Victor Forge
Muscle receptors are a vital component of the human body, providing the central nervous system with information about the mechanical state of the body and facilitating the central control of muscle action. These receptors are sensitive to stretch and arranged in parallel with muscle fibres, responding when the muscle is stretched. Muscle spindles, for example, monitor muscle length and send this information to the CNS through fast-conducting afferent nerve fibres. Additionally, receptors in cardiac muscle cells can slow the heart rate and decrease the force of contraction. Cholinergic receptors, such as the N1 and N2 subtypes, also play a crucial role in neural transmission within the somatic and autonomic nervous systems. The N1 receptor, for instance, is present on skeletal muscle at the neuromuscular junction, while the N2 receptor is found within the peripheral and central nervous systems. Furthermore, muscle sensory receptors like the Golgi tendon organ provide information about muscle force and load.
| Characteristics | Values |
|---|---|
| Function | Sensing position and movement |
| Arrangement | Parallel with muscle fibres |
| Response | Muscle stretch |
| Muscle stretch | Addition of load in the form of weight or resistance |
| Muscle spindle | Located in parallel with extrafusal fibres |
| Golgi tendon organ | Located in series with the muscle |
| Muscle spindle | Monitor muscle length |
| Golgi tendon organ | Provide information about muscle force |
| Motor neurons | Use rate code to signal the amount of force to be exerted by a muscle |
| Muscle | Enters a state called tetanus when successive action potentials no longer produce a summation of muscle contraction |
| Motor neuron size principle | Smaller motor neurons are recruited and fire action potentials before larger motor neurons |
| Group Ia and Group II afferents | Respond differently to different types of muscle movements |
| Nicotinic receptor subtypes | N1 (peripheral or muscle receptor type) and N2 (central or neuronal receptor subtype) |
| N1 receptor | Present on skeletal muscle at the neuromuscular junction |
| N2 receptor | Within the peripheral and central nervous systems |
| Muscarinic receptors | Present throughout the central nervous system |
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What You'll Learn
Muscle spindle afferents
Muscle spindles are stretch receptors within the body of a skeletal muscle that primarily detect changes in the length of the muscle. They convey length information to the central nervous system via afferent nerve fibres. This information can be processed by the brain as proprioception. The muscle spindle has both sensory and motor components. Sensory information is conveyed by primary type Ia sensory fibres, which spiral around muscle fibres within the spindle, and secondary type II sensory fibres.
The primary type Ia sensory fibres of the muscle spindle respond to both changes in muscle length and velocity and transmit this activity to the spinal cord in the form of changes in the rate of action potentials. The Ia afferent signals are transmitted monosynaptically to many alpha motor neurons of the receptor-bearing muscle. The reflexly evoked activity in the alpha motor neurons is then transmitted via their efferent axons to the extrafusal fibres of the muscle, which generate force and thereby resist the stretch. The Ia afferent signal is also transmitted polysynaptically through interneurons (Ia inhibitory interneurons), which inhibit alpha motor neurons of antagonist muscles, causing them to relax.
The secondary type II sensory fibres respond to muscle length changes (but with a smaller velocity-sensitive component) and transmit this signal to the spinal cord. The Group Ia and II afferents differ in their dynamic and static sensitivities to stretch, innervation patterns, and gene expression patterns. The Group Ia afferent fires at a very high rate during the stretch, encoding the velocity of the muscle length; at the end of the stretch, its firing decreases, as the muscle is no longer changing length. The Group II afferent, on the other hand, increases its firing rate steadily as the muscle is stretched.
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Muscle stretch and load
The Golgi tendon organ is another type of receptor located between the muscle and the tendon. Unlike muscle spindles, Golgi tendon organs are arranged in series with the muscle fibres. When force or load is applied to a muscle, the Golgi tendon organ is stretched, causing the collagen fibres within it to squeeze and distort the membranes of the sensory nerve endings. This results in the firing of action potentials that signal the amount of force being applied to the muscle.
The response of muscle receptors to stretch and load can be further understood through experiments that manipulate these variables. For example, studies have investigated the relationship between muscle length and load during stretching. While muscle length is often associated with muscle tension, with increased length leading to decreased tension, the increase in joint range of motion (ROM) observed during stretching may not always be due to increased muscle length. Instead, it could be a result of increased tolerance to stretch, where individuals can withstand greater stretching forces without the muscle elongating.
Loaded stretching is a training technique that combines stretching with resistance or tension to increase muscle mass and strength. It involves holding a stretched position under load for a set duration, typically between 45 and 90 seconds. This method stimulates muscle growth by increasing muscle protein synthesis, inducing occlusion and muscle swelling, and promoting the release of hormones and enzymes that enhance muscle repair and growth. Loaded stretching can be applied to various muscle groups and is often incorporated into workout routines by bodybuilders and athletes.
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Motor neurons and force
Motor neurons play a critical role in muscle function, particularly in terms of force generation and movement. The motor neuron and the muscle fibers it contacts form what is known as a motor unit. The number of muscle fibers innervated by a motor neuron is referred to as its innervation ratio. Motor units are responsible for generating force and facilitating movement.
The force exerted by a muscle is directly related to the rate of action potentials fired by the motor neuron. When a motor neuron fires a single action potential, the muscle twitches slightly and then returns to its resting state. However, if the firing rate of the motor neuron increases, and a second action potential occurs before the muscle relaxes, the second action potential results in a greater force than the first. This phenomenon is known as the rate code, where an increase in the firing rate of the motor neuron leads to an increase in the force generated by the muscle.
The size principle is another important concept in understanding the relationship between motor neuron activity and muscle force. According to this principle, smaller motor neurons are recruited first, followed by larger motor neurons as the strength of the input increases. Smaller motor neurons have a smaller membrane surface area and, consequently, higher resistance. This results in a higher voltage change across the neuronal membrane when an excitatory postsynaptic potential (EPSP) occurs.
Motor unit recruitment refers to the number of motor neurons activated in a particular muscle, which, in turn, determines the number of muscle fibers activated. The force produced by a single motor unit is influenced by the number of muscle fibers in the unit and the frequency of stimulation by the innervating axon. As the motor unit firing rate increases, the force generated by the muscle also increases. This relationship between motor unit firing rate and force production allows for smooth incremental force changes during muscular contractions.
The type of muscle fiber innervated by the motor neuron also influences the force generated. Small motor neurons typically innervate small, "red" muscle fibers that contract slowly and produce smaller forces. On the other hand, larger motor neurons innervate larger, pale muscle fibers that generate more force but are more susceptible to fatigue. This variation in motor neuron and muscle fiber types allows for a range of force generation capabilities, enabling precise control and adaptation to different movement requirements.
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Muscle contraction
Skeletal muscles are attached to bones and provide the body with structure and strength. Skeletal muscle contractions are neurogenic as they require synaptic input from motor neurons. A single motor neuron can innervate multiple muscle fibres, causing them to contract simultaneously. Once innervated, the protein filaments within each skeletal muscle fibre slide past each other to produce a contraction, as explained by the sliding filament theory. The contraction can be described as a twitch, summation, or tetanus, depending on the frequency of action potentials.
Cardiac muscle forms the walls of the heart, facilitating blood pumping through the vasculature. Cardiac muscle tissue is a striated muscle fibre under involuntary control by the body's autonomic nervous system (ANS).
Smooth muscle is found in blood vessels, the gastrointestinal tract, bronchioles, uterus, and bladder. Smooth muscle fibres do not contain sarcomeres but use actin and myosin contraction to constrict blood vessels and move the contents of hollow organs in the body. These fibres are under involuntary control by reflexes and the ANS.
The complex process leading to muscle contraction, known as excitation-contraction coupling, begins with an action potential causing depolarization in the myocyte membrane. This depolarization spreads via transverse (T) tubules, which are invaginations of the muscle cell membrane that help transmit depolarization signals to the entire muscle fibre. Depolarization of the T tubules triggers a conformational change in the dihydropyridine receptors, opening nearby ryanodine receptors on the sarcoplasmic reticulum (SR), the calcium storage site within muscle cells. When calcium is released from the SR, it binds to troponin C, leading to a conformational change that shifts tropomyosin, allowing the myosin heads to attach to the actin filaments and form a cross-bridge. Cross-bridge cycling is initiated when ATP binds to an ATP-binding domain on the myosin head.
Muscle receptors and tendon receptors play a crucial role in sensing position and movement. Muscle receptors are arranged in parallel with muscle fibres, responding when the muscle stretches. Muscle fibres extend when a load is applied in the form of weight or resistance. This stretch of the extrafusal or work muscle fibres simultaneously stretches the intrafusal muscle fibres, which have their own motor and sensory innervation. The Golgi tendon organ, a specialized receptor located between the muscle and tendon, signals information about the load or force applied to the muscle.
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Nicotinic receptors
There are two major types of nicotinic receptors based on their subunit composition: homomeric receptors, which are composed of only one type of subunit, and heteromeric receptors, which are composed of different subunits. The most abundant nicotinic receptors in the brain are composed of α4 and β2 subunits. In muscle-type nAChRs, the acetylcholine binding sites are located at the α and either ε or δ subunits interface. In neuronal nAChRs, the binding site is located at the interface of an α and a β subunit or between two α subunits in the case of α7 receptors.
The activation of nicotinic receptors by nicotine modifies the state of neurons through two main mechanisms. Firstly, the movement of cations causes a depolarization of the plasma membrane, resulting in an excitatory postsynaptic potential in neurons and the activation of voltage-gated ion channels. Secondly, the entry of calcium acts directly or indirectly on different intracellular cascades, leading to the regulation of gene activity or the release of neurotransmitters.
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Frequently asked questions
Muscle receptors are sensory receptors located within skeletal muscles, joints, and the skin. They provide the central nervous system with information about the mechanical and chemical state of the body, thereby assisting in the central control of muscle action.
Muscle spindles, Golgi tendon organs, and free nerve endings are all examples of muscle receptors. Muscle spindles monitor muscle length and send this information to the central nervous system. Golgi tendon organs, on the other hand, are located in the musculo-tendinous junctions and provide information about muscle force. Free nerve endings are sensitive to mechanical, chemical, and nociceptive stimuli.
Muscle receptors respond when the muscle is stretched. When load or resistance is added to a muscle, the extrafusal or work muscle fibres stretch, causing a simultaneous stretch in the smaller intrafusal muscle fibres. This results in a response from the muscle receptors, which then send signals to the central nervous system.
