1. By Shama Rani Paul

2. Definition
 muscle tone (residual muscle
tension or tonus) is the continuous and
passive part contraction of the muscles,
 or the muscle's resistance to passive
stretch during resting state.
 It helps to maintain posture and declines
during REM sleep

3. Types of Muscle Tone
 Normal tone --- means that there is the right amount
of “tension” inside the muscle at rest, and that the
muscle is inherently able to contract on command.
 High tone --- means there is too much tension in the
muscle at rest. In other words, the muscle is tight and
tense even though it is not doing anything. Eg - spastic
cerebral palsy.
 Low tone --- means there is not enough tension in the
muscle when it is at rest. The muscle may have a
slightly mushy or floppy feel to it, and there is a lack of
graded control of the muscle when it is being used
(graded control means that just the right amount of movement and effort
is used as appropriate to the task at hand). Eg - battle to sit
upright at a desk for any period of time, and may
slouch over.

4. Neurophysiology of Muscle Tone
Control of Muscle tone
Spinal
control
Supra spinal
control

5. Motor Unit Types Type A Type B Type C
Size of M.Unit Large Small Intermediate
Diameter of muscle fiber Small Intermediate Small
Capillary Small Intermediate Large
Mitochondrial ATPase Low Medium High
Glycogen content High Medium Low
Contraction Speed Fast Slow Intermediate
Maximum Tetanic tension High Low Intermediate
Fatigability Low Very high High
Post tetanic potentiation of twitch
contraction
Poor Good Good
Post tetanic repetitive activity Absent Present Absent
Electric stimulation of peripheral
nerve, motor cortex
Fascilitation Inhibition
Distribution Flexor Extensor,
Antigravity

6. Spinal Control of Muscle Tone
 Stretch reflex of Sherrington is the basic
mechanism of tonic activity.
 Muscle spindle and alpha and gamma motor
neurons are mainly implicated.

7. Muscle Spindle

8. Muscle Spindle
 Muscle spindle is a
fusiform structure
laying between and
parallel to the muscle
fibres and sharing
their tendinous
attachement

9. Muscle Spindle
 It consisting of about 4 to 12
intrafusal fibres, which have
a smaller diameter than the
extrafusal fibres.
 Intrafusal fibres are of two
types :
Nuclear bag fibres and
Nuclear chain fibres.
 Serve to monitor both the
length of the muscle and the
velocity of its contraction

10. Nuclear Bag Fibres
 These bulge out at the middle,
where they are the most elastic .
 A large diameter myelinated
sensory nerve fibre (Ia) ends at
nuclear bag.
 Motor fibres ( γ efferents) which
subserve contraction of of its
striated portion.
 This is the dynamic component
of the stretch reflex

11. Golgi Tendon Organ
 Net like collection of
knobby nerve endings
among the fascicles of a
tendon.
 Stimulated by passive
stretch & active
contraction of muscle.
 Signals the tension and
provides negative
feedback control of
muscle contraction and
regulates muscle force
rather than length.

12. Muscle Spindle Afferents
 Static response is the discharge at any constant length
of the muscle. The greater the muscle length greater is
the stretch in the spindle and the higher is the static
response of the spindle affrents. Both the primary (Iα)
and secondary II spindle affrents gives static or length
sensitive responses.
 The dynamic response of a spindle affrents refer to the
discharge during stretch of the muscle. If the spindle
affrents gives greater response during a fast stretch
than it dose during a slow stretch (velocity different but
distance of stretch same) it is said to poses a dynamic
response component. Only the primary spindle affrents
gives a dynamic or velocity sensitive response.

13. Afferent and Efferent Pathways
Efferent pathway
• α-motoneurons runs from cell body in ant.
horn to extrafusal muscle fibre.
• γ- motoneurons runs from cell body in ant.
horn to intrafusal muscle spindle.
Afferent pathway
• Ia from nuclear bag fibre passes via dorsal horn to
synapse with α-motoneurons
• II from muscle spindle synapse with interneurons
• Ib from golgi tendon organ ends in nucleus
dorsalis and synapse with interneurons.

14. Mechanism of Tone
 γ- motoneurons activity causes the intrafusal
fibre to contract
 this streches the primary sensory ending,
thus increasing afferent discharge
causing depolarisation of α-motoneurons
supplying the extrafusal muscle, thereby
increasing muscle tone.

15. Supra-spinal control of Muscle Tone
The efferent fibres to the muscle spindle, γ-
motoneurones, receive input form higher
centres via :
 Facilitatory fibres
 Inhibitory fibres

16. Excitatory Supraspinal control
 Medial/Ventral Reticulospinal Pathway
 Vestibulospinal Pathway
Inhibitory Supraspinal Control
 Corticospinal Pathway
 Dorsal/Lateral Reticulospinal Pathway
 Corticoreticular Pathway

17. Disorders of Muscle Tone
 Abnormalities of the tone :
 Hypertonia –
Pyramidal hypertonia (Spasticity)
Extrapyramidal hypertonia (Rigidity)
 Hypotonia

18. Clonus
 Clonus is the phenomenon of involuntary rhythmic contractions in
response to sudden sustained stretch.
 A sudden stretch activates muscle spindles, resulting in the stretch
reflex.
 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.

19. References
 http://www.humanneurophysiology.com/
muscletone.htm
 http://neuroscience.uth.tmc.edu/s3/chapt
er06.html