Reflex

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In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action [1] and nearly instantaneous response to a stimulus. [2] [3]

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The simplest reflex is initiated by a stimulus, which activates an afferent nerve. The signal is then passed to a response neuron, which generates a response. Improved General Reflex Diagram.png
The simplest reflex is initiated by a stimulus, which activates an afferent nerve. The signal is then passed to a response neuron, which generates a response.

Reflexes are found with varying levels of complexity in organisms with a nervous system. A reflex occurs via neural pathways in the nervous system called reflex arcs. A stimulus initiates a neural signal, which is carried to a synapse. The signal is then transferred across the synapse to a motor neuron, which evokes a target response. These neural signals do not always travel to the brain, [4] so many reflexes are an automatic response to a stimulus that does not receive or need conscious thought. [5]

Many reflexes are fine-tuned to increase organism survival and self-defense. [6] This is observed in reflexes such as the startle reflex, which provides an automatic response to an unexpected stimulus, and the feline righting reflex, which reorients a cat's body when falling to ensure safe landing. The simplest type of reflex, a short-latency reflex, has a single synapse, or junction, in the signaling pathway. [7] Long-latency reflexes produce nerve signals that are transduced across multiple synapses before generating the reflex response.

Types of human reflexes

Myotatic reflexes

The myotatic or muscle stretch reflexes (sometimes known as deep tendon reflexes) provide information on the integrity of the central nervous system and peripheral nervous system. This information can be detected using electromyography (EMG). [8] Generally, decreased reflexes indicate a peripheral problem, and lively or exaggerated reflexes a central one. [8] A stretch reflex is the contraction of a muscle in response to its lengthwise stretch.

While the reflexes above are stimulated mechanically, the term H-reflex refers to the analogous reflex stimulated electrically, and tonic vibration reflex for those stimulated to vibration.

Tendon reflex

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.

Reflexes involving cranial nerves

NameSensoryMotor
Pupillary light reflex II III
Accommodation reflex IIIII
Jaw jerk reflex V V
Corneal reflex, also known as the blink reflexV VII
Glabellar reflex VVII
Vestibulo-ocular reflex VIII III, IV, VI +
Gag reflex IX X

Reflexes usually only observed in human infants

Grasp reflex Greifreflex.JPG
Grasp reflex

Newborn babies have a number of other reflexes which are not seen in adults, referred to as primitive reflexes. These automatic reactions to stimuli enable infants to respond to the environment before any learning has taken place. They include:

Other kinds of reflexes

Other reflexes found in the central nervous system include:

Many of these reflexes are quite complex, requiring a number of synapses in a number of different nuclei in the central nervous system (e.g., the escape reflex). Others of these involve just a couple of synapses to function (e.g., the withdrawal reflex). Processes such as breathing, digestion, and the maintenance of the heartbeat can also be regarded as reflex actions, according to some definitions of the term.

Grading

In medicine, reflexes are often used to assess the health of the nervous system. Doctors will typically grade the activity of a reflex on a scale from 0 to 4. While 2+ is considered normal, some healthy individuals are hypo-reflexive and register all reflexes at 1+, while others are hyper-reflexive and register all reflexes at 3+.

GradeDescription
0Absent ("mute")
1+ or +Hypoactive
2+ or ++"Normal"
3+ or +++Hyperactive without clonus, with spread to adjacent muscle groups
4+ or ++++Hyperactive with clonus

Depending on where you are, another way of grading is from –4 (absent) to +4 (clonus), where 0 is "normal".

Reflex modulation

An example of reflex reversal is depicted. Activating the same spinal reflex pathway can cause limb flexion while standing, and extension while walking. Demonstration of reflex reversal.svg
An example of reflex reversal is depicted. Activating the same spinal reflex pathway can cause limb flexion while standing, and extension while walking.

Some might imagine that reflexes are immutable. In reality, however, most reflexes are flexible and can be substantially modified to match the requirements of the behavior in both vertebrates and invertebrates. [9] [10] [11]

A good example of reflex modulation is the stretch reflex. [12] [13] [14] [15] When a muscle is stretched at rest, the stretch reflex leads to contraction of the muscle, thereby opposing stretch (resistance reflex). This helps to stabilize posture. During voluntary movements, however, the intensity (gain) of the reflex is reduced or its sign is even reversed. This prevents resistance reflexes from impeding movements.

The underlying sites and mechanisms of reflex modulation are not fully understood. There is evidence that the output of sensory neurons is directly modulated during behavior—for example, through presynaptic inhibition. [16] [17] The effect of sensory input upon motor neurons is also influenced by interneurons in the spinal cord or ventral nerve cord [15] and by descending signals from the brain. [18] [19] [20]

Other reflexes

Breathing can also be considered both involuntary and voluntary, since breath can be held through internal intercostal muscles. [21] [22] [23]

See also

Related Research Articles

<span class="mw-page-title-main">Nervous system</span> Part of an animal that coordinates actions and senses

In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes that impact the body, then works in tandem with the endocrine system to respond to such events. Nervous tissue first arose in wormlike organisms about 550 to 600 million years ago. In vertebrates, it consists of two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord. The PNS consists mainly of nerves, which are enclosed bundles of the long fibers, or axons, that connect the CNS to every other part of the body. Nerves that transmit signals from the brain are called motor nerves (efferent), while those nerves that transmit information from the body to the CNS are called sensory nerves (afferent). The PNS is divided into two separate subsystems, the somatic and autonomic, nervous systems. The autonomic nervous system is further subdivided into the sympathetic, parasympathetic and enteric nervous systems. The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system is activated when organisms are in a relaxed state. The enteric nervous system functions to control the gastrointestinal system. Nerves that exit from the brain are called cranial nerves while those exiting from the spinal cord are called spinal nerves.

<span class="mw-page-title-main">Somatic nervous system</span> Part of the peripheral nervous system

The somatic nervous system (SNS) is made up of nerves that link the brain and spinal cord to voluntary or skeletal muscles that are under conscious control as well as to skin sensory receptors. Specialized nerve fiber ends called sensory receptors are responsible for detecting information within and outside of the body.

<span class="mw-page-title-main">Muscle spindle</span> Innervated muscle structure involved in reflex actions and proprioception

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 fibers. This information can be processed by the brain as proprioception. The responses of muscle spindles to changes in length also play an important role in regulating the contraction of muscles, for example, by activating motor neurons via the stretch reflex to resist muscle stretch.

<span class="mw-page-title-main">Reflex arc</span> Neural pathway which controls a reflex

A reflex arc is a neural pathway that controls a reflex. In vertebrates, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. This allows for faster reflex actions to occur by activating spinal motor neurons without the delay of routing signals through the brain. The brain will receive the input while the reflex is being carried out and the analysis of the signal takes place after the reflex action.

A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are innervated by sensory neurons that convert mechanical pressure into electrical signals that, in animals, are sent to the central nervous system.

<span class="mw-page-title-main">Sensory neuron</span> Nerve cell that converts environmental stimuli into corresponding internal stimuli

Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded receptor potentials. This process is called sensory transduction. The cell bodies of the sensory neurons are located in the dorsal ganglia of the spinal cord.

<span class="mw-page-title-main">Patellar reflex</span> Monosynaptic reflex

The patellar reflex, also called the knee reflex or knee-jerk, is a stretch reflex which tests the L2, L3, and L4 segments of the spinal cord. Many animals, most significantly humans, have been seen to have the patellar reflex, including dogs, cats, horses, and other mammalian species.

The withdrawal reflex is a spinal reflex intended to protect the body from damaging stimuli. The reflex rapidly coordinates the contractions of all the flexor muscles and the relaxations of the extensors in that limb causing sudden withdrawal from the potentially damaging stimulus. Spinal reflexes are often monosynaptic and are mediated by a simple reflex arc. A withdrawal reflex is mediated by a polysynaptic reflex resulting in the stimulation of many motor neurons in order to give a quick response.

In animals, including humans, the startle response is a largely unconscious defensive response to sudden or threatening stimuli, such as sudden noise or sharp movement, and is associated with negative affect. Usually the onset of the startle response is a startle reflex reaction. The startle reflex is a brainstem reflectory reaction (reflex) that serves to protect vulnerable parts, such as the back of the neck and the eyes (eyeblink) and facilitates escape from sudden stimuli. It is found across many different species, throughout all stages of life. A variety of responses may occur depending on the affected individual's emotional state, body posture, preparation for execution of a motor task, or other activities. The startle response is implicated in the formation of specific phobias.

Motor control is the regulation of movements in organisms that possess a nervous system. Motor control includes conscious voluntary movements, subconscious muscle memory and involuntary reflexes, as well as instinctual taxis.

<span class="mw-page-title-main">Scratch reflex</span> Response to activation of sensory neurons

The scratch reflex is a response to activation of sensory neurons whose peripheral terminals are located on the surface of the body. Some sensory neurons can be activated by stimulation with an external object such as a parasite on the body surface. Alternatively, some sensory neurons can respond to a chemical stimulus that produces an itch sensation. During a scratch reflex, a nearby limb reaches toward and rubs against the site on the body surface that has been stimulated. The scratch reflex has been extensively studied to understand the functioning of neural networks in vertebrates. Despite decades of research, key aspects of the scratch reflex are still unknown, such as the neural mechanisms by which the reflex is terminated.

<span class="mw-page-title-main">Escape reflex</span>

Escape reflex, or escape behavior, is any kind of escape response found in an animal when it is presented with an unwanted stimulus. It is a simple reflectory reaction in response to stimuli indicative of danger, that initiates an escape motion of an animal. The escape response has been found to be processed in the telencephalon.

<span class="mw-page-title-main">Vestibulospinal tract</span> Neural tract in the central nervous system

The vestibulospinal tract is a neural tract in the central nervous system. Specifically, it is a component of the extrapyramidal system and is classified as a component of the medial pathway. Like other descending motor pathways, the vestibulospinal fibers of the tract relay information from nuclei to motor neurons. The vestibular nuclei receive information through the vestibulocochlear nerve about changes in the orientation of the head. The nuclei relay motor commands through the vestibulospinal tract. The function of these motor commands is to alter muscle tone, extend, and change the position of the limbs and head with the goal of supporting posture and maintaining balance of the body and head.

<span class="mw-page-title-main">Stretch reflex</span> Muscle contraction in response to stretching

The stretch reflex, or more accurately "muscle stretch reflex", is a muscle contraction in response to stretching a muscle. The function of the reflex is generally thought to be maintaining 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 criticized as misleading. Tendons have little to do with the response, and some muscles with stretch reflexes have no tendons. Rather, muscle spindles detect a stretch and convey the information to the central nervous system.

<span class="mw-page-title-main">Alpha motor neuron</span>

Alpha (α) motor neurons (also called alpha motoneurons), are large, multipolar lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction. Alpha motor neurons are distinct from gamma motor neurons, which innervate intrafusal muscle fibers of muscle spindles.

The triceps reflex, a deep tendon reflex, is a reflex that elicits involuntary contraction of the triceps brachii muscle. It is sensed and transmitted by the radial nerve. The reflex is tested as part of the neurological examination to assess the sensory and motor pathways within the C7 and C8 spinal nerves.

<span class="mw-page-title-main">Axon reflex</span>

The axon reflex is the response stimulated by peripheral nerves of the body that travels away from the nerve cell body and branches to stimulate target organs. Reflexes are single reactions that respond to a stimulus making up the building blocks of the overall signaling in the body's nervous system. Neurons are the excitable cells that process and transmit these reflex signals through their axons, dendrites, and cell bodies. Axons directly facilitate intercellular communication projecting from the neuronal cell body to other neurons, local muscle tissue, glands and arterioles. In the axon reflex, signaling starts in the middle of the axon at the stimulation site and transmits signals directly to the effector organ skipping both an integration center and a chemical synapse present in the spinal cord reflex. The impulse is limited to a single bifurcated axon, or a neuron whose axon branches into two divisions and does not cause a general response to surrounding tissue.

The Golgi tendon reflex (also called inverse stretch reflex, autogenic inhibition, tendon reflex) is an inhibitory effect on the muscle resulting from the muscle tension stimulating Golgi tendon organs (GTO) of the muscle, and hence it is self-induced. The reflex arc is a negative feedback mechanism preventing too much tension on the muscle and tendon. When the tension is extreme, the inhibition can be so great it overcomes the excitatory effects on the muscle's alpha motoneurons causing the muscle to suddenly relax. This reflex is also called the inverse myotatic reflex, because it is the inverse of the stretch reflex.

<span class="mw-page-title-main">Spinal interneuron</span> Interneuron relaying signals between sensory and motor neurons in the spinal cord

A spinal interneuron, found in the spinal cord, relays signals between (afferent) sensory neurons, and (efferent) motor neurons. Different classes of spinal interneurons are involved in the process of sensory-motor integration. Most interneurons are found in the grey column, a region of grey matter in the spinal cord.

<span class="mw-page-title-main">Golgi tendon organ</span> Proprioceptive sensory receptor organ

The Golgi tendon organ (GTO) is a proprioceptor – a type of sensory receptor that senses changes in muscle tension. It lies at the interface between a muscle and its tendon known as the musculotendinous junction also known as the myotendinous junction. It provides the sensory component of the Golgi tendon reflex.

References

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  2. ↑ Purves (2004). Neuroscience: Third Edition. Massachusetts, Sinauer Associates, Inc. ISBN   0-87893-725-0
  3. ↑ "Definition of reflex". Dictionary by Merriam-Webster. 25 December 2023.
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