The Spinal Cord and Motor Activity

8/7/2019 The Spinal Cord and Motor Activity

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The Spinal Cord and Motor Activity



Spinal Cord

Motor activity

The physical activity of an organism as a behavioural

phenomenon.

The brain controls and plans movement but is dependent on

the spinal cord to provide the neural circuity for rythmic

movement.

Without the special neuronal circuits of the cord, even the most

complex motor control system in the brain couldn¶t cause any

purposeful muscle movement.



Organization Of The Spinal Cord

For Motor Functions

The cord gray matter is the integrative area for the cord

reflexes.

The gray¶s matter neurons are:

1.Posterior sensory neurons

2.Anterior motor neurons

3.Interneurons



Posterior Sensory Neurons

Sensory signals enter the cord almost entirely through the

sensory (posterior) root.

After entering the cord ,every sensory signal travels to twoseparate destinations :

1.One branch of the sensory nerve terminates almost

immediately in the gray matter of the cord and elicits local

segmental cord reflexes and other local effects.

2.Another branches transmits signals to higher levels of the

nervous system ±to higher levels in the cord itself , to the brain

stem or even to the cerebral cortex



Anterior Motor Neurons

They give rise to the nerve fibers that leave the cord by way of

the anterior roots and directly innervate the skeletal muscle

fibers.

The neurons are of two types

1. Alpha motor neurons

2. Gamma motor neurons



Int r neur ns

Interneurons

Neuron with its process exclusively

confined within a certain area.

Interneurons are presents in all areas

of the cord gray matter ±in the dorsal horn

the anterior horns, and the intermediate areas between them.

Synaptic interneurons receive synaptic input from:

1.Primary sensory axons

2.Descending axons from the brain

3.collaterals of lower motor neuron axons



Renshaw Cell Inhibitory System

Renshaw cells are inhibitory interneurons that are associated in

two ways with an alpha motor neuron:

1.They receive an excitatory collateral from the alpha neuron¶s

axon as they emerge from the motor root ( using acetylcholine

as a neurotransmitter),and are thus ³kept informed ́ of how

vigorously that neuron is firing.

2.They send an inhibitory axon (using glycine) to synapse with

the cell body of the initial alpha neuron and/or an alpha motor

neuron of the same motor pool







Renshaw cells remove ³noise´ by dampening the firing

frequency of over-excited neurons with a negative feedback

loop, which prevents weakly excited alpha motor neurons from

firing.

Renshaw cells inhibit the inhibitory interneurons that act on

antagonist motor neurons and inhibit motor neurons that

innervate synergist muscles.

Renshaw cells thus act as ³limiters´ or ³governors´ on the alpha

motor neuron system , thus helping to prevent muscular

damage from tetanus.




Renshaw cells utilize the neurotransmitter glycine as an

inhibitory substance that synapses on the alpha motor neurons

Strychnine specifically acts on these

cell¶s ability to control alpha motor

neuron firing by binding to the glycine

receptors on the motor neuron .



Renshaw Cell Inhi i t r stem

They are also the target of the toxin

of Clostridium tetani , which travels

to the spinal cord where it inhibits

the release of glycine from Renshawcells .As a result ,alpha motor neurons

become hyperactive ,and muscles

constantly contract.




Early dysfunction and/or loss of Renshaw cells has been

suggested to occur in experimental ALS

The reported early loss of Renshaw cells

in ALS mice led Foranai et al to hypothesize

that :

³



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The Spinal Cord and Motor Activity