Reflex Action

Reflex Action After entering the spinal cord, sensory signals may follow one or both of the following:They may ascend to a higher level of the CNS through various sensory tracts and reach the somato-sensory cortex via the thalamus. They may terminate locally in the grey matter of the spinal cord and elicit local segmental responses. A reflex: is an involuntary motor response to an adequate sensory stimulus that occurs subconsciously, without the mediation of the cerebral cortex, e.g. withdrawal reflex

Reflex Action: has the following character:• It has an important role in the protection of the body. • It is responsible for the maintenance of the muscle tone. • It is responsible for the maintenance of the body posture. • Its center can be anywhere except the cerebral cortex. • Its center can be in the spinal cord or in the brain stem. • Reflex action: It is the functional unit of the nervous system. It is an involuntary action in response to a stimulus, carried out through the reflex arc

Classification of the Reflexes Reflexes may be anatomical, physiological and clinical. 1- Anatomical: The reflexes are classified anatomically according to the spinal cord segment involved into:• Segmental reflex ( the reflex arc involves one side of the spinal segment, in other words, the receptors organ, afferent neuron, efferent neuron and effector organ are located on the same side of the spinal segment e.g. stretch reflex). • Intersegmental reflex ( the reflex arc involves both sides of the spinal segment, in other words the efferent neuron and effector organ are on opposite sides of the receptor organ e.g. the crossed extensor reflex). • Suprasegmental (the segments above the level of the reflex arc are involved, e.g. the postural reflex).

• Physiological: Physiological reflexes are classified into:the flexor/ withdrawal reflex extensor / stretch reflex.

Clinical Reflexes: are classified into unconditioned and conditioned reflexes according to whether they were present at birth or developed later. • Conditioned reflexes: are not present at birth but are acquired later on by training, e.g. cycling and swimming. • Unconditioned reflexes: are inborn this means that they are present since birth e.g. suckling reflex. They are of three types:– Superficial reflexes (they are elicited by stimulation of receptor organs that are present superficially in the skin or mucous membrane e.g. conjunctival reflex, abdominal and planter reflex) – Deep reflexes (they are elicited by tapping the tendon of a slightly stretched muscle, e.g. the knee jerk and ankle jerk). – Visceral reflexes: They are concerned with the reflex activity of internal organs. Part of the reflex is made by the autonomic nervous system, e.g. cardiovascular, gastrointestinal, micturition reflexes.

THE STRETCH REFLEX

• Control of skeletal muscle contraction is strongly dependent on the sensory feedback coming from the receptors within the muscles. • Skeletal muscles contain two types of highly specialized proprioceptors specifically concerned with movement. • the tendon organ and the muscle spindle. • The tendon organ responds mainly to muscle tension generated by active contraction. • The muscle spindle is a stretch receptor which is particularly sensitive to lengthening of the muscle.

Stretch reflex: It is a reflex contraction of skeletal muscle in response to passive stretch

Reflex arc of stretch reflex: • Receptor: Muscle spindle. • Afferent nerve: Rapidly conducting Ia & II sensory fibrous. • Center: Motor neuron pool. • Efferent nerve: Rapidly conducting A and motor fibers. • Effector organ: Skeletal Muscle

Muscle Spindle • It is a fusiform structure. • The spindle senses muscle length and changes in length. • It has sensory nerve terminals whose discharge rate increases as the sensory ending is stretched. • It is present in the fleshy parts of the skeletal muscle on a scattered manner. • It lies parallel to muscle fibers = extrafusal. • Each spindle consists of several small muscle fibers = intrafusal. • The intrafusal muscle fibers encapsulated by a connective tissue capsule. • The capsule is attached at its ends to the sides of extrafusal fibers. • The intrafusal muscle is short, slender and less striated. • Each fiber is made up of two peripheral contractile parts and a central noncontractile, multinuclear part.

There are two types of intrafusal fibers: • 1) Nuclear bag. 2) Nuclear chain. • The nuclear bag fibers: are longer (7-8 mm) and thicker (25 mm) and have many nuclei in a dilated, bag like central area. • The nuclear chain fibers: are shorter (3-4 mm), and thinner (12 mm). • - They have nuclei arranged side by side like a chain in its central area. • - They are attached to the sides of the nuclear bag fibers.

Muscle Spindle

Nerve supply to the Muscle spindle Afferent (sensory) and efferent (motor) nerve fibers supply the muscle spindle. 1) Afferent (sensory) is arising from the spindles. 2) Efferent (motor), supplying the spindles (gamma efferent). 1) Afferent (sensory) nerves arising from the spindles. There are two types of afferent nerve that supply the muscle spindle:a) Primary afferent fibers (16 m): • Large, type 1a, terminates by annulospinal endings around the central part of the nuclear bag and nuclear chain stimulated by stretch of central receptor area. b) Secondary afferent fibers (8 m): • Medium, type II, terminates by flower – spray endings around the central parts of the nuclear chain fibers only.

2) Efferent (motor) nerves supplying the spindles (gamma efferent). • There are two types of efferent nerve fibers that supply the peripheral, contractile parts of intrafusal muscle fibers of the muscle spindle. • Dynamic – γ – efferent fibers: They supply the nuclear bag. • Static – γ – efferent fibers: They supply the nuclear chain. • Stimulation of the γ-efferent fibers produces contraction of the peripheral contractile parts of the intrafusal fibers: this leads to stretching of central noncontractile parts of these fibers, which stimulates the stretch receptor.

Functions of Stretch Reflex: • • • • • •

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1- Skeletal Muscle Tone: Maintenance of the erect posture against the force of gravity, by producing a strong muscle tone in the antigravity muscle. 2- Damping (smoothing) function: The signals discharged to a muscle often have varying intensities, and this would result in irregular movements. However through the muscle spindle and alphagamma linkage, the signal are adjusted to produce smooth movements. 3- Servo-Assist Function: servo = force regulator: The stretch reflex assists the brain to produce and regulate the force of muscle contraction as follow when the muscle contract: The a and g motor fibers are activated to the same degree. The extrafusal and intrafusal muscle fibers are equally contracted and shortened to the same degree. So, central part of the intrafusal muscle fibers does not change in length, and the intensity of the stretch receptor remains unchanged. If the muscle tries to left a heavy weight the extrafusal fiber contract isometrically and intrafusal fibers contract at the periphery and lengthen the central part. This potentiates the stretch reflex leading to strong muscle contraction to help lifting the weight. Antigravity Function: To resist the gravity effect which tends to flex the muscle of the lower limbs and trunk. The stretched muscles respond by reflex contraction to maintain the upright position of body against the effect of gravity and prevent its fall down.

Monosynaptic-Stretch Reflex • Consists of only one synapse within the CNS. – Sensory neuron synapses directly with the motor neuron.

• Striking the patellar ligament, passively stretches the spindles. – Stimulates primary endings in spindles, activating sensory neurons. – Synapse with α motor neurons stimulating extrafusal fibers. • Produces isotonic

Golgi Tendon Organ Reflex • Disynaptic reflex. – 2 synapses are crossed in the CNS.

• Sensory neurons synapse with interneurons. – These interneurons have inhibitory synapses with motor neurons.

• Helps prevent excessive muscle contraction or passive muscle stretching.

Reciprocal Innervation • Sensory neuron stimulates motor neuron and interneuron. • Interneurons inhibit motor neurons of antagonistic muscles. • When limb is flexed, antagonistic extensor muscles are passively stretched.

Crossed-Extensor Reflex • Double reciprocal innervation. • Affects muscles on the contralateral side of the cord. • Step on tack: – Foot is withdrawn by contraction of flexors and relaxation of extensors.

• Contralateral leg extends to support body.