Physiology of the Spinal Cord Flashcards
what are the physiological functions of the spinal cord?
initial processing of somatosensory input by the CNS
primary processing of motor output by CNS
what are spinal nerves?
highways for both somatosensory information traffic to the spinal cord (afferent) and motor information from the spinal cord to muscles (effect)
what is the difference between sensation and perception?
sensation = detection by receptors perception = interpretation by spinal cord and brain circuits
what are receptors?
neurons specialised in transduction of energy generated by external stimuli
specific for a narrow range of input
what do neurons in sensory signal systems signal events by?
spatial
rate
temporal codes
impact of convergence and divergence
high spatial resolution required: low convergence
detection of weak signals: high convergence
input used for complex/multiple functions: divergence
2 main kinds of 1st order sensory neurons
bare nerve endings - neuron detects stimulus (most common)
mechanoreceptors - detect stimulus then transmit signal to sensory neuron (found in sense organs)
A beta fibre 1st order neurons
for touch, pressure and vibration
wide diameter, fast
receptor type: mecanoreceptors e.g. Merkell’s cells, Ruffini end-organs, Pacinian corpsucles
A delta fibre 1st order neurons
for touch, pressure, vibration and pain
medium diameter and speed
bare nerve endings
C fibre 1st order neurons
for pain
thin diameter, slow
bare nerve endings
cutaneous mechanosensory receptors
Meissner’s corpuscle
Merkel disc
Ruffini’s corpuscle
Pacinian corpuscle
what do Meissner’s corpuscles detect?
light tough
what do Merkel discs detect?
mechanical pressure and position
what do Ruffini’s corpuscles detect?
tension, folding
what to Pacinian corpuscles detect?
vibration, gross pressure changes
how can sensory stimulation be discriminated?
different types of receptors for same modality
spatial distribution of receptors
windows of response intensity
what is 2 point discrimination?
measure variation in sensitivity of tactile discrimination as a function of location on the body surface
if there is a higher density of mechanoreceptors, smaller distance at which 2 tactile stimuli can be discriminated
where is mechanoreceptor density greatest?
on hands on face
allows detection of stimuli at greater spatial resolution
what is the motor system?
our muscles and neurons that command them
system that gives rise to behaviour
what is the simplest motor system?
spinal cord can generate complex motor patterns independently of the brain
what are lower motor neurons?
final common path for all signals from CNS to skeletal muscles
collected in longitudinally organised columns
each neuron receives motor fibres through more than one ventral root
each (lower motor neuron) column …
- contains larger alpha and smaller gamma motor neurons to one muscle
- extends through more than one segment of the cord
what is the difference between alpha and gamma motor neurons?
alpha: thick axon, high conductance velocity
gamma: thin axon, low conductance velocity
what can destrution of a single ventral root/spinal nerve cause?
muscle weakness = paresis
not paralysis
what is a motor unit?
a single motor neuron and muscle fibre it innervates
what are the simplest reflex responses based on?
an interaction between proprioceptive sensory input and a motor unit
what do proprioceptive sensory neurons detect?
muscle spindles - negative feedback to regulate muscle length
golgi tendon organs - negative feedback to regulate muscle tension
an example of a monosynaptic reflex pathway
stretch reflex - negative feedback loop to regulate muscle length
how does the monosynaptic reflex pathway work?
muscle spindle (sensory receptor) initiates reflex
stretch increases
afferent activity increases
alpha motor neuron activity increases: muscle contracts
an example of a polysynaptic reflex pathway
flexion reflex
how does the polysynaptic reflex pathway work?
stimulation of cutaneous pain receptors in foot
activation of spinal cord local circuits
stimulated extremity is flexed
other extremity extends to provide compensatory support
