Physiology of the spinal cord Flashcards
What are the physiological functions of the spinal cord?
- Initial processing of somatosensory input by the CNS
- Final processing of motor output by the CNS
Describe spinal nerves
Highways of both somatosensory information traffic to the spinal cord (afferent pathways) and motor information traffic from the spinal cord to the muscles (efferent pathways)
Sensation
Detection by receptors
Perception
Interpretation by spinal cord and brain circuits
Receptors
neurons specialised in the transduction of energy generated by external stimuli
Receptors are specific for a narrow range of input
Neurones in sensory systems signal events by
combination of rate, spatial and temporal codes
When is low convergence needed?
for high spatial resolution
e.g. cones and bipolar cells in retina
When is high convergence needed?
for detecting weak signals e.g. rods and bipolar cells in the retina
When is input used for complex or multiple functions?
high divergence (e.g., vestibulo-ocular reflex).
What determines the stimulus specificity of a receptor?
Adequate stimulus: lowest response threshold -> stimulus specificity e.g. light receptors have the lowest threshold to light, so makes them light receptor. Their stimulus specificity and adequate stimulus needed is determined by the lowest response threshold they have.
Describe different types of sensory receptors in the skin
Meissner's corpuscles - sheer force Merkel disk - contact Ruffini's corpuscle - tension, folding Pacinian corpuscle - deep pressure, vibration Free nerve endings - pricking
Describe Pacinian corpuscle
On-off response: action potential when pressure applied, action potential when pressure relieved
Fast adaptation
Afferent signal then propagated to dorsal root
What is two-point discrimination?
Two-point discrimination: to measure variation in the sensitivity of tactile discrimination as a function of location on the body surface.
Principle: the higher the density of mechanoreceptors the smaller the distance at which two tactile stimuli can be discriminated.
e.g. Can’t feel closer than 5cm on calf, but can feel closer than 2mm on fingers
Where is the density of mechanoreceptors greatest?
Hands and face
allowing the detection of stimuli at a much greater spatial resolution
What is the motor system?
Our muscles and the neurons that command them. This is the system that actually gives rise to behaviour.
Anything we do is interaction between nervous system and muscle, and muscle and external environment
What is the evidence that the spinal cord can generate complex motor patterns independently of the brain?
After cord transection:
The hindlimbs are still able to walk on a treadmill
Patterned electrical activity can be recorded from the muscles
What are lower motor neurones?
Motor neurons present in the spinal cord
Final common path for all signals (neuronal information) from the CNS to skeletal muscles (“keys on a piano”)
How are lower motor neurons organised?
Lower motor neurons are collected in longitudinally organised columns
Each column contains the larger, alpha (thick axon, high conductance velocity), and smaller, gamma (thin axon, low conductance velocity), motor neurons to one muscle (or a few functionally similar muscles).
Each column extends through more than one segment of the cord
Each muscle receives motor fibres through more than one ventral root and spinal nerve
Alpha neurons
Larger
thick axon
high conductance velocity
Gamma neuron
smaller
thin axon
low conductance velocity
What is a motor unit?
The motor unit: a single motor neuron and the muscle fibres it innervates
Each motor neuron synapses with multiple muscle fibres. The motor neuron and the fibres it contacts defines the motor unit.
Simplest reflex responses
based on an interaction between a proprioceptive sensory input and a motor unit
Muscle spindles
negative feedback regulation of muscle length
Golgi tendon organs
negative feedback regulation of muscle tension
What type of afferent neurone is in the clasp-knife reflex?
Ib
synapses onto inhibitory interneuron which synapses onto motor neuron
in tendon organs
What are Ia afferent neurons?
Synapses from stretch receptor in spindle
onto motor neuron
knee jerk reflex
What do tendon organs and spindles respond to?
Tendon organs respond to muscle tension (due to contraction), spindles respond to length (due to passive stretch), but in a manner modified by the activity of their own contractile elements.
Describe the stretch reflex circuitry
A monosynaptic reflex pathway
- Muscle spindle is the sensory receptor which initiates the reflex
- Stretch leads to increased Ia afferent activity -> increased alpha motor neuron activity -> contraction of the same muscle
(Ia afferents also excite motor neurons that innervate synergistic muscles, and inhibit motor neurons of the antagonist muscles) - Stretch reflex: negative feedback loop to regulate muscle length - desired length is preset by descending pathways
What is the flexion reflex?
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
Spinal cord circuitry is responsible for the flexion reflex
It is an example of a polysynaptic reflex pathway
Describe the somatotopic organisation of lower motor neurons in the ventral horn at the cervical level of the spinal cord
Motor neurons innervating axial musculature (i.e. postural trunk) most medial
Motor neurons innervating distal musculature (e.g. hands) are most lateral
What is notable about the somatic sensory cortex?
The amount of somatic sensory cortex devoted to the hands and face is much larger than the relative amount of body surface in these regions.
Anterior cord syndrome
Bilateral lower motor neuron paralysis and muscle atrophy in segment of lesion (due to damage to lower motor neurons)
Bilateral spastic paralysis below the level of the lesion (loss of anterior descending tracts)
bilateral loss of pain, temperature and light touch sensations below level of the lesion (loss of anterior and lateral spinothalamic tracts)
But: tactile discrimination and vibratory and proprioceptive sensations are preserves because posterior (dorsal) white columns on both sides are undamaged
Brown-Sequard or cord hemisection syndrome
Ipsilateral lower motor neuron paralysis and muscular atrophy in the segment of the lesion (due to damage to lower motor neurons).
Ipsilateral spastic paralysis below the level of the lesion (due to loss of anterior descending tracts).
Ipsilateral band of cutaneous anesthesia in the segment of the lesion (due to loss of dorsal root).
Ipsilateral loss of tactile discrimination and of vibratory and proprioceptive sensations below the level of the lesion (due to loss of ascending tracts in the dorsal white column on the side of the lesion).
Contralateral loss of pain, temperature and light touch (due to loss of crossed lateral spinothalamic tracts on the side of the lesion).
But: Discriminative touch pathways travelling in the ascending tracts in the contralateral dorsal white column remain intact.
Complete cord transection
Complete loss of sensation and voluntary movement below the level of the lesion.
Bilateral lower motor neuron paralysis and muscular atrophy in the segment of the lesion.
Bilateral spastic paralysis below the level of the lesion (due to loss of descending tracts).
Bilateral loss of all sensations below the level of the lesion (due to loss of ascending tracts).
Bladder and bowel functions no longer under voluntary control (due to loss of descending autonomic fibres)
Sadly, no buts.
(Or maybe?? Stem cell research….? Other new emerging treatments..? Brain-computer interface…?)
