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Reflexes are the body's automatic and involuntary responses to external stimuli. They are mediated by the nervous system and can involve muscles and movement, or internal processes such as digestion and blood flow. While some reflexes are simple, involving a single synapse, others are more complex, requiring multiple parts of the nervous system. The term reflex was coined in the 19th century by English physiologist Marshall Hall, who described involuntary movements triggered by external stimuli and mediated by the spinal cord and nervous system. These reflexes can be further classified into superficial reflexes, which are elicited by stroking the skin or mucous membranes, and deep tendon reflexes, which involve a stronger stimulus such as tapping or striking a tendon. Deep tendon reflexes are commonly used in clinical examinations to assess the health of the nervous system and can indicate the presence of certain medical conditions.
| Characteristics | Values |
|---|---|
| Definition | Reflexes are automatic and involuntary actions the body produces in response to certain stimuli. |
| Involvement of nervous system | Reflexes are found with varying levels of complexity in organisms with a nervous system. |
| Involvement of muscles | While some reflexes can involve muscles and movement, others involve internal processes within the body. |
| Types | Somatic reflexes, autonomic reflexes, superficial reflexes, deep tendon reflexes, stretch reflexes, short-latency reflexes, PRM reflexes, H-reflexes, F-reflexes, etc. |
| Examples | The escape reflex, the withdrawal reflex, the startle reflex, the feline righting reflex, the patellar reflex, the Jendrassik manoeuvre, etc. |
| Functions | Reflexes are usually protective in nature and are fine-tuned to increase an organism's survival and self-defence. They also help to stabilise posture. |
| History | The concept of reflexes was introduced by René Descartes in the 17th century and later by Marshall Hall in the 19th century. |
| Abnormalities | Abnormal reflex responses can indicate upper motor neuron level (UMNL) involvement. Peripheral neuropathy is a common cause of absent reflexes. |
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Tendon reflexes
DTRs were first described in 1875 and are used to evaluate neurological diseases affecting afferent nerves, spinal cord synaptic connections, motor nerves, and descending motor pathways. The technique and interpretation of results are crucial in differentiating between upper and lower motor neuron pathologies, such as multiple sclerosis (MS), amyotrophic lateral sclerosis (ALS), spinal cord injuries, and spinal muscular atrophies.
To elicit a DTR, a tendon is tapped briskly, causing the passive stretch of the associated muscle. This stretch is detected by the muscle spindle, which then stimulates the alpha motor neuron, resulting in muscle contraction. The amplitude of tendon jerks can be increased by having the patient contract a different muscle, a process known as reinforcement. The Jendrassik manoeuvre is a type of reinforcement where the patient locks their hands together and pulls them apart as the tendon in the lower extremity is tapped.
The National Institute of Neurological Disorders and Stroke (NINDS) proposed a grading scale from 0 to 4 for evaluating DTRs, with 1 being a small reflex and 4 being an enhanced reflex. Asymmetry of reflexes between the left and right sides of the body suggests abnormality.
DTRs are an important tool in neurological examinations, helping to distinguish between different types of neurological pathologies and providing valuable information about the nervous system.
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Stretch reflexes
The stretch reflex, or more accurately, the muscle stretch reflex, is a neural reflex involving a receptor and effector neurone. It is a muscle contraction in response to stretching a muscle. The function of the reflex is to maintain the muscle at a constant length, but the response is often coordinated across multiple muscles and even joints. The older term "deep tendon reflex" is now criticised as misleading, as tendons have little to do with the response, and some muscles with stretch reflexes have no tendons.
The stretch reflex can be either phasic or tonic. A tonic stretch reflex is one in which a stimulus produces a prolonged asynchronous discharge of motor neurons, causing sustained muscle contraction for the maintenance or alteration of posture. In contrast, a phasic stretch reflex consists of a synchronous motor neuron discharge caused by brief stimulation of muscle spindles or their afferent nerve pathways. The tendon jerk is a phasic stretch reflex. The active contraction provoked in a muscle by continuous stretch is a tonic reflex and is the basis of muscle "tone".
The stretch reflex is accomplished through several different structures. In the muscle, there are muscle spindles, whose intrafusal muscle fibres lie parallel to the muscle and sense changes in length and velocity. The afferent sensory neuron carries the signal from the muscle to the spinal cord, specifically to the dorsal root ganglion of the spinal cord. The efferent motor neuron then carries the signal from the spinal cord back to the muscle. This impulse does not need to travel to the brain, and therefore simply travels from the muscle spindles to the spinal cord, and back to the muscle. The entire reflex process takes place over a few milliseconds.
The patellar reflex (knee jerk) is an example of the stretch reflex and is commonly performed by doctors to test the integrity of the peripheral nerves and the reflex arc components. The patient is asked to completely relax their leg, and a tendon hammer is used to tap on the patellar tendon. The striking of the tendon will cause a slight stretch, resulting in the contraction of the quadriceps femoris muscle. The patient should give a small kicking motion caused by knee extension.
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Superficial reflexes
Reflexes are the body's intrinsic stimulus-response systems for maintaining homeostasis. They can be divided into four groups, one of which is superficial reflexes. Superficial reflexes are motor responses that occur when the skin or mucous membranes are stroked. They are typically graded as either present or absent.
The cremasteric reflex is a superficial reflex found in human males. It is elicited when the inner part of the thigh is stroked, causing the cremaster muscle to contract and pull up the testicle toward the inguinal canal. This reflex is believed to play a role in preserving the thermoregulation of the testicles, raising and lowering them to control their temperature. In a cold environment, the cremaster muscle pulls the testicles closer to the body. During fight or flight and sexual arousal, it moves the testicles into a more protected location closer to the body. The female counterpart of the cremasteric reflex is the Geigel reflex, which involves the contraction of muscle fibres along the upper part of the Poupart or inguinal ligament.
Another example of a superficial reflex is the superficial abdominal reflex, which is elicited by lightly stroking the four quadrants of the abdomen near the umbilicus. The normal response is the contraction of the abdominal muscles, causing the umbilicus to move toward the area being stroked.
The Babinski, Chaddock, and Oppenheim reflexes all evaluate the plantar response. The normal reflex response is flexion of the great toe. An abnormal response, indicating an upper motor neuron lesion, is slower and consists of the extension of the great toe with fanning of the other toes and often knee and hip flexion.
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Deep tendon reflexes
The mechanism of eliciting a deep tendon reflex involves tendons, muscles, and the reflex arc. When a muscle tendon is tapped briskly, the muscle immediately contracts due to a two-neuron reflex arc involving the spinal or brainstem segment that innervates the muscle. The afferent neuron, whose cell body lies in a dorsal root ganglion, innervates the muscle or Golgi tendon organ associated with the muscles. The efferent neuron is an alpha motoneuron in the anterior horn of the cord. The muscle spindle, a sensory proprioceptor, detects the stretch of the muscle fibre and identifies its length. Ia afferent sensory fibres in the muscle spindles produce action potentials in response to the stretch and stimulate the alpha motor neuron that goes to the homonymous muscle extrafusal fibres. The extrafusal muscle fibres then contract to resist the stretch.
There are five primary deep tendon reflexes: biceps, brachioradialis, triceps, patellar, and ankle. These reflexes are important in evaluating neurologic diseases affecting afferent nerves, spinal cord synaptic connections, motor nerves, and descending motor pathways. Proper technique and interpretation of results are crucial in achieving a proper distinction between upper and lower motor neuron pathologic processes.
The National Institute of Neurological Disorders and Stroke (NINDS) proposed a grading scale from 0 to 4 for evaluating deep tendon reflexes in 1993. This scale has been validated and is universally accepted. A deep tendon reflex is considered "normal" based on the patient's history and past documented reflex grade. Asymmetry of reflexes suggests abnormality, and the presence of hyporeflexia or hyperreflexia is considered a 'hard sign' of neurologic dysfunction.
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H-reflexes
Reflexes are the body's intrinsic stimulus-response systems for maintaining homeostasis. One of the most studied reflexes in humans is the H-reflex, also known as Hoffmann's reflex, which was first described by Paul Hoffmann in 1910. The H-reflex is a valuable tool for assessing motoneuronal excitability in single muscles and neurologic conditions.
The H-reflex is a reflectory reaction of muscles after electrical stimulation of sensory fibres (Ia afferents stemming from muscle spindles) in their innervating nerves. For example, those located behind the knee. The H-reflex test is performed using an electric stimulator that delivers a short-duration, small-amplitude square-wave current, and an EMG set to record the muscle response. The response is typically a clear wave, known as an H-wave, which occurs 28-35 ms after stimulation. This is distinct from an F-wave or an early M-wave response, which occurs 3-6 ms after stimulation. As the stimulus increases, the amplitude of the F-wave increases slightly, while the H-wave decreases and eventually disappears at supramaximal stimulus.
The H-reflex is analogous to the mechanically induced spinal stretch reflex, such as the knee-jerk reflex. However, the primary difference is that the H-reflex bypasses the muscle spindle, making it a valuable tool for assessing monosynaptic reflex activity in the spinal cord. The H-reflex provides quantitative information about reflex arc activity and is particularly useful for comparing performances between different subjects. Additionally, it can be used to evaluate neurologic function in various populations, including athletes with sports injuries.
The H-reflex is also influenced by external factors such as gravity. For instance, after spending about five days in zero gravity, the H-reflex diminishes significantly due to a reduction in spinal cord excitability. Upon returning to Earth, recovery occurs within the first day, but it can take up to ten days to return to normal H-reflex levels.
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Frequently asked questions
Reflexes are automatic and involuntary actions the body produces in response to certain stimuli. They are usually protective in nature.
Some examples of reflex actions include breathing, digestion, and the maintenance of the heartbeat. Other examples include the startle reflex, which is an automatic response to an unexpected stimulus, and the feline righting reflex, which reorients a cat's body when falling to ensure a safe landing.
A tendon reflex is the contraction of a muscle in response to striking its tendon. The Golgi tendon reflex is the inverse of a stretch reflex.
The stretch reflex, also known as the muscle stretch reflex or deep tendon reflex, is a somatic reflex that keeps muscles at a certain length. When a muscle starts to lengthen or stretch too much, this reflex triggers a muscle contraction to return the muscle to its regular length.
Reflexes can be tested as part of a neurological examination, often if there is an injury to the central nervous system. To test the reflex, the muscle should be in a neutral position. The Jendrassik manoeuvre is a technique used to augment hypoactive reflexes.
