Properties Of Skeletal Muscle 5k6n5c

Dr. Niranjan Murthy H L Asst. Prof., Dept. of Physiology Sree Siddhartha Medical College & Hospital, Tumkur

PROPERTIES OF SKELETAL MUSCLE 1. EXCITABILITY 3. CONTRACTILITY 5. CONDUCTIVITY 7. 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 (mv)

STRENGTH-DURATION CURVE

C 2R RHEOBASE U.T DURATION IN ms

• 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 • •

Strength of stimuli- Quantal summation 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) fatigue c) 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

The Skeletal muscle can be Tetanized – Why ? – 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

• • • • • • • •

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 ive stretching of muscle • Rigidity • Spasticity