The stretch reflex is the contraction of a musculus that occurs in response to its stretch. It is not controlled by higher functioning centre i.eastward. the encephalon, and is a monosynaptic response that is transmitted to the spinal cord. Our body needs to be able to respond without our cortical input. In this article nosotros volition talk over the stretch reflex, the anatomy that underpins information technology, as well as the clinical relevance.
Contents
Overview
Microanatomy
Machinery
Functions
Clinical testing of reflexes
Clinical points
Nervous system lesions
Multiple sclerosis
'Charcot Marie Tooth' disease
Stroke
Sources
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Overview
Reflexes are automated, subconscious responses to changes within or exterior the body. They function to maintain the homeostasis (autonomic reflexes), which include breathing, blood force per unit area regulation and heartbeat. We also have reflexes that conduct out automated things we don't need to call up about to perform e.g. swallowing, sneezing, coughing and vomiting. We require reflexes to maintain our posture and trunkal rest. These are known equally spinal reflexes. Finally we accept brainstem reflexes, e.thou. the eye move reflexes.
The stretch reflex is likewise referred to as the deep tendon reflex or myotatic reflex. Information technology is a uncomplicated pre-programmed response by the human trunk in response to the musculus being passively stretched e.thousand. by a tendon hammer, or a sudden change in the ground surface. The stretch reflex can exist activated past external forces such equally a load placed on the muscle or internal forces i.eastward. the motor neurons existence stimulated from inside. An example of the former is someone holding a plate, and someone serving them some food. An instance of the latter is shivering that occurs when the person is common cold, and is instigated by the internal neurons of the muscle.
Microanatomy
Biceps femoris muscle (posterior view)
Embedded inside a musculus are the muscle spindles. Musculus spindles are composed of a few intrafusal fibres (nuclear bag and nuclear chain are the subtypes), which in fact lack the contractile proteins of normal muscle (actin and myosin), they are non contractile, and they serve as receptive surfaces. Two types of afferent nerve endings penetrate muscle spindles, these are primary sensory fibres of type Ia and secondary sensory fibres of type II. The gamma efferent fibres innervate these regions. It is worth remembering, that the contractile skeletal musculus fibres are 'extrafusal' and are thus innervated by the blastoff efferent nerves.
The static component of the stretch reflex is in place as long as the muscle is stretched. The dynamic stage occurs merely when the muscle is stretched east.m. when the tendon is struck with a tendon hammer. When the muscle is stretched, they send an impulse via the sensory neurons to the relevant spinal cord segment. The nerve synapses with a second nervus within the spinal cord, with the alpha motor neuron to contract. This impulse does not need to travel to the brain, and therefore simply travels from the musculus spindles to the spinal string, and back to the musculus. The entire reflex process takes identify over a few milliseconds.
There is a second set of motoneurons called Golgi tendon organs. Golgi tendon organs are sensory receptor organs present where the musculus is connected to the tendon, and which regulate the tension within the muscle. Alpha gamma coactivation maintains the tension in the musculus spindles when a musculus is in a state of contraction.
Information technology's almost time to start revising what you lot've learned about the stretch reflex. Don't forget the importance of active call up when learning anatomy!
Machinery
Evolutionarily, the stretch reflex was designed every bit a protective measure out for the muscles, in gild to forbid tearing that can occur due to vigorous movement. Once the musculus spindle is stretched, the impulse is sent back to the musculus very apace, and protects information technology from existence pulled forcefully or across its normal range of motion. When a reflex takes place, all of the synergistic muscles (those that cause the same motility) likewise contract while combative muscles are inhibited. The decrease in the simultaneous contraction of the opposing muscles reduces the likelihood of injury.
When a stretch reflex is activated, it not only causes contraction of the synergistic muscles, but as well caused relaxation i.due east. has an inhibitory effect on the antagonist muscles. In one case a stretch reflex occurs, the impulse is sent from the stretched muscle spindle, to the alpha motor neuron. The blastoff motor neuron is dissever. Hence information technology is able to cause contraction in the synergistic grouping, and relaxation in the antagonistic group. If the combative group did not relax, both groups of muscles would contract, resulting in no corrective motion.
The stretch reflex is an instance of a circuit that skips the brain, and follows the simple neural loop connecting the muscle to the spinal cord and back. This enables a rapid response. This item response is crucial. For example, if the muscle is working against a load and shortening during contraction and a subsequent extra load is added, the muscle is able to recognize the stretch immediately and tin compensate with a stronger contraction. This likewise protects the inhibited adversary muscles from being injured from excessive stretching.
Spinal cord (anterior view)
Functions
The starting time major function of the stretch reflex is muscle protection. When a musculus length increases, the musculus spindle within that muscle stretches, and its nerve activity will increase. Resulting from this is increased alpha motor neuron activeness. These neurons volition crusade the muscle to contract, and therefore reduce the stretching of the muscle. The whole organisation functions as an autonomic regulation of muscle length.
Posture is also a very of import consequence of the stretch reflex. It is relevant, as leaning to one side will result in the musculus spindles of the leg and spinal muscles of the contrary side beingness stretched. This is rapidly countered by the stretch reflex, which volition keep usa upright. Some other example is if we meet a sudden change in the ground nosotros are standing/walking on. If we are to foreclose ourselves falling from the force of gravity, we will require rapid correction, mediated by the stretch reflex. The motor neurons travel from the spinal cord itself, and return to the muscle in a loop of neural circuitry. When nosotros brand wittingmovements, we send impulses from our brains down our spinal cords, and so back to the brain for cerebral processing.
Clinical testing of reflexes
Mandibular nerve (lateral-left view)Jaw jerk reflex (CN V- Mandibular partitioning of the Trigeminal nervus): It is a stretch reflex used to test the cranial nervus V. The mandible is struck just below the lower lips while the mouth is slightly open. This causes the masseter musculus to contract, and the jaw to shut. In a normal person, the reflex is pocket-size or absent. In a person with an upper motor neuron lesion, it may be brisk.
Biceps reflex C5/C6: This is performed by striking the biceps tendon within the cubital fossa. This will cause elbow flexion.
Brachioradialis reflex C6: This is performed by striking the brachioradialis tendon at the wrist, just earlier it inserts into the radial styloid process. This will crusade supination of the forearm.
Extensor digitorum reflex C6/C7: This reflex can be elicited by striking the extensor digitorum muscle when the fingers are flexed or one-half flexed. Extension of the fingers should event.
Patellar ligament (lateral-right view) Triceps reflex–C7-viii: This 3-headed muscle causes elbow extension. Its tendon is located merely superior to the olecranon procedure of the elbow.
Patellar reflex L2-four (knee-jerk): This is very commonly performed by doctors who wish to test the integrity of the peripheral fretfulness, 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 hitting of the tendon will cause a slight stretch, which will result in stretch, and contraction of the quadriceps femoris musculus . The patient should give a small kick motion acquired by knee extension.
Ankle wiggle reflex S1-2: This test will appraise the integrity of the sciatic nerve mainly. It is an ankle plantar flexor, and therefore will cause the patient to point their toes, when its tendon (Achilles tendon) is struck.
Clinical points
Nervous organisation lesions
Lesions of the nervous system can exist divided into either upper or lower (some disease processed can affect both). Causes of upper motor neuron damage include:
increased tone
hyper-reflexia
squeeze-knife spasticity
atomic number 82-pipe rigidity
Multiple sclerosis
This disease is acquired by degeneration of the myelin (oligodendrocytes) of the central nervous system i.e. the brain, cerebellum, spinal string. It is associated with brisk reflexes. Symptoms include:
leg weakness
spasticity
ataxia
double vision
gaze palsy
dementia in its late stages
'Charcot Marie Tooth' disease
This is a affliction caused by degeneration of the peripheral motor and sensory neurons i.e. a lower motor neuron disease. It is associated with diminished reflexes. Symptoms usually brainstorm in machismo, but may begin in childhood. They include:
muscle weaknes
reduced muscle tone
loss of sensation.
Stroke
This is the archetypal upper motor neuron lesion and the one that presents almost often clinically. The underlying crusade is the disturbance of claret flow to a region of the brain (like to a myocardial infarction). The precise mechanism is either ischaemic (ordinarily from an atherotic embolus) or haemmorhagic (commonly from a perforating vessel of the brain e.g. the middle cerebral artery). Symptoms volition vary co-ordinate the anatomical site. Reflexes are brisk.
Sources
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References:
Frank H.Netter Md: Atlas of Human Anatomy, 5th Edition, Elsevier Saunders, Chapter 1 Head and Cervix.
Chummy S.Sinnatamby: Last's Beefcake Regional and Applied, 12th Edition, Churchill Livingstone Elsevier, Chapter half-dozen Head and Neck and Spine, Page 375.
Richard L. Drake, A. Wayne Vogl, Adam. W.G. Mitchell: Gray'south Beefcake for Students, 2nd Edition, Churchill Livingstone Elsevier, Chapter 8 Head and Neck: Nasal Cavities, Page 1014 to 1020.
Richard L. Drake, A. Wayne Vogl, Adam. W.K. Mitchell: Dorland's Grey's Pocket Atlas of Anatomy, Churchill Livingstone Elsevier , Affiliate 8 Head and Neck, Page 361.
Manji H: Oxford Handbook of Neurology, 2nd Edition. Affiliate 5, Neurological disorders. page 310.
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Kim Bengochea, Regis University, Denver
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