BIOCHEMISTRY - L. Stryer - 1984

VOLUME 3

PART V. MOLECULAR PHYSIOLOGY

CHAPTER 37. EXCITABLE MEMBRANES AND SENSORY SYSTEMS

Summary

Action potentials in nerve Cell axon membranes are mediated by transient changes in permeability to Na⁺ and K⁺. Both Na⁺ and K⁺ channels are regulated by transmembrane potential differences. Tetrodotoxin and saxitoxin are specific blockers of Na⁺ channels. The transmission of nerve impulses across synapses is mediated in most cases by chemical Neurotransmitters. Acetylcholine, which serves as a neurotransmitter at many synapses and motor end plates, is synthesized from acetyl-CoA and Choline. Acetylcholine is stored in synaptic vesicles that fuse with the presynaptic membrane upon the arrival of a Nerve Impulse; the released acetylcholine diffuses across the synaptic cleft and binds to specific protein receptors on the postsynaptic membrane. The resulting increase in permeability to Na⁺ and K⁺ leads to depolarization of the postsynaptic membrane. The Acetylcholine Receptor has a high affinity for α-bungarotoxin and other neurotoxins; this property has facilitated the Isolation and Purification of this integral membrane protein. Following the Hydrolysis of acetylcholine by acetylcholinesterase, the postsynaptic membrane undergoes repolarization. Inhibitors of acetylcholinesterase include organic phosphates, notably DFP, which can cause death due to respiratory paralysis. Other neurotransmitters include catecholamines (epinephrine, norepinephrine, and dopamine) and γ-aminobutyrate (GABA).

Excitation of retinal rod Cells can be triggered by a single photon. The outer segment of a rod cell contains about a thousand stacked disks, which are closed bilayer membranes densely packed with molecules of the transmembrane protein rhodopsin. Rhodopsin is a photoreceptor protein; its chromophore is 11-cis-retinal, which is derived from all-trans-retinol (vitamin A). 11-cis-retinal is attached to a specific Lysine residue in opsin, forming a Schiff base. The primary event in visual excitation is the isomerization of the 11-cis-retinal moiety to all-trans-retinal. The final stage of visual excitation is hyperpolarization of The Plasma Membrane of the outer segment, caused by a decrease in Na+ permeability. The absorption of a single photon by a dark-adapted rod blocks the influx of more than a million sodium ions. Second messengers (likely Ca²⁺ or cyclic GMP) transmit the signal from photolyzed rhodopsin to the plasma membrane. Color Vision is mediated by Three types of photoreceptors, each containing 11-cis-retinal. Remarkably, 11-cis-retinal serves as the chromophore in all known visual systems, providing a striking example of convergent evolution.

Motile Bacteria are capable of directed movement toward an attractant (such as glucose) or away from a repellent (such as Fatty acids). Chemotaxis sensors are localized in the periplasmic space or on the plasma membrane. Bacterial flagella are driven by rotary "motors" that utilize the energy of the proton gradient across the plasma membrane. Bacteria swim smoothly for about a second, then tumble As a result of a reversal in the direction of flagellar rotation from counterclockwise to clockwise. When a bacterium is moving toward an attractant or away from a repellent, its tumbles become less frequent. Bacteria detect temporal rather than spatial concentration gradients. Within the bacterial cell, information from chemosensors is transmitted to three methyl-accepting chemotaxis Proteins (MCPs) and subsequently to the flagella. Reversible methylation of MCPs regulates the sensitivity of the detection system.