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Biology 232
Human Anatomy and Physiology
Chapter 15 Lecture Outline
Sensory and Motor Pathways
sensation – conscious or subconscious awareness of internal or external stimuli
perception – conscious awareness and interpretation of sensations (occurs in
thalamus and cerebral cortex)
Basic Sensory Pathway
1) sensory receptor – specialized cell or dendrites that detect stimuli
stimulus – change in internal or external environment
specificity – most receptors are most sensitive to a particular type of
stimulus (modality)
receptive field – area in which a stimulus can be detected
varies in size for different receptors and body regions
2) transduction – stimuli produce graded potentials
receptor potentials – size depends on strength of stimulus
3) generation of an action potential
first-order neuron – conducts sensory impulse from PNS to CNS
4) integration of sensory input in CNS
spinal cord – spinal reflexes
brain stem – cranial reflexes, autonomic reflexes
lower brain regions – subconscious motor regulation
cerebral cortex – precise perception and complex integration
Sensory Adaptation – amplitude of graded potentials produced by some receptors
decreases during constant stimulation and perception of stimulus decreases
fast-adapting receptors – detect changes in stimulus (eg. touch, smell)
slow-adapting receptors – continue signaling as long as stimulus persists
(eg. pain, proprioception, visceral receptors)
(central adaptation – occurs in sensory pathways in the CNS)
SENSORY RECEPTORS
Structural Classifications of Sensory Receptors:
free nerve endings – bare dendrites (eg. pain, temperature, touch)
encapsulated nerve endings – dendrites enclosed in capsule of connective
tissue (eg. touch, pressure)
specialized cells – cells that produce receptor potentials which trigger
release of neurotransmitters at a synapse with the first-order neuron
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Locational Classifications of Sensory Receptors:
exteroceptors – at or near body surface; sensitive to external stimuli
consciously perceived (SNS)
interoceptors – within deep tissues and visceral organs; sensitive to internal
stimuli; not usually consciously perceived (ANS)
proprioceptors – in muscles, tendons, joints, inner ear; detect body position
and movements (SNS)
FUNCTIONAL CLASSIFICATIONS OF SENSORY RECEPTORS – based on the
type of stimulus that excites them
1) nociceptors – detect pain sensations
free nerve endings in most tissues (fewer in visceral organs)
stimulated mainly by chemicals released by tissue damage
fast pain (acute or sharp pain) – conduct rapidly by Type A axons
well localized sensations, mainly superficial
slow pain (aching or throbbing pain) – brief delay, then intensity increases
conducted by Type C axons
not well localized
referred pain – pain felt in another area served by the same spinal cord segment
occurs mainly with visceral pain that cannot be localized well due to
fewer receptors and less well-established pain pathways
slow-adapting receptors – pain continues until tissue damage ends
2) thermoreceptors – detect temperature
free nerve endings with in skin, muscles, hypothalamus
cold receptors – detect temperatures ranging from 50-105 degrees F
warm receptors – detect temperatures ranging from 90-118 degrees F
lower or higher temperatures mainly stimulate pain receptors
fast-adapting receptors – detect mainly changes in temperature
3) mechanoreceptors – detect distortions of cell membrane (touch, pressure, vibration)
adaptation speed varies
Meissner (tactile)corpuscles – encapsulated nerve endings in dermal papillae of
skin
hair root plexuses – free nerve endings around hair follicles
detect touches that disturb hairs
Merkel discs (tactile discs) – flattened free nerve endings that contact Merkel cells
Ruffini corpuscles – encapsulated receptors in deep dermis; detect stretching
lamellated (Pacinian) corpuscles – encapsulated receptors; widely distributed in
internal and external tissues; detect pressure sensations
proprioceptors – mechanoreceptors that monitor location of body parts
located in muscles, tendons, joint capsules, (inner ear)
slow-adapting
kinesthesia – perception of movement of body part in relation to another
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muscle spindle – nerve endings wrapped around 3-10 intrafusal muscle
fibers and enclosed in a capsule; measure muscle length;
stimulated by stretching
brain regulates sensitivity to maintain muscle tone
tendon organs – in tendon/muscle junction; nerve endings entwined with
collagen fibers and wrapped in a capsule; stimulated by tension
joint capsule receptors – variable nerve endings in joint capsules
stimulated by pressure, tension, or movement
baroreceptors – mechanoreceptors in blood vessels and organs
detect pressure in vessels or organs – stimulated by stretch of walls
mainly function in autonomic reflexes
4) chemoreceptors – detect concentrations of specific chemicals
mainly function in autonomic reflexes
special senses – smell, taste
5) photoreceptors – detect light; function in vision
SENSORY PATHWAYS TO CEREBRAL CORTEX
1) first-order neurons – from somatic receptors to CNS (spinal cord or brain stem)
cranial nerves – areas of face and mouth
spinal nerves – neck, body, posterior head
2) second-order neurons – from brain stem or spinal cord to thalamus
decussate (cross over) in medulla or spinal cord before ascending to
thalamus
all sensory information from one side of the body goes to the
cerebral cortex on the opposite side
3) third-order neurons – from thalamus to cerebral cortex
primary somatosensory area – in postcentral gyri of parietal lobes
sensory homunculus – each region of cortex receives input from specific
body area; size of cortical region depends on number of sensory
receptors in body part
visual cortex, auditory cortex, olfactory cortex, gustatory cortex
Not all sensory inputs reach the sensory cortex and are perceived.
proprioceptive inputs to cerebellum – subconscious movements
visceral (autonomic) sensations
central adaptation
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Somatic Motor Pathways – from cerebral cortex and subconscious motor centers of
lower brain to skeletal muscles
primary motor area – in precentral gyri of frontal lobes
motor homunculus – each region of cortex controls muscles of specific area; size
of cortical region depends on number of motor units in area
Motor Pathway Neurons:
lower motor neurons (LMNs) – cell bodies in brain stem or spinal cord;
axons in cranial nerves or spinal nerves; extend to skeletal muscle
upper motor neurons (UMNs) – cell bodies in primary motor cortex,
motor association areas, or subconscious motor centers of lower
brain
plan and initiate voluntary movements
regulate muscle tone, posture and balance
basal ganglia neurons – help initiate and terminate movements;
influence muscle tone; suppress unwanted movements
cerebellar neurons – monitor motor commands (UMNs)
compare with actual movements (proprioceptors, inner ear, eyes)
provide feedback to UMNs to correct movements;
coordinate movements, help maintain posture and balance
direct motor pathways – UMNs from cortex synapse directly with LMNs; 90%
decussate in medulla, other 10% in spinal cord
control voluntary body movements
motor impulses from cortex control skeletal muscles on opposite side of
body
indirect motor pathways – UMNs from brain form complex pathways with
input from motor cortex, basal ganglia, thalamus, cerebellum, reticular
formation and nuclei in brain stem
control subconscious movements
responsible for muscle tone, posture and balance, coordination
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