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Physiology 3120 Lecture Notes - Afferent Nerve Fiber, Receptive Field, Myelin

Course Code
Tom Stavraky

of 2
Human Physiology
Wednesday, October 21, 2009
All rapidly adapting receptors are associated with connective tissue
Somatic Sensory System
6 different touch receptors (therefore touch is a hugely important sensation/stimulus)
Some slowly adapting (free nerve ending, Merkel’s, Ruffini)
Can detect vibrations of stimuli (fast adapting receptors) very accurately
Free nerve endings part of the sensation of many modalities; different sensation mechanisms between
Touch faster than pain
Touch receptors have axons that conduct signals at about 80m/s; pain receptors (at the fastest)
have axons that conduct signals at about 20m/s
May reflect an increased need for tactile sensation in mammals compared with lower-order
Pain: two types
Slow pain/ache: goes through the C pathway
Fast/sharp pain: goes through alpha-delta pathway
Clusters of axons in a nerve: classification by conduction velocity
Alpha conduct signals faster than beta axons; different diameters produce different conduction
velocities; larger diameter = faster conduction; all of these fibres are “A” fibres
Unmyelinated fibres are “C” axons
Muscle afferents have a different classification
= knowledge of location & movement of limbs in space without vision
Receptive field
The area on the surface of the skin to which an afferent nerve is responsive & produces action
You need 2 representations in the brain to detect 2 different points of sensations (i.e. fingers more
sensitive in this respect than the back)
Anterolateral (spinothalamic) system (phylogenetically old pathway)
Afferents synapse in the spinal cord
They cross-over in the spinal cord & travel up to the brain, where they synapse on the ventral
posterior lateral nucleus of the thalamus
Axons from thalamus synapse on the somatosensory cortex
Large receptive fields
Many neurons in this pathway have convergence of different modalities
Detects pain, itch, crude touch, etc. (slow conduction; more primitive, second-order neurons)
Dorsal column medial lemniscal system (phylogenetically new pathway)
First order afferents enter spinal cord & enter the dorsal column
Axons travel up dorsal column, where they synapse with the dorsal column nuclei in the medulla
From here, they cross over via the medial lemniscus in the medulla, and then they synapse in the
VPL nuclei in the thalamus
Discriminative touch (using touch to recognize objects), proprioception, & vibrations
Reaches its highest development in man & monkeys
4 characteristics of neurons in the DCML system
Small receptive fields
Respond to only one modality; each axon responds as if it’s a different kind of receptor
Faithful transmission (995Hz on skin will cause Pacinian corpuscle to fire at 995Hz)