PHYSILOGY HOT TOPIC DISCUSSION

Physiology Hot Topic Discussion

Skeletal Muscle Contractions – Characteristics

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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

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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.