11.3.2- Resting Potential, Ionic Concentrations, and Channels NEURONS 11.3.1- Membrane Potentials 11.3.2- Resting Potential, Ionic Concentrations, and Channels

What are neurons? a.k.a “nerve cells” Responsible for the signaling processes done by the brain and thus enabling the brain to do complex tasks Generally has two classes: Nerve cell – primarily involved in the signaling process Neuroglial cell – provides support to the nerve cells Composed of 4 major regions: Cell body – contains nucleus and other apparatus to nourish the cell Dendrites - receptive surfaces of the neuron that receive signals from thousands of other neurons passively and without amplification Axon – serves as a transmission line to move information from one neuron to another - ranges in length from 1 meter in the human spinal cord to a few millimeters in the brain - the larger the diameter of the axon, the faster the signal travels

Presynaptic Terminals - the transmitting unit of the neuron which, when stimulated, release a neurotransmitter that flows across a gap of approximately 20 nanometers to an adjacent cell where it interacts with the postsynaptic membrane and changes its potential

How do neurons communicate? Communication is done through the ff: Through a neurotransmitter that changes membrane properties or membrane potentials Information and/or action potentials are envisioned as pulses that travel through the axon without decreasing in amplitude All action potentials that move through the axon propagate through each branch to the presynaptic terminal

Membrane Potentials The neuron, like other cells in the body, has a separation of charge across its external membrane. The cell membrane is positively charged on the outside and negatively charged on the inside This separation of charge, due to the selective permeability of the membrane to ions, is responsible for the membrane potential The potential difference across the cell membrane is approximately 60mV to 90mV, depending on the specific cell

The figure on the left shows a cell membrane with positive ions along the outer surface of the cell membrane and negative ions along the inner surface of the cell membrane. The figure on the right further illustrates separation of charge by showing that only the ions along the inside and outside of the cell membrane are responsible for membrane potential (negative ions along the inside and positive ions along the outside of the cell membrane)

By convention, the outside is defined as 0mV (ground), and the resting potential is Vm = vi-vo = -60 mV. Most signaling involves changes in this potential across the membrane. Signals such as action potentials are a result of electrical perturbations of the membrane. By definition, if the membrane is more negative than resting potential (i.e., -60 to -70 mV), it is called hyperpolarization, and an increase in membrane potential from resting potential (i.e., -60 to -50 mV) is called depolarization

To create a membrane potential of 60mV does not require the separation of many positive and negative charges across the membrane. The actual number, however, can be found from the relationship Cdv = dq, or C∆v = ∆ q (∆ q = the number of charges times the electron charge of 1.6022 x 10-19 C) Ex. With C = 1µF/cm2 and ∆ v = 60 x 10-3, the number of charges equals approximately 1 x 108per cm2. These charges are located within 1 mm distance of the membrane

Graded Response and Action Potentials A neuron can change the membrane potential of another neuron to which it is connected by releasing its neurotransmitter. The neurotransmitter crosses the synaptic cleft or gap, interacts with receptor molecules in the postsynaptic membrane of the dendrite or cell body of the adjacent neuron, and changes the membrane potential of the receptor neuron The change in membrane potential at the postsynaptic membrane is due to a transformation from neurotransmitter chemical energy to electrical energy. The change in membrane potential depends on how much neurotransmitter is received and can be depolarizing or hyperpolarizing. This type of change in potential is typically called a graded response since it varies with the amount of neurotransmitter received

Note that a signal from a neuron is either inhibitory or excitatory, but specific synapses may be excitatory and others inhibitory, providing the nervous system with the ability to perform complex tasks. The result: The net result of activation of the nerve cell is the action potential w/c is a large depolarizing signal of up to 100mV that travels along the axon & lasts approximately 1 to 5 ms

Resting Potential, Ionic Concentrations, and Channels A resting membrane potential exists across the cell membrane because of the differential distribution of ions in and around the membrane of the nerve cell. The cell maintains these ion concentrations by using a selectively permeable membrane w/ ion channels Channels allow ions to pass through the membrane, are selective, and are either passive or active.

Passive channels are always open and are ion specific thus allow only one ion type to pass through the membrane and prevents all other ions from crossing the membrane through that channel Active channels, or gates, are either opened or closed in response to an external electrical or chemical stimulation. The active channels are also selective and allow only specific ions to pass through the membrane. Typically, active gates open in response to neurotransmitters and an appropriate change in membrane potential.