Victor Forge
The human body is an intricate machine with over 600 muscles that work together to produce movement. These muscles are broadly categorized into three types: skeletal, cardiac, and smooth muscles. Skeletal muscles, also known as voluntary muscles, are attached to the bones and allow us to perform a wide range of movements. They are controlled by our somatic nervous system, allowing us to decide how and when they work. On the other hand, cardiac and smooth muscles are involuntary, working automatically to keep our organs functioning without conscious thought. The coordination of these muscles is a complex process influenced by neurological, muscular, and external factors, with the cerebellum, motor cortex, and basal ganglia playing key roles in controlling and refining these movements.
| Characteristics | Values |
|---|---|
| Muscle contraction and relaxation | Nerve impulses are transmitted across neuromuscular junctions to the membrane covering each muscle fiber |
| Muscle type | Skeletal muscles are attached to bones and produce movement at the joints |
| Innervation | Efferent motor nerves, and sometimes efferent sympathetic and parasympathetic nerves |
| Body movement aspects | Force, speed, and position |
| Receptors | Proprioceptors, muscle spindles, and tendon organs |
| Nervous system | The central nervous system is continuously reported to by proprioceptors |
| Neurons | The cerebellum contains over 50% of the brain's neurons in a dense cellular layer called the cerebellar cortex |
| Cerebellum function | Movement planning, motor learning, and motor movement regulation |
| Cerebellum zones | Vermis/vermal zone, intermediate zone, and lateral zone |
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What You'll Learn
The cerebellum's role in movement
The cerebellum, or "little brain", is a structure located at the back of the brain, underlying the occipital and temporal lobes of the cerebral cortex. It accounts for approximately 10% of the brain's volume but contains over 50% of the total number of neurons in the brain.
The cerebellum is primarily responsible for muscle control, including balance and movement. It plays a major role in coordinating the timing and force of different muscle groups to produce fluid limb or body movements. For example, when reaching for a cup, an extensor muscle must be activated to start the movement, and the corresponding flexor muscle must be activated at the end of the movement to stop it. The cerebellum is involved in timing these muscle contractions and adjusting the force necessary for each contraction based on variables such as the amount of load placed on a muscle and its inherent properties.
The cerebellum is also important for motor learning. It helps the body learn and fine-tune movements that require practice, such as learning to ride a bicycle or play a musical instrument. This is achieved through a trial-and-error process, where the cerebellum detects the difference or "motor error" between the intended movement and the actual movement, and then modifies the motor commands to make movements more adaptive and accurate.
In addition to its role in motor control, the cerebellum is also involved in certain cognitive functions, such as language processing and memory. Researchers are still working to fully understand the extent of the cerebellum's functions beyond motor control.
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Muscle contraction and relaxation
The movement of the body is made possible by the contraction and relaxation of muscles. The human body has three types of muscles: skeletal, cardiac, and smooth. Skeletal muscles are attached to bones and produce movement at the joints. They are innervated by efferent motor nerves and sometimes by efferent sympathetic and parasympathetic nerves.
Muscle contraction usually occurs when nerve impulses are transmitted across neuromuscular junctions to the membrane covering each muscle fiber. The sequence of events that result in the contraction of an individual muscle fiber begins with a signal—the neurotransmitter, ACh—from the motor neuron innervating that fiber. The local membrane of the fiber will depolarize as positively charged sodium ions (Na+) enter, triggering an action potential that spreads to the rest of the membrane, including the T-tubules. This triggers the release of calcium ions (Ca++) from storage in the sarcoplasmic reticulum (SR). The Ca++ then initiates contraction, which is sustained by ATP. As long as Ca++ ions remain in the sarcoplasm to bind to troponin, which keeps the actin-binding sites "unshielded", and as long as ATP is available to drive the cross-bridge cycling and the pulling of actin strands by myosin, the muscle fiber will continue to shorten to an anatomical limit.
A cross-bridge forms between actin and the myosin heads, triggering contraction. In cardiac muscles, Ca binds to cardiac troponin C, which moves the troponin complex away from the actin-binding site. Removal of the troponin complex frees actin, which becomes bound by myosin and initiates contraction. Intracellular Ca is then removed by the SR, dropping the concentration of intracellular Ca. This decrease in intracellular Ca concentration returns the troponin complex to its inhibiting position on the active site of actin, ending contraction as the actin filaments return to their initial position, relaxing the muscle.
Smooth muscle relaxation occurs as free Ca in the cytosol decreases when Ca is pumped out of the cell or back into the SR. Ca unbinds from calmodulin. Myosin phosphatase removes phosphate from myosin, decreasing myosin ATPase activity and muscle tension. Muscle contraction usually stops when signaling from the motor neuron ends, which repolarizes the sarcolemma and T-tubules, and closes the voltage-gated calcium channels in the SR. Ca++ ions are then pumped back into the SR, which causes the tropomyosin to reshield (or re-cover) the binding sites on the actin strands. A muscle may also stop contracting when it runs out of ATP and becomes fatigued.
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The nervous system's role in movement
The nervous system plays a role in everything we do, including movement. It is made up of nerve cells, or neurons, that carry messages between the brain and the rest of the body. The brain reads signals from the nerves to regulate how we think, move and feel.
There are two parts to the nervous system: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and the spinal cord, while the PNS is made up of a network of nerves that branch out from the spinal cord. The PNS relays information from the CNS to the organs, arms, legs, fingers and toes.
The somatic nervous system, a part of the PNS, guides our voluntary movements. For example, when we touch a hot plate, we reflexively pull back our hand, and our nerves simultaneously send pain signals to the brain. The autonomic nervous system, another part of the PNS, regulates involuntary movements, such as breathing, heartbeat and metabolic processes. It receives signals from the brain and passes them on to the body, and it can also send signals back to the brain.
Movements of the body are brought about by the contraction and relaxation of selected muscles. Contraction occurs when nerve impulses are transmitted across neuromuscular junctions to the membrane covering each muscle fibre. Most muscles are not continuously contracting but are kept in a state of readiness to contract. The slightest movement or even the intention to move results in widespread activity of the muscles of the trunk and limbs.
Movements can be organised at several levels of the nervous system. At the lowest level are movements of the viscera, which are controlled by neurons of the autonomic nervous system within the viscera themselves. Movements of the trunk and limbs occur at the next level of the spinal cord. If the spinal cord is severed, certain movements of the trunk and limbs below the level of the injury can still occur. At a higher level, respiratory movements are controlled by the lower brainstem, while the upper brainstem controls muscles of the eye, bladder and basic movements of walking and running.
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Skeletal muscles
The skeletal muscles serve various purposes beyond just movement. They play a role in sustaining body posture and position, maintaining body temperature, storing nutrients, and stabilizing joints. Additionally, they are associated with the diaphragmatic, esophageal, and eye muscles, and are involved in processes such as breathing and swallowing.
Skeletal muscle fibres can be classified into three types based on their contractile properties: Type I, or slow oxidative fibres, are slow-twitching fibres with low fatigability and a low rate of contraction, making them suitable for endurance activities like marathon running. Type IIa, or fast oxidative fibres, are fast-twitching and have an intermediate rate of fatigue, making them ideal for moderate-movement actions like walking. Type IIb, or fast glycolytic fibres, are also fast-twitching fibres.
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The brain's role in movement
The brain is the command centre of the human body, controlling thought, memory, emotion, touch, motor skills, vision, respiration, and every process that regulates the body. The brain controls the body's movements and can instruct the body to move in just a split second. While some movements are easy or automatic, such as blinking, other movements are more challenging, such as choosing between two toys.
Movements of the body are brought about by the harmonious contraction and relaxation of selected muscles. Contraction occurs when nerve impulses are transmitted across neuromuscular junctions to the membrane covering each muscle fibre. Most muscles are not continuously contracting but are kept in a state of readiness to contract. The slightest movement or even the intention to move results in widespread activity of the muscles of the trunk and limbs.
At a high level, the brain can be divided into the cerebrum, brainstem, and cerebellum. The cerebrum (front of the brain) comprises grey matter (the cerebral cortex) and white matter at its centre. The largest part of the brain, the cerebrum initiates and coordinates movement and regulates temperature. Other areas of the cerebrum enable speech, judgement, thinking and reasoning, problem-solving, emotions, and learning.
The brainstem (middle of the brain) connects the cerebrum with the spinal cord. The brainstem includes the midbrain, the pons, and the medulla. The midbrain is a very complex structure with a range of different neuron clusters, neural pathways, and other structures. These features facilitate various functions, from hearing and movement to calculating responses and environmental changes. The pons is the origin of four of the 12 cranial nerves, which enable a range of activities such as tear production, chewing, blinking, focusing vision, balance, hearing, and facial expression.
Each brain hemisphere (parts of the cerebrum) has four sections, called lobes: frontal, parietal, temporal, and occipital. The frontal lobe is the largest lobe of the brain, located at the front of the head, and is involved in personality characteristics, decision-making, and movement. The parietal lobe, in the middle of the brain, helps a person identify objects and understand spatial relationships. The occipital lobe is involved in the processing of visual information.
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Frequently asked questions
The cerebellum is located at the back of the brain and is involved in movement planning and motor learning. It coordinates and influences eye movement and the vestibulo-ocular reflex, as well as influencing postural muscles to maintain an upright posture and horizontal head position.
The cerebellum has three zones: the vermis/vermal zone (medial), the intermediate zone (paravermal zone), and the lateral zone. The vermal zone is involved in maintaining balance and regulating tone, posture, locomotion, and equilibrium. The intermediate zone governs skilled movements, posture, and tone of the ipsilateral extremities, while the lateral zone governs skilled limb movement.
The nervous system plays a crucial role in movement. Movements of the body are a result of the contraction and relaxation of selected muscles, which occur when nerve impulses are transmitted across neuromuscular junctions. The central nervous system receives continuous feedback from proprioceptors, which are receptors sensitive to position, posture, and equilibrium, allowing for adjustments in force, speed, and position during movement.
Proprioceptors are sensory receptors that provide feedback to the central nervous system about body movement, including information on muscle contractions, tendon tension, and joint position. There are two types of proprioceptors: muscle spindles and tendon organs. Muscle spindles are more complex and help sense changes in muscle length, while tendon organs detect tension in tendons.
The cerebellum receives information from the cerebral cortex via the cerebrocerebellar loop and processes it to adjust and fine-tune movements. It integrates sensory inputs and motor commands to coordinate trunk and limb movements. The processed information is then sent to the thalamus, red nucleus, spinal cord, and cortex.
