Acetylcholine Receptors

Ach receptors in the post junctional membrane of the motor endplate are of the nicotinic type. A normal neuromuscular endplate on average contains about 50 million Ach receptors. They are situated at the crests of the junctional folds. The nicotinic Ach receptor is a protein with a molecular weight of about 250 000 Daltons and is made up of five polypeptide subunits: two identical a subunits, one P, one y (replaced by s in adult mammals) and one 5 subunit. Each subunit is encoded by a different gene and has similar overall structure. The five subunits are arranged in a cylindrical fashion surrounding a central funnelshaped pore, which is the ion channel (Figure MP.15). The receptor spans the membrane and possesses both hydrophilic and hydrophobic regions.

COMPARISON OF TYPE I, II AND III MUSCLE FIBRES

Property

Type I

Type II

Type III

Colour

Red

White

Red

Diameter

Small

Large

Intermediate

Formation of ATP

Oxidative phosphorylation

Glycolysis

Oxidative phosphorylation

Mitochondria

Many

Few

Many

Glycogen content

Low

High

Intermediate

Glycolytic enzymes

Low

High

Intermediate

Myosin-ATPase activity

Low

High

High

Contraction speed

Slow

Fast

Fast

Myoglobin content

High

Low

High

Fatigue rate

Slow

High

Intermediate

Mitochondria

Many

Few

Figure MP.13

Figure MP.14 Structure of the NMJ

Aih rflepprgr*

Figure MP.14 Structure of the NMJ

When Ach molecules bind to the receptors, they do so by binding onto specific binding sites on the a subunits. As a result, they induce a change in the configuration to the receptor so that the central pore opens, allowing Na+ and other ions to pass down their concentration and electrical gradients. This sudden influx of Na+ ions into the cell results in depolarization.

Was this article helpful?

0 0
Dealing With Impotence Naturally

Dealing With Impotence Naturally

Put an end to the disappointment, frustration and embarrassment of... Erectile Dysfunction. Have Sex Like You are 18 Years Old Again. Have a natural powerfully stiff penis. Eject volumes of semen. Do it again and again night after night. Never make another excuse for not being able to get it up.

Get My Free Ebook


Post a comment