BIOL 2305
Peripheral Nervous System - Afferent Division - Part I
Peripheral Nervous System - Afferent
Peripheral Nervous System – all neural structures outside the brain and spinal cord
Includes sensory receptors, peripheral nerves, associated ganglia, and motor endings
PNS provides links to and from the external environment
Properties of Sensory Systems
All afferent pathways have in common:
Stimulus (physical energy – internal or external)
Sensory receptor (transducer – converts stimulus into AP)
Sensory neuron (carries AP along afferent pathway to CNS)
Destination: CNS Integration
Some travel to cerebral cortex for conscious perception
Some reach only spinal cord or lower levels of cerebrum, cerebellum, & brain stem for
unconscious perception and/or reflexive response
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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 – the process on converting stimulus energy into electrical impulses that can be sent to
the CNS
Sensory receptors – sensory nerve endings that responds to changes in the environment around
them by transducing stimuli into electrical impulses
Ion channels or second messengers initiate a change in membrane potential of receptor
Local depolarizations (graded potentials) trigger electrical impulses (action potentials) that travel
to the CNS (brain and spinal cord)
The realization of these stimuli (sensation and perception) occur in the brain
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Sensory Receptor Types
Receptor Classification
Mechanoreceptors – respond to touch, pressure, vibration, stretch, and itch e.g., Pacinian corpuscles
in skin, baroreceptors in aorta
Thermoreceptors – sensitive to changes in temperature
Photoreceptors – respond to light energy e.g., rods and cones of the retina
Chemoreceptors – respond to chemicals e.g., olfactory neurons (smell), taste buds, carotid and aortic
bodies (changes in blood chemistry)
Nociceptors – sensitive to pain-causing stimuli e.g., free nerve endings
Osmoreceptors – detect changes in concentration of solutes, osmotic activity (primarily found in the
hypothalamus)
Receptor Characteristics
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
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Types of Graded Potentials
Generator potentials
Occur in specialized nerve endings of sensory neurons
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 which propagates to CNS
Receptor potentials
Occur in separate receptor cells at distal end of sensory neurons
Stimulus opens ion channels in receptor causing graded membrane potential
Receptor cell releases chemical messengers
Chemical messenger opens ion channels in afferent neuron AP generating region
If threshold reached, AP is generated which propagates to CNS
Sensory Pathways
Stimuli exist in a variety of energy forms or modalities – heat, light, sound, pressure, chemical, etc.
Stimulus as physical energy makes contact with sensory receptor
Receptor acts as a transducer
Intracellular signal: usually change in membrane potential
Stimulus  generator potential  threshold  action potential to CNS
Integration in CNS: cerebral cortex or acted on subconsciously in brain stem or spinal cord
Transduction
The process of converting energy forms into electrical signals via a generator potential which triggers
an action potential if it is large enough to reach threshold. A generator potential is a type of graded
potential similar to an EPSP (excitatory postsynaptic potential)
Somatic Senses – Internal Stimuli
Vision
Hearing
Taste
Smell
Equilibrium
Somatic Senses
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Somatic Senses – Internal Stimuli
Somatic Pathways
First-order neurons (1o)
soma reside in dorsal root or cranial ganglia
conduct impulses from the skin to the spinal cord or brain stem
Second-order neurons (2o)
soma reside in the dorsal horn of the spinal cord or medullary nuclei
transmit impulses to the thalamus or cerebellum
Third-order neurons (3o)
located in the thalamus
conduct impulses from thalamus to the somatosensory cortex of the cerebrum
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Sensory Coding
Sensory systems code 4 aspects of a stimulus:
Modality – type of stimulus
chemo-, thermo-, noci-, mechano-, osmo-, photoLocation
Physical site of the stimulated receptor
Acuity - precision of stimulus location
Larger receptive field size and field overlap decreases acuity
Lateral inhibition increases acuity
Intensity
Stronger stimuli result in higher frequency of receptor potentials leading to a higher
frequency of action potentials
Stronger stimuli also affect a larger area and leads to recruitment of a larger number of
receptors and their corresponding sensory neurons
Duration/Adaptation
Tonic receptors – generally do not adapt
Phasic receptors – adapt readily
Receptive Fields of Sensory Neurons
Receptive Field: Two-point discrimination
Receptive field – area within which a receptor can detect a stimulus
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Lateral Inhibition
Lateral Inhibition - a process by which information from neurons at the edge of a stimulus is inhibited;
a means of increasing acuity
To facilitate localization and sharpen contrast, the most strongly activated pathway at the center
inhibits the less excited pathways from the fringe areas
Sensory Coding: Stimulus Intensity & Duration
Intensity - coded by number of receptors activated and frequency of action potentials
Duration - coded by duration of action potentials;, although some receptors can adapt or cease to
respond
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Adaptation
Adaptation occurs when sensory receptors are subjected to an unchanging stimulus
Receptor membranes become less responsive
Receptor potentials decline in frequency or stop
Adaptation occurs in the receptor, not the CNS
Tonic receptors – do not adapt or adapt very slowly
Important when maintaining information about a stimulus is valuable – stretch, pain receptors
Phasic receptors – readily adapt
Useful in situations where it is important to signal a change in stimulus – tactile (touch)
receptors
Sensory Adaptation
Tonic receptors (pain):
Do not adapt or adapt very slowly
Produce constant rate of firing as long as stimulus is applied
Phasic receptors:
Adapt very quickly
Burst of activity but quickly reduce firing rate (adapt) if stimulus maintained.
Sensory adaptation: cease to pay attention to constant stimuli.
Tonic receptors
Phasic receptors
Receptor Types and 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
Adaptation mechanisms
Mechanical – specialized receptor ending consists of concentric layers of connective tissue.
Sustained pressure causes layers to slip, dissipating stimulus intensity
Chemical – Na+ channels that initial opened are slowly inactivated
Touch (pressure)
Mechanoreceptors:
Free nerve endings
Enclosed 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
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Proprioceptors
Muscle spindles
In muscles
Detect stretch (change in the length of the muscle)
Golgi tendon organs
In tendons
At the insertion of skeletal muscle fibers into their respective tendons
Sense force and stretch
Joint receptors
In the synovial junctions between bones
Sense position & pressure
Muscle Spindle Structure
Consist of collections of specialized
muscle fibers known as intrafusal fibers
Lie within spindle-shaped
connective tissue capsules parallel
to extrafusal fibers
Each spindle has its own private
efferent and afferent nerve supply
Play key role in stretch reflex
Muscle spindles monitor changes
in length of muscle by responding
to the rate and degree of change
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Tendon Organs (Golgi tendon organs)
Consists of sensory fiber penetrating a thin capsule of connective tissue and entwining around a few
collagen fibers
Found at the junctions of a tendon with a muscle
Help protect tendons and associated muscles from damage due to excessive tension or stretching
Golgi Tendon Organ
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
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Pain
Nociceptors
Reflexive path
Fast pain (Aδ)
Slow pain (C)
Pain Receptors
Three types of receptors (nociceptors): mechanical, thermal, polymodal
All are naked nerve endings and do not adapt
All can be sensitized by prostaglandins (increase pain)
Prostaglandins derived from lipid bilayer of membrane released from damaged tissues
Mechanical (crushing, cutting, pinching) and thermal (extreme temperatures) are transmitted over
small myelinated A-delta fibers – 30m/sec fast pain pathway
Polymodal respond to all kinds of damaging stimuli and is carried by small unmyelinated C-fibers
12m/sec slow pain pathway
Nociceptive Transmission Pathway & Fibers
A-Delta Fibers (Aδ)
“Fast pain”
Small, thinly myelinated
10 % sensory pain fibers
Conduct at 5-30 m/sec
Mechanical and thermal stimuli
Sensations of sharp, pricking pain
C Fibers
“Slow pain”
Small, unmyelinatd fibers.
90% of afferent sensory fibers.
Conduct at 0.5-2.0 m/sec.
Mechanical, thermal, chemical.
Long lasting, burning pain.
Aδ and C Nociceptors Mediate Pain
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Neurotransmitters in Spinal Cord
Substance P:
Key nociceptor transmitters
Released from first order sensory neurons
Activates ascending pathways that transmit nociceptor impulses
Glutamate:
Make up majority of excitatory synapses in brain/spinal cord
Modifiable Synapses (can increase or decrease excitability)
Binds to AMPA receptors, increases permeability, increasing likelihood of AP
Binds to NMDA receptors increases excitability of dorsal horn neurons.
Spinal Cord: Excitatory Transmitters
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