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The intrafusal fibers are specialized fibers; their central regions are not contractile. The sensory endings spiral around the central regions of the intrafusal fibers and are responsive to stretch of these fibers. Gamma motor neurons innervate the contractile polar regions of the intrafusal fibers. Contraction of the intrafusal fibers pulls on the central regions from both ends and increases the sensitivity of the sensory endings to stretch. B, The muscle spindle contains three types of intrafusal fibers: dynamic nuclear bag, static nuclear bag, and nuclear chain fibers. A single group Ia afferent fiber innervates all three types of intrafusal fiber, forming a primary ending. A group II afferent fiber innervates chain and static bag fibers, forming a secondary ending. Two types of efferent axons innervate different intrafusal fibers. Dynamic gamma motor axons innervate only dynamic bag fibers; static gamma motor axons innervate various combinations of chain and static bag fibers.
If the tension developed in a strongly contracting muscle becomes excessive, it is not inconceivable that the tendon could pull free from the bone, certainly an undesirable situation.
Muscle tone roohi
By- Dr. Harshdeep Kaur
Guide- Dr. Himanshu Dua
• Tone is defined as state of muscle contraction
• It is functionally defined as resistance to
• Two types- phasic and postural
• Phasic tone is due to rapid stretching of
muscle and muscle spindle. The response is
rapid and short lived.
• Postural tone is due to steady strained stretch
on tendons and attached muscle with
contraction of involved muscle.
• The word TONUS was first used to designate the
state of contraction of resting muscle by Muller in
• Vulpian defined Tone as a state of permanent
• Muscle tone is usually described as the resistance
of a limb to passive movement (Foster 1892).
• Stretch reflex of Sherrington is the basic
mechanism of tonic activity.
• Muscle spindle is a fusiform
structure laying between
and parallel to the muscle
fibres and sharing their
• It consisting of about 4 to 12
intrafusal fibres, which have a
smaller diameter than the
• Intrafusal fibres are of two
Nuclear bag fibres and
Nuclear chain fibres.
• Serve to monitor both the
length of the muscle and the
velocity of its contraction
GOLGI TENDON ORGAN
• These receptors are
sensitive to the changes
in tension generated by
muscles as they
• Signals the tension and
feedback control of
muscle contraction and
regulates muscle force
rather than length
• Ia from muscle spindle
passes via dorsal horn to
synapse with α-
• II from muscle spindle
synapse with interneurons
• Ib from golgi tendon
organ ends in nucleus
dorsalis and synapse with
• α-motoneurons runs
from cell body in ant.
horn to extrafusal
• γ- motoneurons runs
from cell body in ant.
horn to intrafusal
• γ- motoneurons activity causes the intrafusal
fibre to contract this stretches the
primary sensory ending, thus increasing
afferent discharge causing depolarisation
of α-motoneurons supplying the extrafusal
muscle, thereby increasing muscle tone.
• The Myotatic (Stretch) Reflex
When a muscle is stretched, the
spindles in that muscle are also
Stretch stimulates afferent fibers and
send impulses into the cord.
Some fibers synapse directly on
alpha motor neurons supplying
the same muscle which was
initially stretched. This causes the
muscle to contract and shorten,
relieving the initial stretch. Such
neurons are called homonymous
alpha motor neurons.
stretch" is known as the myotatic
or stretch reflex.
• Stretch reflex is due to stimulation of muscle
• it is a monosynaptic reflex involving one
motor, one sensory neuron and one synapse
in the pathway.
• Reciprocal inhibition is also present
• Polysynaptic reflex
• Operates as feedback mechanism to control
• Receptor is golgi tendon organ.
• Increase tension to muscle spindle- stimulation of
tendon organ- nerve impulse transmitted by
sensory neuron –synapse with inhibitory
association neuron-inhibits motor neuron
associated with muscle- relaxation of agonist
• Motor neuron synapse with stimulatory
association neuron-stimulate motor neuron of
antagonist muscle-contraction of antagonist
• The efferent fibres to the muscle spindle, γ-
motoneurones, receive input form higher
centres via :
• Facilitatory fibres and
• Inhibitory fibres
• The motor tracts can be functionally divided into two
• Pyramidal tracts – These tracts originate in the
cerebral cortex, carrying motor fibres to the spinal cord
and brain stem. They are responsible for the voluntary
control of the musculature of the body and face.
• Extrapyramidal tracts – These tracts originate in the
brain stem, carrying motor fibres to the spinal cord.
They are responsible for the involuntary and
automatic control of all musculature, such as muscle
tone, balance, posture and locomotion
• Corticospinal tracts – supplies the
musculature of the body.
• Corticobulbar tracts – supplies the
musculature of the head and neck.
• There are two vestibulospinal pathways; medial and lateral. They
arise from the vestibular nuclei. The tracts convey this balance
information to the spinal cord, where it remains ipsilateral.
• Fibres in this pathway control balance and posture by innervating
the ‘anti-gravity’ muscles (flexors of the arm, and extensors of the
2.Reticulospinal Tracts -
• The two recticulospinal tracts have differing functions:
• The medial reticulospinal tract arises from the pons. increases
• The lateral reticulospinal tract arises from the medulla. reduces
• The rubrospinal tract originates from the red nucleus, a midbrain
structure. As the fibres emerge, they decussate , and descend into
the spinal cord. They have a contralateral innervation.
• play a role in the fine control of hand movements
• This pathway begins at the superior colliculus of the midbrain. The
superior colliculus is a structure that receives input from the optic
nerve. The neurones then quickly decussate, and enter the spinal
cord. They terminate at the cervical levels of the spinal cord.
• The tectospinal tract coordinates movements of the head in
relation to vision stimuli
• Aka extensor posturing.
• In decerebrate posturing, the head is arched
back, the arms are extended by the sides, and
the legs are extended.
• A hallmark of decerebrate posturing is
• The arms and legs are extended and rotated
• The patient is rigid, with the teeth clenched.
• A complete transection of
the brain stem between
the superior and inferior
colliculi permits the brain
stem pathways to function
independent of their input
from higher brain
structures. This is called a
lesion is below the red
• This lesion interrupts all input from the cortex
(corticospinal and corticobulbar tracts) and red nucleus
(rubrospinal tract), primarily to distal muscles of the
• The excitatory and inhibitory reticulospinal pathways
(primarily to postural extensor muscles) remain intact.
• The excitatory reticulospinal pathway leads to
hyperactivity in extensor muscles in all four extremities
which is called decerebrate rigidity.
• Aka flexor posturing. Patients with decorticate
posturing present with the arms flexed, or
bent inward on the chest, the hands are
clenched into fists, and the legs extended and
feet turned inward.
• There are two parts to decorticate posturing.
• The rubrospinal tract facilitates motor
neurons in the cervical spinal cord supplying
the flexor muscles of the upper extremities.
The rubrospinal tract and reticulospinal tract
based flexion outweighs the vestibulospinal
and pontine reticulospinal based extension in
the upper extremities.
Cerebellum and muscle tone
• The cerebellum does not seem to have a direct effect on
muscle tone determining spinal reflex pathways as
there is no direct descending cerebellospinal tract.
• The cerebellum mainly influences muscle tone through
its connections with the vestibular and brain stem
• Pure cerebellar lesions classically produce hypotonia.
DISORDERS OF MUSCLE TONE
• Abnormalities of the tone :
Pyramidal hypertonia (Spasticity)
Extrapyramidal hypertonia (Rigidity)
METHOD TO ELICIT TONE
The determination of tone is subjective and prone to
The most important part of the examination of tone is
determination of the resistance of relaxed muscles to
passive manipulation as well as the range of motion.
METHOD TO ELICIT TONE
• Inspection : Attitude of the limb at rest.
• Palpation : Feel of the muscle – normal, firm or flabby.
-Range of movement at the joints.
-Passive movement - first slowly and through complete range of motion and then at
-Shake the distal part of the limb.
-Brace a limb and suddenly remove support.
-Bilateral examination of homologous parts helps compare for differences in tone on
the two sides of the body.
• Spasticity is a motor disorder characterized by
a velocity-dependent increase in tonic stretch
reﬂexes (‘‘muscle tone‘‘) with exaggerated
• Involves mainly antigravity muscles-flexors in
upper limb and extensors in lower limb
MECHANISMS OF SPASTICITY
1.Disynaptic reciprocal Ia inhibition-
Reciprocal inhibition ensures that antagonist
muscles remain relaxed when the agonist
muscles are activated during voluntary
Reduced reciprocal inhibition may therefore
contribute to the development of
hyperreﬂexia and spasticity.
• Rigidity is characterized by an increase in muscle
tone causing resistance to external movements.
• It is not velocity dependent and can be elicited at
very low speeds of passive movement.
• It is felt in both agonist and antagonist muscles.
• 'Cogwheel' rigidity and 'leadpipe' rigidity are two types.
• 'Leadpipe' rigidity results when an increase in muscle tone causes
a sustained resistance to passive movement throughout the whole
range of motion, with no fluctuations.
• 'Cogwheel' rigidity occurs due to contraction of agonists and
antagonists alternately so that increased tone is not felt throughout
range of motion but intermittently.
• Basal ganglia structures are clearly implicated in pathophysiology of
• Clonus is the phenomenon of involuntary rhythmic contractions in
response to sudden sustained stretch.
• A sudden stretch activates muscle spindles, resulting in the stretch
• Tension produced by the muscle contraction activates the Golgi
tendon organs, which in turn activate an ‘inverse stretch reflex’,
relaxing the muscle.
• If the stretch is sustained, the muscle spindles are again activated,
causing a cycle of alternating contractions and relaxations.
• It is a state in which muscle contraction
continues beyond a period of time. There is
delay in relaxation.
• Eg. Myotonic dystrophy and myotonia
WEAKNESS WEAKNESS<HYPOTONIA WEAKNESS>/=HYPOTONIA
COGNITION AFFECTED NOT AFFECTED
DTR’s BRISK ABSENT
DEVELOPMENT DELAY PRESENT NOT PRESENT
PULL TO SIT HEAD LAG MARKED HEAD LAG
IN PRONE AND SUPINE
FACIAL DYSMORPHISM PRESENT ABSENT