Dr. Niranjan Murthy H L

Asst. Prof., Dept. of Physiology

Sree Siddhartha Medical College & Hospital, Tumkur

PROPERTIES OF SKELETAL MUSCLE

1. EXCITABILITY

2. CONTRACTILITY

3. CONDUCTIVITY

4. TONICITY

I.EXCITABILITY

Def: It is the change in potential and the consequent responses inherent to the tissues, in response to a stimulus.

Stimulus: It is the change in the external environment bringing about excitation in an excitable tissue.

TYPES OF STIMULUS

• Electrical- commonly used in labs

• Mechanical

• Thermal

• Chemical

• Electro-magnetic

QUALITY OF STIMULUS

• Strength of stimulus- subminimal, minimal (threshold), submaximal, maximal and supramaximal

• Duration of stimulus

STRENGTH-DURATION CURVE

U.T

RHEOBASE

C

2R

DURATION IN ms

Str

engt

h (

mv)

• Rheobase: the minimum strength of the current acting on the muscle for a variable period that can bring about a response.

• Utilization time: the minimum duration for which a current of rheobase strength is applied to excite an excitable tissue

• Chronaxie: is defined as the shortest duration of stimulus required to excite a tissue by a current strength equal to twice of rheobase voltage.

• Chronaxie of a tissue is a definite measure of its excitability.

II.CONTRACTILITY

• Def: internal events of the muscle which are manifested by shortening or development of tension or both.

• Types of contraction:

1) isotonic contraction

2) isometric contraction

• Muscle-twitch (simple muscle curve): The contraction and relaxation of skeletal muscle in response to a single adequate stimulus

–All or None response

•An individual muscle fiber exhibits contraction of an uniform intensity once their particular threshold has been reached

FACTORS AFFECTING CONTRACTION

a) Strength of stimuli- Quantal summationb) Effect of multiple stimuli:

1)effect of 2 successive stimuli: i) beneficial effect

ii) superposition iii) summation

2) more than two stimuli: i) clonus

ii) tetanus iii) treppe

iv) fatiguec) Effect of temperature

– Super-position• The second contraction develops a greater

tension than the first one if the second stimulus is applied before the relaxation is complete in the first one.

– Availability of more Ca++

– Ca ++ from the first contraction is not completely pumped into the SR

– Second stimulus releases Ca ++ and adds to the remaining Ca ++ from the first contraction

–Treppe or Stair case effect

•After a period of rest, sudden series of stimulation results in a series of contraction that increases in amplitude until a steady state is reached.

•Due to redistribution of intra cellular Ca ++ / more Ca ++ availability

FATIGUE

• Def: it is a decrease in the performance due to continuous and prolonged activity

• Site of fatigue: CNS is the first site of fatigue even though the muscle itself can undergo fatigue.

• Nerve is indefatigable

– Changes in excitability of a muscle during contraction relaxation coupling• Duration of Skeletal muscle AP = 5 ms• The muscle like nerve has

– ARP– RRP– Supra normal period – long negative AP

during which the muscle is hyper excitable

The Skeletal muscle can be Tetanized – Why ?

• Effect of load on muscle contraction

• 1. Free-load

• 2. After-load

• Effect of temperature on muscle contraction

• 1. Heat rigor

• 2. Cold rigor

• 3. Calcium rigor

• 4. Rigor mortis

III. CONDUCTIVITY

• velocity of action potential conduction across skeletal muscle is 5m/sec

• in nerves it is up to 120m/sec

• conduction is along the sarcolemma and moves along the T-tubules

IV.TONICITY

• Def: it is the state of partial contraction of the muscle

• Reflex phenomenon

• Resistance encountered on passive stretching of muscle

• Rigidity

• Spasticity