Motor System and Muscle Contraction

Organization of Muscle

Skeletal muscle is organized into several hierarchical structures that contribute to its function. A muscle as a whole is composed of muscle bundles that contain groups of fascicles. Each fascicle consists of muscle fibers, which are long and fibrous; these muscle fibers are the individual muscle cells. Each muscle fiber contains myofibrils, which are further divided into functional units known as sarcomeres. The muscle cell membrane is referred to as the sarcolemma. Within the sarcomere, there are clear demarcations known as Z lines that delineate the boundaries of each sarcomere.

Muscle Contraction Mechanism

Muscle contraction occurs when myosin motors move along actin filaments within the sarcomere. This action is driven by the hydrolysis of ATP and the role of calcium ions (Ca++). The interaction between actin and myosin leads to a contraction at the level of the muscle fiber, which collectively results in the contraction of the entire muscle.

Anatomy of the Neuromuscular Junction (NMJ)

The NMJ is composed of both pre-synaptic and post-synaptic elements. The motor neuron's axon terminals synapse with the muscle cell near the T tubules. T tubules are extensions of the muscle cell membrane that dive deep into the muscle, facilitating rapid action potential transmission and calcium concentration regulation. In cardiac muscle, calcium activates ryanodine receptors located on the sarcoplasmic reticulum, leading to contraction. In contrast, skeletal muscle involves a direct link between L-type calcium channels and ryanodine receptors, allowing calcium release from the sarcoplasmic reticulum without requiring an influx of Ca++ as in cardiac muscle. This close association facilitates synchronized calcium release throughout the muscle fiber, enhancing contraction force. T tubules are associated with terminal cisternae, forming structures known as triads in skeletal muscles.

Effects of Acetylcholine (ACh) at the NMJ

Acetylcholine (ACh) is the neurotransmitter at the NMJ. Transmission is terminated by various mechanisms, such as stopping motor neuron stimulation, blocking calcium channels, or depleting ACh stores. The arrival of an action potential (AP) at the NMJ results in ACh release, binding to receptors on the muscle cell, leading to sodium influx, depolarization, and the propagation of the AP into T tubules.

Sequence of Muscle Contraction

The muscle contraction process begins with an action potential generated in the sarcolemma, following these steps:

1. The AP travels down through the motor neuron.

2. ACh is released at the NMJ.

3. ACh binds to its receptors, causing Na+ influx and depolarizing the muscle cell.

4. The AP propagates into the T tubule, triggering Ca++ influx.

5. This influx signals the release of more Ca++ from the sarcoplasmic reticulum.

6. Released Ca++ activates troponin, which influences myosin ATPase activity and muscle contraction.

7. Myosin heads move along actin filaments in a cyclical process, resulting in sarcomere contraction.

Motor Units and Motor Pools

A motor unit consists of a motor neuron and the muscle fibers it innervates. The spinal cord contains motor pools, which consist of all the motor units responsible for contracting a single muscle. Each muscle fiber is innervated by a single motor neuron, which can have multiple axonal projections, influencing the muscle's overall activity.

Size Principle in Motor Neurons

The response of motor neurons to synaptic input is influenced by their size. This relationship can be understood through Ohm's law: larger motor neurons, with lower resistance, require a higher input to generate an action potential than smaller motor neurons, which can achieve significant voltage changes with less current input. This size principle explains the motor recruitment during muscle force generation.

Motor Pathways and Spinal Cord Thickness

The pathway from the primary motor cortex to the spinal cord involves synaptic connections that vary in thickness across different spinal cord regions due to the density of neuronal inputs and outputs. Thicker areas indicate a higher concentration of motor neuron bodies and associated synaptic activity necessary for muscle control.

Sensory Feedback in Motor Control

Sensory feedback plays a crucial role in motor activity. It allows the integration of visual and tactile information necessary for interaction with the environment. For example, a tennis player uses visual feedback to track a moving ball and tactile feedback from their grip on the racket to execute swings effectively. Reflexes, such as the knee-jerk reflex, exhibit sensory feedback control without conscious awareness, demonstrating how sensory pathways facilitate coordinated muscle contractions.

Monoaminergic Modulation of Motor Neurons

Monoamines, including norepinephrine, serotonin, and dopamine, modulate motor neuron firing rates. Norepinephrine influences motor activity depending on vigilance, while serotonin helps maintain long-term potentiation and skill execution. Dopamine enhances motor learning by adjusting the signal-to-noise ratio and promoting outputs. This neuromodulation occurs at pre- and post-synaptic levels, impacting both projection neurons and interneurons.

Central Pattern Generators (CPGs)

Central pattern generators are neural circuits that generate rhythmic motor patterns. Experimental evidence from invertebrates such as crustaceans has shown that these rhythms can occur independently of sensory input or motor neuron activity, demonstrating intrinsic pacemaker rhythms in individual cells. The study's advantages include a smaller number of easily identifiable neurons and accessible recording capabilities, aiding in understanding CPG dynamics.