2.4 Nervous system and motor unit recruitment

The nervous system plays a crucial role in strength and conditioning. It controls muscle contractions through motor neurons and motor units. Understanding how the nervous system recruits and coordinates muscle fibers is key to optimizing training and performance.

Neural adaptations are a major factor in early strength gains from training. These include increased motor unit recruitment, improved synchronization, and faster firing rates. Over time, these changes lead to more efficient and powerful muscle contractions.

Nervous System Structure and Function

Central and Peripheral Nervous System

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• The nervous system is divided into two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS)
• The CNS consists of the brain and spinal cord, which process and integrate sensory information, make decisions, and initiate motor responses
• The PNS consists of all the nerves that connect the CNS to the rest of the body, including sensory receptors, motor neurons, and autonomic nerves
  • Sensory (afferent) neurons carry information from sensory receptors (mechanoreceptors, chemoreceptors, photoreceptors) to the CNS for processing
  • Motor (efferent) neurons carry signals from the CNS to effector organs (muscles, glands) to initiate a response
  • The autonomic nervous system (ANS) is part of the PNS and regulates involuntary functions (heart rate, digestion, respiratory rate). It is divided into the sympathetic and parasympathetic divisions

Neuron Structure and Communication

• The basic functional unit of the nervous system is the neuron, which consists of a cell body, dendrites, and an axon
• Neurons communicate with each other through electrical and chemical synapses
• Electrical synapses allow direct and rapid transmission of signals between neurons through gap junctions
• Chemical synapses involve the release of neurotransmitters (glutamate, GABA, dopamine) from the presynaptic neuron, which bind to receptors on the postsynaptic neuron, triggering a response

Motor Neurons in Muscle Contraction

Motor Neuron Structure and Function

• Motor neurons are specialized nerve cells that carry signals from the CNS to skeletal muscles, causing them to contract and generate movement
• The cell body of a motor neuron is located in the spinal cord or brainstem, and its axon extends out to the muscle fibers it innervates
• At the neuromuscular junction (NMJ), the axon terminal of the motor neuron releases the neurotransmitter acetylcholine (ACh), which binds to receptors on the muscle fiber membrane, initiating a series of events that lead to muscle contraction

Motor Neuron Size and Innervation

• The size of the motor neuron and the number of muscle fibers it innervates determine the precision and force of the resulting movement
  • Smaller motor neurons innervate fewer muscle fibers and are recruited for fine, precise movements (hand muscles)
  • Larger motor neurons innervate more muscle fibers and are recruited for gross, powerful movements (leg muscles)
• The activation of motor neurons is regulated by descending signals from the motor cortex and other brain regions, as well as by sensory feedback from muscle spindles and Golgi tendon organs

Motor Units and Recruitment Patterns

Motor Unit Structure and Function

• A motor unit consists of a single motor neuron and all the muscle fibers it innervates. It is the smallest functional unit of the neuromuscular system
• The number of muscle fibers innervated by a single motor neuron varies depending on the muscle's function. Muscles involved in fine, precise movements have smaller motor units (extraocular muscles), while those involved in gross, powerful movements have larger motor units (quadriceps)

Motor Unit Recruitment and Synchronization

• Motor units are recruited in a specific order during muscle contraction, known as the size principle. Smaller motor units, which generate less force, are recruited first, followed by progressively larger motor units as the force requirement increases
• The recruitment of motor units is regulated by the nervous system to match the force output to the task demands. This is achieved through the modulation of motor neuron firing rates and the number of motor units recruited
• During sustained muscle contractions, motor units are recruited and de-recruited in a cyclical manner to prevent fatigue, a process known as motor unit rotation
• The synchronization of motor unit firing rates can also contribute to increased force output during high-intensity muscle contractions

Neural Adaptation in Strength Training

Neural Adaptations and Strength Gains

• Neural adaptation refers to the changes in the nervous system that occur in response to strength training, leading to improved muscle force production and coordination
• In the early stages of strength training, neural adaptations are the primary contributors to strength gains, while muscle hypertrophy becomes more significant in later stages
• Neural adaptations include increased motor unit recruitment, improved motor unit synchronization, and enhanced firing rates of motor neurons
  • Strength training can increase the number of motor units recruited during a muscle contraction, allowing for greater force production
  • Synchronization of motor unit firing rates can also improve, leading to more efficient and coordinated muscle contractions
  • The firing rates of individual motor neurons can increase, allowing for more rapid and forceful muscle contractions

Motor Learning and Coordination

• Neural adaptations also involve changes in the organization and efficiency of the motor cortex and other brain regions involved in motor control
• Improved intermuscular coordination, or the coordination between different muscle groups (agonists, antagonists, synergists), is another neural adaptation that contributes to enhanced performance in complex movements
• The extent and rate of neural adaptations vary depending on factors such as training intensity, volume, frequency, and the individual's training status