D’YOUVILLE COLLEGE
PMD 604 - ANATOMY, PHYSIOLOGY, PATHOLOGY II
Lecture 14: Neuron Circuits, Sensory Pathways & Somatosensory Cortex
G & H chapters 45 - 48
1.
Review Synaptic Functions:
• synaptic transmission & summation: chemical synapses release
neurotransmitter that binds to chemically gated channel on postsynaptic cell
(paracrine function):
- postsynaptic cell (usually another neuron) may become depolarized (EPSP),
which moves it closer to threshold (= facilitated) (fig. 45 - 9 & ppts.1 & 2)
- postsynaptic cell may become hyperpolarized (IPSP), which moves it further
from threshold (= inhibited) (fig. 45 - 9 & ppts. 1 & 2)
- many EPSPs are required to reach threshold (fig. 45 - 10 & ppt. 3);
summation must occur (usually via currents passing through soma to initial segment of
axon)
- signal strength (frequency of APs) reflects magnitude & duration of
summated PSPs that exceed threshold (fig. 46 - 2 & ppt. 4)
2.
Sensory Receptors, Afferent Neurons: (chapter 46)
• part of afferent pathway of reflex arc: receptor  sensory neuron  CNS
• types of sensory receptor (table 46 – 1):
- mechanoreceptors: senses respond to touch, pressure, vibration, tickle &
itch (tactile senses); hearing & equilibrium (special senses); visceral pressure, stretch
- thermoreceptors: respond to temperature changes on body surfaces or
deeper tissues (somatic)
- chemoreceptors: respond to chemicals dissolved in mucus, tissue fluid,
etc.; senses of smell & taste; level of chemicals in body fluids (osmoreception)
- photoreceptors: respond to electromagnetic energy (visible light); sense of
vision
- nociceptors: pain receptors respond to various sensory modalities that are
extreme: mechanical, thermal, & chemical; tissue damage is involved
• labeled line principle: sensory modality is not a property of a receptor, but is
determined by destination in sensory areas of brain, e.g., touch is experienced
because receptor stimulates sensory fiber to tactile area of brain
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- p. 2 -
• sensory adaptation (fig. 46 – 5 & ppt. 5): chronic stimulation causes decline
in sensitivity to stimulus (reducing signal strength in response to continuing stimulation)
often to zero sensitivity; speed of adaptation varies widely
- zero adaptation is characteristic of most pain receptors
- slowly adapting receptors detect persistent stimuli (tonic receptors);
examples include receptors monitoring musculoskeletal activity, blood pressure or
chemistry, static equilibrium
- rapidly adapting receptors detect rates of stimulation, vibrations,
movements across sensory field; rate receptors help anticipate expected changes in
body position so adjustments can be made (e.g. standing on a moving bus or train),
e.g., receptors monitoring tactile stimuli, rate of change of position, dynamic
equilibrium
• receptor potential: sensory receptors respond to their own modality by
transducing one form of energy (physical, chemical, etc.) to electrical energy (change in
membrane potential); generates graded potential or action potential in sensory
nerve fiber
• classification (velocity of transmission) of nerve fibers (fig. 46 – 6 & ppt. 6):
variability based on axon diameter and state of myelination or non-myelination
- A fibers (myelinated) – fast neurons; fastest = A least fast = A
- C fibers (non-myelinated) are slow neurons
• alternative classification (certain sensory nerve fibers) (fig. 46 – 6 & ppt. 6):
also based on axon diameter and state of myelination or non-myelination
- IA fibers (myelinated) – correspond to A orfaster A
- II & III fibers (myelinated) – correspond to slower A A A
- IV fibers (non-myelinated) – correspond to C fibers
• signal strength: stronger stimuli generate stronger receptor potentials, which are
of prolonged duration; if sensory neuron is raised to threshold  AP results; if sensory
neuron is kept above threshold  volley of APs results (signal) (figs. 46 - 7, 46 - 8 &
ppts. 7 & 8)
3.
Neuron Pools: (chapter 46)
• patterns of signals in neuron pools: synaptic connections from input cells
range from a few to hundreds or to thousands of presynaptic terminals; postsynaptic
(output) cells may have elaborate dendritic branching (extends # of postsynaptic receptors)
- signal pattern that passes within CNS is based upon # of input synapses
firing at a given postsynaptic (output) cell (figs. 46 – 9, 46 - 10 & ppts. 9 & 10): if input
produces subthreshold potential, output cell is facilitated; if input produces
summation above threshold, output cell is excited (i.e. discharges APs)
• some typical patterns (figs. 46 – 11 to 46 – 14 & ppts. 11 - 14):
- divergence of output: may result in amplification or distribution to
multiple destinations
- convergence: designed to maximize temporal &/or spatial summation
- lateral inhibition: input transmits to excitatory and parallel inhibitory
output  sharpens output signal
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- p. 3 -
- after discharge: slowly acting neurotransmitters or synapses with
presynaptic cells may produce prolongation of output signal
- reverberating circuits exploit self-stimulation to produce a continuous output
for prolonged period of time
- rhythmic outputs result from collaboration of reverberating circuits with
excitatory vs. inhibitory outputs, e.g., respiratory centers (fig. 47 - 17 & ppt. 15)
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4.
- p. 4 -
Sensory Pathways: (chapter 47)
• tactile and position senses: free nerve endings (cornea), Meissner’s
corpuscles (non-hairy skin, fingertips), Merkel’s discs (fingertips), hair end organs,
Ruffini’s end organs, Pacinian corpuscles (fig. 46 - 1 & ppt. 16)
- most are rapidly adapting types that detect movement across skin and/or
vibrations
- slowly adapting (e.g., Ruffini’s) respond to continuous touch or
continuous pressure
- transmit via A fibers or A fibers (some free nerve endings)
• dorsal column – medial lemniscal system (figs. 47 – 2 to 47 – 4 & ppts. 17 19):
- two fascicles (gracilis – medial & cuneatus – lateral) reside in the white
matter posterior to the grey matter of the cord (dorsal columns)
- first order fibers synapse in cord grey matter & send fibers up dorsal
column – (mostly fast A fibers) to medulla where they synapse with second order
neurons (fig. 47 - 2 & ppts. 17 - 19)
- second order fibers cross over to opposite side (decussate) & ascend
medial lemniscus to the thalamus (nuclei of ventrobasal area) (fig. 47 - 3 & ppt. 20)
- third order fibers from thalamus relay signals to somatosensory cortex
(fig. 47 - 4 & ppt. 21)
- high degree of spatial orientation of fast fibers (e.g., lower sensory
fibers enter medially {fasciculus gracilis}, higher sensory fibers enter laterally
{fasciculus cuneatus}); pathway adapted for precision mechanoreception at high velocity of
transmission
- divergent circuitry including some lateral inhibition facilitates precision
in detection of stimulus strength as well as point for point discrimination (figs 47 9, 47 - 10 & ppts. 22 & 23)
• tickle & itch senses: mostly free nerve endings in superficial skin; transmit
via slow, type C fibers
• anterolateral system (spinothalamic tracts) (figs. 47 – 2, 47 – 13 & ppts. 17, 24
& 25): two extensive tracts (lateral to lateral grey matter & anterior to anterior horn)
- carries slower fibers – A fibers & C fibers
- first order fibers synapse in dorsal horn of grey matter
- second order fibers cross over to opposite side of cord to enter anterolateral
system
- higher order fibers terminate in various brainstem regions (mainly reticular
nuclei) as well as thalamus (ventrobasal area -- mostly tactile fibers; intralaminar nuclei -mostly pain fibers); some thalamic fibers ascend to somatosensory cortex
- slower, less organized tract that transmits crude touch, tickle and itch;
also transmits warmth, cold, sexual sensations & pain signals
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5.
- p. 5 -
Pain: (chapter 48)
• free nerve endings located abundantly in skin, sparser in deep tissues
- respond to three modalities: mechanical, thermal (in skin) & chemical
(mostly in deeper tissues, which sense all three)
- two types: fast (skin) and slow (more often deep)
- receptors are essentially non-adapting; may even intensify signals as time
passes
• fast pain (acute pain, sharp pain, pricking pain) is experienced in a fraction
of a second & is usually of brief duration
- transmitted by A fibers, which synapse (glutamate) in dorsal horn of grey
matter, sending second order fibers to opposite side of cord to enter
neospinothalamic tract within anterolateral system (fig. 48 – 2 & ppt. 26)
- fibers ascend to brainstem regions and mostly to ventrobasal area of
thalamus, which relays signals to somatosensory cortex (fig. 48 – 3 & ppt. 27);
localization is good, so long as fast pain signals are accompanied by tactile signals
via dorsal column-medial lemniscal system
• slow pain (aching, throbbing) is not experienced until after a second or
longer and is of prolonged duration
- transmitted by C fibers, which synapse (substance P) at least twice in grey
matter of cord; third order or higher fibers pass to opposite side of cord to enter
paleospinothalamic tract within anterolateral system (fig. 48 – 2 & ppt. 26)
- fibers mostly terminate in reticular areas of brainstem &/or intralaminar
nuclei of thalamus (fig. 48 – 3 & ppt. 27) (both arousal systems); accounts for
interference with sleep
- not well localized, fewer thalamic termini than fast pain
- slow pain usually associated with tissue damage that releases chemicals
(especially bradykinins); mechanical trauma, ischemia, muscle spasm are most likely
causes of tissue damage
- rate of tissue damage governs intensity of pain signals
• analgesia system: pain signals may be blocked by inhibitory synapses in
cord grey matter
- brain analgesic system: nuclei in periaqueductal grey matter &
periventricular grey matter of midbrain send fibers to synapse (enkephalin) with
raphe magnus nucleus of brainstem; raphe magnus fibers descend & synapse
(serotonin) with dorsal horn fibers, which in turn, release enkephalin to inhibit pain
fibers (both A & C) (fig. 48 – 4 & ppt. 28)
- alternate inhibition by A fibers carrying tactile signals from nearby
areas of skin may also block pain; possible basis for massage &/or acupuncture
effects
• referred pain: pain perceived to originate from a body surface area different
from the source (usually visceral)
- pain fibers (C fibers) from viscera appear to synapse with same second
order neurons as pain fibers from body surface (fig. 48 – 5 & ppt. 29); clinical
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- p. 6 -
interpretations of various body surface referred pains facilitate diagnosis of internal
tissue/organ damage (fig. 48 – 6 & ppt. 30)
- pain usually caused by diffuse stimulation of pain receptors in viscera
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6.
- p. 7 -
Somatosensory Cortex: (chapter 47)
• located in post-central gyrus of parietal lobe of cerebrum (immediately
posterior to central sulcus) (fig. 47 – 5 & ppt. 31)
- localization of somatosensory experiences is organized according to body
region of origin: lower extremity and genitalia most medial, jaw & pharynx most
lateral
- density of sensory receptors of a body region reflected by size of region
in cortex – produces disproportionate map (e.g., large hand, head & lips; small lower
extremity – caricature) of body regions (figs. 47 – 6, 47 – 7 & ppt. 32)
• layers of neurons: cortex features six layers of neurons, each with specific
roles (figs. 47 - 8, 57 - 1 & ppt. 33); three types of neurons (granular or stellate,
pyramidal & fusiform) are included in distinct layers
- IV receives incoming signals from sensory input via thalamus and
distributes them to other layers of the cortex; stellate cells
- I, II & III, perform communication with other cortical areas; granular
and pyramidal cells
- I & II receive facilitatory signals from brainstem (probably reticular
areas); I sends signals to somatosensory association area
- II & III communicate with adjacent cortical areas & with contralateral
hemisphere via corpus callosum
- V sends signals to basal nuclei, brainstem & spinal cord; large
pyramidal cells
- VI sends signals mostly to thalamus; these are inhibitory and serve to
modulate intensity of sensory input; fusiform cells
- columns (each 0.3 - 0.5 mm. wide, with about 10,000 neurons) extending
through the layers are dedicated to specific modalities for various points of origin
on body surface (e.g., anterior columns receive from muscle spindles and tendon
organs & send information to adjacent motor cortex in precentral gyrus for control of
muscles)
• somatosensory association area – located more posteriorly in parietal lobe
combines somatosensory inputs with inputs from visual and auditory cortices
enabling interpretation of sensory information (i.e. perception of form, texture) as a
meaningful experience or sense of form, e.g., ball, knife, cloth, awareness of own
body, etc.)