By Shama Rani Paul

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

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.

Neurophysiology of Muscle Tone

Control of Muscle tone

Spinal control

Supra spinal control

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

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.

Muscle Spindle

Muscle Spindle

Muscle spindle is a

fusiform structure

laying between and

parallel to the muscle

fibres and sharing

their tendinous

attachement

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

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

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.

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 affrentsgives a dynamic or velocity sensitive response.

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.

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.

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

Excitatory Supraspinal control

Medial/Ventral Reticulospinal Pathway

Vestibulospinal Pathway

Inhibitory Supraspinal Control

Corticospinal Pathway

Dorsal/Lateral Reticulospinal Pathway

Corticoreticular Pathway

Disorders of Muscle Tone

Abnormalities of the tone :

Hypertonia –

Pyramidal hypertonia (Spasticity)

Extrapyramidal hypertonia (Rigidity)

Hypotonia

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.

References

http://www.humanneurophysiology.com/

muscletone.htm

http://neuroscience.uth.tmc.edu/s3/chapt

er06.html