Posture And Equilibrium Definition
- Subconscious adjustment of tone in the different musos in relation to every movement is known as the posture The significance of posture is to make the movement smooth and accurate and to maintain the line of gravity constant or to keep the body in equilibrium with the line of gravity.
- Posture is not an active movement. It is the passive movement associated with the redistribution of tone in different groups of related muscles.
Basic Phenomena Of Posture
The basic phenomena for the maintenance of posture are the muscle tone and stretch reflex.
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Read And Learn More: Medical Physiology Notes
Muscle Tone Definition: Muscle tone is defined as the state of continuous and passive partial contraction of the muscle with certain vigor and tension. It is also called tonus. It is also defined as resistance offered by the muscle to stretch.
Significance of Muscle Tone: Muscle tone plays an important role in the maintenance of posture. Change in muscle tone enables movement of different parts of the body. Muscle tone is present in all the skeletal muscles. However, tone is greater in the antigravity muscles such as extensors of lower limb, trunk muscles, and neck muscles.
Development of Muscle Tone
- Gamma motor neurons and muscle spindle are responsible for the development and maintenance of muscle tone.
- The muscle tone is purely a reflex process. This reflex is a spinal segmental reflex. It is developed by continual synchronous discharge of motor impulses from the gamma motor neurons present in the anterior gray horn of the spinal cord.
- Sequence of events
- The impulses from the gamma motor neurons cause contraction of end portions of intrafusal fibers (stimulus)
- This stretches and activates the central portion of the intrafusal fibers which initiates the reflex action for the development of muscle tone by discharging the impulses
- The impulses from the central portion of intrafusal fibers pass through the primary sensory nerve fibers (afferent fibers) and reach the anterior gray horn of the spinal cord
- These impulses stimulate the alpha motor neurons in the anterior gray horn (center)
- The alpha motor neurons in turn, send impulses to extrafusal fibers of the muscle through spinal nerve fibers (efferent fibers)
- These impulses produce partial contraction of the muscle fibers resulting in the development of muscle tone (response).
- When the frequency of the discharge from gamma motor neurons increases, the activity of muscle spindle is increased and the muscle tone also increases.
- Stimulation of gamma motor neurons increases the muscle tone. Lesion in gamma motor neurons leads to loss of tone in muscles.
- Sequence of events
Regulation of Muscle Tone: Though the muscle tone is developed by discharges form gamma motor neurons, it is maintained continuously and regulated by some supraspinal centers situated in different parts of brain. Some of these centers increase the muscle tone by sending facilitatory impulses while other centers decrease the muscle tone by inhibitory impulses.
- Supraspinal facilitatory centers: The supraspinal centers which increase the muscle tone are
- Motor area 4 in cerebral cortex
- Cerebellum
- Descending facilitatory reticular system
- Red nucleus
- Vestibular nucleus
- Supraspinal inhibitory centers: The supraspinal centers which decrease the muscle tone are
- Suppressor areas of cerebral cortex
- Basal ganglia
- Descending inhibitory reticular system
- Role of motor area of cerebral cortex – Coactivation
- The motor area of the cerebral cortex influences the activity of lower motor neurons by sending motor impulses through the pyramidal tract fibers. The motor impulses from cerebral cortex stimulate both α motor neurons and γ motor neurons simultaneously. This type of simultaneous stimulation is called coactivation.
- It is also called α-γ coactivation. Stimulation of γ motor neurons causes contraction of extrafusal fibers. Stimulation of γ motor neurons causes contraction of intrafusal fibers which leads to increase in muscle tone.
- Role of cerebellum and basal ganglia: It is interesting to find that cerebellum and basal ganglia influence the muscle tone without sending direct fibers to γ motor neurons. These parts of brain influence the muscle tone indirectly through brainstem centers.
- Role of brainstem centers: Brainstem centers that influence the y motor neurons are in reticular formation, red nucleus, and vestibular nucleus. These centers modulate the discharge from y motor neurons by receiving signals from cerebral cortex, cerebellum, and basal ganglia.
Stretch Reflex
- The basic reflex involved in the maintenance of posture is the stretch reflex.
- This reflex is normally present and serves particularly to maintain the body in an upright position. Such reflexes are, therefore more pronounced in extensor muscles.
Postural Reflexes
- Postural reflexes are the reflexes that are responsible for the maintenance of posture. The afferent impulses for the maintenance of posture arise from proprioceptors, vestibular apparatus, and retina of the eye and reach the centers in central nervous system.
- The centers, which maintain the posture, are located at different levels of central nervous system particularly the cerebral cortex, cerebellum, brainstem, and spinal cord. These centers send motor impulses to the different groups of skeletal muscles so that appropriate movements occur to maintain the posture.
Classification Of Postural Reflexes: The postural reflexes are generally classified into two groups
- Static reflexes
- Statokinetic reflexes
1. Static Reflexes: Static reflexes are the postural reflexes that maintain posture at rest. Static reflexes are of four types:
- General Static Reflexes or Righting Reflexes
- General static reflexes are otherwise called righting reflexes because these reflexes help to maintain an upright position of the body. Righting reflexes help to govern the orientation of the head in space, the position of the head in relation to the body, and appropriate adjustment of the limbs and eyes in relation to the position of the head, so that the upright position of the body is maintained.
- When a cat, held with its back downwards, is allowed to fall through the air, it lands upon its paws, with the head and body assuming the normal attitude in a flash.
- A fish resists any attempt to turn it from its normal position and if it is placed in water upon its back, it flips almost instantly into the normal swimming position. All Urose actions occur because of the righting reflexes.
- The righting reflexes consist of a chain of reactions That occur one after another in an orderly sequence.
- Each reflex causes the development of the succeeding one
- The righting reflexes are divided into five types
- Labyrinthine reflexes acting upon the neck muscles
- Neck righting reflexes acting upon the body
- Body righting reflexes acting upon the head
- Body righting reflexes acting upon the body
- Optical righting reflexes.
- The first four reflexes are easily demonstrated on a thalamic animal or a normal animal, which is blindfolded.
- Labyrinthine righting reflexes acting upon the neck muscles
- When a thalamic animal (rabbit) is suspended by holding at the pelvic region, the head turns up, until it assumes its normal position. It is because of the reflexes arising from the labyrinth, the sensory organ concerned with the equilibrium of the head, in regard to the position of the body.
- Turning the body of the animal through the air into different positions is followed by compensatory movements of the head. After the extirpation of the labyrinths, the head shows no compensatory movements when the rabbit is suspended. It hangs simply like that of a dead rabbit.
- Neck righting reflexes acting on the body:
- It is noticed that during labyrinthine righting reflexes, the head raises up to the normal position. It is because of the contraction of the neck muscles.
- Now, the contraction of neck muscles produces proprioceptive impulses, which act on the body and rotate the body in relation to the position of the head. This reflex is well noticed if the animal is laid down in resting position on its side on a table.
- Body righting reflexes acting on the head: Labyrinthine righting reflexes are not the only reflexes acting on the neck muscles to cause rotation of head. If the animal is laid down upon its side on a table, the unequal distribution of pressure on that particular side of the body stimulates exteroceptors on the skin. The impulses thus generated by the exteroceptors, act on the neck muscles and rotate the head.
- Body righting reflexes acting on the body: When the same animal is laid down on the table on its side, with the head held down to the table, to eliminate labyrinthine and neck righting reflexes, the body attempts to right itself by raising the lower parts. It is because of the impulses from the exteroceptors on that side of the body acting on the body itself.
- Optical righting reflexes: Optical righting reflexes are initiated through the retinal impulses. The center for optical righting reflexes is in the occipital lobe of cerebral cortex.
- So, these reflexes are absent in thalamic animals. The optical righting reflexes help to correct the position of the body or head with the help of sight.
- It is proved in a labyrinthectomized animal. When such an animal is suspended, it rotates its head to its normal position with the help of sight. But, the movements of the head do not occur if the eyes of the animal are closed.
- Labyrinthine righting reflexes acting upon the neck muscles
- Summary of righting reflexes: Following are the sequential events of righting reflexes
- When the animal is placed upon its back, the labyrinthine reflexes acting upon the neck muscles turn the head into its normal position in space, in relation to the body
- The proprioceptive reflexes of the neck muscles then bring the body into its normal position in relation to the position of head
- When resting upon a rigid support, these reflexes are reinforced by the body righting reflexes on head as well as on the body
- If the animal happens to be a labyrinthectomized one, then it makes an attempt to recover its upright position as a result of operation of the optical righting reaction.
- If the optical righting reflexes are abolished by covering the eyes, the righting ability is lost. Optical righting reflexes are also demonstrated in a 3 or 4 weeks old baby. When laid down on belly, i.e. prone position, the baby tries to raise the head to a vertical position.
- Centers for righting reflexes: The centers for the first four righting reflexes are in the red nucleus situated in the midbrain. The center for the optical righting reflexes is in the occipital lobe of cerebral cortex.
- Local Static Reflexes or Supporting Reflexes
- Local static reflexes or the supporting reactions support the body in different positions against the gravity and protect the limbs against hyperextension or hyperflexion.
- The supporting reactions are classified into two types:
- Positive supporting reflexes
- Positive supporting reflexes are the reactions, which help to fix the joints and make the limbs rigid like pillars so that limbs can support the weight of the body against gravity.
- It is brought about by the simultaneous reflex contractions of both extensor and flexor muscles and other opposing muscles.
- The impulses for these reflexes arise from proprioceptors present in the muscles, joints, and tendons and the exteroceptors, particularly pressure receptors present in deeper layers of the skin of sole. While standing, the positive supporting reflexes are developed in the following manner
- When an animal stands on its limbs, the pressure of the animal’s paw upon the ground produces proprioceptive impulses from the flexor and extensor muscles of the limbs – particularly in terminal segments of the limbs like the digits, ankle, or wrist. The proprioceptive impulses cause a reflex contraction of the muscles of limbs making the limbs rigid.
- Excessive extension at the joints is checked or guarded by the myotatic reflexes setting up In the flexor muscles. When the flexor rnuselas are simultaneously contracting, extensor muscles cannot be stretched beyond the physiological limits. Similarly, over activity of the flexor muscles is prevented by the stretch reflexes developed in the extensor muscles.
- Impulses arise even from exteroceptors while standing. While standing, the sole remains in contact with the ground. It causes stimulation of the pressure receptors, which are present in deeper layers of the skin. These impulses from pressure receptors reinforce the rigidity of the limbs caused by proprioceptive impulses.
- Negative supporting reflexes
- Relaxation of the muscles and unfixing of the joints enable the limbs to flex and-move to a new position. It is called negative supporting reaction.
- It is brought about by raising the leg off the ground and plantar flexion of toes and ankle. When the leg is lifted off the ground, the exteroceptive impulses are stopped. When the toes and ankle joints are plantar flexed, the stretch stimulus for the plantar muscles is stopped.
- So, unlocking of the limbs occurs. Moreover, by the plantar flexion of the toes and ankle, the dorsiflexor muscles are stretched, causing relaxation of the extensors and contraction of the flexors of the knee.
- The positive and negative supporting reactions are demonstrated well on a decorticate animal. The centers for the supporting reflexes are located in the spinal cord.
- Segmental Static Reflexes
- The segmental static reflexes are very essential for walking. During walking, in one leg, the flexors are active and the extensors are inhibited.
- On the opposite leg, the flexors are inhibited and extensors are active. Thus, the flexors and extensors of the same limb are not active simultaneously. It is known as crossed extensor reflex.
- It is due to the reciprocal inhibition and the neural mechanism responsible for this reflex is called Sherrington’s reciprocal innervation.
- The segmental static reflexes are demonstrated in the spinal animals. And, the centers for these reflexes are situated in the spinal cord.
- Statotonic or Attitudinal Reflexes: Statotonic or attitudinal reflexes are developed according to the attitude of the body and are of two types
- Tonic labyrinthine and neck reflexes acting on the limbs
- These reflexes decrease or increase the tone of the skeletal muscles of the limbs in accordance to the attitude or position of the head.
- These reflexes are best studied in decerebrate animal. The proprioceptors concerned with these reflexes are in the labyrinthine apparatus. Whenever the position of the head is altered, the receptors present in the labyrinth are stimulated and generate impulses.
- The impulses are also generated from the neck muscles when the position of the head is altered. The impulses from the labyrinth produce the same effect on all four limbs. But the impulses from neck muscles cause opposite effects in the forelimbs and hind limbs.
- The labyrinthine reflexes are particularly effective on extensor muscles. When the head is dorsiflexed, all in four limbs are extended maximally and, when the head is ventriflexed, all the four limbs are flexed.
- In a labyrinthectomized animal where only neck reflexes are operated, during dorsiflexion of the head, there is an extension of the forelimbs and flexion of the hind limbs. The ventriflexion of the head causes flexion of the forelimbs and extension of the hind limbs.
- The importance of these reflexes is understood well while observing the movements during change in the attitude of a normal animal.
- When an animal turns to one side, the limbs of that side become rigid in order to support the weight of the body. A cat looking upwards, keeps the hind limbs flexed but the forelimbs remain extended. It gives a suitable inclination to the back of the animal, which improves the positions of the head and eyes.
- When the cat looks down, forelimbs are flexed and hind limbs are extended, giving the proper supported inclination at the neck region.
- Labyrinthine and neck reflexes acting upon the eyes
- According to the changes in the position of the head and neck, the eyes are also moved. These reflexes arise from labyrinth and neck muscles. Turning the head downward causes upward movement of the eyes. The eyes remain in this position as long as the position of the head is retained.
- When the head is moved down, the tone in the superior recti and inferior oblique are increased, and tone of the inferior recti and superior oblique are reduced, so that the eyeballs move upwards. When the head is turned to one side, a corresponding compensatory movement of the eyes occurs.
- When the head is turned to one side, the eyes deviate outward or inward in relation to the head. The e/es are moved in a direction opposite to that of the head movement. It is because of external and internal
Vne centers for the somatotonic reflexes are present in the medulla oblongata.
- Tonic labyrinthine and neck reflexes acting on the limbs
- Positive supporting reflexes
Statokinetic Reflexes
- Statokinetic reflexes are the postural reflexes that maintain posture during movement. These reflexes are concerned with both angular (rotatory), and linear (progressive) movements.
- The vestibular apparatus is responsible for these reflexes. So, it is essential to study the structure and functions of vestibular apparatus to understand the autokinetic reflexes.
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