MOTOR SYSTEM IV REGULATION OF POSTURE
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Transcript of MOTOR SYSTEM IV REGULATION OF POSTURE
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DR NILESH KATE
MBBS,MD ASSOCIATE PROF
DEPT. OF PHYSIOLOGY
REGULATION OF
POSTURE.
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POSTURE Postural reflexes help to
maintain the body in upright and balanced position
They also provide adjustments necessary to maintain a stable posture during voluntary activity
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Reflex Arc Of Postural Reflex Afferent Pathway- comes from the eyes, the
vestibular apparatus and the proprioceptors
Integrating Centres- are formed by neuronal network in the brain stem and spinal cord
Efferent Pathway- α-motor neurons supplying the various skeletal muscles i.e. the effector organ
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TYPES OF POSTURAL REFLEXES
Postural reflexes are of two types-
(A) Static Reflexes (B) Statokinetic Reflexes
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(A) Static Reflexes Static reflexes are involved in adjustments to
displacements produced by gravity
It is of three types-
(1) Local static reflexes (2) Segmental static reflexes (3) General static reflexes
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(1) Local static reflexes They exert their effect on the same limb from
which the stimulus was initiated
The centre -- Located in Spinal Cord
Some important local static reflexes include: i) Stretch reflex ii) Positive supporting reaction iii) Negative supporting reaction
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I) STRETCH REFLEX This is the most
important local static reflex which controls the tone in those extensor muscles which keep the body upright (antigravity muscles)
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II) POSITIVE SUPPORTING REACTION
It is characterized by simultaneous reflex contraction of both extensors and flexors of a limb
It plays an important role of steadying the ankle joint in standing position
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III) NEGATIVE SUPPORTING REACTION
It refers to disappearance of positive supporting reaction
It is initiated by stretch of the extensor muscles
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(2) SEGMENTAL STATIC REFLEXES
It is characterized by a bilateral reflex response when stimulus is applied to one limb
The best example of segmental static reflex is crossed extensor reflex response component of withdrawal reflex
The centre of this reflex is in spinal cord
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(3) GENERAL STATIC REFLEXES
It is characterized by a generalized effect from many muscle groups in the body in response to a stimulus that arises at one side of the body
General static reflexes can be divided into three groups:
a) Attitudinal reflexes b) Long loop stretch reflexes c) Righting reflexes
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A) ATTITUDINAL REFLEXES
These reflexes are initiated when the attitude of the body is changed i.e. while standing on an inclined plane
These reflexes are of two types
1) Tonic Labyrinthine Reflex 2) Tonic Neck Reflex
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1) TONIC LABYRINTHINE REFLEX
These reflexes are produced in response to alternation in POSITION OF HEAD relative to horizontal plane e.g. while standing on an inclined plane
Pathway of reflex arc- Stimulus- is gravity Receptors- Otolith organ (Utricle and Saccule
of labyrinth)
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1) TONIC LABYRINTHINE REFLEX
Afferents- From receptors travel along the vestibular nerve
Centre- vestibular and reticular nuclei present in spinal cord
Efferents- vestibulospinal and reticulospinal tracts which end on α-motor neurons of spinal cord
Reflex reponse- contraction of extensor muscles of limb
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1) TONIC LABYRINTHINE REFLEX
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2) TONIC NECK REFLEX These reflexes are produced in response to
alternation in the position of head relative to the body
Pathway of reflex arc- Stimulus- stretch of neck muscles Receptors- pacinian corpuscles in the ligaments of
the cervical joint and muscle spindle of neck muscles Centre- in the Medulla oblongata Efferent- corticospinal tracts
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2) TONIC NECK REFLEX Reflex response- depends on the position of the
head in relation to the body:
Turning down of head causes flexion of the forelimb and extension of the hind limb
Turning up of head causes reverse of the above
Turning the head sideways produces flexion of the ipsilateral limbs and extension of contralateral limbs
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2) TONIC NECK REFLEX
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B) LONG LOOP STRETCH REFLEXES
They are also called Functional stretch reflex
These are polysynaptic reflexes with the centre of reflex arc in cerebral cortex
These reflexes are continuously active in erect posture
They bring about continuous correction of the sways that occur from moment to moment during standing
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C) RIGHTING REFLEXES
Righting reflexes help to correct the position of the body when it goes off balance and falls down
These reflexes consists of a chain of reactions following one another in an orderly sequence
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C) RIGHTING REFLEXES For example, if an
animal is laid on it’s side or back,
head rights itself followed by body and
animal finally resumes upright posture.
The sequence of righting reflexes will be as follows :
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Head righting reflex or Labyrinthine righting reflex
It is initiated when animal’s head is in lateral position
Impulses arising from the saccules reflexly stimulate the appropriate muscles to bring head back to upright position
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2. BODY RIGHTING REFLEX When the animal lies on
the ground, the side in contact with the ground is constantly stimulated while the other side is not
This differential stimulation of the deep structures in the body wall reflexly rights the head
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3. NECK RIGHTING REFLEX The head is righted by above two reflexes
but the body still remains in lateral position
This leads to twisting of neck and this brings thorax and lumbar region successively into upright posture
If the righting of head is prevented , impulses from the body surface may cause righting of the body directly (Body on body righting reflex)
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4. LIMBS RIGHTING REFLEX
Impulses arising from the limb muscles leads to attainment of appropriate posture of limbs
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5. OPTICAL RIGHTING REFLEXES
Optical impulses also cause righting of the head in animals with intact visual cortex
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CENTERS OF RIGHTING REFLEXES
Chief center for all righting reflexes, except the optical righting reflex is Red Nucleus lying in the mid brain
The center for optical righting reflex is in the Visual Cortex
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(B) Statokinetic Reflexes
1. Vestibular placing reaction Stimulus- linear acceleration Receptors- in utricle and saccule Center- cerebral cortex Response- as soon as foot comes in contact of
any firm surface, it is reflexly placed on surface in position to support the body
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(B) STATOKINETIC REFLEXES
2. Visual placing reaction Stimulus- visual cues Receptors- eyes Center- cerebral cortex Response – as in above
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(B) STATOKINETIC REFLEXES 3. Hopping reaction Stimulus- lateral displacement while standing Receptors- muscle spindle Center- cerebral cortex Response- hopping movement that keeps the
limb in position to support the body when standing animal is pushed laterally
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DECEREBRATE ANIMAL
Decerebrate animal is one in whom the brain stem is transected at intercollicular level i.e. between superior and inferior colliculi
Characteristic features- (i) Decerebrate rigidity It refers to marked increase in tone of extensors
i.e. antigravity muscles Occurs immediately after decerebration
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DECEREBRATE ANIMAL Features of decerebrate rigidity
- hyperextension of all four limbs - dorsiflexion (hyperextension) of
tail and head - extreme hyperextension of the
spine(opisthotonus) produces concave configuration of the back
- the animal can be made to stand on four limbs but is easily toppled by slight push
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MECHANISM OF DECEREBRATE RIGIDITY
Rigidity occurs due to increased activity of motor neurons that facilitate stretch reflex
Facilitation of stretch reflexes occur due to increased rate of γ motor neuron discharge
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MECHANISM OF DECEREBRATE RIGIDITY
(ii) No spinal shock
(iii) Postural reflexes are those which have their centre in spinal cord, medulla or pons. These include-
- stretch reflexes, positive and negative supporting reaction, crossed extensor reflex, tonic neck and tonic labyrinthine reflex
(iv) Righting reflexes are absent
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DECORTICATE ANIMAL Decorticate animal is one in whom the whole cerebral
cortex is removed but the basal ganglia and brain stem are left intact
Characteristic features: (i) Moderate rigidity is present Cortex inhibit medullary reticulospinal tract
Therefore, removal of cortex results in facilitation of γ motor neuron discharge
Seen only when the animal is at rest
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CHARACTERISTIC FEATURES:(ii) Typical posture in
decorticate man consists of
- full extension of legs - arms lying across the chest - semiflexion at elbow - slight pronation of forearm - flexion of wrist and fingers
(iii) Righting reflexes are present
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Spinal animalPreparation: Prepared by sectioning the cord at the cervical level.
Significance: Segmental and intersegmental reflexes can be
studied.
Eg:Stretch reflex, Positive and negative supporting reaction, Crossed extensor reflex.
Characteristics: Absence of reflexes for a period of days or weeks. Spinal man cannot stand unsupported.
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Thalamic animalPreparation: Thalamus intact in this animal. The rest of the cerebral hemispheres are removed. This preparation can be with the cerebellum or without
the cerebellum.
Significance: Used for studying righting reflexes.
Characteristics: Optical and limb righting reflexes are lost.
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Vestibular apparatus & equlibrium
Influence on labyrinthine reflexes
-senses gravity and rotational movement
- Consist of i)SCC ii)Otolith organs
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SEMICIRCULAR CANALS THE AMPULLA
HAIR CELLSNERVE
CELLS
kinociliumsterocilia
cupula
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THE AMPULLAMECHANISM OF ACTIVATION AND TRANSDUCTION
HAIR CELLSBENT
NERVECELLSFIRE
INERTIAL FLUID MOVEMENT
MOVEMENT OF HEAD
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THE UTRICLE & SACCULUSotolith organ
HAIR CELLSNERVE
CELLS
OTOLITHICMEMBRANE-GELATINOUSLAYER
OTOCONIA(CALCIUMCARBONATECRYSTALS)
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THE UTRICLE & SACCULUS MECHANISM OF ACTIVATION AND TRANSDUCTION
HAIR CELLSNERVE
CELLS
OTOLITHICMEMBRANE-GELATINOUSLAYER
OTOCONIA(CALCIUMCARBONATECRYSTALS)
HEAD MOVEMENT
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Otolithic membane
Hair cell
Anatomy: Maculae of Utricle or SacculePhysiology: Linear acceleration of head
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Effect of Gravity on Utricular Receptor Cells
Figure 15.36
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Vestibular reflexes
Vestibulo-Ocular reflex (VOR)Vestibulo-Spinal reflex (VSR)
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Function of vestibular reflexes The VOR stabilizes gaze during fast head
movement
The VSR sends information regarding gravity and linear acceleration to body muscles
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Vestibulo-Spinal Reflex (VSR) – maintaining posture
STIMULUS =Gravity linear acceleration
Efferent = Spinal nervesEffector = proximal limb Neck and
body musclesSensory = Vestibular HC
Afferent = vestibular nerve
Center
increases extensor(antigravity) tone to maintainposture
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Vestibulo-Ocular Reflex (VOR)
STIMULUS =Head movement
Efferent = oculomotor nervesEffector = Extra-ocular muscles
Sensory= Vestibular HC
Afferent = vestibular nerve
Center
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Objectives
Posture. Muscle tone & reflex arc. Gamma motor neuron. Decerebrate rigidity. Spinal animal. Thalamic animal. Postural reflexes. equlibrium & vestibular apparatus. Vestibular reflexes.
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Thank You