NERVE & MUSCLE l Physiology MCQs for dental students

 NERVE & MUSCLE l Physiology MCQs (multiple choice question) for dental students 

 

PHYSIOLOGY OF THE NERVE & MUSCLE

 

INTRODUCTION

 

Organization of the nervous system:

• The nervous system is anatomically and functionally divided into two distinct parts:

1. Central nervous system (CNS):

• Brain
• Spinal cord
• It is an integration and command center.

2. Peripheral nervous system (PNS) :

• A complex network of nerves which resides or extends outside the central nervous system.
• They allow the brain and spinal cord to communicate with the body tissues and organs and regulate their activity.

 

The anatomical unit of the nervous system: the nerve cell or neuron:

• The neuron is made up of a cell body or soma and cell processes including dendrites and axon (nerve fiber).
• The human nervous system contains billions of neurons.
• The functional unit is formed at least of two neurons.
• The axon (nerve fiber):
• is a single elongated cytoplasmic extension,
• branches freely near its termination on:

1.         the surface of the soma and dendrites of another neuron (neuroneural) junction or synapse,
2.         may end at a muscle (neuromuscular)
3.         on a gland (neuro-epithelial) junctions.

• The axon ends in synaptic knobs or buttons.

Nerve and muscle are called excitable tissues because they respond to stimuli (electrical, mechanical or chemical) when they are excited.

 

NERVE:

• is an excitable tissue which receives stimuli, processes them, and transmits signals to target tissues to integrate the functions of the whole body.

 

MUSCLE:

• is an excitable tissue which contracts in response to specific stimuli. Its function is to produce force and cause motion, either locomotion or movement within internal organs.

 

Resting Membrane Potential (RMP)

 Definition:

• RMP is the potential difference created across the cell membrane by the metabolic processes of the fiber during rest.

 

Explanation:

•  All the points on the outer surface of the membrane of a resting excitable tissue {nerve or muscle} are equipotential or isopotential i.e. When 2 microelectrodes connected with a special voltmeter capable of measuring very small potentials are placed on the outer surface of a resting excitable tissue, no potential difference is recorded.

• A peculiar characteristic of all living cells is that there is always an electric potential difference between the outer and inner surfaces of its membrane. i.e. If a microelectrode is inserted in a living cell and another electrode is applied to its outer surface , a current flows from outside to the inside of the fiber at rest, which shows that the outside of the fiber is positively charged relative to the inside i.e. the membrane is in the polarized state.

 

Value:

1- nerve:

• -70 mv i.e. the potential inside the nerve fiber for example is 70 mv more negative than the potent ial in the extracellular fluid on the outside of the nerve fiber.

2- muscles:

• skeletal muscle: -90 mv
• smooth muscle: -50mv

Cause of Resting Membrane Potential:

• RMP is caused by unequal distribution of ions on both sides of the membrane with prevalence of cations at outer surface and anions at inner surface.

 

1- Selective permeability across the cell membrane:

 

a- Na+ and K+ ions pass through membrane through leak channel with a matter of selection:

• during rest sodium ions try to pass from extracellular to intracellular space and potassium ions do the reverse according to the concentration gradient.

• Hydrated form of K+ ions are smaller than the hydrated form of Na+ ions, therefore the permeability of the membrane to K+ ions is 50-100 times more than its permeability to Na+ ions. But K movement is slightly limited and repelled by +ve outer charge.

• In the resting state, the negative interior of the cell is due to much greater diffusion of K+ out of the cell than Na+ diffusion into the cell. So, potassium ions play the most important role in generating the resting membrane potential.

 

b- C1-, HCO3-:

• The membrane is freely permeable to C1- and HCO3-, which diffuse from outside to inside according to their concentration gradient, but limited by -ve inner charge.

 

c- Proteins:

• The membrane is impermeable to proteins (creating negative charge)
• N.B. Migration of ions across the membrane till the state of equilibrium is reached (polarized state) i.e. +ve charge at the outer surface and –ve charge at innerside.

 

2- Sodium-Potassium Pump (electrogenic pump):

 

it is an active transport mechanism :

• by carrier protein against both concentration and electrochemical gradient (i.e. transports sodium ions to the exterior and pumping of K+ ions in the opposite direction.

 

carrier protein formed of:

•     2 separate globular proteins.
•     Receptor sites:

1.         3 receptor sites for binding sodium ions towards the interior of the cell
2.         2 receptor sites for potassium ions towards the outer surface.

•    The inner part has adenosine triphosphatase activity.

 

Mechanism of transport:

• 2 K+ and 3 Na+ ions bound to the receptor sites → Activation of ATPase (at inner surface of the carrier) → Conformational change in the protein carrier molecule, extruding the 3 Na+ ions to the outside and the 2 K+ ions to the inside

 

Importance of Na-K Pump:

• It maintains the sodium and potassium concentration gradients across the cell membrane i.e. the fact that Na-K pump moves 3 Na ions to exterior for every 2 K ions to interior means that a net of 1 +ve charge is moved from the interior to the exterior surface for each cycle of the pump. This will create +ve charge outside and –ve charge inside.

• Na-K pump is said to be electrogenic as it establishes a negative electrical potential inside the cells, which helps to transmit signals throughout the nervous system Maintenance of the normal level of intracellular K+ is necessary for protein metabolism. It keeps the osmotic equilibrium. Without function of this pump, most cells of the body would swell until they burst.