Physiology of the spinal cord Flashcards
List the physiological functions of the spinal cord
Initial processing of somatosensory input by the CNS
Final processing of the motor output in the CNS
What is sensation?
Detection by receptors
What is perception?
Interpretation by spinal cord and brain circuits
What are receptors?
Neurons specialised in the transduction of energy generated by external stimuli
How do neurons in the sensory system signal events?
By rate, spatial and temporal codes
Receptors are specific for…
A narrow range of input
What receptor type detects touch, pressure and vibration?
Mechanoreceptors
Bare nerve endings
What is the afferent fibre type and conduction velocity of mechanoreceptors?
Abeta
Wide diameter and fast
What is the afferent fibre type and conduction velocity of bare nerve endings?
Adelta
Medium diameter and speed
What are the receptor types that detect pain?
Bare nerve endings - fast pricking pain
Bare nerve endings - slow burning pain and itch
What is the afferent fibre type and conduction velocity of fast pricking pain bare nerve endings?
Adelta
Medium diameter and speed
What is the afferent fibre type and conduction velocity of slow burning pain and itch bare nerve endings?
C fibres
Thin diameter
Slow
Describe some mechanisms of sensory stimulus discrimination
Different types of receptors for the same modality
Spatial distribution of receptors
windows of response intensity
Describe the spatial distribution of receptors
The higher the density of mechanoreceptors the smaller the distance at which two tactile stimuli can be discriminated
The density of mechanoreceptors is much greater on the hand and face than elsewhere, allowing the detection of stimuli at a much greater spatial resolution.
Name a mechanoreceptor
Pacinian corpuscle
What is the motor system?
Our muscles and the neurons that command them. This is the system that actually gives rise to behaviour.
What are the lower motor neurons?
Final common path for all signals (neuronal information) from the CNS to skeletal muscles (“keys on a piano”)
How are lower motor neurons organized?
Collected in longitudinally organized columns
What does each column contain?
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.
Where does each column extend?
Through more than one segment of the cord
What does each muscle receive?
Motor fibres through more than one ventral root and spinal nerve
What will destruction of a single ventral root or single spinal nerve lead to?
Will not produce paralysis, only weakness (paresis)
What is the motor unit?
A single alpha motor neuron and the muscle fibres it innervates
Each motor neuron synapses with multiple motor fibres.
What are the simplest reflexes based on?
Interaction between a proprioceptive sensory input and a motor unit
Name some proprioceptive sensory organs
Muscle spindles - negative feedback regulation of muscle length
Golgi tendon organs- negative feedback regulation of muscle tension
Describe stretch reflex circuitry
(A) Muscle spindle: the sensory receptor that initiates the reflex.
(B) Stretch increased Ia afferent activity increased α motor neuron activity leading to contraction of the same muscle.
(Ia afferents also excite motor neurons that innervate synergistic muscles, and inhibit motor neurons of the antagonist muscles).
(C) Stretch reflex: negative feedback loop to regulate muscle length. ‘Desired’ length is preset by descending pathways.
What is the Stretch reflex circuitry an example of?
a monosynaptic reflex pathway
What is spinal cord circuitry responsible for?
Flexion reflex
What is the flexion reflex an example of?
Polysynaptic reflex pathway
Describe the flexion 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.
Where are motor neurons innervating axial musculature located?
Medially
Where are motor neurons innervating distal musculature located?
Most laterally
Describe anterior cord syndrome
Bilateral lower motor neuron paralysis and muscular atrophy in the segment of the lesion (due to damage to lower motor neurons).
Bilateral spastic paralysis below the level of the lesion (due to loss of anterior descending tracts).
Bilateral loss of pain, temperature and light touch sensations below the level of the lesion (due to loss of anterior and lateral spinothalamic tracts).
Tactile discrimination and vibratory and proprioceptive sensations are preserved because the posterior (dorsal) white columns on both sides are undamaged.
Describe Brown – Séquard 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).
Discriminative touch pathways travelling in the ascending tracts in the contralateral dorsal white column remain intact.
Describe complete cord transection syndrome
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)
