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How can lower motor neuron excitability be influenced?
By upper motor neurons and local circuits in the spinal cord.
What are the lesion locations that can cause upper motor neuron injury?
Lesions in the brain or spinal cord affecting UMNs.
What are the lesion locations that can cause lower motor neuron injury?
Lesions in the spinal cord affecting LMNs or peripheral nerves.
What causes spinal cord infarction?
Usually due to occlusion of the anterior spinal artery, often from aortic dissection or thromboemboli.
What is the role of spinal cord circuitry in locomotion?
It contributes to rhythmic movements and is not limited to reflexive responses to sensory inputs
What are central pattern generators (CPGs)?
Neural circuits that control the timing and coordination of rhythmic movement patterns
How is locomotion characterized?
As a cycle of stance phase and swing phase
What evidence supports the existence of CPGs in animals?
Research with spinalized cats showed rhythmic stepping of hindlimbs even without sensory input
What is required for bipedal locomotion beyond spinal cord circuitry?
Supraspinal (upper motor neuron) pathways input for postural control
What initiates goal-directed movement in humans?
Motor cortex and basal ganglia, with assistance from the cerebellum and brainstem
What are the two types of circuitry involved in human locomotion?
Central pattern generators and reflex circuits
What is the role of the periaqueductal gray matter?
Modulation of pain, sympathetic responses, and decussating interneurons
What is the function of the motor nuclei in the ventral horn?
Innervate skeletal muscle of the neck, trunk, and limbs
What is the role of the commissural nucleus?
Modulates motor output to skeletal muscle
Where are CPGs primarily located?
In the intermediate and ventral gray matter of the spinal cord.
What functions do CPGs serve beyond locomotion?
They also facilitate movements such as scratching in dogs and swimming in animals.
What role do cutaneous sensations play in locomotion?
They help facilitate extensor muscle activity during stance.
How does proprioception influence CPGs?
CPGs are facilitated by input from proprioceptors, such as muscle spindles and Golgi tendon organs (GTOs).
What is the significance of hip extension in locomotion?
Hip extension is critical for transitioning from stance to swing during walking.
What happens during the late stance phase of locomotion?
Calf muscle stretch facilitates the transition from stance to swing.
How does load affect locomotion?
Increased muscle tension in weight-bearing conditions stimulates GTOs, facilitating extensor activation.
What are the five functions of supraspinal areas in relation to CPGs?
1. Activating spinal CPGs, 2. Controlling CPG intensity, 3. Maintaining equilibrium, 4. Adapting limb movement, 5. Coordinating locomotion with other motor acts.
Which brain regions are involved in supraspinal influences on locomotion?
Sensorimotor cortex, cerebellum, basal ganglia, mesencephalon, mesencephalic locomotor region (MLR), and reticulospinal neurons.
What is the role of the mesencephalic locomotor region (MLR)?
It triggers locomotion and adjusts movement speed by modulating spinal cord input.
How does the cerebellum contribute to locomotion?
It refines muscle activation timing by receiving feedback from CPG outputs and peripheral motor apparatus.
What functions do the basal ganglia serve in locomotion?
They are involved in the planning, initiation, execution, and termination of motor programs.
What impact does spinal cord injury have on CPGs?
It disrupts input from supraspinal centers, but reorganization in spared pathways may occur.
Why is task-specific training important after spinal cord injury?
It is essential for improving locomotion despite diminished sensory input.
What is the relationship between weight-bearing and gait?
Weight-bearing and a step-through gait pattern are critical for facilitating locomotion.
What are 'motor neuron pools' in the context of locomotion?
They are groups of neurons in the spinal cord that contribute to locomotion control.
What is the structure of a central pattern generator?
A CPG consists of two units, called half-center oscillators, that produce flexion and extension on opposite sides.
What types of input do CPGs receive?
CPGs receive descending input from the cortex, sensory feedback from muscle spindles, and connections to the opposite half-center oscillator.