Transcript Peripheral NS: Sensory processing & receptors

Peripheral Nervous System
& Reflex Activity
Part A
Prepared by Janice Meeking & W. Rose.
Figures from Marieb & Hoehn 8th , 9th ed.
Portions copyright Pearson Education
Peripheral Nervous System (PNS)
All neural structures outside the brain
• Sensory receptors
• Peripheral nerves and associated ganglia
• Motor endings
Central nervous system (CNS)
Peripheral nervous system (PNS)
Sensory (afferent)
division
Motor (efferent) division
Somatic nervous
system
Autonomic nervous
system (ANS)
Sympathetic
division
Parasympathetic
division
Figure 13.1
Sensory Receptors
Specialized structures which respond to changes in
their environment (stimuli)
Some receptors are simply ends of sensory nerve
fibers. Other receptors are cells adjacent to sensory
nerrve fibers. Other receptors are sensory nerve
fiber endings plus specialized supporting cells and/or
extracellular material
Receptor activation results in graded potentials that
may trigger action potentials
Sensation (awareness of stimulus) and perception
(interpretation of the meaning of the stimulus) occur
in the brain
Classifying receptors
Receptors have been classified according to:
– The type of stimulus they detect (best)
– Receptor location in the body
– Structural complexity of the receptor
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)
Classification by Location
Exteroceptors
Respond to stimuli arising outside the body: receptors in the skin
for touch, pressure, pain, and temperature; also most special sense
organs (eyes, ears, etc)
Interoceptors (visceroceptors)
Respond to stimuli arising in internal viscera and blood vessels:
chemical environment, tissue stretch, temperature
Proprioceptors
Respond to stretch in skeletal muscles, tendons, joints, ligaments,
and connective tissue coverings of bones and muscles; inform the
brain of one’s movements
Classification by Structural Complexity
Complex receptors: Special sense organs
Vision, hearing, equilibrium, smell, taste (ch. 15)
Simple receptors: General sensation
Tactile sensations (touch, pressure, stretch, vibration),
temperature, pain, and muscle sense
Unencapsulated (free) dendritic endings
Encapsulated dendritic endings
Unencapsulated (free) dendritic endings
Table 13.1
Encapsulated dendritic endings
Table 13.1
From Sensation to Perception
• Survival depends upon sensation and
perception
• Sensation: awareness of changes in the
internal and external environment
• Perception: conscious interpretation of
those stimuli
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
Figure 13.2
Processing at the Receptor Level
• Different receptors respond to different stimuli
Light, pressure, temperature, chemicals, etc.
• Receptive field
Physical region over which each receptor can detect stimuli
• Transduction occurs
Stimulus energy is converted into a graded potential called
a receptor potential
Receptor processing in general sense receptors
(receptors in which there are not separate receptor cells)
Stimulus

Receptor Potential (graded potential)
at distal end of afferent neuron

Action Potential(s)
(if threshold is reached)
Receptor processing in special sense receptors
stimulus

receptor potential in receptor cell

release of neurotransmitter

generator potential in first-order sensory neuron

action potentials (if threshold is reached)
Adaptation of Sensory Receptors
Change in response in the presence of a constant
stimulus
• Receptor membranes become less responsive
• Receptor potentials decline in frequency or stop
Phasic (fast-adapting) receptors signal beginning or
end of stimulus
• Examples: receptors for pressure, touch, smell
Tonic receptors adapt slowly or not at all
• Examples: nociceptors; most proprioceptors
Processing at the Circuit Level
• 3-neuron pathway conducts sensory impulses
upward to appropriate brain regions
• First-order neurons (cell body in DRG or other
peripheral ganglion): conduct impulses from
receptor to second-order neurons in the CNS
• Second-order neurons (in CNS): transmit
impulses to thalamus or cerebellum
• Third-order (thalamic) neurons: conduct
impulses from the thalamus to the
somatosensory cortex (perceptual level)
Processing at the Perceptual Level
• Identification of the sensation depends on the
specific location of the target neurons in the
sensory cortex
• Aspects of sensory perception
• Stimulus detection: requires multiple impulses
• Magnitude estimation: intensity coded by frequency
of APs & number of neurons active
• “Spatial” discrimination: identify the site or pattern
of stimulus (e.g. two-point discrimination test)
Further Processing at the Perceptual Level
• Feature abstraction—identification of more
complex aspects and several stimulus properties
• Quality discrimination: identification of
submodalities of a sensation (e.g., sweet or sour
tastes)
• Pattern recognition: identification of familiar or
significant patterns in stimuli (face, melody, etc.)
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
Figure 13.2
Nerve Structure
• Bundle of myelinated and unmyelinated
peripheral axons enclosed by connective tissue
• Connective tissue coverings, from inside to
outside:
– Endoneurium, perineurium, epineurium
Endoneurium
Axon
Myelin sheath
Perineurium
Epineurium
Fascicle
Blood
vessels
(b)
Figure 13.3b
Classification of Nerves
• Most nerves are mixed: afferent and efferent
fibers and somatic and autonomic (visceral)
fibers
• Pure sensory (afferent) or motor (efferent)
nerves are rare
• Cranial and spinal nerves (12 pair cranial,
Roman numerals; 31 pair spinal, named for
the nearby vertebra, e.g. C5 or L4)
Ganglion (plural: ganglia)
• A group of neuron cell bodies outside the
CNS (analogous to nuclei inside the CNS)
• associated with nerves
Examples
• dorsal root ganglia (sensory, somatic; ch. 12)
• autonomic ganglia, such as sympathetic
trunk ganglia (motor, visceral; ch.14)