2.3 Spinal Cord Mechanisms in Motor Control Flashcards
Where do lower motor neurons originate
Lower motor neurons exit from the ventral horn (grey matter) of the spinal cord segment
What are some inputs into the motor neuron at the ventral horn.
Upper motor neurons from the brain, segmental reflex nerves from the periphery and inhibitory/excitatory interneurons in the spinal cord
What are the lower motor neurons that innervate skeletal muscle known as
Alpha motor neurons
Describe the organisation of these motor neurones
Each alpha motor neurone innervates many muscle fibres, forming a motor unit. Each muscle is innervated by many motor neurones, forming the motor neurone pool for the muscle. The motor neurone pool for each muscle will come from a column in the ventral horn. Collections of motor neurones in the ventral horn are called motor nuclei. There is segmental distribution of these motor neuron spinal columns, the cervical spinal cord innervating the arms and upper body, the thoracic innervating the trunk and the lumbar and sacral innervating the legs.
What are 2 ways the strength of contraction can be graded by
The frequency of the muscle contraction - is there summation/tetany
How many motorneurones are recruited (the most important way)
What is a reflex
An involuntary, stereotyped response to a stimulus
What are tendon jerk reflexes
A spinal reflex where muscle contractions are triggered by a muscle stretch. They can be useful clinically for assessing neurological function. It is a monosynaptic reflex. This is because the 1a afferents activated by the muscle stretch synapse directly onto the alpha motor neurone (at the spinal cord) that innervates the same muscle
What is the stretch reflex
The stretch reflex uses the same monsynaptic circuitry as the tendon jerk reflex. However it is happening continuously to maintain position and balance of the joints during movement. The sensitivity of the stretch reflex is modified for different muscles where some muscles are very responsive to movement (stretch) and stiff but others are less responsive to movement and can be looser
How can stretch reflex control be disrupted
Disorders such as stroke, spinal cord injury and basal ganglia disorders can disrupt descending control of reflexes. This can lead to spasticity which is increased muscle tone or hyperflexia which is exaggerated tendon jerk reflexes.
Describe 1a reciprocal inhibition
Where detection of stretch in the agonist muscle is detected but afferents send a signal to an inhibitory interneuron so the antagonist muscle does not contract. It is reciprocal as this can also work the other way around
What are the 2 descending pathway systems
The lateral pathways which controls voluntary movement in the distal limbs
The ventromedial pathway controls posture and balance in the proximal and axial muscles.
Describe the intrinsic circuits for rhythmic activity
Intrinsic circuits of the spinal cord produce rhythmic activity to driving stepping/walking. It is done by alternating flexor and extensor muscles that are coordinated across opposite limbs.
