PHYSILOGY HOT TOPIC DISCUSSION

Skeletal Muscle Contractions – Characteristics

SKELETAL MUSCLE CONTRACTIONS -CHARACTERISTICS

Characteristics include:

All or none law.
Stimulus-response relationship.
Frequency of stimulus.
Motor unit recruitment.
Starling’s Law.
Isometric contractions
Muscle Tone.
Denervation hypersensitivity.

1. “All or none law”:

Motor unit is “Motor neuron collectively with all its peripheral branches & innervated muscle fibers”.
Motor unit obey all or none law.
Ie., On stimulation, either all motor unit fibers will contract maximally or not contract at all.
This depends upon stimulus intensity (threshold/subthreshold).

Thus,

Motor unit is “Unit of contraction”.
Nerve fiber is “Unit of activation”.

Concentration of motor unit:

3-6 fibers in a motor unit:
Seen with muscles for fine, graded & precise movement.
Eg: Fingers & eye.

600-1000 muscle fibers per unit:

Seen with muscles for gross movements.
Eg: Leg.

2. Stimulus-Response relationship:

Contractile response depends on stimulus strength.
With increasing stimulus strength → Number of contracting motor units increases.
Hence,
Increased no. of contracting motor units→ Stronger muscle contraction.

3. Frequency of stimulus:

Increased frequency of stimulation → Increases contraction strength & frequency of activation of motor units.

This explains,

Simple muscle twitch:

Contractile response of a skeletal muscle to a single brief stimulus.

Tetanus:

Due to summation of twitches.

Partial/Incomplete tetanus:

State of repetitive muscle contractions separated by partial relaxation, at slower frequency of stimulation.

Complete tetanus:

State of sustained muscle contraction, at higher frequency of stimulation.

Post-tetanic potentiation:

Phenomenon of “Repetitive stimulation enhancing force development due to increased intracellular Ca2+.

Mechanism behind:

Ca2+concentration in sarcoplasm determines muscle tension to produce tetanus.

During single twitch:

Ca2+ released into sarcoplasm insufficient for tetanic tension.

On rapid & successive muscle stimulation:

With each stimulus Ca2+ efflux into sarcoplasm.
Thus, progressive sarcoplasmic Ca2+ accumulation.

On maximum sarcoplasmic Ca2+ levels:

Muscle tension created.

For tetanic response:

Tetanic tension is about four times twitch tension.

4. Motor unit recruitment:

Start of muscle contraction – Smallest motor units contract first.
On insufficient power generation, larger motor units are recruited.
“Henneman principle”/”Size principle”
Order of recruitment from smaller to larger motor unit.
Increases contraction strength

5. Starlings law:

Explains that,

There is an optimal length at which force generated by muscle is maximal.
Ie., Upto a certain limit,
Greater the initial length/length at relaxed state ——> Greater is contraction force.

6. Isotonic Vs isometric contraction

Isotonic contraction:

Contraction with change of length at constant tension.
Tension is equal to weight lifted during muscle contraction.

Isometric contraction:

Contraction with increased tension & constant length.
Generates more contraction force.

Exercise:

Gym exercises are isotonic type.
Involves change in muscle length with constant tension.
Requires greater energy than isometric contraction.
Thus, isotonic exercise best increases muscle strength.

7. Tonus:

State of muscles in partial contraction.
Defined as “Muscle resistance to passive stretch”.
Involves γ-motor neuron activity for contraction.
Assessed by observing resistance offered by muscle to passive stretch.

8. Denervation hypersensitivity:

Destruction of skeletal muscle nerve supply causes –
Abnormal muscle excitability.
Increased sensitivity to circulating acetylcholine.

“Fibrillation”:

Fine irregular contraction of individual fiber.
Classically in lower motor neuron lesion.
Not visible grossly.
On motor nerve regeneration, fibrillation disappears.

Fasiculations –

Jerky, visible contractions.
Occurs with group of muscle fibers/complete motor unit supplied by motor neuron.

9. Muscle fiber type:

Type I/slow motor units:

Have early recruitment.

Type II motor units:

Type IIa/“Fast-Fatigue Resistant” (FR) motor units.
Type IIb/“Fast-Fatigable” motor units.

Factors increasing force of muscle contraction:

Increased number of motor units.
Increasing frequency of stimulus (Tetanic stimulus).
Increasing stimulus strength.
Appropriate initial length.
Larger motor unit recruitment (Henneman principle).
Type II/fast unit.
Isometric contraction

SKELETAL MUSCLE CONTRACTIONS -CHARACTERISTICS

Characteristics of skeletal muscle contractions include:
All or none law.
Stimulus-response relationship.
Frequency of stimulus.
Motor unit recruitment.
Starling’s Law.
Isometric contractions
Muscle Tone.
Denervation hypersensitivity.
Motor unit obey all or none law.
Unit of activation – Nerve fiber.
Unit of contraction – Motor unit.

3-6 fibers in a motor unit

Seen with muscles for fine graded & precise movement
Eg: Fingers & eye.

Contractile response depends on strength of stimulus.
With increasing stimulus strength → Number of contracting motor units increase.
Larger number of motor units contracting → Stronger muscle contraction.
Strength of contraction is increased by increasing frequency of stimulation.
This, in turn increases frequency of activation of motor units.
Tetanus is due to summation of twitches.
On stimulating muscle in rapid succession, there is progressive sarcoplasmic Ca2+ accumulation.
Tetanic tension is reached when sarcoplasmic Ca2+ levels reach their maximum.

Tetanic tension is about four times the twitch tension.

Larger motor units recruited on insufficient power generation.
“Henneman principle”/ “Size principle”:
Order of recruitment from smaller to larger motor unit.
This increases contraction strength
According to Starlings law, there is an optimal length at which force generated by a muscle is maximal.
Isotonic contraction: Contraction in which there is change of length at constant tension.
Isometric contraction: Contraction in which there is constant length with increased tension.
Hence, generates more force of contraction/tension.
Muscle strength is best increased by isotonic exercise.
Exercises one does in the gym are isotonic exercises as muscle length changes in each step but not tension.
Tonus involves γ-motor neuron activity leading to muscular contraction.
Fine irregular contraction of individual fiber appears, referred as “Fibrillation”.
Fasiculations – Jerky, visible contractions of muscle group.

Type II motor units:

Type IIa/”Fast-Fatigue Resistant” (FR) units.
Type IIb/”Fast-Fatigable” units.

Factors increasing force of muscle contraction include:

Increased number of motor units.
Increasing frequency of stimulus (Tetanic stimulus)
Larger motor unit recruitment (Henneman principle)

Nerve Conduction

NERVE CONDUCTION

Neuron – Basic unit & functional unit of nervous tissue.
Specialized for function of reception, integration, & transmission of information within body.

Structure:

Nerve cell has cell body “Soma” with 5-7 small processes called ‘Dendrites’.
Continuing as “Axon hillock” – Thickened area of cell body.
Origination point for long process “Axon”.
First portion of axon is called “Initial segment”.
Nerve fibers may be myelinated/unmyelinated.
Schwann cells:
Found both in myelinated & non-myelinated nerve fibers of PNS.
ln myelinated nerves – Provide structural support & form myelin sheath.
In non-myelinated nerves – Provide only structural support.

Functional division of neuron:

4 zones –

1. Receptor zone:

Dendrites & cell body – Soma.

2. Transmitter zone:

Transmits nerve impulse – Axon.

3. Generator area:

Point of impulse origination/generation.
Mainly at Axon hillock of body & initial segment of axon.
Due to their lowest excitation threshold.
Also contain higher density of voltage-gated sodium channels.

4. Release zone:

Release neurotransmitters – Nerve terminals.

“Orthodromic” vs “Antidromic” conduction:

Experimentally, an axon can conduct impulse in either direction.
When an AP initiated in middle of axon, impulse shall travel in both direction.
One along axon towards its terminal knobs.
Another (opposite) towards cell body soma & dendrites.

“Orthodromic conduction”:

In Natural situation(intact body), impulses conducted unidirectionally only.
I.e. From synaptic junction/receptors along axons to their termination.

“Antidromic conduction”:

Conduction opposite to physiological direction.
Very rare & seen only in muscle tissue.

NERVE CONDUCTION PROCESS:

In nerve fibers, AP propagation is unidirectional (orthodromic) to synapse/NM junction.

Reason:

Presence of neurotransmitters at presynaptic terminal producing required effect.

Factors favoring nerve conduction:

Low axoplasmic resistance (Ri).
Low external longitudinal resistance (Ro).
High membrane resistance (Rm).
Low membrane capacitance (Cm).
Thick (large) nerve → Linear relation with conduction.
Myelination of nerve.
High space & time constant.

Myelination & nerve conduction:

Main purpose of myelin sheath:

1. To increase impulse conduction speed.

Conduction is faster in myelinated nerve than in unmyelinated.
Impulses jump from one node of Ranvier to next node; Hence, faster.
Referred as “Saltatory conduction” or “Propagation by Saltation”.
Note: In unmyelinated fibers, impulses move continuously as waves.

2. Nerve myelination decreases membrane capacitance.

Allows faster depolarization →↑es AP speed propagation.
Membrane capacitance & nerve conduction:
Measure of quantity of charge that must be moved across a unit area of membrane producing unit change in membrane potential.

Variations:

Membrane with high capacitance → Ions crossing membrane is high →Slower AP conduction.
With low capacitance → Ions crossing is less→ faster AP conduction.

Exam Question

NERVE CONDUCTIONPoint of impulse origination:

In a motor neuron, axon hillock & initial segment of axon have lowest excitation threshold.
Because they have a much higher density of voltage-gated sodium channels.
Axon hillock of body & initial segment of axon→ Generator area (Nerve impulse is generated).
Schwann cells are found both in myelinated & non-myelinated nerve fibers of peripheral nervous system.
ln myelinated nerves Schwann cells provide structural support & form myelin sheath.
Schwann cells are derived from neuroectoderm

CONDUCTION PROCESS:

Experimentally, an axon can conduct impulse in either direction.
In Natural situation, impulses are conducted in one direction only (in an intact body)
I.e. From synaptic junction or receptors along axons to their termination.
Such conduction is called “Orthodromic conduction”.

“Antidromic conduction”:

Conduction in opposite direction (i.e., opposite to physiological direction).

Direction of flow in nerve fiber:

In nerve fibers, action potential propagation is unidirectional (orthodromic)
I.e., In natural situation, impulses travel only orthodomically.

Reason for unidirectional flow:

Conduction is unidirectional at synapses or in NM junction
Because, transmission across synapses & NM function is unidirectional
Inturn due to presence of neurotransmitters at terminal end of axon (Presynaptic terminal)
FACTORS AFFECTING NERVE CONDUCTION:
Propagation of action potential (nerve conduction) is favored by,
Low axoplasmic resistance (Ri).
High membrane resistance (Rm).
Low membrane capacitance (Cm).
Thick (large) nerve → Linear relation with conduction.
Myelination of nerve.

1. Effect of myelination on conduction of AP:

The rate of propagation of nerve impulse is faster in a myelinated nerve fiber than in unmyelinated nerve fiber.

Main purpose of myelin sheath:

Is to increase in speed at which impulses propagate along myelinated fiber.
In myelinated fibers, they jump from one node of Ranvier to next node.
So propagation is much faster.
This type of conduction of nerve impulse in myelinated nerves is called “Saltatory conduction” or “Propagation by Saltation”.
Myelination of nerve decreases membrane capacitance.
Allows depolarization to occur very fast.
So, myelination increase speed of action potential propagation.

2. Membrane capacitance:

Measure of quantity of charge that must be moved across a unit area of membrane to produce a unit change in membrane potential.
If membrane has low capacitance → Number of ions (charge) crossing membrane is less.
Hence, faster AP conduction.

Notes on NEET PG