Athena Vale
Muscle receptors play a crucial role in the body's motor control system. These receptors provide the central nervous system (CNS) with information about the body's mechanical state, enabling the CNS to coordinate and adapt movements. There are several types of sensory receptors located within skeletal muscles, including muscle spindles, Golgi tendon organs, and free nerve endings. Muscle spindles are highly sensitive receptors that monitor muscle length and velocity, while Golgi tendon organs are located at the musculo-tendinous junctions and provide information about muscle force and load. Free nerve endings, on the other hand, are sensitive to mechanical, chemical, and nociceptive stimuli. Additionally, acetylcholine receptors on muscle fibers play a key role in propagating action potentials when acetylcholine is released by motor neurons. Understanding the structure and function of these muscle receptors is essential for comprehending motor control and movement regulation in the body.
| Characteristics | Values |
|---|---|
| Receptor type | Muscle spindles, Golgi tendon organs, free nerve endings |
| Receptor function | Provide information to the CNS about the mechanical state of the body, assist in the central control of muscle action |
| Muscle spindle function | Signal muscle length and velocity to the CNS, monitor muscle length |
| Golgi tendon organ function | Signal information about load or force being applied to the muscle, provide information about muscle force |
| Free nerve ending function | Sensitive to mechanical, chemical, and nociceptive stimuli |
| Specific receptor types | Acetylcholine, nicotinic |
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What You'll Learn
Muscle spindles
The muscle spindle consists of several modified muscle fibres enclosed in a sheath of connective tissue. These modified fibres are called intrafusal fibres and are oriented parallel to the regular, power-producing extrafusal muscle fibres. The muscle spindle has both sensory and motor components. The sensory component involves two types of specialised sensory fibres, namely Group Ia (primary) and Group II (secondary) afferent fibres, which spiral around the central portion of the intrafusal fibres. These fibres have stretch receptors that respond to changes in muscle length, transmitting this information to the CNS.
The motor component of the muscle spindle is provided by motor neurons, particularly gamma motor neurons or fusimotor neurons. These neurons activate the intrafusal muscle fibres, changing their firing rate and stretch sensitivity. The activation of these neurons causes a contraction and stiffening of the end parts of the muscle spindle fibres. The function of gamma motor neurons is to modify the sensitivity of the muscle spindle sensory afferents to stretch. When a muscle spindle's associated muscle is stretched, the spindle detects the change in length and signals the muscle to contract, resisting further stretching. This stimulation of a reflexive muscle contraction is known as the stretch or myotatic reflex.
The muscle spindle's structure and function have been studied through various methods, including immunostaining, sequencing, biomechanical modelling, and electrophysiological recordings. Its function has been compared to feedback elements in engineered control systems, highlighting the importance of feedback for precision in action. The muscle spindle works in conjunction with the Golgi tendon organ, another proprioceptor, to regulate muscle stiffness and flexibility. While the muscle spindle stimulates muscle contraction, the Golgi tendon organ interrupts contraction, causing muscle relaxation.
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Golgi tendon organs
Muscle receptors provide the central nervous system with information about the body's mechanical state, thereby assisting in the central control of muscle action. One such receptor is the Golgi tendon organ (GTO), a proprioceptor that senses changes in muscle tension. GTOs are located in the musculotendinous junctions, which are the interfaces between muscles and tendons. GTOs are nearly as common in most muscles as muscle spindles.
The GTO is a fusiform receptor enclosed within a thin perineural capsule containing packed tendinous collagen, with several muscle fibres attached to one pole. The collagen bundles pass through the organ, with collagen fascicles attached to muscle fibres at one end and merging with the tendon at the other. The capsule is approximately 1 mm long and has a diameter of about 0.1 mm.
The GTO is innervated by primary afferents called Group Ib fibres, which have specialised endings that weave in between the collagen fibres. When force is applied to a muscle, the GTO is stretched, causing the collagen fibres to squeeze and distort the membranes of the primary afferent sensory endings. This results in the afferent being depolarised, and it fires action potentials to signal the amount of force.
The GTO is considered a muscle tension receptor rather than a muscle stretch receptor. When the GTO is stimulated by a prolonged stretch, it causes the stretched muscle to relax through the inverse stretch reflex. This reflex connects high force in the GTO with relaxation, which is the opposite of the myotatic reflex, in which stretch elicits a reflex contraction.
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Acetylcholine receptors
Acetylcholine is a neurotransmitter that intervenes in numerous physiological functions, including regulating cardiac contractions and blood pressure, intestinal peristalsis, glandular secretion, and skeletal muscle contraction. Acetylcholine receptors are found in both the central nervous system (CNS) and peripheral nervous system.
There are two types of acetylcholine receptors: nicotinic and muscarinic. Nicotinic receptors are ion channels for sodium and calcium, while muscarinic receptors are coupled with G proteins. The muscular subtype of nicotinic receptors (N1) is found on the surface of muscle cells at the neuromuscular junction.
Active acetylcholine receptors prevent the atrophy of skeletal muscles and favour reinnervation. Denervation of skeletal muscles can induce severe muscle atrophy, but activation of nicotinic acetylcholine receptors (nAChRs) by the release of acetylcholine from motoneurons can prevent the changes induced by denervation.
In addition, acetylcholine receptors play a role in memory, including long-term and working memory, memory formation, consolidation, and retrieval. Acetylcholine is also involved in motivation, arousal, attention, learning, and promoting rapid eye movement (REM) sleep.
Disorders such as myasthenia gravis involve a rapid weakening of skeletal muscles due to interference with acetylcholine receptors at the neuromuscular junction. Cholinesterase inhibitors, which block the breakdown of acetylcholine, can be used to treat this condition.
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Stretch receptors
The muscle spindle, for example, is a stretch receptor that signals muscle length and velocity to the CNS (Central Nervous System). It does this through two types of specialised sensory fibres that innervate the intrafusal fibres. These sensory fibres have stretch receptors that open and close as a function of the length of the intrafusal fibre. Group Ia afferents, or primary afferents, wrap around the central portion of all three types of intrafusal fibres and provide information about both length and velocity. Group II afferents, or secondary afferents, on the other hand, only innervate the ends of the nuclear chain fibres and static nuclear bag fibres, providing information about muscle length.
The Golgi tendon organ is another example of a stretch receptor. It is located between the muscle and the tendon and signals information about the load or force being applied to the muscle. When force is applied to a muscle, the Golgi tendon organ is stretched, causing the collagen fibres to squeeze and distort the membranes of the primary afferent sensory endings. This results in the afferent being depolarised, and it fires action potentials to signal the amount of force.
In the lungs, stretch receptors located within the smooth muscle of airway walls respond to changes in lung inflation. As the lung inflates, receptor discharge increases. This is known as the Hering-Breuer reflex and is responsible for apnea in animals. Stretch receptors in the chest wall play a role in fine-tuning ventilation and preventing dyspnea. They are part of a reflex arc that adjusts the output of respiratory muscles if the desired degree of muscular work has not been achieved.
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Sensory receptors
Muscle fibres are innervated by sensory receptors that detect changes in the muscle and its environment, providing feedback to the central nervous system. These receptors are essential for maintaining muscle health and function, as they provide information about muscle length, tension, and force, helping to
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Frequently asked questions
Muscles contain sensory receptors that provide the central nervous system with information about the mechanical state of the body, assisting in the central control of muscle action.
Muscle spindles and Golgi tendon organs are two of the most studied muscle receptors.
Muscle spindles are highly sensitive sensory receptors that monitor and signal information about muscle length and velocity to the central nervous system.
Golgi tendon organs are located in the musculo-tendinous junctions and provide information about the load or force being applied to the muscle.
Free nerve endings with slow-conducting, thinly myelinated or unmyelinated nerve afferents are also found in muscles and are sensitive to mechanical, chemical, and nociceptive stimuli.
