Transcript Sensory Receptors

Marieb Chapter 13 Part A PNS
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Peripheral Nervous System (PNS)
• All neural structures outside the brain
• Sensory receptors
• Peripheral nerves and associated ganglia
• Motor endings
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Central nervous system (CNS)
Peripheral nervous system (PNS)
Sensory (afferent)
division
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Motor (efferent) division
Somatic nervous
system
Autonomic nervous
system (ANS)
Sympathetic
division
Parasympathetic
division
Figure 13.1
Sensory Receptors
• Specialized to respond to changes in their
environment (stimuli)
• Activation results in graded potentials that
trigger action potentials
• Sensation (awareness of stimulus) and
perception (interpretation of the meaning of
the stimulus) occur in the brain
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Classification of Sensory Receptors
• Based on:
• Stimulus type
• Location
• Structural complexity
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Classification by Structural Complexity
1. Complex receptors (special senses)
•
Vision, hearing, equilibrium, smell, and taste
(Chapter 15; we won’t cover these)
•
More than one type of cell that work together
2. Simple receptors for general senses:
•
Tactile sensations (touch, pressure, stretch,
vibration), temperature, pain, and muscle
sensation
•
Unencapsulated (free) or encapsulated
dendrites as sensors
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Unencapsulated Dendrites
• Thermoreceptors
• Cold receptors (10–40ºC);more numerous,
in superficial dermis
• Heat receptors (32–48ºC);
in deeper dermis
•
Also located in muscles, liver,
hypothalamus, etc.
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Unencapsulated Dendritic Endings
• Nociceptors (PAIN
receptors!)
• Respond to:
• Pinching/mechanical force
• Chemicals from damaged tissue
(inflammation chemicals)
• Temperatures outside the range of
thermoreceptors (extremes)
• Other chemicals [Capsaicin (hot pepper extract)]
• Located in skin, periosteum, joint capsules, tendons,
meninges, blood vessel walls, etc.
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Unencapsulated Dendrites
• Light touch receptors
• Tactile (Merkel) discs
• Hair follicle receptors
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Table 13.1
Encapsulated Dendrites
• All are mechanoreceptors
•
Meissner’s (tactile) corpuscles— touch
•
Pacinian (lamellated) corpuscles—deep
pressure and vibration
•
Ruffini endings—deep continuous pressure
•
Muscle spindles—muscle stretch
•
Golgi tendon organs—stretch in tendons
•
Joint kinesthetic receptors—stretch in
articular capsules (a proprioceptor)
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Table 13.1
Classification by Stimulus Type
• Mechanoreceptors—respond to touch, pressure,
vibration, stretch, and itch
• Thermoreceptors—sensitive to changes in temperature
• Photoreceptors—respond to light energy (e.g., retina)
• Chemoreceptors—respond to chemicals (e.g., smell,
taste, changes in blood chemistry)
• Nociceptors—sensitive to pain-causing stimuli (e.g.
extreme heat or cold, excessive
pressure, inflammatory chemicals)
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Classification by Location
1. Exteroceptors
•
Respond to stimuli arising outside the body
•
Receptors in the skin for touch, pressure,
pain, and temperature
•
Most special sense organs in this class
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Classification by Location
2. Interoceptors (visceroceptors)
•
Respond to stimuli arising in internal viscera
and blood vessels
•
Sensitive to chemical changes, tissue
stretch, and temperature changes
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Classification by Location
3. Proprioceptors
•
Respond to stretch in skeletal muscles,
tendons, joints, ligaments, and connective
tissue coverings of bones and muscles
•
Inform the cerebellum and cortex of our
position in space
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From Sensation to Perception
• Survival depends upon sensation and
perception
• Sensation: the awareness of changes in the
internal and external environment
• Perception: the conscious interpretation of
those stimuli
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Sensory Integration
• Input comes from exteroceptors,
proprioceptors, and interoceptors
• Input is relayed toward the head, but is
processed along the way
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Sensory Integration
•
The signal can be processed and altered at
three different levels:
1. Receptor level—the sensor receptors
2. Circuit level—ascending pathways
3. Perceptual level—neuronal circuits in the
cerebral cortex
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Perceptual level (processing in
cortical sensory centers)
3
Motor
cortex
Somatosensory
cortex
Thalamus
Reticular
formation
Pons
2 Circuit level
(processing in
Spinal
ascending pathways) cord
Cerebellum
Medulla
Free nerve
endings (pain,
cold, warmth)
Muscle
spindle
Receptor level
(sensory reception Joint
and transmission
kinesthetic
to CNS)
receptor
1
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Figure 13.2
Processing at the Receptor Level
• Stimulus energy is converted into a graded potential
called a receptor potential (don’t pay attention to the
term generator potential- only used with special
senses)
• In general sense receptors, it works like this:
stimulus

receptor potential in afferent neuron

action potential at first node of Ranvier
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Processing at the Receptor Level
• In special sense organs:
stimulus

receptor potential in receptor cell

release of neurotransmitter

generator potential in first-order sensory neuron

action potentials (if threshold is reached)
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Adaptation of Sensory Receptors
• Adaptation is a change in sensitivity in the
presence of a constant stimulus
• Receptor membranes become less responsive
• So the receptor potentials decline in frequency
or stop
• Why does this happen? Is it a good thing?
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Adaptation of Sensory Receptors
• Phasic (fast-adapting) receptors adapt
• Examples: receptors for pressure, touch, and
smell
• Tonic receptors adapt very slowly or not at all
• Examples: nociceptors and most
proprioceptors
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Adaptation - What Happens to Signaling?
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Processing at the Circuit Level
• Ascending pathways of three neurons conduct
sensory impulses to the appropriate brain regions
• First-order neurons
• Conduct impulses from the receptor level to the
second-order neurons in the CNS
• Second-order neurons
• Transmit impulses to the thalamus or cerebellum
• Third-order neurons
• Conduct impulses from the thalamus to the
somatosensory cortex (perceptual level)
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Perceptual level (processing in
cortical sensory centers)
3
Motor
cortex
Somatosensory
cortex
Thalamus
Reticular
formation
Pons
2 Circuit level
(processing in
Spinal
ascending pathways) cord
Cerebellum
Medulla
Free nerve
endings (pain,
cold, warmth)
Muscle
spindle
Receptor level
(sensory reception Joint
and transmission
kinesthetic
to CNS)
receptor
1
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Figure 13.2
Perception of Pain
• Definition: an unpleasant sensory and emotional
experience associated with actual or potential tissue
damage
• Warns that you are “at the edge of a cliff!”
• Stimuli include:
•
extreme pressure
•
extreme temperature
•
histamine, K+, ATP, acids, and bradykinin
(chemicals released during inflammation)
• Some pain impulses are blocked by inhibitory
endogenous opioids
• Is pain necessary?
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Referred Pain
• Visceral pain afferent fibers travel along the
same pathway as somatic pain fibers
• Referred Pain = pain stimuli arising in the
viscera are perceived as somatic in origin
• Examples:
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Referred Pain
Heart
Lungs and
diaphragm
Liver
Gallbladder
Appendix
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Heart
Liver
Stomach
Pancreas
Small intestine
Ovaries
Colon
Kidneys
Urinary
bladder
Ureters
Pain
• Does everyone have the same pain threshold?
• Does everyone have the same pain tolerance?
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Pain
• Pain tolerance is influenced by many factors:
•
•
•
•
•
•
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How Is Pain Processed?
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Analgesia
• Defined as “
“
• Major Analgesics
•
•
• Other agents that can act as pain relievers
•
•
•
•
•
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•
Classification of Nerves
• Peripheral nerves classified as cranial or spinal nerves
• Most nerves are mixtures of afferent and efferent
fibers and somatic and autonomic (visceral) fibers
(carry sensory + motor = mixed nerves)
• Pure sensory (afferent) or motor (efferent) nerves are
rare (which cranial nerves are purely sensory nerves?)
• Types of fibers in mixed nerves:
• Somatic afferent and somatic efferent
• Visceral afferent and visceral efferent
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Regeneration of Nerve Fibers
• Mature neurons can’t divide
• If the soma of a damaged nerve is intact, its axon will
regenerate
• Involves coordinated activity among
• Macrophages
• Schwann cells
• Axons
• CNS oligodendrocytes bear growth-inhibiting proteins
that prevent CNS fiber regeneration (UGH!)
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