 Sensation (perception): conscious awareness of stimuli
received by sensory receptors
 Steps to Sensation:
– Sensory receptors detect the stimulus
– Send action potential by nerves to CNS
 Steps to sensation :
– Within the CNS, nerve tracts convey action potentials to
the cerebral cortex and to other areas of the CNS
– In CNS, AP are translated so the person can be aware of
the stimulus
Three ways to classify Sensory receptors:
Type of stimuli detected
 By location
 Structural Complexity
 Mechanoreceptors: respond to touch, pressure,
vibration, stretch, and itch, hearing and balance
 Chemoreceptors: respond to chemicals (e.g. smell,
taste)
 Thermoreceptors: respond to changes in
temperature
 Photoreceptors: respond to light: vision
 Nociceptors: respond to pain-causing stimuli
Exteroceptors:
 Respond to stimuli arising outside the body
 Found near the body surface
 Contains touch, pressure, pain, and temperature receptors in
skin
 And receptors of special sense organs (vision, hearing, taste,
smell)
 Visceroreceptors:
 Respond to stimuli arising within the body
 Found in viscera organs and blood vessels
 Sensitive to chemical changes, stretch, and temperature
changes
 Proprioceptors:
 Found in skeletal muscles, tendons, joints, ligaments, and
connective tissue coverings of bones and muscles
 Respond to degree of stretch of the organs they occupy
 Receptors are structurally classified as:
 Simple receptors
 Complex receptors
 Simple Receptors: Are distributed throughout the body and
associated with general senses
Divided into two groups:
-- Somatic senses (provides sensory information about the body
and environment): touch, pressure, temperature, proprioception,
pain
– Visceral senses (information about internal organs): pain and
pressure
Complex Receptors: Receptors are present in
specific organs and associated with special senses
– Special senses are smell, taste, sight, hearing, balance
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Free nerve endings
Merkel (tactile) discs
Hair follicle receptors
Pacinian corpuscles
(lamellated corpuscles)
Meissner’s corpuscles
(tactile corpuscles)
Ruffini’s end organs
Muscle spindles
Golgi tendon organs
• Simplest, most common sensory receptor
• In all body tissues (esp. epithelium)
– Detect pain, temp., and pressure
• Free nerve with disc shaped endings
• Found in deep epidermis
• Detect light touch and superficial pressure
• Hair follicle Receptors
– Respond to slight bending
of hair as occurs in light
touch
• Pacinian Corpuscles
– In hypodermis of skin,
periostea, ligaments, joint
capsules, fingers, soles of
feet, external genitalia and
nipples
– Deep pressure and
stretching
– Respond only when
pressure first applied
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Numerous in dermal papillae of hairless skin (lips,
nipples, fingertips)
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Ability to detect simultaneous stimulations at two points
on the skin (Two-point discrimination)
• Primarily in dermis of fingers
• Respond to continuous touch or pressure
• 3-10 specialized skeletal muscle cells
• Provide information about length of muscles
• Involved in stretch reflex
• Proprioceptors associated with tendons
• Respond to increased tension on tendon
• Sensory receptor generates Graded potential or Receptor
potential when interacts with stimulus
– Primary receptors: axons conduct action potentials in
response to graded potential
eg. Simple sensory receptors
• Secondary receptors: Have no axons or have short
axon like projections
• Causes release of neurotransmitters that bind to receptors on
a neuron causing a receptor potential eg. Smell, taste,
hearing, balance
• SC and brain stem contains no. of
sensory pathway
• They transmit action potentials from
periphery to various parts of brain
• Each pathway involved with specific
modality (type of information
transmitted)
• Names of ascending pathway or tracts
in CNS indicate their origin &
termination
• First half of word indicates origin,
second half indicates termination
• Located in the anterior and lateral white
columns of spinal cord
• Convey sensory information such as pain,
temp, light touch, pressure, tickle, and itch
• Consist of three Neuron system:
– Primary neuron: Relay sensory input
from periphery to posterior horn of SC
– And synapse with interneurons
– Interneurons synapse with sec. neuron
– Secondary neuron : cross to opposite
side of SC & enter spinothalamic tract,
ascend to thalamus
– Tertiary neuron: thalamus to somatic
sensory cortex
• Carries sensations of two-point
discrimination, proprioception,
pressure, vibration to cerebrum, and
cerebellum
• Primary neurons: Located in dorsal
root ganglion
• Axons enter spinal cord and ascend to
the medulla oblongata where they
synapse with secondary neurons
• Secondary neurons: axons travel
contralaterally and ascend to thalamus
• Tertiary neurons: thalamus to
somatic sensory cortex
• Fibers of Trigeminothalamic tract join the spinothalamic tract
in the brainstem
• Made up of afferent (sensory) fibers from Cranial nerve V
• Carries similar information to that of the spinothalamic and
dorsal-column/medial-lemniscal system, but from the face,
nasal cavity and oral cavity
• Carries proprioceptive information to
cerebellum
• Information concerning actual
movements is monitored
• Two spinocerebellar tracts extend
through spinal cord:
• Posterior spinocerebellar tract :
– Carries information from thoracic
and upper lumbar regions to
cerebellum
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Anterior spinocerebellar tract:
– Carries information from lower trunk
and lower limbs to cerebellum
• Sensory Areas:
– Primary somatic sensory cortex
(general sensory area): posterior to
the central sulcus, Post central gyrus
area
– General sensory input for pain,
pressure, temperature
– Taste area: located at inferior end of
post central gyrus
– Olfactory cortex: at inferior surface
of frontal lobe
– Primary auditory cortex: superior
part of temporal lobe
– Visual cortex: occipital lobe
• Association areas: Involved in process
of recognition
Somatic sensory Association Area:
– posterior to primary somatic sensory
cortex
Visual Association area:
– anterior to visual cortex:
– Where present visual
information is compared to past
visual experience
• Motor system of brain and SC: maintains posture and
balance; moves limbs, trunk, head, eyes; facial expression,
speech
• Voluntary movements: consciously activated to achieve a
specific goal, eg. walking
• Voluntary movements depend on two neurons:
– Upper motor neurons: directly or through interneurons
connect to lower motor neurons
– Lower motor neurons: Have axons that leave the CNS,
and supply to skeletal muscles
1.
Initiation of voluntary movement
begins in the premotor areas of the
cerebral cortex and results in the
stimulation of upper motor neurons
2.
The axons of the upper motor
neurons form the descending nerve
tracts. They stimulate lower motor
neurons which stimulate skeletal
muscles to contract
3.
The cerebral cortex interacts with the
basal nuclei and cerebellum in the
planning, coordination and execution
of movements
• Precentral gyrus (primary motor
cortex, primary motor area): 30%
of upper motor neurons. Another
30% in premotor area, rest in
somatic sensory cortex
• Premotor area: ant. to primary
motor cortex. Motor functions
organized here & initiation takes
place in motor cortex area
• Prefrontal area: motivation,
foresight to plan and initiate
movements takes place
• Also involved in emotional
behavior, mood
• Axons carry AP from cerebrum or
cerebellum to brain stem or SC
• Descending motor fibers are divided
into two groups:
• Direct pathways (pyramidal
system): Involved in maintenance of
muscle tone, controlling speed and
precision of skilled movements
• Indirect pathways (extrapyramidal
system): Involved in less precise
movements, such as posture
• Control muscle tone and conscious
fine, skilled movements in the face
and distal limbs
• Direct synapse of upper motor
neurons of cerebral cortex with lower
motor neurons in brainstem or spinal
cord
• Tracts
– Corticospinal: direct control of
movements below the head
– Corticobulbar: direct control of
movements in head and neck
• Axons of upper motor neurons
descend through internal
capsules and cerebral
peduncles to pyramids of
medulla oblongata
• 75-85% decussate and descend
in the lateral Corticospinal
tracts. Supply all levels of body
• Remaining fibers descend
uncrossed in anterior
Corticospinal tracts but
decussate near level of synapse
with lower neurons. Supply
neck; upper limbs
• Innervate the head
• Upper neurons enter the
cranial nerve nuclei after
forming the reticular formation
• Lower motor neurons control
eye and tongue movement,
mastication, facial expression,
palatine, pharyngeal, and
laryngeal movements
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Control conscious and unconscious muscle
movements in trunk and proximal limbs
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Synapse in some intermediate nucleus rather
than directly with lower motor neurons
• Tracts
– Rubrospinal: upper neurons synapse in
red nucleus
– Regulates fine motor control of muscles in
distal part of upper limb
– Vestibulospinal: Originate in vestibular
nuclei and synapse in SC
– influence neurons innervating extensor
muscles in trunk and proximal portion of
lower limbs; help maintain upright posture
– Reticulospinal: Present in reticular
formation of pons and medulla and
synapse in SC
– maintenance of posture
• Important in planning, organizing,
coordinating movements and posture
• Complex neural circuits between basal nuclei,
thalamus, and cerebral cortex
– Stimulatory circuit : facilitate muscle activity like
rising from a chair
– Inhibitory circuit : inhibit activity in antagonistic
muscles
• Helps maintain muscle tone in postural
muscles, helps control balance during
movement, and coordinate eye movements
• Cerebellum compares the intended movement
with the actual movement
• e.g. Touching the nose
• All ascending and descending pathways pass through
the brainstem
• Nuclei of cranial nerves II-XII located here
• Many reflexes important to survival located here: heart
rate, blood pressure, respiration, sleep, swallowing,
vomiting, coughing, and sneezing
• Reticular activating system (RAS)- controls sleep/wake
cycle
• RAS receives input from cranial nerves II (optic), V
(trigeminal), VIII (Vestibulocochlear), ascending tactile
sensory pathways and descending neurons from the
cerebral cortex
• Speech Area normally is in left cerebral cortex
• Wernicke's area: sensory speech area- Understanding written and
spoken language
• Broca's area: motor speech area - sending messages to the appropriate
muscles to actually make the sounds
• Aphasia: absent or defective speech or language comprehension. Caused
by lesion in the language area of the cortex
• Right: controls muscular activity in and receives sensory information from
left side of body
• Left: controls muscular activity in and receives sensory information from
right side of body
• Sensory information of both hemispheres shared through commissures:
corpus callosum
• Language, and possibly other functions like artistic activities, not shared
equally
– Left: mathematics and speech
– Right: three-dimensional or spatial perception, recognition of faces,
musical ability
• Electroencephalogram (EEG): record of brain’s electrical activity
Summation of all of the action potentials occurring at a particular moment
sensed by electrodes placed on the scalp
• Brain wave patterns
– Alpha: Resting state with eyes closed
– Beta: During intense mental activity
– Theta: Occur in children but also in adults experiencing frustration or brain
disorders
– Delta: Occur in deep sleep, infancy, and severe brain disorders
• There are two major types of sleep:
– Non-rapid eye movement (NREM)
– Rapid eye movement (REM)
• One passes through four stages of NREM during the first
30-45 minutes of sleep
• REM sleep occurs after the fourth NREM stage has been
achieved
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Sensory: very short-term retention of sensory input received by brain
Short-term: information retained for few seconds to minutes
Long-term: Two types
Explicit or declarative memory:
– Retention of facts, such as names, dates
– Accessed by hippocampus, part of temporal lobe (actual memory)
– and amygdaloid nucleus – emotional, such as like or dislike
• Implicit (procedural; reflexive) memory: development of skills such as
riding a bicycle, playing a piano
• Influences emotions, visceral responses to emotions, motivation,
mood, sensations of pain and pleasure
• Limbic system is associated with Basic survival instincts: acquisition
of food and water; reproduction
• Major source of sensory input in Limbic System is Olfactory nerve
• Pheromones: molecules released by one organism that have an
effect on another organism; e.g., females release pheromones that
affect menstrual cycle of other women
• Cingulate gyrus: satisfaction center
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Gradual decline in sensory and motor function
Reflexes slow
Size and weight of brain decrease
Decreased short-term memory in most people
Long-term memory unaffected or improved
• Changes in sleep patterns