Logan Steele
The human body has over 600 muscles that help us move, breathe, swallow, and survive. The muscular system is responsible for body movement, and about 700 named muscles are attached to the bones of the skeletal system. These muscles are controlled by the brain and nervous system, with the brain stem controlling involuntary muscles and the cerebral motor cortex and cerebellum controlling voluntary muscles. The strength of a muscle's contraction depends on the number of motor units involved and the stimulus from the nervous system. The muscles surrounding synovial joints are responsible for moving the body in space, and muscle actions are often paired, like flexion and extension or abduction and adduction.
| Characteristics | Values |
|---|---|
| Number of muscles in the human body | Over 600 |
| Muscle composition | Skeletal muscle tissue, blood vessels, tendons, and nerves |
| Types of muscle tissue | Visceral, cardiac, and skeletal |
| Types of muscle contractions | Isotonic, isometric, and isotonic-isometric |
| Types of muscle fiber | Type I (slow oxidative), Type IIa (fast oxidative), and Type IIb (fast glycolytic) |
| Muscle movement | Contraction and relaxation |
| Brain regions involved in muscle movement | Cerebral cortex, basal nuclei (basal ganglia), and cerebellum |
| Muscle movement control | Voluntary and involuntary |
| Muscle memory | Refers to the automatic nature of learned movements |
| Muscle actions | Flexion and extension, abduction and adduction, supination and pronation, elevation and depression |
| Types of muscles | Prime movers (agonists) and antagonists, synergists, and stabilizers |
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What You'll Learn
Voluntary vs involuntary movement
Human movements can be broadly categorized into voluntary and involuntary movements. Voluntary movements are self-generated, willed actions performed as a result of cognitive processes. They are rather complex and involve multiple areas within the central (CNS) and peripheral nervous systems (PNS). These movements are consciously initiated and then carried out almost automatically. Examples include walking upright, riding a bike, or bringing a spoon to your mouth. The more we practice a skill, the more automatic it becomes, a phenomenon commonly referred to as "muscle memory".
Voluntary movements depend on upper motor neurons (UMN) and lower motor neurons (LMN). The cell bodies of upper motor neurons are found in the cerebral cortex, where planning, initiation, and coordination of movement occur. The upper motor neurons then synapse with lower motor neurons in the cranial nerve nuclei or in the anterior horn of the spinal cord. The planning step includes forming an idea of what you want to do and organizing the sequence of events to accomplish the movement. In the initiation step, action potentials are sent to the upper motor neurons of the primary motor area, which initiates the movement.
In contrast, involuntary movements are non-intentional and can be further classified into normal non-intentional movements, reflexes, and pathological non-intentional movements. Normal non-intentional movements often occur without causing problems in daily life, such as associate movements, mirror movements, or yawning. Reflexes are responses to external signals or stimuli, such as spinal tendon reflexes or startle responses. Pathological non-intentional movements, also known as "involuntary movements" in clinical practice, are abnormal movements that can be harmful to the patient's daily life and require treatment. Involuntary movements can be caused by situational factors such as caffeine consumption, medication side effects, or anxiety, or neurological factors such as brain injuries, seizures, or Parkinson's disease.
The ability to perform tasks smoothly and precisely is essential, and involuntary movements, tremors, or shakiness can interfere with activities that require accuracy, such as writing or using a touch screen. Involuntary movements originate from anywhere in the output pathways or modulation loops and can be divided into four major groups: tremor, myoclonus, ballism/chorea, and dystonia/athetosis.
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Motor neurons and muscle contractions
The process of muscle contractions can be broken down into several steps. Firstly, planning takes place in the pre-frontal or motor association area of the brain, where the desired movement is formed as an idea. This is followed by initiation, which involves sending action potentials to the upper motor neurons in the primary motor area of the brain. These upper motor neurons then synapse with lower motor neurons in the spinal cord, which directly innervate skeletal muscles and cause contractions.
The strength of muscle contractions can vary depending on the required force. For example, lifting a feather requires minimal effort, while lifting a car demands a much stronger contraction. This variation in force is achieved by stimulating more or fewer muscle fibres within a motor unit. Each motor neuron innervates a certain number of muscle fibres, and by increasing the number of stimulated fibres, the force of contraction can be modulated.
The mechanism of muscle contraction involves the release of neurotransmitters, specifically acetylcholine, from motor neurons. Acetylcholine binds to receptors on muscle fibre membranes, triggering a chemical reaction that reorganises the proteins within the fibres, resulting in contraction. This process is known as excitation-contraction coupling, where neural action potentials are converted into cross-bridge cycling, leading to muscle fibre contraction.
The nervous system plays a crucial role in muscle contractions, generating signals that initiate the process. These signals, known as action potentials, travel through motor neurons and result in the release of acetylcholine at the neuromuscular junction. The release of acetylcholine triggers a series of chemical reactions that ultimately lead to muscle contraction. When the nervous system signal ceases, the chemical process reverses, the muscle fibres rearrange, and the muscle relaxes.
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Planning, initiation, and execution
The human body has over 600 muscles, which, along with bones and joints, form the musculoskeletal system. This system enables us to perform everyday physical activities. The brain and nervous system coordinate and control the movements of the muscles. The cerebral cortex, basal nuclei (basal ganglia), and cerebellum work together to control and facilitate the desired movement.
The process of muscle movement can be broken down into three steps: planning, initiation, and execution. The planning step involves forming an idea of the desired movement in the pre-frontal or motor association area of the brain and then organising and coordinating the sequence of events in the premotor area to accomplish the movement. The initiation step involves sending action potentials to the upper motor neurons of the primary motor area in the precentral gyrus, which initiates the movement. The execution step involves relaying the command from the central nervous system (CNS) through the peripheral nervous system (PNS) to the muscles involved in the movement.
The muscles surrounding synovial joints are responsible for moving the body in space. These muscle actions are often paired, like flexion and extension or abduction and adduction. For example, the biceps brachii, brachialis, and brachioradialis flex the elbow, while the triceps brachii and anconeus extend the elbow. The strength of a muscle's contraction can vary from weak to very strong. It is controlled by the number of motor units involved in the contraction and the amount of stimulus from the nervous system.
The muscles can be further categorised into prime movers, antagonist muscles, synergists, and stabilisers. Prime movers, or agonists, are the muscles that provide the primary force driving an action. Antagonist muscles oppose the prime movers by providing resistance or reversing a movement. Synergists assist the prime movers, while stabilis ers keep bones immobile when needed, such as when maintaining posture.
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Types of muscle contractions
Muscle contractions are essential for the human body to produce movement, maintain body temperature, and store nutrients. There are three types of muscle contractions: isotonic, isometric, and eccentric.
Isotonic contractions are those that generate force by changing the length of the muscle. This can be further classified into two types: concentric contractions and eccentric contractions. Concentric contractions cause muscles to shorten, thereby generating force. For example, when lifting a heavy weight, a concentric contraction of the biceps causes the arm to bend at the elbow, lifting the weight towards the shoulder. On the other hand, eccentric contractions cause muscles to elongate in response to a greater opposing force. An example of this is the controlled lowering of a heavy weight.
Isometric contractions generate force without changing the length of the muscle. This is typical of muscles found in the hands and forearms. For instance, when gripping an object, the joints of the hand do not move, but the muscles generate sufficient force to prevent the object from being dropped.
Eccentric contractions can be both voluntary and involuntary. An involuntary eccentric contraction may occur when a weight is too great for a muscle to bear, and it is slowly lowered while still under tension.
The strength of muscle contractions can vary from weak to very strong. The main mechanism for increasing the force of contraction is to stimulate more muscle fibers.
Additionally, muscles can be categorized into three types based on their fiber composition: Type I, Type IIa, and Type IIb. Type I fibers are slow-twitching fibers with a low rate of fatigue, making them suitable for endurance activities such as marathon running. Type IIa fibers are fast-twitching and are suited for moderate-movement actions like walking. Type IIb fibers are also fast-twitching and are designed for short-duration, intense movements such as sprinting and weight-lifting.
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Skeletal muscle functions
Skeletal muscles are voluntary muscles, meaning that we can control how and when they move and work. They are the most common type of muscle in the body, comprising around 30% to 40% of our total body mass. They are attached to bones via tendons, and they allow us to perform a wide range of movements and functions.
Skeletal muscles are composed of flexible muscle fibres that can contract (tighten) in response to a stimulus, allowing the muscles to move the bones. Each muscle can contain thousands of fibres, and these fibres usually span the length of the muscle. The strength of muscle contractions can vary from weak to very strong. For example, lifting a feather requires less effort than lifting a car.
There are three layers of connective tissue that enclose skeletal muscles and provide structure: the outermost layer is called the epimysium, the middle layer is the perimysium, and the innermost layer surrounding individual muscle fibres is the endomysium. All these connective tissues merge to form a tendon that attaches the muscle to bones.
The main functions of skeletal muscles include:
- Producing movement
- Sustaining body posture and position
- Maintaining body temperature
- Storing nutrients
- Stabilizing joints
From a mechanical standpoint, skeletal muscles convert chemical energy into mechanical energy, generating force and power. From a metabolic standpoint, they contribute to basal energy metabolism and serve as a storage site for essential substrates such as carbohydrates and amino acids.
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Frequently asked questions
Muscles are pieces of soft tissue that help the body move, breathe, swallow, and perform other essential functions to keep us alive. There are over 600 muscles in the human body, and they make up about half of a person's body weight.
Muscles move body parts by contracting and then relaxing. They can pull bones but can't push them back to their original position. So, they work in pairs of flexors and extensors. The flexor contracts to bend a limb at a joint. Then, when the movement is completed, the flexor relaxes and the extensor contracts to extend or straighten the limb at the same joint.
The movements our muscles make are coordinated and controlled by the brain and nervous system. The voluntary muscles are regulated by parts of the brain known as the cerebral motor cortex and the cerebellum. When we decide to move, the motor cortex sends an electrical signal through the spinal cord and peripheral nerves to the muscles, causing them to contract.
Muscle movements include flexion and extension, abduction and adduction, supination and pronation, and elevation and depression. Flexion and extension refer to movements forward and backward from the body, such as nodding the head. Abduction and adduction refer to side-to-side movements, such as moving the arm laterally when doing jumping jacks.
