
– within the periostea of bones
– around the walls of blood
vessels
• Free nerve endings with
large receptive fields
Figure 15–2
Type A and Type C Fibers
(Type B fibers are found in the ANS)
• Carry painful sensations
Myelinated Type A Fibers
• Carry sensations of fast pain, or prickling
pain, such as that caused by an injection or
a deep cut
• Sensations reach the CNS quickly and often
trigger somatic reflexes
• Relayed to the primary sensory cortex and
receive conscious attention
Type C Fibers
• Carry sensations of slow pain, or burning
and aching pain
• Cause a generalized activation of the
reticular formation and thalamus
• You become aware of the pain but only
have a general idea of the area affected
Thermoreceptors
• Also called temperature receptors
• Are free nerve endings located in:
–
–
–
–
the dermis
skeletal muscles
the liver
the hypothalamus
Thermoreceptors
• Also called temperature receptors
• Are free nerve endings located in:
–
–
–
–
the dermis
skeletal muscles
the liver
the hypothalamus
Mechanoreceptors
• Sensitive to stimuli that distort their cell
membranes
• Contain mechanically regulated ion
channels whose gates open or close in
response to:
–
–
–
–
stretching
compression
twisting
or other distortions of the membrane
3 Classes of Mechanoreceptors
• Tactile receptors:
– provide the sensations of touch, pressure, and
vibration:
• touch sensations provide information about shape or
texture
• pressure sensations indicate degree of mechanical
distortion
• vibration sensations indicate pulsing or oscillating
pressure
3 Classes of Mechanoreceptors
• Baroreceptors:
– detect pressure changes in the walls of blood
vessels and in portions of the digestive,
reproductive, and urinary tracts
• Proprioceptors:
– monitor the positions of joints and muscles
– the most structurally and functionally complex
of general sensory receptors
Proprioceptors
(not shown – Joint kinesthetic
receptors.)
Fine Touch and
Pressure Receptors
• Are extremely sensitive
• Have a relatively narrow receptive field
• Provide detailed information about a source of
stimulation, including:
–
–
–
–
–
its exact location
shape
size
texture
movement
Crude Touch and
Pressure Receptors
• Have relatively large receptive fields
• Provide poor localization
• Give little information about the stimulus
Tactile Receptors
Figure 15–3
Baroreceptors
• Monitor change in pressure
• Consist of free nerve endings that branch
within elastic tissues in wall of distensible
organ (such as a blood vessel)
• Respond immediately to a change in
pressure, but adapt rapidly
Chemoreceptors
• Respond only to water-soluble and lipidsoluble substances dissolved in surrounding
fluid
• Receptors exhibit peripheral adaptation over
period of seconds, central adaptation may
also occur
Organization of the Primary Motor and
Somatosensory cortices
• The Primary Motor Cortex is located on the
precentral gyrus of the cerebral cortex.
• The Primary Somatosensory cortex is located on
the postcentral gyrus
• The same areas of the body are represented in
both hemispheres but they are connected
contralaterally
Somatic Sensory Pathways
The pathways to the sensory areas of the
cerebral cortex can be organized according to
the following hierarchy
• First-order neurons: somatic receptors to
spinal cord or brain stem
• Second-order neurons: brain stem or spinal
cord to thalamus (decussation occurs here)
• Third-order neurons: thalamus to cortex
3 Major Somatic
Sensory Pathways
Figure 15–4
Fig. 15.05
The homunculus
Posterior
Column
Pathway
• Carries sensations of
highly localized
(“fine”) touch,
pressure, vibration,
and proprioception
Figure 15–5a
Sensory Homunculus
Figure 15–5a, b
Sensory
Homunculus
Figure 15–5c
Sensations
Bound
for Cerebral
Cortex
• Ascend within the
anterior or lateral
spinothalamic tracts:
– the anterior
spinothalamic tracts
carry crude touch and
pressure sensations
Figure 15–5b
Sensations
Bound
for Cerebral
Cortex
• The lateral
spinothalamic tracts
carry pain and
temperature
sensations
Figure 15–5c
Feeling Pain
• An individual can feel pain in uninjured part
of body when pain actually originates at
another location
Strong Visceral Pain
• Sensations arriving at segment of spinal
cord can stimulate interneurons that are part
of anterolateral pathway
• Activity in interneurons leads to stimulation
of primary sensory cortex, so an individual
feels pain in specific part of body surface:
– also called referred pain
Referred Pain
• The pain of a heart attack is frequently felt in the left arm
• The pain of appendicitis is generally felt first in the area
around the navel and then in the right lower quadrant
Figure 15–6
The Spinocerebellar
Pathway
• Cerebellum receives
proprioceptive information
about position of skeletal
muscles, tendons, and joints
Figure 15–7
Spinocerebellar Tracts
• Axons of these second-order neurons ascend in 1 of
the spinocerebellar tracts:
– the posterior spinocerebellar tracts:
• contain axons that do not cross over to the opposite side of the
spinal cord:
– axons reach cerebellar cortex via inferior cerebellar
peduncle of that side
Spinocerebellar Tracts
– the anterior spinocerebellar tracts:
• dominated by axons that have crossed over to opposite side of
spinal cord
• contain significant number of uncrossed axons as well:
– sensations reach the cerebellar cortex via superior
cerebellar peduncle
– many axons that cross over and ascend to cerebellum then
cross over again within cerebellum, synapsing on same
side as original stimulus
Spinocerebellar Tracts
Table 15–1
Main Aspects of Sensory
Perception
• Perceptual detection – detecting that a
stimulus has occurred and requires
summation
• Magnitude estimation – how much of a
stimulus is acting
• Spatial discrimination – identifying the site
or pattern of the stimulus
Main Aspects of Sensory
Perception
• Feature abstraction – used to identify a
substance that has specific texture or shape
• Quality discrimination – the ability to
identify submodalities of a sensation (e.g.,
sweet or sour tastes)
• Pattern recognition – ability to recognize
patterns in stimuli (e.g., melody, familiar
face)
Motor Commands
• Issued by the CNS
• Distributed by somatic nervous system
(SNS) and autonomatic nervous system
(ANS):
– SNS, or the somatic motor system, controls
contractions of skeletal muscles
– ANS, or the visceral motor system, controls
visceral effectors, such as smooth muscle,
cardiac muscle, and glands
Somatic Motor Pathways
• Always involve at least 2 motor neurons:
– upper motor neuron:
• cell body lies in a CNS processing center
– lower motor neuron
• cell body lies in a nucleus of the brain stem or
spinal cord
Upper Motor Neuron
• Synapses on the lower motor neuron
• Innervates a single motor unit in a skeletal
muscle:
– activity in upper motor neuron may facilitate or
inhibit lower motor neuron
Lower Motor Neuron
• Triggers a contraction in innervated muscle:
– only axon of lower motor neuron extends
outside CNS
– destruction of or damage to lower motor neuron
eliminates voluntary and reflex control over
innervated motor unit
Concious and Subconscious
Motor Commands
Figure 15–8
Concious and Subconscious
Motor Commands
• Control skeletal muscles by traveling over 3
integrated motor pathways:
– corticospinal pathway
– medial pathway
– lateral pathway
Corticospinal
Pathway
Figure 15–9
Corticospinal Pathway
• Sometimes called the pyramidal system
• Provides voluntary control over skeletal
muscles:
– system begins at pyramidal cells of primary
motor cortex
– axons of these upper motor neurons descend
into brain stem and spinal cord to synapse on
lower motor neurons that control skeletal
muscles
3 Pairs of Descending Tracts
• The corticospinal pathway contains 3 pairs
of descending tracts:
– coricobulbar tracts
– lateral corticospinal tracts
– anterior corticospinal tracts
Corticobulbar Tracts
• Provide conscious control over skeletal
muscles that move the eye, jaw, face, and
some muscles of neck and pharynx
• Innervate motor centers of medial and
lateral pathways
The Pyramids
• As they descend, corticospinal tracts are
visible along the ventral surface of medulla
oblongata as pair of thick bands, the
pyramids
Crossing Over
• At spinal segment it targets, an axon in
anterior corticospinal tract crosses over to
opposite side of spinal cord in anterior
white commissure before synapsing on
lower motor neurons in anterior gray horns
Fig. 15.05
The homunculus
Motor
Homunculus
Figure 15–9
Proportions of
Motor
Homunculus
Figure 15–5a
Somatic Motor Commands
• Several centers in cerebrum, diencephalons,
and brain stem may issue somatic motor
commands as result of processing
performed at subconscious level
Primary Functions
• These nuclei and tracts are grouped by their
primary functions:
– components of medial pathway help control
gross movements of trunk and proximal limb
muscles
– components of lateral pathway help control
distal limb muscles that perform more precise
movements
Medial Pathway
• Primarily concerned with control of muscle
tone and gross movements of neck, trunk,
and proximal limb muscles
Upper Motor Neurons
of Medial Pathway
• Are located in:
– vestibular nuclei
– superior and inferior colliculi
– reticular formation
Vestibular Nuclei
• Receive information over the
vestibulococlear nerve (VIII) from receptors
in inner ear that monitor position and
movement of the head:
– primary goal is to maintain posture and balance
– descending fibers of spinal cord constitute
vestibulospinal tracts
Superior and Inferior Colliculi
• Are located in the roof of the
mesencephalon, or the tectum:
– colliculi receive visual (superior) and auditory
(inferior) sensations
– axons of upper motor neurons in colliculi
descend in tectospinal tracts
– these axons cross to opposite side, before
descending to synapse on lower motor neurons
in brain stem or spinal cord
Lateral Pathway
• Primarily concerned with control of muscle
tone and more precise movements of distal
parts of limbs:
– axons of upper motor neurons in red nuclei
cross to opposite side of brain and descend into
spinal cord in rubrospinal tracts
Lateral Pathway
Table 15–2
Basal Nuclei and Cerebellum
• Responsible for coordination and feedback
control over muscle contractions, whether
contractions are consciously or
subconsciously directed
Basal Nuclei
• Provide background patterns of movement
involved in voluntary motor activities
Cerebellum
• Monitors:
– proprioceptive (position) sensations
– visual information from the eyes
– vestibular (balance) sensations from inner ear
as movements are under way
Sensory and Motor
Pathway Patterns
• All sensory and motor pathways involve a
series of synapses, one after the other
• General pattern:
– spinal and cranial reflexes provide rapid,
involuntary, preprogrammed responses that
preserve homeostasis over short term
Cranial and Spinal Reflexes
• Control the most basic motor activities
Integrative Centers in the Brain
• Perform more elaborate processing
• As we move from medulla oblongata to
cerebral cortex, motor patterns become
increasingly complex and variable
Primary Motor Cortex
• Most complex and variable motor activities
are directed by primary motor cortex of
cerebral hemispheres
Reticular Formation
• Loosely organized network of neurons that
extends throughout brain stem:
– axons of upper motor neurons in reticular
formation descend into reticulospinal tracts
without crossing to opposite side
The RAS
Next
Higher Brain & ANS Function