T1L18 physiology of the spinal chord Flashcards
spinal nerves
highways of both somatosensory (afferent) pathways to sc and motor (efferent) pathways from sc to muscles
sensation;
perception;
sensation- detection by receptors
perception- interpretation by sc and brain
convergence and divergence
- receptors transduce energy generated by externals stimuli
- receptors specific to a narrow range of input
- signal events by combination of RATE, SPATIAL AND TEMPORAL codes
- high spatial resolution required (eg cones in retina) > low convergence
- detection of weak signals (eg rods in retina) > high convergence
2 kinds of primary sensory recpetors
pic s6
most the stimulus directly forms ap in neurone in some (taste, eqbm, hearing), a separate cell is effected by the stimulus which causes chemical transmission to sensory neuron
modaility, receptor, afferent fibre type and velocity:
touch, pressure, vibration
- mechanorecptors eg Pacinian corpuscle
fibres: Abeta, wide diameter, fast - bare nerve endings (as in uncovered)
fibres: Adelta, medium diameter and speed
modaility, receptor, afferent fibre type and velocity:
pain
fast, pricking pains > bare nerve endings
fibres: Adelta, medium speed and diameter
slow, burning pains, itch
fibres: C fibes, low diameter and velocity
receptors for: shear contact pricking tension, folding deep pressure, vibration hair movement
shear- Meissner's corpuscle contact- merkel disk pricking- free nerve endings tension, folding- ruffinis corpuscle deep pressure, vibration- Pacinian corpuscle hair movement- idk
Pacinian corpuscle
pressure and vibration
afferent signals to dorsal root ganglion
pressure starts> generator potential > ap
pressures stops> generator potential> ap
on/off system
density of mechanoreceptor
- much denser on hand and face
- greater spatial resolution
- measured by 2 point discrimination
the spinal chord can —– independently to the brain
generate complex motor patterns independently of the brain
motor neurons in spinal chord are called
lower motor neurons
lower motor neurons
- first common path for all signals from cns to muscles
- in longitudinally organised columns
each column contains:
- large alpha fibres (thick and fast)
- small gamma fibres (thin and slow)
and goes to a muscle or functionally similar tissues
each muscle receives motor fibres through more than one ventral root in spinal nerve, so destruction of one ventral root or spinal nerve causes weakness rather than paralysis
motor unit
a single alpha motor neuron and the muscle fibres it innervates
- each motor neuron synapses to multiple fibres
more = more spatial resolution
- higher density of mechanoreceptors
eg on face and hand
simple reflexes
- interaction between a proprioceptive sensory and a motor unit
proprioception = self detection
proprioceptive sensory organs:
- muscle spindles (-ve feedback loop by muscle length (eg to catch a ball, force is exerted as the ball exerts force)
- Golgi tendon organs (-ve feedback loop by muscle tension)
clasp knife and knee jerk reflexes
s20, 21
- muscle spindles (-ve feedback loop by muscle length (eg to catch a ball, force is exerted as the ball exerts force) eg KNEE JERK
- Golgi tendon organs (-ve feedback loop by muscle tension) eg CLASP-KNIFE
stretch reflex circuitry
- monosynaptic reflex pathway
A) muscle spindle- sensory receptor initiates reflex
B) stretch > increase Ia afferent > increased alpha motor neuron > contraction of muscle
C) stretch reflex- negative feedback loop to the desired muscle length. the desired length is set by descending pathways
s22
flexion reflex
eg standing on a lego
polysynaptic reflex pathway
Stimulation of cutaneous pain receptors in the foot leads to activation of spinal cord local circuits that withdraw (flex) the stimulated extremity and extend the other extremity to provide compensatory support.
s23
origins of different motor neurons in spinal chord
medial = middler muscles eg abdominals
lateral = extremities eg hands
s24
homunculus
figure with proportions based on density of neuronal coverage
s25
anterior chord syndrome
bilateral lower motor neuron paralysis:
- muscular atrophy
- bilateral spastic paralysis below lesion level due to cut of anterior descending tracts
- bilateral loss of pain, temperature control, light touch sensations due to loss of anterior and lateral spinothalamic tracts
cord hemisection syndroms (brown-sequard syndrome)
ipsilateral lower motor neuron paralysis:
- ipsilateral cutaneous anaesthesia in the segment of the lesion (complete sensation loss)
- ipsilateral loss of vibration, proprioception and tactile discrimination below level of lesion
- contralateral loss of paint, temp and light touch
contralateral = other side ipsilateral = same side
complete cord transection syndrome
complete loss of sensation and voluntary movement below level of lesion
- bilateral spastic paralysis below lesion due to descending tracts
- bilateral loss of all sensation below lesion due to ascending tracts
- bladder and bowel no longer under voluntary control due to loss of descending autonomic fibres
