Download Mammalian Physiology Sensory Nervous System

Mammalian Physiology
Sensory Nervous System
UNLV
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UNIVERSITY OF NEVADA LAS VEGAS
PHYSIOLOGY, Chapter 7
Berne, Levy, Koeppen, Stanton
Objectives
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Describe the classifications of sensory receptors
Describe sensory nerve pathways
Describe the operation of a sensory neuron
Describe the function and operation of the muscle spindle
Describe the function and operation of the Golgi tendon organ
Basic Functions of the Nervous System
Sensory Input - provides the central nervous system with information about
the internal and external environment
Integration - CNS takes all the incoming information, processes it, then
selects an appropriate action
Motor Output – effects the physical responses dictated by the central
nervous system
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Sensory Receptors
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Mechanoreceptors
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Skin tactile sensibilities – free
nerve endings,
Deep tissue sensibilities –
Pacinian corpuscles, muscle
spindles, Golgi tendon organ
Hearing – sound receptors of
cochlea
Equilibrium – vestibular
receptors
Arterial pressure –
baroreceptors
Mechanoreceptors respond to mechanical
stimuli and are either fast or slow adapting
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Sensory Receptors
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Thermoreceptors
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Cold
Warm
Both types are slow
adapting – discharge
spontaneously and are
active over a broad
temperature range
Receptors are reciprocal
Cold receptors become
inactive at skin temps
above 37º C
Warm receptors are
inactive above 45º C –
cannot sense heat pain
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Sensory Receptors
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Nociceptors
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Pain – free nerve endings
Mechanical
Polymodal
Sensitization
Sensitization – damage to cell → release of
proteolytic enzymes leads to production of
bradykinen which binds to receptor and
increases receptor’s sensitivity to subsequent
stimuli
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Sensory Receptors
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Nociceptors
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Pain – free nerve endings
Inflammation
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Rubor
Calor
Rumor
Dolor
Activation of the receptor
releases peptides, substance
P, CGRP into the skin P and CGRP cause
vasodilation and increased
capillary permeability,
augmenting other factors
released from damaged cells
– histamines, etc
Sensory Receptors
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Electromagnetic receptors
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Chemoreceptors
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Vision – rods and cones
Taste – taste buds
Smell – olfactory
epithelium
Arterial oxygen – aortic
and carotid bodies
Osmolality – anterior
hypothalamus
Blood carbon dioxide –
CNS (medulla)
Sensors in the Skin
For all sensory receptors, excitation by a stimulus results in a change in
the receptor’s membrane electrical potential – the receptor potential
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Mechanisms of Receptor Potentials
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Mechanical deformation of the receptor
– Stretches the receptor membrane & opens ion channels
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Application of a chemical to the membrane
– Opens ion channels
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Change in temperature of the membrane
– Altars permeability of the membrane
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Effects of electromagnetic radiation
– Directly or indirectly changes membrane characteristics allowing
ions to flow through membrane channels
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Receptor Potential – Action Potential
Action potentials
appear when receptor
potential reaches
threshold potential
Frequency of action
potentials increases,
the more receptor
potential rises above
threshold
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Pacinian Corpuscle
Mechanical deformation of receptor initiates an action potential
Corpuscle contains gel-like substance which redistributes after initial
response (pressure applied equally on all sides of central nerve) –
decreasing sensor output (adaptation)
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Pacinian Corpuscle
Receptor output in response
to mechanical stress in a
pacinian corpuscle
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Adaptation
Mechanoreceptors
Rapid adapting receptor
-discharges at the onset of the
stimulus, ceases if stimulus
persists at a steady rate, may
also discharge at the end of
stimulus
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Adaptation
Mechanoreceptors
Slow adapting receptor
-discharge continues as long as
stimulus persists
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Sensor Adaptation
When a continuous sensory stimulus is applied, the receptor responds
initially with a high impulse rate, which declines until the rate of action
potentials is few or none (accommodation).
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Spatial Summation
Receptive fields define degree of spatial discrimination – sensor responds to any
stimulus in the receptive field
Strength of sensory signal is a function of the number of fibers activated
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Spatial Discrimination
The horizontal bars represent the
minimum distance at which two
points can be perceived as
distinct at various locations over
the body
Spatial discrimination depends on
-Receptive field size
-Density of the receptors
The higher the density of
receptors, the finer the
discrimination
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Temporal Summation
Strength of signal is a function of the frequency of impulses in any
given nerve fiber
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Proprioceptros
Muscle Spindle and Golgi Tendon Organ
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Muscle Spindle Apparatus
More than one type of motor neuron
- alpha motor neuron
- gamma motor neuron
More than one type of sensory neuron
- primary afferent
- secondary afferent
Many interneurons
Two types of muscle fibers
- intrafusal muscle fibers
- extrafusal muscle fibers
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Muscle Spindle
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Muscle spindle
– Changes in muscle length - spatial position
– Rate of change in muscle length - stretch reflex
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Primary afferents – fast
– Rate of stretch - Phasic stretch
– Function: react to oppose stretch
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Secondary afferents – slow
– Final length of muscle - Tonic stretch
– Function: maintain muscle tone, posture, positional awareness
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Muscle Spindle – Sensory End of System
Primary sensory nerve fibers
(1a afferent fibers)
Found integrated within the
intrafusal muscle fiber and
respond to distortion
(rate of stretch)
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Muscle System – Motor End of System
Alpha motor (α) neurons
innervate the extrafusal
muscle fibers. When the
alpha nerve fires the
motor unit generates
tension and/or shortens.
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Stretch Reflex
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Quick stretch of muscle distorts nuclear bag
Afferent signal via primary sensory nerve
Monosynapse in spinal cord with α-motorneuron
Efferent signal via α-motor neuron
Muscle contraction results to oppose stretch
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Reciprocal innervation
– Efferent signal to antagonist → relaxation
– Inhibitory interneuron (Renshaw cell)
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Schema of Basic Stretch Reflex
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Coactivation – the Gamma Loop
Gamma motor (γ) neurons
innervate the intrafusal
muscle fibers. When the
gamma nerve fires the
intrafusal muscle fiber
generates tension and/or
shortens.
α-motor neuron and γ-motor
neuron are activated in
parallel
Muscle spindle shortens in
proportion to muscle fiber
shortening – maintains
constant rate of signaling.
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Golgi Tendon Organ
–Monitors tension
developed in muscle
–Prevents damage
during excessive
force generation
–Stimulation results
in reflex relaxation of
muscle
–Inhibitory
interneuron, not
mono-synapse
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Golgi Tendon Organ
Inhibitory
Interneuron
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Sensory Pathways
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1st order neurons
– Primary afferents
• Peripheral processes – receptors
• Central process – collateral branches
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2nd order neurons
– Transmit information to thalamus
– Cross-over to contralateral thalamus
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3rd order neurons
– Project to somatosensory regions of
the cerebral cortex
Dorsal column-medial lemniscus tract
Spinothalamic tract
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Sensory Pathways
White matter –
Myelinated fibers
bundled into large
fibers and tracts
Commissural –
connect corresponding
gray areas
Association – connect
different parts of
same hemisphere
Projection – connect cortex to lower centers
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Sensory Pathways
Ascending Tracts
Specific pathways – dorsal column
medial lemniscus tract
Sensory input from precise
locations
Discriminatory touch,
vibrations
Spinocerebellar tracts information from muscle or
tendon stretch to cerebellum
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Somatotopic Arrangement of Neurons
Dorsal column medial lemniscus pathway
Somatosensory cortex
Leg- medial aspect
VPL thalamic nucleus
Arm-lateral aspect
Face-dorsal to lateral aspect
VPM nucleus
Distortion – regions that have
more dense innervation, use
more cortical circuitry
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Sensory homunculus
Sensory Pathways
Ascending Tracts
Non-specific pathways
anterolateral pathways
Lateral spinothalamic tract
Sensory for pain, temperature,
and course touch
Non-discriminating localization
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Motor Pathways
Descending Tracts
Pyramidal tracts direct pathways from
pyramidal neurons to
spinal cord to motor
neurons
Regulates fast & fine
motor skills
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Motor Pathways
Descending Tracts
Extrapyramidal tract –
All motor pathways except
pyramidal pathways including
brain stem motor nuclei
Balance, course limb
movements, - heavy dependence
on reflex activity
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Integration of Input and Output
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