Dominic Stone
The neuromuscular system combines the nervous system and the muscles, allowing the body to move and perform important functions such as breathing. Neurons are the cells that make up the brain and nervous system, sending and receiving signals to and from the brain through the spinal cord to the muscles in the body. Motor neurons stimulate muscles to contract, conveying impulses from the central nervous system to a muscle or gland. Each skeletal muscle fibre is controlled by a motor neuron, which conducts signals from the brain or spinal cord to the muscle.
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What You'll Learn
Motor neurons stimulate muscle contraction
Motor neurons play a crucial role in stimulating muscle contraction, which is essential for the body's movements. Each skeletal muscle fibre is controlled by a motor neuron, which transmits signals from the brain or spinal cord to the muscle. This signal transmission is facilitated by the release of neurotransmitters, specifically acetylcholine (ACh), at the neuromuscular junction.
The process begins with an action potential travelling down the motor neuron's axon. This action potential causes the release of ACh from the synaptic terminal into the synaptic cleft. From there, ACh diffuses across the cleft to the motor end plate, where it binds to ACh receptors. This binding opens ion channels, allowing Na+ ions to enter the muscle cell. This influx of Na+ ions leads to depolarization, reducing the voltage difference between the inside and outside of the cell.
The depolarization then spreads along the sarcolemma, generating an action potential as adjacent sodium channels sense the change in voltage and open. This action potential triggers the release of calcium ions (Ca2+) from the sarcoplasmic reticulum into the sarcoplasm, initiating muscle contraction. The calcium ions enable synaptic vesicles to move and bind to the presynaptic membrane, facilitating the release of more neurotransmitters.
The rate of firing of the motor neuron also influences the strength of muscle contraction. If the motor neuron fires rapidly, resulting in successive action potentials before the muscle relaxes back to its baseline, the subsequent action potentials produce a greater force. This phenomenon is known as the summation of muscle contraction. However, once the muscle reaches its maximum state of contraction, further action potentials do not lead to additional contraction, and the muscle enters a state called tetanus.
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Neurotransmitters and muscle fibre
The neuromuscular system combines the nervous system and muscles, allowing nerves and muscles to work together to make the body move as desired and manage important functions such as breathing. The nerves serving the muscles are called neurons. Neurons carry messages to and from the brain through the spinal cord to the muscles in the body. Outgoing messages from the brain travel along the motor pathways to activate the muscles of the body. The neurons that make up these pathways are called motor neurons. Incoming messages are sent from the senses back to the spinal cord and brain along the sensory pathways. These are called sensory neurons.
Each skeletal muscle fibre is controlled by a motor neuron, which conducts signals from the brain or spinal cord to the muscle. The area of the sarcolemma on the muscle fibre that interacts with the neuron is called the motor end plate. The end of the neuron's axon is called the synaptic terminal, and it does not actually touch the motor end plate. A small space called the synaptic cleft separates the synaptic terminal from the motor end plate. Electrical signals travel along the neuron's axon, branching through the muscle and connecting to individual muscle fibres at a neuromuscular junction.
Neurotransmitters are released when an action potential travels down the motor neuron's axon, resulting in altered permeability of the synaptic terminal membrane and an influx of calcium. The Ca2+ ions allow synaptic vesicles to move to and bind with the presynaptic membrane (on the neuron), and release neurotransmitters from the vesicles into the synaptic cleft. Acetylcholine (ACh) is a neurotransmitter released by motor neurons that binds to receptors in the motor end plate. Once released by the synaptic terminal, ACh diffuses across the synaptic cleft to the motor end plate, where it binds with ACh receptors. As a neurotransmitter binds, these ion channels open, and Na+ ions cross the membrane into the muscle cell. This reduces the voltage difference between the inside and outside of the cell, which is called depolarization. As ACh binds at the motor end plate, this depolarization is called an end-plate potential.
The depolarization then spreads along the sarcolemma, creating an action potential as sodium channels adjacent to the initial depolarization site sense the change in voltage and open. When acetylcholine binds to receptors on the muscle fibre membrane, the process that contracts a relaxed muscle fibre begins. Open channels allow an influx of sodium ions into the cytoplasm of the muscle fibre. The sodium influx also sends a message within the muscle fibre to trigger the release of stored calcium ions. The calcium ions diffuse into the muscle fibre. The relationship between the chains of proteins within the muscle cells changes, leading to the contraction. When the stimulation of the motor neuron providing the impulse to the muscle fibres stops, the chemical reaction that causes the rearrangement of the muscle fibres' proteins is stopped, and the muscle relaxes.
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Alpha and gamma motor neurons
Motor neurons are cells that carry messages from the brain along the motor pathways to activate the muscles of the body. Each skeletal muscle fibre is controlled by a motor neuron, which conducts signals from the brain or spinal cord to the muscle.
Motor neurons are divided into two groups: alpha motor neurons and gamma motor neurons. Alpha motor neurons are larger and more abundant than gamma motor neurons. They innervate extrafusal fibres, which are the highly contracting fibres that supply the muscle with its power. When the central nervous system sends out signals to alpha neurons to fire, signals are also sent to gamma motor neurons to do the same.
Gamma motor neurons innervate intrafusal fibres, which contract only slightly. The function of intrafusal fibre contraction is not to provide force to the muscle; instead, gamma activation of the intrafusal fibre is necessary to keep the muscle spindle taut and, therefore, sensitive to stretch over a wide range of muscle lengths. This process, called alpha-gamma coactivation, ensures that muscle spindles maintain sensitivity to stretch over a wide range of muscle lengths.
The presence of myelination in gamma motor neurons allows a conduction velocity of 4 to 24 meters per second, which is significantly faster than non-myelinated axons but slower than in alpha motor neurons.
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Excitation-contraction coupling
The human body's neuromuscular system consists of all the muscles and the nerves serving them. Neurons carry messages to and from the brain through the spinal cord to the muscles in the body. The neurons that make up these pathways are called motor neurons. Each skeletal muscle fibre is controlled by a motor neuron, which conducts signals from the brain or spinal cord to the muscle.
The first phase of ECC involves depolarisation and the spread of an action potential (AP) along the sarcolemma and the propagation of the AP into the transverse tubules. An action potential is a separation of electrical charge that is capable of doing work. It is measured in volts, and the transmembrane potential is considerably smaller (0.07 V), so the value is expressed in millivolts (70 mV). The inside of a cell is negative compared to the outside, so a minus sign signifies the excess of negative charges inside the cell.
The second phase of ECC is the release of calcium from the sarcoplasmic reticulum. When an action potential occurs, voltage-gated calcium channels allow calcium into the cell, activating calcium release from ryanodine receptors at the sarcoplasmic reticulum, and causing contraction. The calcium binds to troponin molecules on the thin filament. This binding causes a configurational change, removing tropomyosin from its blocking position on the actin filament.
The third phase of ECC is the cross-bridging cycle, which describes the cyclic events necessary for the generation of force or tension within the myosin heads during muscle contraction. The generation of tension within the contractile elements results from the binding of the myosin heads to actin and the subsequent release of stored energy in the myosin heads.
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Action potentials
An action potential is a rapid sequence of changes in the voltage across a cell membrane. It is a nerve impulse or "spike" when in a neuron. It is a series of quick changes in voltage across a cell membrane. Action potentials occur in several types of excitable cells, including animal cells like neurons and muscle cells, as well as some plant cells.
In neurons, action potentials play a central role in cell–cell communication by providing for—or with regard to saltatory conduction, assisting—the propagation of signals along the neuron's axon toward synaptic boutons situated at the ends of an axon; these signals can then connect with other neurons at synapses, or to motor cells or glands. In muscle cells, an action potential is the first step in the chain of events leading to contraction.
In the context of neurons and muscle interaction, excitation–contraction coupling is the link (transduction) between the action potential generated in the sarcolemma and the start of a muscle contraction. Each skeletal muscle fiber is controlled by a motor neuron, which conducts signals from the brain or spinal cord to the muscle. The end of the neuron’s axon is called the synaptic terminal, and it does not actually touch the muscle fiber. A small space called the synaptic cleft separates the synaptic terminal from the motor end plate. Electrical signals travel along the neuron’s axon, which branches through the muscle and connects to individual muscle fibers at a neuromuscular junction.
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Frequently asked questions
Motor neurons stimulate muscles to contract. Each skeletal muscle fibre is controlled by a motor neuron, which conducts signals from the brain or spinal cord to the muscle. The transmission of a nerve impulse to the synapse causes the muscle fibres to contract.
Alpha motor neurons innervate extrafusal fibres, the highly contracting fibres that supply the muscle with its power. Gamma motor neurons innervate intrafusal fibres, which contract only slightly. The function of intrafusal fibre contraction is not to provide force to the muscle; rather, it is necessary to keep the muscle spindle taut and therefore sensitive to stretch.
The refractory period is the period immediately following the transmission of an impulse in a nerve or muscle, in which a neuron or muscle cell regains its ability to transmit another impulse.
