Nerve Muscle Physiology—Important Questions and Answers

Nerve Muscle Physiology Important Topics 

• Axonal Transportation
• Origin of Resting Membrane Potential
• Donnan's Effect
• Nernst Equation
• Equilibrium Potential of Ions in Various Mammalian Cells
• Action Potential
• Different Parts of Action Potential
• Permeability of different ions during Action Potential
• Conduction of Action Potential
• Classification of Nerve Fibres
• Muscle Physiology
• Structure of Sarcomere
• Regulatory Protein
• Molecular Mechanism
• Length-Tension Relationship
• Tetanizing Frequency (TF)
• Mechanism of Smooth Muscle Contraction

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Important question with answers 

Q. Which connective tissue layer forms the blood neuron barrier?

Ans. Perineurium contains connective tissue cells and minimal gaps exist between these connective tissue cells. Tight junctions (occludin and claudin) bridge these gaps. The combination of perineurium and tight junctions forms the blood neuron barrier. Recent research indicates that capillary endothelial cells and tight junctions are also part of the blood neuron barrier.

Q. Who forms the blood-brain barrier?

Ans. Structurally, the endothelial cells lining the capillaries of the brain & tight junctions.

Q. Graded electrogenesis occurs at which part of the neuron?

Ans. Graded electrogenesis is a local potential development on neurons that originates at the level of dendrites and soma.

Q. The action potential initiation occurs in which part of the neuron?

Ans. Dendrites and soma cannot generate action potentials. Action potential initiation relies on voltage-gated Na (sodium) channels. The presence of the maximum voltage-gated Na channels determines the action potential initiation site. In the case of motor neurons, action potential is initiated at the initial segment of a neuron and in case of segment and in the case of sensory neurons, it is the Node of Ranvier.

Q. Where are the maximum number of voltage-gated Na channels present?

Ans. The maximum number of voltage-gated sodium channels are present in the Node of Ranvier.

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Q. Where are the least number of voltage-gated Na channels present?

Ans. The least number of voltage-gated sodium channels are present on the surface of the myelin sheath.

Q. How is the difference between ECF and ICF maintained and why are they not equalized over time?

Ans. There are various pumps and carrier proteins that constantly eject certain ions outside and inject ions inside to maintain this difference of ions between ECF and ICF. This difference is maintained by Donnan's effect. Donnan's effect is due to large impermeable molecules that are present mainly on one side of the cell. The concentration of protein in the ECF is very low but very high in ICF. This means that intracellular fluid contains a huge amount of protein as compared to ECF.

Q. For how long does this diffusion occur?

Ans. Till equilibrium state is established. For example, if 5 molecules of K+ travel outside the cell, then the positive charges present outside the cell will cause a repulsive action towards the sixth molecule that is trying to pass through.

• When enough positive charges move out of the cell, extracellular positivity or electrical gradient, will push the extra positive charge in the reverse direction.
• The concentration gradient pushes positively charged K+ outside, and the electrical gradient directs it inside after some time. This cycle is continued until the equilibrium state is established.
• The equilibrium state is developed by electrochemical gradient or electrochemical forces.
• At the equilibrium state, the net movement of potassium through the leak channel is zero.
• As we know, some amounts of positive and negative charges have already been deposited at the outer and inner leaflets of the cell; the measured voltage difference at the cell's equilibrium state is termed the equilibrium potential of the cell or isoelectric potential of the cell.

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Q. Equilibrium potential for an ion is calculated using? 

Ans.  Goldman equation - RMP

Q. Resting membrane potential of a nerve is equal to the equilibrium potential of?

Ans. Cl-

Q. Resting membrane potential is close to the isoelectric potential of ?

Ans. K+

Q. If more and more insulated covering is given on the surface of the membrane, then what happens to the transmembrane resistance?

Ans. The transmembrane resistance in such a case increases.

Q. Which type of stimulus produces an action potential easily?

Ans. 

It is stimulus A. If it is a rapidly rising stimulus, there will be depolarization of the membrane. Depolarization will open up 2 voltage-gated channels; one is a voltage-gated Na+ channel and one is a voltage-gated K+ channel. The voltage-gated sodium channel is very fast and the voltage-gated potassium channel is very slow. A rapidly rising stimulus prompts the simultaneous opening of fast Na+ and slow K+ channels. Due to the faster opening of sodium channels, their number surpasses that of potassium channels. The resulting higher net influx of sodium compared to potassium leads to an elevated intracellular voltage, generating an action potential.

If a slow-rising stimulus is given, the sodium channel opening will also be slower, and the potassium channel is itself slow. If both of these channels are opening up in the same manner, the inward positive charge of sodium will be neutralized by the outward positive charge of potassium. This is the reason why action potential cannot be generated by a slow-rising stimulus.

This kind of property where a slowly rising stimulus does not produce action potential because of inadequate depolarization and failure to attain the threshold voltage. This is known as Accommodation property.

Getting back to the question, it can be understood that the best stimulus is A. Stimulus B is the slowest-rising stimulus, making it the least effective stimulus. C will be effective; thus the sequence of stimulus will be A > C > D > B.

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Important Points to Remember

• Tropomyosin is present in smooth muscle to enhance the actin-myosin interaction.
• Calponin & caldesmon prevent actin-myosin interaction in the resting condition of smooth muscles.

• MLC Phosphorylation causes increased angles of Myosin Heads & Increased Myosin ATPase Activity in smooth muscle.
• Rate of Cross-bridge cycling in smooth muscle is (1/10)th of that in skeletal muscle (5 cross-bridge cycles/sec)
• The sustained tonic contraction of smooth muscle, even after dephosphorylation, is known as Latch Bridge Phenomenon.
• Rigor Mortis → sustained contractile state in skeletal muscle → pathological condition.
• Latch Bridge Phenomenon → smooth muscle → physiological phenomenon.
• In our body, the contraction occurs in a complete tetanizing fashion in most cases. The motor nerve that is supplying the skeletal muscle, from the level of the anterior region of the spinal cord up to the neuromuscular junction is also known as Lower Motor Neuron. Electromyography (EMG) is the summated version of all MUPs generated inside the skeletal muscle. Fasciculation is the contraction of a single motor unit. Fibrillation is the contraction of a single muscle fiber. Every skeletal muscle in the body is a combination of red fiber and white fiber.
• The small release of calcium from the cisternal region is called a calcium spark.
• Volume or the size of the Sarcoplasmic reticulum → skeletal muscle > myocardium.
• Size of the T-tubule → myocardium > skeletal muscle.\
• Myosin type II is responsible for muscle contraction.
• Myosin type V and I are positive end-directed molecular motors.
• Myosin type VI is a negative end-directed molecular motor.

Also read: Nerve Muscle Physiology

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