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A muscle reflex is an involuntary and nearly instantaneous movement in response to a stimulus, such as a tap or stretch. It is an automatic response that does not require conscious thought or input from the brain. The muscle stretch reflex, also known as the deep tendon reflex or myotatic reflex, is the most basic reflex pathway in the body. It is a simple pre-programmed response to the muscle being passively stretched or a sudden change in the ground surface. This reflex can be tested by striking the tendon with a single, brisk stroke, and the response should be a contraction of the muscle. The strength of the reflex can be graded from 0 (no response) to 4+ (clonus). Reflexes are important for maintaining homeostasis and protecting the body from injury.
| Characteristics | Values |
|---|---|
| Definition | A muscle reflex is an involuntary response of a muscle to a stimulus, such as a tap or stretch. |
| Purpose | Reflexes are the body's intrinsic stimulus-response systems for maintaining homeostasis. |
| Types | Stretch reflex, monosynaptic reflex, polysynaptic reflex, withdrawal reflex, righting reflex, occulopelvic reflex, abdominal reflex, cremasteric reflex, corneal reflex, plantar reflex, tendon jerk, tonic stretch reflex, tonic vibration reflex, H reflex, patellar reflex, Achilles reflex, knee jerk reflex, clasp-knife response, Babinski reflex, etc. |
| Testing | Reflexes can be tested as part of a neurological examination, often if there is an injury to the central nervous system. |
| Grading | Reflex strength can be graded from 0 (no response) to 4+ (clonus). 2+ (brisk response) is normal. |
| Abnormalities | Abnormalities in reflexes may indicate localized disorders in the peripheral nerves or other diseases. |
| Enhancement | Reflexes can be enhanced by techniques like the Jendrassik manoeuvre. |
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What You'll Learn
Muscle stretch reflex
A muscle reflex is an involuntary and nearly instantaneous movement in response to a stimulus. Muscle stretch reflex, also known as the myotatic reflex, is a muscle contraction in response to stretching a muscle. The function of the reflex is to maintain the muscle at a constant length, and the response is often coordinated across multiple muscles and joints.
The older term for this reflex, deep tendon reflex, is now considered misleading. Tendons are not significantly involved in the response, and some muscles with stretch reflexes have no tendons. Instead, muscle spindles detect the stretch and convey this information to the central nervous system. As a spinal reflex, it results in a fast response that involves an afferent signal into the spinal cord and an efferent signal out to the muscle.
The stretch reflex can be a monosynaptic reflex, which provides automatic regulation of skeletal muscle length. It can also include a polysynaptic component, as in the tonic stretch reflex. When a muscle lengthens, the muscle spindle is stretched, increasing nerve activity and causing the muscle fibres to contract and resist the stretching. A secondary set of neurons also causes the opposing muscle to relax.
Gamma motoneurons regulate the sensitivity of the stretch reflex by tightening or relaxing the fibres within the spindle. Alpha-gamma co-activation keeps the spindles taut when a muscle is contracted, preserving stretch reflex sensitivity as the muscle fibres become shorter. This reflex has the shortest latency of all spinal reflexes.
The stretch reflex is a simple, pre-programmed response to the muscle being passively stretched, for example, by a tendon hammer or a sudden change in the ground surface. It can be activated by external forces, such as a load placed on the muscle, or internal forces, such as the motor neurons being stimulated from within.
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Monosynaptic reflex
A muscle reflex is an involuntary and nearly instantaneous movement in response to a stimulus. Reflexes are the body's intrinsic stimulus-response systems for maintaining homeostasis.
The monosynaptic reflex originates from primary sensory endings in muscle spindles. Muscle spindles detect a stretch and convey the information to the central nervous system. The afferent sensory neuron carries the signal from the muscle to the spinal cord. The signal travels from the dorsal root ganglion of the spinal cord to the muscle down the alpha motor neuron. The monosynaptic reflex can be stimulated naturally, for example, by a very quick muscle stretch produced by a tendon tap.
The two versions of the monosynaptic reflex are the tendon tap reflex (or T-reflex) and the H-reflex. The typical delay for a monosynaptic reflex in the human arm and leg muscles is between 20-40 ms. Monosynaptic reflexes are phasic, meaning they produce quick, transient twitch muscle contractions. They are inborn and can be observed in newborn babies.
Examples of monosynaptic reflex arcs in humans include the patellar reflex, the Achilles reflex, and the biceps and triceps reflexes.
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Tonic stretch reflex
A muscle reflex is an involuntary response to a stimulus, such as a tap or stretch. The stretch reflex (myotatic reflex) is a muscle contraction in response to a muscle stretch. The tonic stretch reflex is a type of stretch reflex that involves a prolonged asynchronous discharge of motor neurons, resulting in sustained muscle contraction for the maintenance or alteration of posture.
The tonic stretch reflex is a spinal reflex that occurs when a muscle lengthens, causing the muscle spindle to stretch and increasing its nerve activity. This increased nerve activity results in the activation of alpha motor neurons, leading to muscle fibre contraction and resistance to the stretching. The tonic stretch reflex is crucial in maintaining posture and muscle tone. It is often coordinated across multiple muscles and joints.
The tonic stretch reflex can be influenced by various factors, including the velocity of the stretch and the length of the muscle fibres. The dynamic fusimotor fibres can alter the sensitivity of the receptors in the muscle spindles independently of the static fusimotor fibres that regulate the response to muscle length. Gamma motoneurons also play a role in regulating the sensitivity of the stretch reflex by tightening or relaxing the fibres within the spindle.
The Tonic Stretch Reflex Threshold (TSRT) is a metric used to characterise the velocity-sensitivity of the reflex. It is defined as the joint angle or muscle length at which muscle activation begins. In spasticity, the TSRT lies abnormally inside the biomechanical joint range. The TSRT is determined by measuring the Dynamic Stretch Reflex Thresholds (DSRTs) by stretching the resting muscle at different velocities.
The tonic stretch reflex is a valuable tool in understanding and evaluating muscle function, particularly in the presence of disorders or injuries. It is often used as a biomarker of deficits in motor control and spasticity, helping to identify and treat conditions affecting the central nervous system.
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Muscle weakness
Voluntary muscle contractions are typically generated when the brain sends signals through the spinal cord and nerves to a muscle. If this process is disrupted by injury or disease, it can result in muscle weakness. For example, nervous system conditions such as certain types of migraines, autoimmune conditions, or inflammatory diseases can cause muscle weakness. Additionally, electrolyte imbalances, chronic conditions, or infections can also lead to muscle weakness.
In some cases, muscle weakness may be a sign of a more serious health condition. For instance, in anterior horn cell disease, muscle weakness occurs before the reflexes disappear. Similarly, in primary muscle diseases, weakness is usually more pronounced before the loss of reflexes. Prolonged use of certain drugs, sleep disorders, and extended periods of inactivity can also contribute to muscle weakness.
It is important to seek medical attention if muscle weakness lasts more than a few days or interferes with daily activities. Treatment options may include physical therapy, medication, or assistive devices, depending on the underlying cause and severity of symptoms. Strength training and knowing one's physical limits can help prevent and manage muscle weakness.
Therefore, muscle weakness can have a range of causes, from temporary and treatable conditions to more serious underlying health issues. It is important to consult a healthcare provider to determine the cause and appropriate course of action.
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Hyperreflexia and hyporeflexia
A muscle reflex is an involuntary and nearly instantaneous movement in response to a stimulus. It is an automatic response to a stimulus that does not require conscious thought as it occurs through a reflex arc. Reflexes are the body's intrinsic stimulus-response systems for maintaining homeostasis.
Hyperreflexia refers to hyperactive or repeating (clonic) reflexes. It happens when muscles have an increased or overactive reflex response. This is usually indicated by an interruption of corticospinal and other descending pathways that influence the reflex arc due to a suprasegmental lesion, or a lesion above the level of the spinal reflex pathways. This can be caused by damage to motor neurons (nerve cells) that transmit signals from the brain to the spinal cord, resulting in an upper motor neuron (UMN) lesion. Certain neurological conditions, like amyotrophic lateral sclerosis (ALS) and multiple sclerosis (MS), can cause hyperreflexia. Additionally, non-neurological conditions such as anxiety disorders and hyperthyroidism can also lead to hyperreflexia. The patellar reflex, or knee-jerk reflex, is a well-known example of a deep tendon reflex (DTR) used to check for hyperreflexia. When the knee is tapped with a rubber hammer, the patellar tendon and the muscle in the thigh are stretched, resulting in a brisk and forceful contraction of the lower leg.
On the other hand, hyporeflexia refers to a condition in which the muscles exhibit a decreased or absent reflex response. It is often associated with lower motor neuron deficits, such as spinal muscular atrophy (SMA), Guillain-Barré syndrome (GBS), and chronic inflammatory demyelinating polyneuropathy (CIDP). Hypothyroidism, or low levels of thyroid hormones, can also cause hyporeflexia due to muscle weakness and slow nerve conduction. The diagnosis of hyporeflexia involves a neurological examination that evaluates deep tendon reflexes (DTRs) using a reflex hammer. Common reflexes tested include the biceps, brachioradialis, and triceps reflexes in the arm, as well as the patellar and Achilles reflexes in the leg. With hyporeflexia, there is a weakened or diminished muscle contraction in response to the tap, and in the case of no contraction, it is termed areflexia.
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Frequently asked questions
A muscle reflex is an involuntary response to a stimulus, such as a tap or stretch. It is a protective measure for the muscles to prevent tearing that can occur due to vigorous movement.
A stretch reflex is a muscle contraction in response to stretching a muscle. It is also referred to as the deep tendon reflex or myotatic reflex. It is the most basic reflex pathway in the body.
A monosynaptic reflex is a simple reflex involving only one synapse between the sensory and motor neurons. An example of this is the knee jerk reflex.
