Physiology of the Spinal Cord Flashcards
4 physiological functions of the spinal cord:
- acts as a conduit between the brain
and the rest of the body - initial processing of somatosensory
input to the CNS - final processing of motor output from
the CNS - can act without brain signals; reflex
activity
Sensation definition:
the detection of a stimulus by a receptor
Perception definition:
the interpretation of that stimulus by the brain with existing emotions and memories
Sensory Receptors:
- are
- two types
- specialised endings of the peripheral
process of the sensory neuron - mechanoreceptors (pressure or
distortion) - nociceptors: chemical, thermal,
mechanical
Mechanoreceptors in skin:
- hair follicle receptors
- merkell’s receptor
Receptors Mediating Tactile Senses:
insert diagram
All somatosensory processing receptors are located in the
dermis
epidermis = dead skin = no sensory fibers
Pancinian Corpuscle:
- distortion due to pressure = stimulus
- causes all or nothing action potential
- depolarisation of dorsal root ganglion
- deep pressure, fast vibration
What are C fibre pains?
slow pain, might have a delay
slow phase
Mechanosensory (proprioception/vibration/light touch): ascend in dorsal columns on the
same side to ssynapse with second order neuron in medulla
Pain & temperature: synapse at level of entry with second order neuron which
crosses midline and ascends in lateral spinothalamic tract
Sensory information is integrated at all levels of the nervous system;
excitatory and inhibitory modulation
whether stimulus is passes to brain
Descending pathways can modulate sensory input:
green and red are descending trunks
different neurotransmitters modulate the transmission at spinal level
Mechanisms of sensory stimulus discrimination:
- different receptor types allow
differentiation between different
sensations - spatial distribution of receptors
- windows of response intensity
Spatial Distribution of Receptors:
- two point discrimination to measure
variation in the sensitivity of tractile
discrimination as a function of location
on the body surface - the higher the density of the
mechanoreceptors,
the smaller the distance at which two
tactile stimuli can be discriminated
eg higher density of mechanoreceptors on the
hand and face, allowing the detection of
stimuli at a much greater spatial resolution
Motor Processing:
descending pathway, short dendrons, long axons, decussation at brain level
Ventral horn of the cord has anterior motor neurons: give rise to nerve fibers innervating muscles:
- alpha motor neurons
- gamma motor neurons
What is a motor unit?
a motor neuron and the muscle fibres it synapses with
Merkell’s receptor:
- type of receptor
- what sensory information
- mechanoreceptor
- touch and pressure; low threshold
Meissner’s Corpuscle:
- type of receptor
- what sensory information
- mechanoreceptors
- texture, slow vibration
- just under the epidermis
Painless burns
can be deeper than dermis
Ruffini’s Ending:
- type of receptor
- what sensory information
- mechanoreceptor
- skin stretch
- superior dermis
Mechanisms of Sensory Stimulus Discrimination:
insert diagram
How are lower motor neurons collected?
longitudinally in organised columns
No muscle is innervated by a single spinal cord level:
each column of lower motor neurons extend through more than one segmented
each muscle receives motor fibers through more than one ventral root
hence damage to a single nerve will not cause complete paralysis
Renshaw Cells:
groups of cells sitting close to motor neurons
inhibitory to motor neurons
affected by descending pathways through the spinocerebellar tract
lateral inhibition of lower motor neurons; preventing innervation of muscles that are not needed
Can more than one muscle be innervated by the same spinal nerve root?
Link with lateral inhibition.
more than one muscle can be innervated by the same spinal nerve root
lateral inhibition via renshaw cells prevents for example 2 muscles moving when not needed
Effect of lateral inhibition on signals?
Lateral inhibition sharpens or focusses signals
2 types of reflex integration:
- simple reflex
- complex polysynaptic reflex causes
bilateral withdrawal
both occur solely in the spinal cord
Muscle Sensory Organs: Mechanoreceptors:
- detect proprioception = forces
generated within the body - sensory nerve fibers
- muscle spindles
- golgi tendon organs
- continuous feedback; integrate
between several muscles - control muscle at conscious and
unconscious levels: spinal cord,
cerebellum, cerebral cortex
What do Muscle Spindles do?
Muscle sensory organs found in bodies of muscle, which respond to muscle length
What do Golgi Tendon Organs do?
Muscle sensory organs found in bodies of muscle, which respond to tension of muscle
Muscle Spindles:
- are
- when are impulses created
- sensory fibers stimulated by stretch
- spindles emit impulses continually!!!
- with stretch = impulses increase
- with contraction = impulses decrease
Muscle Spindles:
- dynamic response
a sudden increase in length, causes impulses to suddenly increase
Muscle Spindles:
- gradual increase
impulses increase proportionally to the stretch
Muscle Spindles:
- static response
impulses continue after stretch
What is shown in the diagram below?
Muscle spindle
Monosynaptic Pathway:
interaction of sensory and motor nerves at a spinal cord level: simple circuit
Knee Jerk Reflex:
- dynamic stretch reflex
- sudden stretch leads to strong reflex
contraction - static stretch reflex, leads to weaker,
more prolonged phase - due to muscle spindles
Golgi Tendon Organ:
- is
- does
- effect
- function
- sensory receptor through which muscle
tendon fibers pass - detects muscle tension
- effect is inhibitory
- protects against excess tension due to
prolonged contraction of muscle
What is show below?
Golgi Tendon Organ
Clasp Knife Reflex:
- golgi tendon organ responds to
tension, not passive stretch - activation inhibits the alpha motor
neuron - leads to reduction in activity in the
same muscle - sometimes called the inverse stretch
reflex
What is the reflex circuitry for the contraction of muscle below?
- bicep contract, triceps relax to flex
elbow to maintain distance - stretch of muscle leads to lengthening
of muscle spindle - activates the alpha motor neuron to
contract the muscle - negative feedback loop regulates
muscle length - desired length set by descending
pathways; circuits do not exist in
isolation
The flexor reflex is a monosynaptic or polysynaptic reflex pathway?
Polysynaptic
Flexor Reflex:
- pain leads to flexion of muscle
- allows muscle withdrawal
- other circuits are also activated to
inhibit antagonistic muscles - effects on opposite side of the body:
crossed extensor reflex
Knee Jerk Reflex can assess how senstive the
stretch reflexes are
If a reflex is reduced, then which motor neurons are afffected?
lower motor neurons
If a reflex is exaggerated, then which motor neurons are affected?
upper motor neurons
