Motor unit--a motor neuron and the muscle fibers that
it innervates
- Innervation ratio 150-1 to 3-1
- Ratio related to complexity of movement
- Effects of synapse with motorneuron
- Facilitation--spatial summation
- Inhibition from contralateral activity
Motor Neuron Pool
- Dendrites and cell bodies of motor neurons in the anterior horn of spinal cord
- All motor systems that influence muscles must do so in this
area or higher up
- Lateral corticospinal tract
- Ventral corticospinal tract
- Rubrospinal tract
- Reticulospinal tract
- Reflex connections
- Direct, or via:
- Interneurons from same segment
- Interneurons from other segments
- Degree of muscle contraction is a function of total activity in motor neuron pool
- Inhibitory influence on this area due to Renshaw cells
Kinds of Reflexes
Stretch reflex--monosynaptic reflex arc
- Monosynaptic reflex arc
- Simplest
- Receptor from central region (nuclear bag) of intrafusal fiber
- Intrafusal fibers scattered about between the extrafusal fibers that do work
- Intrafusal fibers are in parallel with extrafusal fibers
- When intrafusal fibers are stretched, they fire at a rate that correlates somewhat with the degree of stretch
- Reflex that resists all movement
- Primary sensory neuron to spinal cord
- Synapse directly on motorneurons
- Fibers contract, decreasing stretch
- Higher centers not required
- Movement - inhibition of this reflex
Disynaptic reflex arc
- One connecting neuron
- Golgi tendon organ
- Action is inhibitory
- Higher threshold than monosynaptic
- Can over-ride stretch reflex
- Change detectors
Multisynaptic reflex arcs (flexion)
- Touch and pain receptors to spinal cord
- Connecting neuron
- Motorneurons
- Suprasegmental reflex involves more segments of spinal cord (cooperative reflex)
Reciprocal innervation (inhibition) via Renshaw cells
Other reflexes
- Crossed extensor--weight adjustment
- Clasp knife--resist injury
- Extensor thrust--pressure on feet
- Grasp reflex
- Placing, hopping, and righting
Influence of Higher Centers
Muscle tone
- Descending reticular activating system
- Via reticulospinal tract
- Motorneuron pool
- Activate alpha motorneurons of antagonistic muscles directly or activate gamma efferent motorneurons
Gamma motorneurons innervate distal portions of intrafusal fiber causing contraction
- When distal ends contract, the primary afferent from nuclear bag region fires setting up the stretch reflex
- Analogous to a thermostat in a house
Somatotopic organization
- Pyramidal system--instigates movement
- Extrapyramidal system--sustains movement
- Cerebellar influence--muscle synergism (all influence motorneurons in motorneuron pool of spinal cord)
- Area related to specific structure
- Sherrington thinks in terms of movements, not muscles
Damage and Recovery
Damage
- Area 4 without COEPS--flaccidity occurs
- COEPS fibers--responsible for spasticity
- Pyramidotomy--flaccidity; grasp increases (hung on bars)
- Since pyramidal tract was given inhibitory role, its loss accounts for flaccidity
Recovery of voluntary power depends on:
- Extent of cortex removed -- bilateral is more paralyzing than unilateral removal
- Time between operations--if much delay between stages, control is regained
- Age--in infant, animal is at first little affected but deficit appears with development
- Retraining
- Time after operation
Mechanisms of recovery
- Compensation--use of previous minor innervation; doubtful vicariation occurs (innervation by previously non-effective neural structure)
- Release phenomena--what is released?
- The cause is not the lesion (only antecedent condition; e. g., handbrake on car does not cause it to go forward) but there is loss of structure
- For spasticity to appear, some facilitatory tract from brain stem must remain functional
Terms to Know
| Extrafusal fibers |
Rubrospinal tract |
Ventral spinocerebellar tract |
| Intrafusal fibers |
Stretch reflexes |
Suprasegmental reflexes |
| Reciprocal innervation |
Extensor thrust |
Crossed extension reflex |
| Renshaw fibers |
Flexion reflex |
Brachium conjunctivium |
| Isotonic |
Innervation ratio |
Decerebrate rigidity |
| Gamma efferents |
Alpha motorneuron |
Monosynaptic reflex |
| Isometric |
Muscle spindle |
Segmental reflex |
| Nuclear bag |
Golgi tendon organ |
Motor neuron pool |
| Motor unit |
Clasp knife reflex |
Stretch receptor (spindle) |
