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Muscle firing is a process that involves the brain and the muscles. When a motor neuron sends a signal to a muscle, it is said to be firing, which then causes the activation or contraction of the motor units in the muscles. This process is also known as muscle activation or muscle contraction. The contraction of a skeletal muscle is a complex process, and the activation order of muscle groups can vary between individuals. As muscles are made up of thousands of small fibres, different fibres are activated depending on the type of exertion, and as people get stronger, more fibres are used.
| Characteristics | Values |
|---|---|
| Definition | Muscle firing refers to the activation or contraction of muscles through signals sent by motor neurons. |
| Process | When a motor neuron sends a signal, it activates or contracts motor units in the muscle it controls. This results in muscle fibres stretching and pressing together, leading to movement. |
| Control | Each bundle of muscle fibres has a control unit that sends out signals to turn on or off specific fibres. The control pattern is learned and reinforced through repetition. |
| Variability | Muscle firing patterns can vary between individuals, with no consistent pattern observed in asymptomatic individuals. |
| Recruitment | Motor units are recruited in order of their size, starting with small and weak units and progressively adding larger and stronger units, resulting in a smooth increase in muscle strength. |
| Frequency | The firing frequency increases with greater muscle contraction strength. Facial muscles have shorter recruitment intervals and higher frequencies compared to extremity muscles. |
| Number of Muscles | The human body has over 600 muscles that enable various movements and support vital functions such as breathing. |
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What You'll Learn
Muscle activation and contraction
The second step involves the chemical reactions that lead to muscle fibres reorganising themselves to shorten the muscle, resulting in a contraction. This occurs when acetylcholine binds to receptors on the muscle fibre membrane, opening membrane channels. This allows sodium ions to enter the muscle fibre, sending a message to release calcium ions. The calcium ions then diffuse into the muscle fibre, leading to a contraction as the chains of proteins within the muscle cells rearrange.
The third step is the reversal of the chemical process, leading to muscle relaxation. When the nervous system signal stops, the chemical reaction is halted, and the muscle fibres return to their original state, relaxing the muscle.
The sliding filament theory explains how muscle contractions occur. The actin and myosin filaments within the muscle fibres bind to create cross-bridges and slide past each other, resulting in a contraction. This process is fuelled by ATP, an energy compound used by all cells.
Muscle firing refers to the process of muscle activation and contraction. When a motor neuron sends a signal to the muscles, it is said to be "firing", activating and contracting the motor units in the muscles. The degree of muscle firing can vary, depending on the strength of the muscle and the type of movement.
Muscle contractions can be described in terms of force and length. Muscle tension refers to the force exerted by the muscle on an object, while load refers to the force exerted by an object on the muscle. Isometric contractions occur when muscle tension changes without any change in muscle length. Conversely, isotonic contractions involve changes in muscle length while tension remains constant.
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Motor neurons and units
Motor neurons, also known as efferent neurons, are nerve cells that carry signals from the central nervous system to muscles, glands, and organs, causing voluntary or involuntary movement. They are the most common structure for neurons and are typically multipolar, meaning they have a single axon and multiple dendrites. The axon sends impulses away from the soma, or cell body, while the dendrites carry incoming information.
There are two types of motor neurons: upper and lower. Upper motor neurons originate in the cerebral cortex (primary motor cortex) and travel to the brainstem or spinal cord, where they synapse. Lower motor neurons, on the other hand, originate in the brainstem or spinal cord and project to innervate muscles and glands throughout the body. They are also known as somatic motor neurons and are further divided into alpha, beta, and gamma categories. Alpha motor neurons are the primary means of skeletal muscle contraction, innervating extrafusal muscle fibres. Beta motor neurons innervate both extrafusal and intrafusal fibres, while gamma motor neurons innervate muscle spindles and dictate their sensitivity.
The motor neuron and its associated muscle fibres make up a motor unit. Fine muscles, such as extraocular muscles, have small motor units and can be controlled more precisely compared to larger muscles like the biceps. When a motor neuron sends a signal to a muscle, it activates or contracts the motor units in the muscle that it controls. This process is known as muscle activation or contraction and involves the release of neurotransmitters and the activation of muscle fibres.
The degree of muscle activation depends on how much of the muscle is involved and which specific muscle fibres are activated. As an individual gets stronger, more muscle fibres are utilised. This control pattern is learned, and the patterns that work are reinforced.
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Muscle fibres
There are three types of muscle fibres: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). Most skeletal muscles in the human body contain all three types, but in varying proportions. The speed of contraction depends on how quickly myosin's ATPase hydrolyzes ATP to produce cross-bridge action. Fast fibres hydrolyze ATP approximately twice as rapidly as slow fibres, resulting in much quicker cross-bridge cycling.
Skeletal muscle fibres are classified into two types: type 1 and type 2. Type 2 is further broken down into subtypes 2A and 2B. Type 1 fibres utilize oxygen to generate energy for movement and have a higher density of energy-generating organelles called mitochondria, which makes them dark. Type 2A fibres can also use oxygen to generate energy but contain fewer mitochondria, making them light. Type 2B fibres do not use oxygen to generate energy; instead, they store energy that can be used for short bursts of movement and appear white.
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Muscle recruitment patterns
Motor unit recruitment follows a specific order, with smaller motor neurons being activated before larger ones. This is known as Henneman's size principle, which suggests that smaller motor neurons have a smaller surface area and higher membrane resistance, resulting in higher voltage changes during activation. The three types of motor units, as categorised by Burke, are S (slow-twitch), FR (fast-twitch, fatigue-resistant), and FF (fast-twitch, fatigable). However, modern research indicates that human motor units may be more complex and might not fit neatly into these categories.
During shoulder flexion, for example, the rotator cuff muscles exhibit direction-specific recruitment patterns. Studies have shown that the anterior deltoid, pectoralis major, supraspinatus, infraspinatus, serratus anterior, and upper and lower trapezius muscles are activated at similar moderate levels. The subscapularis muscle, on the other hand, is activated at significantly lower levels. The supraspinatus muscle appears to consistently initiate flexion, with its activity onset occurring simultaneously with the anterior deltoid and prior to movement.
It is important to note that muscle recruitment patterns can be influenced by various factors, such as the complexity of the task, individual muscle strength, and previous learning experiences. For instance, when performing a lift or any exertion, there is a "rotation" of which muscle fibres are utilised. As an individual gets stronger, more muscle fibres are recruited, and the control patterns are "learned", leading to improved performance.
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Voluntary and involuntary movements
Human movements are classified into two main types: voluntary and involuntary. Voluntary movements, such as running and dancing, are intentional and consciously controlled. On the other hand, involuntary movements occur without conscious control and are divided into normal non-intentional movements, reflexes, and pathological non-intentional movements.
Voluntary movements are mediated by several descending motor pathways, which are modified and regulated by two main loops: the basal ganglia loop and the cerebellar loop. These loops influence the final motor commands that drive voluntary actions.
Involuntary movements, on the other hand, are produced by non-intentional, pathological activation within the final common pathways or the two loops mentioned above. They can be further categorized into four major groups: tremor, myoclonus, chorea/ballism, and dystonia/athetosis. Tremor refers to oscillations that may occur at a single site or across multiple loops. Myoclonus is characterised by sudden, brief, shock-like movements that can originate anywhere from the cortex to the muscle. Chorea/ballism refers to sudden, irregular, and phasic movements, while dystonia/athetosis involves sustained muscle contractions that may include torsion components.
Reflexes, a type of involuntary movement, are automatic muscle responses to specific stimuli. They help maintain balance and safety. For example, when you touch something hot, your hand rapidly withdraws, thanks to the activation of sensory receptors in your skin, joints, or muscles. This response is rapid and doesn't involve the brain or conscious attention; instead, it relies on spinal cord neurons. Another example of a reflex is the "knee jerk" response, where a physician strikes the tendon below the knee, causing a rapid leg lift.
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Frequently asked questions
Muscle firing refers to the process of muscle activation, where a motor neuron sends a signal to the muscles, causing them to contract and generate force. Each bundle of muscle fibres has a control unit that sends out signals to turn on certain fibres and switch off others.
There are two types of muscle movements: voluntary and involuntary. Voluntary movements are actions that you consciously control, such as scrolling on your phone or sprinting. Involuntary movements occur automatically without conscious thought, such as your heart beating or breathing.
A common misconception is that muscle firing is related to laser beams or generating maximum contraction. Additionally, people often believe that they have inherent weaknesses when their muscles are held back in new situations, but this is a natural response from the brain.
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