Proprioceptive Reflexes: Mechanisms and Functions

• Proprioceptors are the receptors that give a sense of the position of joints in 3-D space. Eg:- 
• There are three major types of proprioceptors:-
  • Muscle Spindle
  • Golgi Tendon Organ(GTO)
  • Various Joint Capsule Receptors - Most of the joint capsule receptors are slow-adapting receptors.

Muscle Spindle Receptor

It is present inside the skeletal muscle and they are spindle-shaped. It detects the length of the muscle to sense the position of the joint.

• The spindle-shaped receptor inside the main muscle bulk/belly is known as the muscle spindle. Multiple such muscle spindles are present in skeletal muscle.
• The muscle spindle is also composed of skeletal muscle fibre but slightly modified. Thus, the red fibre that forms the muscle spindle is known as intrafusal fibre. 
• The white fibre around it is known as the extrafusal fibre of skeletal muscle. It is the contractile fibre.
• There are up to 50,000 muscle spindles in our whole body.
  • A single arm has approximately 7000 muscle spindles.
  • A single leg has approximately 4000 muscle spindles.

Golgi Tendon Organ(GTO)

Another receptor, the Golgi Tendon Organ Receptor (GTO), is present at the junction of muscle and tendon in the Musculotendinous junction.

• Intrafusal fibres are of two types:-
  • Nuclear Bag fibre
  • Nuclear chain fibre

Nuclear Bag Fiber

• If the single fibre contains multiple nuclei together inside a bag-like region, it is known as the nuclear bag fibre of the muscle spindle.
• They are 2-3 in number.
• According to their functions, they are further classified into 2 types:-
  • Dynamic nuclear bag fibre: It detects the real-time shortening/lengthening of a muscle, i.e., the velocity of a muscle(length/time).
  • Static nuclear bag fibre: They only detect the increased length of a muscle.

• If the single fibre contains multiple nuclei together inside a bag-like region, it is known as the nuclear bag fibre of the muscle spindle.
• They are 2-3 in number.
• According to their functions, they are further classified into 2 types:-
  • Dynamic nuclear bag fibre: It detects the real-time shortening/lengthening of a muscle, i.e., the velocity of a muscle(length/time).
  • Static nuclear bag fibre: They only detect the increased length of a muscle.

Nuclear Chain Fibre

• If these nuclei are spread throughout the muscle at regular intervals, it is known as the nuclear chain fibre.
• They are 5-9 in number.
• They are always static in nature.
• They, too, only detect the increased length of a muscle.

Innervation Of Muscle Spindle

• Although sensory receptors indicate that there is the presence of afferent fibre, it is also supplied by efferent fibre from CNS. Eg:- Muscle spindle, hair cells at the organ of Corti.

Afferent Connections Of The Muscle Spindle

• A nerve fibre originates from the centre region of the muscle spindle, and it is Ⅰ_a fibre. It makes a synapse at the centre region of the muscle spindle.
  • This synapse is a spiral connection. Thus, it is also known as the Annulo-Spiral Ending or Primary Afferent/Ending of the Muscle Spindle.
  • Ⅰ_a fibre is connected to all three i.e., dynamic and static bag fibre and chain fibre. Thus, all three fibres get their afferent supply.
  • Ⅰ_a fibre can detect the velocity and length of the muscle.
• Another afferent fibre originating from the peripheral region of the muscle spindle is known as the Ⅱ_a fibre.
  • The nerve fibre spreads instead of making a spiral synapse with the muscle spindle and it is known as the Flower-spray ending or Secondary ending of muscle spindle.
  • It doesn't connect the dynamic bag fibre. It will only connect the static bag and static chain fibre.
  • It cannot detect the velocity of the muscle.

Stretch Reflex/ Myotatic Stretch Reflex

• Consider the afferent neuron of a skeletal muscle i.e., Ⅰ_a fibre that goes to the level of the spinal cord. 
• It makes another synapse inside the spinal cord with another neuron known as the ?-motor neuron(causes contraction of muscle). It particularly supplies extrafusal fibre of the muscle.
• Thus when this extrafusal fibre is stimulated, there will be contraction of the muscle.
• Suppose the length of the muscle is increased i.e., the muscle is stretched suddenly and passively, it causes an increase in spindle length following which, the muscle spindle is activated.
• Thus, Ⅰ_a fibre is activated to bring the sensation to the spinal cord and release neurotransmitters to activate the ?-motor neuron.
• The action potential travels down to cross the neuromuscular junction and activates the white fibre. This leads to the contraction of the whole muscle.
• Thus, a sudden and passive stretch on the skeletal muscle leads to reflex contraction known as the stretch reflex/myotatic stretch reflex. Eg:- Catching a ball through contraction since passive stretching occurs when the ball falls on our hand.
• The stretch reflex is the most important postural reflex in the body. Eg:- A passenger in a bus may tilt his body when the vehicle turns/brakes. The skeletal muscle stretches to activate the muscle spindle and send signals to the spinal cord. ?-motor neuron is activated to bring sensation back to the muscle and the muscle contracts to correct the posture. This happens within a fraction of second because conduction velocity at the level of  Ⅰ_a fibre is 70-120 m/s.
• Since there is only one synapse in the shown pathway, the stretch reflex pathway is a monosynaptic pathway.
• The receptor for this pathway is the Muscle spindle.
  • Afferent from this receptor is Ⅰ_a neuron.
  • Efferent from this receptor is ?-motor neuron.
• The time taken for this reflex contraction to occur when a muscle is stretched is known as the reaction time of the stretch reflex pathway.
• This reaction time is 19-24 ms. Within this time, 0.5 ms is required to cross the synapse.
  • Deep Tendon Reflexes(DTR)( Knee jerk reflex)

Efferent Connections Of The Muscle Spindle

• The efferent connection of the muscle spindle originates from a neuron known as the A?.
• It is connected only at the terminal-most region of the muscle spindle.
• If the A? fibre is activated, there will be activation of the terminal-most portion of the muscle spindle.
• This is the only portion of the muscle spindle that contains actin & myosin.
• Intrafusal fibre is a non-contractile fibre i.e., contains very less actin & myosin that is concentrated in the terminal-most region, i.e., the portion where A? fibre is connected.
• Thus when stimulated, the actin-myosin cross-bridge is formed because of which, there is contraction of the terminal-most region of the muscle spindle.
• However, this contraction doesn’t shorten the muscle spindle. This is because the spindle is connected with extrafusal fibre on the outside.
• Extrafusal fibres are very strong and actin-myosins are few in number. Thus, the ends/terminal regions remain fixed.
• The shortening occurs in opposite directions and lengthens the middle portion of the muscle spindle since the ends cannot stretch.
• Ⅰ_a fibre is activated as it is connected in the middle portion. Since Ⅰ_a fibre is connected to ?-motor neuron, Muscle contraction occurs indirectly.
• A? fibre activation also leads to increased sensitivity of the muscle spindle, which exaggerates the stretch reflex.
• This activation can be performed by Jendrassik Maneuver.
  • Interlocking hands and, pulling fingers, clenching teeth strongly can activate A? fibre.
• It can also be activated in cases of anxiety, fever, pain, activated sympathetic system, etc.
• When such actions are performed, the brain sends signals toward the spinal cord to activate A? fibre and ?-motor neuron(stimulated by the cerebral cortex due to voluntary contraction). This simultaneous activation of A? fibre and ?-motor neuron is known as ?-?-coactivation.
• This is necessary because sole ?-motor neuron activation will lead to contraction, but the muscle will be loose and lax. The muscle spindle will not be active.
• Thus, if the muscle spindle is required to be active even during contraction, then A? fibre must be stimulated along with ?-motor neuron activation to prevent the relaxation of the muscle spindle.
• During muscle contraction for posture correction, the muscle spindle will be loose. If a second immediate disturbance of posture occurs, the muscle spindle will not act immediately. Thus, the muscle spindle must be kept active for any sudden requirement of posture correction and it is only done through ?-?-coactivation.
• Upon stretching a muscle, it will contract while the opposite/antagonist muscle relaxes.
• While Ⅰ_a fibre originates from the muscle spindle to reach and activate the ?-motor neuron, another fibre originates from the Ⅰ_a neuron to stimulate an interneuron that is inhibitory in nature.
• This inhibitory interneuron makes a synapse with the ?-motor neuron that belongs to the antagonist muscle. It causes relaxation of this muscle.
• This relaxation of the antagonist muscle is known as the Reciprocal Innervation Pathway. It was first discovered by Sherrington and is also called Sherrington’s Law.
• Reciprocal innervation pathway is a Bisynaptic pathway. The inhibitory interneuron of this pathway is also known as the Golgi Bottle Neuron.

Golgi Tendon Organ(GTO)

• GTO is located at the muscle-tendon junction in 95% of the cases. It is located within the tendon in the rest 5% of the cases.
• There are 3-25 fibres or 10-15 fibres on average within a single GTO.
• The stimulus for activation of GTO is increased tension at the tendon of the muscle.
• The afferent neuron from the Golgi Tendon Organ Receptor is the Ⅰ_b neuron. It does not make a synapse directly with the ?-motor neuron.
• It takes the help of another inhibitory interneuron that makes a synapse with the ?-motor neuron of the same muscle.

Inverse Stretch Reflex Or Autogenic Inhibition

• Upon activation of GTO and Ⅰ_b neuron, the inhibitory interneuron releases neurotransmitters to inhibit the ?-motor neuron. The extrafusal fibre will relax rather than contract, leading to muscle relaxation.
• GTO is activated through sudden, strong muscle contractions that cause increased tension at the tendon. This activates GTO and ultimately leads to muscle relaxation. 
• Thus, sudden contraction of the muscle leads to reflex relaxation through this protective response that prevents tendon rupture. This reflex is known as Inverse Stretch Reflex or Autogenic Inhibition.
• The receptor for this reflex is GTO.
  • The afferent for this reflex is Ⅰ_b neuron.
  • The efferent for this reflex is ?-motor neuron.
• It is a Bisynaptic Reflex pathway.

Clonus

• When the upper motor neuron is damaged.There are certain neurons that arrive from the brain and have control over the reflex pathway. In resting conditions, this control is inhibitory.
• If this neuron is damaged, it is known as upper motor neuron damage.
• It causes an exaggeration in the stretch reflex and inverse stretch reflex.
• An important clinical sign is performed in this condition for neurological examination known as Clonus.
• Clonus occurs due to alternate activation of stretch and inverse reflex in case of upper motor neuron damage.

Withdrawal Reflex

• It is a polysynaptic reflex pathway.
• The stimulus for the withdrawal reflex is generally
  • Afferent: Group Ⅲ & Ⅳ(painful stimulus) and group Ⅱ(pressure)
• Component of withdrawal reflex:
  • Flexor reflex(withdrawing the limb from a stimulating object)
  • Reciprocal Inhibition
  • Crossed Extensor Reflex (0.2-0.5 sec after stimulus)
  • After-discharge(contraction of muscle persists for long duration due to strong stimulus)

• During withdrawal, the flexor muscle contracts, and the extensor muscle relaxes.
• The painful stimulus is given to the hand, and the pain fibre brings the pain sensation to the spinal cord, where it takes the help of multiple small interneurons. It ultimately stimulates the flexor group of muscle and inhibits the extensor/opposite group of muscle. This reflex is on the same side of the body and is known as the Flexor Reflex.
• When a nail pricks the sole of the right foot, there is a withdrawal of the right foot and contraction of the left foot. 
• This happens because the reflex pathway also communicates with the other side of the body via certain interneurons.
• It causes excitation of the extensor muscles and inhibition of the flexor muscles. This response, which takes place on the opposite side of the body(balances and supports the body weight), is called the Crossed Extensor Reflex.

This is a really complex topic, and therefore, it is recommended that you watch the video from the PrepLadder application's physiology section with it. This is a volatile topic for all the major competitive exams, such as FMGE, NEET-PG, and INICET.

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