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 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 Intensity 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 Duration Amplitude Time (sec) 5 10 Threshold (a) (b) Stimulus 20 -20 -40 -60 -80 potential (mV) Membrane Longer and stronger stimulus 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): Phasic receptors: 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 Golgi tendon organ Joint receptors 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 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

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. 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. Thalamus Sensory Receptor axon Spinal cord DRG

Fibers A-Delta C Fibers 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 First pain Second Time Pain intensity C-fiber Ad fiber

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