8.4 Voluntary movements all processes involved

Voluntary movements all processes involved

• Upper motor neurons (UMNs)

• Basal ganglia & cerebellum

• Local circuit neurons (interneurons)

• Lower motor neurons (LMNs)

Upper motor neurons (UMNs):

• Modulate the activity of lower motor neurons

• They “order” the movements

• Soma in motor cortex (or in some brainstem centers).

• Axons synapse directly with lower motor neurons & with the local circuit interneurons.

Basal ganglia & cerebellum

• help regulate UMNs, ensuring that movements are performed correctly (initiation, force, coordination).

Local circuit neurons (interneurons

• They receive input from UMNs & sensory neurons.

• They provide higher integration & can participate in reflex arcs.

Lower motor neurons (LMNs)

• Soma in spinal cord or brainstem

• Axons synapse with the skeletal muscle to activate contraction.

VOLUNTARY movement control involves

• Decision-making & planning

• Initiation of the movement

• Execution of the movement

Motor unit

• Lower motor neuron + skeletal muscle fibers innervated by the same motor neuron

muscles for fine motor actions containing 

• few fibers for precise control

muscles for gross motor actions

• thousands of fibers allowing fro powerful simultaneous contraction.

Neuromuscular Junction

• is the synaptic connection between the lower motor neuron & a muscle cell → resulting in a muscle contraction

Neuromuscular junction neurotransmitters

• ACETYLCHOLINE

Neuromuscular junction Receptor

• Nicotinic Ach receptor

Neuromuscular junction Cessation of response

• Acetylcholinesterase

Neuromuscular junction process →

• The acetylcholine released from axon endings of the LMN diffuses across the synaptic cleft & binds to the ACh receptors located on the plasma membrane of the muscle cell, thereby stimulating the muscle cell & inducing a muscular action potential.

The neuromuscular synapse triggers the

• excitation-contraction coupling

Excitation-Contraction coupling process

1. Motor neuron action potential

2. Action potential on skeletal muscle cell

3. Contraction (response)

Threshold stimulus for excitation-concentration

• Minimum level of stimulation for a fiber to contract.

• Subthreshold stimuli have no effect on the fiber contraction.

• After the contraction, the muscle fiber immediately returns to the relaxed state.

Tetanization

• The action potential in skeletal muscle does NOT have a refractory period

• If a series of stimuli arrive in rapid succession, the muscle does not have time to fully relax before the next phase of contraction begins

Tetanus

• When a motor unit is stimulated at such a high frequency that the individual muscle twitches fuse together → sustained contraction.

Mechanisms to increase the intensity of muscle contraction

• Multi-fiber summation

• Frequency summation (rate coding)

 Frequency summation (rate coding)

• Increasing the frequency of the action potential, until tetanization

Multi-fiber summation 

• Increasing the number of motor units that contract simultaneously.

Contractile Unit:

• SARCOMERE (skeletal muscle)

SARCOMERE

• Thin filaments (actin)

• Thick filaments (miosin)

• Regulatory proteins

SARCOPLASMIC RETICULUM

• The specialized endoplasmic reticulum of muscle cells, wrapping around the filaments.

• Concentrates and stores Calcium (Ca2+), whose release will play a key role in contraction.

T-TUBULE

• Invaginations in the plasma membrane of muscle cells, they help propagation of the action potential (depolarization) into the interior of the muscle fiber

MUSCLE CONTRACTION MECHANISM

1. Action potential arrives at the motor neuron.

• The motor neuron releases ACh.

• ACh binds to the nicotinic receptor in the muscle fiber.

2. Entry of Na+ through Ach nicotinic receptor channels initiates a muscle action potential.

• Propagation of action potential into the muscle fiber by the T-tubules

3. Depolarization opens channels in the sarcoplasmic reticulum & Ca2+ is released into cytoplasm.

4. Ca2+ ions cause the thick & thin filaments to slide against each other, which constitutes the process of contraction

5. Ca2+ ions are pumped back into the sarcoplasmic reticulum by means of a calcium pump, & the filaments go back to the relaxed position.

MUSCLE TONE

• muscles in tension even when we are at rest

Maintaining posture

• Flaccid muscles (hypotonia)

• Spastic muscles (hypertonia)

Flaccid muscles (hypotonia)

•  Lower tone than normal

Spastic muscles (hypertonia)

• Higher tone than normal

Muscle tone is controlled by

•  negative feedback mechanisms in the spinal cord (motor medullary reflex)