Physiology of Peripheral Nerves I - Angelfire

28 Pages·2006·0.87 MB·English

by  Amelyn R. Rafael

PPhhyyssiioollooggyy ooff PPeerriipphheerraall
NNeerrvveess II
Amelyn R. Rafael, MD
©
TTrraannssppoorrtt ooff IIoonnss aanndd MMoolleeccuulleess
tthhrroouugghh tthhee cceellll mmeemmbbrraannee
• DIFFUSION (Passive transport)
• ACTIVE TRANSPORT
DDIIFFFFUUSSIIOONN
• Means random molecular movement of
substances molecule by molecule either
through intermolecular spaces in the
membrane or in combination with a carrier
protein
• The energy that causes diffusion is the
energy of the normal kinetic motion of matter
– the continual movement of molecules
among one another in liquids, or in gases
DDiiffffuussiioonn ssuubbttyyppeess
• 1. Simple diffusion – molecular kinetic
movement of molecules or ions
• Occurs through a membrane opening
or through intermolecular spaces
without the necessity of binding with
carrier proteins in the membrane
• The rate of diffusion is determined by
the amount of substance available, by
the velocity of kinetic motion, and by the
number of openings in the cell
membrane through which the molecules
can move
SSiimmppllee ddiiffffuussiioonn
• Occur through the cell membrane by
two pathways:
• Through the interstices of the lipid
bilayer
• The rate of diffusion of lipid soluble
substances is directly related to
their lipid solubility
• Through watery channels in some of
the transport proteins
PPrrootteeiinn cchhaannnneellss
• Distinguished by 2 important
characteristics:
• Selective permeability
• Gating
• Voltage gating
• Chemical gating
• Ex. acetylcholine
SSeelleeccttiivvee ppeerrmmeeaabbiilliittyy
• Results from the characteristic of the
channel itself: diameter, shape, and the
nature of the electrical changes along
its inside surfaces
ex. Sodium channels
GGaattiinngg
• Provides a means for controlling the
permeability of the channels
• Gate-like extensions of the transport
molecule
• Can close over the opening of the
channel or can be lifted away from
the opening by a conformational
change in the shape of the protein
molecule itself
VVoollttaaggee ggaattiinngg
• The molecular conformation of the gate
responds to the electrical potential
across the cell membrane
• Sodium channels
• The inner surfaces are strongly
negatively charged – postulated to
pull small dehydrated sodium ions
into these channels
• Specifically selective for the passage
of sodium ions