Download Afferent (Sensory) Division Part 1

PNS – Afferent Division
Sensory Physiology
Part I
Peripheral Nervous System
• PNS – all neural structures outside the
brain and spinal cord
• Includes sensory receptors, peripheral
nerves, associated ganglia, and motor
endings
• Provides links to and from the external
environment
Organization of the Nervous System
Figure 8-1: Organization of the nervous system
Properties of Sensory Systems
• Stimulus
– Internal
– External
– Energy source
• Receptors - Afferent pathway
– Sense organs
– Transducer
• CNS integration
From Sensation to Perception
• Survival depends upon sensation and
perception
• Sensation is the awareness of changes
in the internal and external
environment
• Perception is the conscious
interpretation of those stimuli
Sensory Receptors: Transducers
• Transduction - stimulus energy converted
into information processed by CNS
• Sensory receptors are structures specialized
to respond to stimuli, activation results in
– Ion channels or second messengers that initiate
membrane potential change is sensory receptors
– Depolarizations trigger impulses to the CNS
• The realization of these stimuli, sensation
and perception, occur in the brain
Sensory Receptor Types
Receptor Classification
• 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
• Osmoreceptors – detect changes in concentration of
solutes, osmotic activity
Receptor
• The receptor must have specificity for
the stimulus energy
• The receptor’s receptive field must be
stimulated
• Stimulus energy must be converted
into a graded potential
• A generator potential in the associated
sensory neuron must reach threshold
Conversion of Receptor and Generator
Potentials into Action Potentials
•
Generator potentials
–
–
–
–
•
Occur in specialized nerve endings
Stimulus opens ion channels in receptor causing local current flow
Local current flow opens ion channels in afferent neuron AP generating region
If threshold reached, AP is generated
Receptor potentials
–
–
–
–
Occur in separate receptor cells
Stimulus opens ion channels in receptor causing graded membrane potential
Receptor cell releases chemical messenger
Chemical messenger opens ion channels in afferent neuron AP generating
region
– If threshold reached, AP is generated
Receptor Potential
Generator Potential
Sensory Pathways
• Stimulus as physical energy  sensory
receptor
– Receptor acts as a transducer
• Intracellular signal  usually change in
membrane potential
• Stimulus > threshold  action potential
to CNS
• Integration in CNS  cerebral cortex or
acted on subconsciously
Sensory Pathways – External Stimuli
•
•
•
•
•
•
Vision
Hearing
Taste
Smell
Equilibrium
Somatic Senses
Somatic Senses – Internal Stimuli
•
•
•
•
Touch
Temperature
Pain
Proprioception
Figure 10-10: The somatosensory cortex
Somatic Pathways
•
•
•
First-order neurons –
soma reside in dorsal
root or cranial ganglia,
and conduct impulses
from the skin to the
spinal cord or brain
stem
Second-order neurons –
soma reside in the
dorsal horn of the
spinal cord or medullary
nuclei and transmit
impulses to the
thalamus or cerebellum
Third-order neurons –
located in the thalamus
and conduct impulses
to the somatosensory
cortex of the cerebrum
Figure 10-9: Sensory pathways cross the body’s midline
Sensory Coding
• Modality – type of stimulus
• Location
– Coded by site of the stimulated receptor
– Precision of location called acuity,
• Receptive field
• Lateral inhibition
• Intensity
– Increased stimulus results in a larger
receptor potential leading to a higher
frequency of action potential
– Stronger stimuli also affect a larger area and
recruit a larger number of receptors
• Duration - Adaptation
– Tonic receptors
– Phasic receptors
Receptive Fields of Sensory Neurons
Figure 10-2
Receptive Field: Two-point discrimination
Lateral Inhibition
Figure 10-6: Lateral inhibition
Sensory Coding: Stimulus Intensity & Duration
•
•
•
Intensity - coded by number of receptors activated and frequency of
action potentials
Duration - coded by duration of action potentials
Some receptors can adapt or cease to respond
Amplitude
Membrane
potential (mV)
(a) Stimulus
(b) Longer and
stronger
stimulus
Membrane
potential (mV)
Duration
20
0
-20
-40
-60
-80
20
0
-20
-40
-60
-80
Threshold
0
5
10
0
5
10
Time (sec)
0
5
10
0
5
10
Threshold
0
5
10
0
5
10
Figure 10-7
Figure 10-7: Sensory coding for stimulus intensity and duration
Adaptation
• Adaptation occurs when sensory receptors
are subjected to an unchanging stimulus
– Receptor membranes become less responsive
– Receptor potentials decline in frequency or stop
• Tonic receptors – do not adapt or adapt
very slowly
• Phasic receptors – readily adapt
Sensory Adaptation
• Tonic receptors (Pain):
– Produce constant rate of
firing as long as stimulus
is applied
• Phasic receptors:
– Burst of activity but
quickly reduce firing rate
(adapt) if stimulus
maintained.
– Sensory adaptation:
cease to pay attention to
constant stimuli.
Adaptation
• Receptors responding to pressure,
touch, and smell adapt quickly
• Receptors responding slowly include
Merkel’s discs, Ruffini’s corpuscles
• Pain receptors and proprioceptors do
not exhibit adaptation
Touch (pressure)
• Mechanoreceptors
• Free nerve endings
– Lamellated (Pacinian) corpuscles - rapidly adapting skin receptor
that detects pressure and vibration.
– Corpuscle of touch (Meissner‘s) - receptor for discriminative touch
– Type I cutaneous (Merkel) receptors for discriminative touch
– Type II cutaneous(Ruffini) receptor for continuous touch sensation
– Baroreceptors – receptors to detect pressure changes
Proprioceptors
• Muscle spindle
– In muscles
– Sense stretch
• Golgi tendon
organ
– Near tendon
– Sense force
• Joint receptors
– Sense position
& pressure
Muscle Spindle Structure
• Consist of collections
of specialized muscle
fibers known as
intrafusal fibers
– Lie within spindleshaped connective
tissue capsules parallel
to extrafusal fibers
– Each spindle has its
own private efferent and
afferent nerve supply
– Play key role in stretch
reflex
Stretch Reflex
• Primary purpose is to resist tendency
for passive stretch of extensor muscles
by gravitational forces when person is
standing upright
• Classic example is patellar tendon, or
knee-jerk reflex
Pain
•
•
•
•
Nociceptors
Reflexive path
Fast pain
Slow pain
Nociceptive Transmission Pathway
•
A-Delta
–
–
–
–
Small, thinly myelinated.
10 % sensory pain fibers.
Conduct at 5-30 m/sec.
Mechanical and thermal
stimuli.
– Sensations of sharp,
pricking pain.
•
Sensory
Receptor
C Fibers
– Small, unmyelinatd fibers.
– 90% of afferent sensory
fibers.
– Conduct at 0.5-2.0 m/sec.
– Mechanical, thermal,
chemical.
– Long lasting, burning pain.
DRG
Sensor
axon
y
Spinal
cord
Thalamus
Fibers
• A-Delta
–
–
–
–
–
Small, thinly myelinated.
10 % sensory pain fibers.
Conduct at 5-30 m/sec.
Mechanical and thermal stimuli.
Sensations of sharp, pricking pain.
• C Fibers
–
–
–
–
–
Small, unmyelinatd fibers.
90% of afferent sensory fibers.
Conduct at 0.5-2.0 m/sec.
Mechanical, thermal, chemical.
Long lasting, burning pain.
 Ad
and C Nociceptors Mediate Pain
C-fiber
Ad fiber
First
pain
Second
pain
Pain
intensity
Time
Neurotransmitters in Spinal Cord
• Key nociceptor transmitter is
substance P.
– Activates ascending pathways that
transmit nociceptor impulses.
• Glutamate:
– Binds to AMPA receptors, increases
permeability, increasing likelihood of AP.
– Binds to NMDA receptors increases
excitability of dorsal horn neurons.
Spinal Cord: Excitatory Transmitters
Spinothalamic
Tract
DRG
1o Afferent fiber
Substance P
Glutamate
2nd Order
Neuron