Weeks 4 & 5 - Circuits and Movement

How is information encoded in the pattern of action potential discharge?

To achieve the task of producing movement, motoneurons respond by recruiting (activating) an increased number of motor units within a muscle to gradually generate greater force or increase rate of action potential discharge overtime to gradually generate greater force.

Neuronal pool

A collection of neurons that work together to perform a specific function.

Circuit

Pathways that determine how information moves between or within neuronal pools.

Reflex

An action that is performed unconsciously in response to stimuli.

What is the difference between withdrawal and stretch reflex?

Although both are types of reflexes, withdrawal reflex is a simple circuit reflex which is an involuntary and rapid response that occurs when the body encounters a painful or harmful stimulus (e.g. touching a hot pan) and stretch reflex is a polysynaptic reflex which is an involuntary response where the muscle contracts when muscle stretches.

What is the difference between simple (monosynaptic) reflex and polysynaptic reflex?

A simple circuit involves a direct synapse from sensory to motor neuron bypassing the brain whereas a polysynaptic reflex involves one or more interneurons and multiple synapses.

How do the axons of alpha motoneurons exit the spinal cord and travel to skeletal muscle and which types of skeletal muscles do these neurons innervate?

Alpha motoneurons axons exit the spinal cord through the ventral root, join the spinal nerve, and travel to skeletal muscle fibres via peripheral nerves. Alpha motoneurons innervate extrafusal muscle fibres, which generate tension.

How do the axons of alpha motoneurons exit the brain stem and which types of skeletal muscles do these neurons innervate?

Alpha motoneurons axons exit the brain stem through the motor roots of cranial nerves and innervate extrafusal muscle fibres, which generate tension.

How do alpha motoneurons receive stimulation (input) from the brain?

Alpha motoneurons receive stimulation from the brain via descending input pathways down to the spinal cord.

What are the six motor tracts?

• Corticospinal tract

• Corticobulbar tract

• Tectospinal tract

• Reticulospinal tract

• Vestibulospinal tract

• Rubrospinal tract

Corticospinal tract (function and origin + end)

Corticospinal tract is responsible for goal-directed limb movement. Originates from primary motor cortex to alpha motoneurons in ventral horn.

Corticobulbar tract (function and origin + end)

Corticobulbar tract is responsible for control of skeletal muscles innervated by cranial nerves. Originates from primary motor cortex to alpha motoneurons in midbrain or medulla oblongata.

Tectospinal tract (function and origin + end)

Tectospinal tract is responsible for orientation of head towards visual target. Originates from superior colliculus to alpha motoneurons in ventral horn.

Reticulospinal tract (function and origin + end)

Reticulospinal tract is responsible for maintenance of standing posture by ⬆ or ⬇ reflex control of antigravity muscles. Originates from pontine reticular formation to alpha motoneurons in ventral horn.

Vestibulospinal tract (function and origin + end)

Vestibulospinal tract is responsible for head stability. Originates from vestibular nucleus to alpha motoneurons in ventral horn.

Rubrospinal tract (function and origin + end)

Rubrospinal tract’s potential function is to be involved in primitive types of movement, however, role has been diminished with evolution. Originates from red nucleus to alpha motoneurons in ventral horn.

Neuroplasticity

Structural and/or functional reorganisation in the brain in response to learning, experience, or injury.

What is the mechanism that underlies neuroplasticity?

Reorganisation which refers to changes in synaptic efficacy (or strength).

What is the difference between long-term potentiation (LTP) and long-term depression (LTD)?

LTP occurs when pre-synaptic neuron is stimulating the post-synaptic neuron when the post-synaptic neuron is already strongly depolarised (⬆ synaptic efficacy) whereas LTD occurs when pre-synaptic neuron is stimulating the post-synaptic neuron when the post-synaptic neuron is already weakly depolarised (⬇ synaptic efficacy).

What is the key brain structure involved in speech?

Primary motor cortex because it sends movement commands down to the >100 muscles (in the mouth, pharynx, larynx, and thoracic cavity) involved in speech.

What is the importance of the larynx in sound production?

Air passes through rima glottidis, which is an opening between the vocal cords, causing vibrations which produces sound.

Compared to control of limb movement, how does control of speech differ?

Control of limb movement is primarily motor and is often oriented to physical objects compared to control of speech, which is more complicated and mechanisms are still not fully understood which involves integration of many senses and brain regions.

Muscle fatigue

Exercise-induced decline in voluntary force or power.

What are the general central mechanisms that underlie muscle fatigue?

Occurs in the nervous system, which accounts for progressive decline in voluntary activation of muscle.

What are the general peripheral mechanisms that underlie muscle fatigue?

Occurs within skeletal muscle fibres, which accounts for majority of the decline in voluntary force or power.