Chapter 4 How do neurons use electrical signals to transmit information?

Maintaining the Resting Potential

1)Because the membrane is relatively impermeable to large molecules, the negatively charged proteins remain inside the cell. 2)Ungated potassium and chloride channels allow potassium and chloride ions to pass freely, but gates on sodium channels keep out positively charged sodium ions. 3)Na+-K+ pumps extrude Na+ from the intracellular fluid and inject K+.

Inside the Cell

1)Large protein anions are manufactured inside cells. 2)No membrane channels are large enough to allow these proteins to leave the cell, and their negative charge alone is sufficient to produce transmembrane voltage, or a resting potential.

Outside the Cell

1)Sodium ions are kept out to the extent that about 10 times as many sodium ions reside outside of the axon membrane than inside it. 2)The difference in concentrations of sodium also contributes to the membrane's resting potential. 3) Gates on the sodium ion channels in the cell membrane are ordinarily closed, blocking the entry of most sodium ions.

Inside the Cell

3)Cells accumulate positively charged potassium ions to the extent that about 20 times as many potassium ions cluster inside the cell as outside it. 4) Because the concentration of potassium is much higher inside than outside the cell, potassium ions are drawn out of the cell by their concentration gradient.

Outside the Cell

4)Chloride ions move in and out of the cell through open channels in the membrane. 5)The equilibrium point, at which the concentration gradient equals its voltage gradient, is approximately the same as the membrane's resting potential, and so chloride ions ordinarily contribute little to the resting potential.

Tools for Measuring a Neuron's Electrical Activity: Voltmeter

A device that measures the difference in electrical potential between two bodies

Tools for Measuring a Neuron's Electrical Activity: The oscilloscope

A device that serves as a sensitive voltmeter Used to record voltage changes on an axon

Electricity

A flow of electrons from a body that contains a higher charge (more electrons) to a body that contains a lower charge (fewer electrons)

Tools for Measuring a Neuron's Electrical Activity: Volt

A measure of a difference in electrical potential

Summation of Inputs

A neuron sums all inputs that are close in time and space. It provides an indication of the summed influences of multiple inputs. If the summed ionic inputs exceed the threshold (approximately −50 mV) at the axon hillock, an action potential will be initiated.

Tools for Measuring a Neuron's Electrical Activity: Microelectrodes

A set of electrodes small enough to place on or in an axon Can be used to: measure a neuron's electrical activity. deliver an electrical current to a single neuron (stimulation).

Refractory Periods and Nerve Action

Although an action potential can travel in either direction on an axon, refractory periods prevent it from reversing direction and returning to its point of origin. Refractory periods thus produce a single discrete impulse that travels away from the point of initial stimulation. When an action potential begins near the cell body, it usually travels down the axon to the terminals.

Saltatory Conduction and the Myelin Sheath

An axon is insulated by (A) oligodendroglia in the CNS and (B) Schwann cells in the PNS. Each myelin sheath segment is separated by a gap, or node of Ranvier.

Tools for Measuring a Neuron's Electrical Activity: Electrical potential

An electrical charge measured in volts; the ability to do work through the use of stored potential electrical energy

The Versatile Neuron

Because the cell body membrane does not contain voltage-sensitive channels, a typical neuron does not initiate action potentials on its dendrites. In some neurons, however, voltage-sensitive channels on dendrites do enable action potentials.

Role of Voltage-Sensitive Ion Channels:5

Both sodium and potassium voltage-sensitive channels are attuned to the threshold voltage of about −50 mV.

Excitatory postsynaptic potential (EPSP)

Brief depolarization of a neuron membrane in response to stimulation Depolarized neuron is more likely to produce an action potential.

Inhibitory postsynaptic potential (IPSP)

Brief hyperpolarization of a neuron membrane in response to stimulation Hyperpolarized neuron is less likely to produce an action potential.

Acetylcholine

Chemical transmitter that the axon terminal releases at the muscle end plate Attaches to transmitter-sensitive channels Channels open, allowing Na+ and K+ ions across the muscle membrane to depolarize the muscle to the threshold. Muscles then generate action potentials to contract.

Voltage-Sensitive Ion Channels

Closed at resting potential; ions cannot pass through. When the membrane reaches threshold, channels open briefly, enabling ions to pass through, then close again to restrict their flow.

Depolarization

Decrease in electrical charge across a membrane (more positive) Usually due to the inward flow of sodium

Role of Voltage-Sensitive Ion Channels: 2

Depolarization due to Na+ influx With tetrodotoxin (to block sodium channels), a slightly different action potential due entirely to the efflux of potassium is recorded.

How the Movement of Ions Causes Electrical Charges: Voltage gradient

Difference in charge between two regions that allows a flow of current if the two regions are connected Opposite charges attract. Similar charges repel. Ions will move down a voltage gradient from an area of higher charge to an area of lower charge.

How the Movement of Ions Causes Electrical Charges: Concentration gradient

Differences in concentration of a substance among regions of a container allow the substance to diffuse from an area of higher concentration to an area of lower concentration.

The Toilet Flushing Analogy

During the flush, the toilet is absolutely refractory: another flush cannot be induced. During refilling of the bowl, the toilet is relatively refractory, meaning that flushing is possible but harder.

Excitatory and Inhibitory Postsynaptic Potentials

EPSPs and IPSPs last only a few milliseconds before they decay and the resting potential is restored.

Electrical Activity of a Membrane: Resting potential

Electrical charge across the cell membrane in the absence of stimulation A store of negative energy on the intracellular side relative to the extracellular side The inside of the membrane at rest is −70 millivolts relative to the extracellular side.

Electrical stimulation studies: Galvani (18th century)

Electrical current applied to a dissected nerve induced a twitch in the muscle connected to the nerve; Galvani concluded that electricity flows along the nerve.

Electrical Stimulation Studies:Fritsch and Hitzig (mid-nineteenth century)

Electrical stimulation of the neocortex causes movement (arms and legs).

Equilibrium

Equilibrium occurs when the concentration gradient of chloride ions on the right side of the beaker is balanced by the voltage gradient of chloride ions on the left. At equilibrium the concentration gradient is equal to the voltage gradient.

How Sensory Stimuli Produce Action Potentials

Example is touch Each hair on our body allows us to detect the slightest displacement. Dendrite of a touch neuron is wrapped around the base of each hair. Hair displacement opens stretch-sensitive channels in the dendrite's membrane. When channels open, they allow an influx of Na+ ions sufficient to depolarize the dendrite to its threshold level.

Electrical Stimulation Studies:Bartholow (1874)

First report of human brain stimulation: "Passed an insulated needle into the left posterior lobe so that the non-insulated portion rested entirely in the substance of the brain. The reference was placed in contact with the dura mater. When the circuit was closed, muscular contraction in the right upper and lower extremities ensued."

Electrical Recording Studies:Caton (early nineteenth century)

First to attempt to measure electrical currents of the brain using a voltmeter and electrodes on the skull

Electrical Recording Studies:von Helmholtz (nineteenth century)

Flow of information in the nervous system is too slow to be a flow of electricity Nerve conduction: 30-40 meters/second Electricity: 3 × 108 meters/second

Resting Potential

Four charged particles take part in producing the resting potential. Sodium (Na+) and chloride (Cl−) Higher concentration outside cell Potassium (K+) and large proteins (A−) Higher concentration inside cell

Voltage-sensitive ion channels

Gated protein channel that opens or closes only at specific membrane voltages Sodium (Na+) and potassium (K+) Closed at membrane's resting potential Na+ channels are more sensitive than K+ channels and therefore open sooner.

Electroencephalogram

Graph that records electrical activity through the skull or from the brain and represents graded potentials of many neurons

Role of Voltage-Sensitive Ion Channels: 3

Hyperpolarization due to K+ efflux With TEA surrounding the axon and blocking potassium channels, a smaller-than-normal action potential due entirely to a Na+ influx is recorded.

The Myelin Sheath

If myelin is damaged, a neuron may be unable to send any messages over its axons. In multiple sclerosis (MS), the myelin formed by oligodendroglia is damaged, which disrupts the functioning of neurons whose axons it encases.

Role of Voltage-Sensitive Ion Channels:6

If the cell membrane changes to reach this voltage, both types of channels open to allow ion flow across the membrane.

Graded Potentials

If the concentration of any of the ions across the unstimulated cell membrane changes, the membrane voltage changes. These graded potentials are small voltage fluctuations across the cell membrane.

Hyperpolarization

Increase in electrical charge across a membrane (more negative) Usually due to the inward flow of chloride ions or outward flow of potassium ions

Electrical Recording Studies: (Bernstein, 1886).

It is not the ions themselves that travel along the axon but rather a wave of charge

The axon hillock

Junction of cell body and axon Rich in voltage-sensitive channels Where EPSPs and IPSPs are integrated Where action potentials are initiated

Maintaining the resting potential

Large A− molecules cannot leave cell: make inside negative. Ungated channels allow K+ and Cl− to move into and out of cell more freely, but gated sodium channels keep out Na+ ions.

Action potential

Large, brief reversal in polarity of an axon Lasts approximately 1 millisecond (ms)

Positive pole

Location to which electrons flow; lower charge

How the Movement of Ions Causes Electrical Charges: Diffusion

Movement of ions from an area of higher concentration to an area of lower concentration through random motion

Tools for Measuring a Neuron's Electrical Activity: Giant Axon of the Squid

Much larger in diameter than human axons Humans: 1 to 20 micrometers Squid: Up to 1 millimeter (1000 micrometers) Easier subject of experiments Used by Hodgkin and Huxley in the 1930s and 1940s

Anions

Negatively charged ions Examples: chloride (Cl−), protein molecules (A−)

Role of Voltage-Sensitive Ion Channels: 1

Occurs when a large concentration of first Na+ ions, then K+ ions crosses the membrane rapidly.

Node of Ranvier

Part of an axon that is not covered by myelin Tiny gaps in the myelin sheath Enables saltatory conduction

Electrical stimulation

Passing an electrical current from the tip of an electrode through brain tissue, resulting in changes in the electrical activity of the tissue

Cations

Positively charged ions Examples: sodium (Na+), potassium (K+)

Myelin

Produced by oligodendroglia in the CNS and Schwann cells in the PNS Speeds up neural impulse

Nerve impulse

Propagation of an action potential on the membrane of an axon Refractory periods produce a single discrete impulse that travels along the axon in one direction only. Size and shape of action potential remain constant along the axon. All-or-none law

Spatial summation

Pulses that occur at approximately the same place on a membrane are summed.

Temporal summation

Pulses that occur at approximately the same time on a membrane are summed.

Saltatory conduction

Saltare: to dance (Latin) Propagation of an action potential at successive nodes of Ranvier

Role of Voltage-Sensitive Ion Channels:8

Sodium channels have two gates. Once the membrane depolarizes to about −30 mV, one of the gates closes.

How Nerve Impulses Produce Movement

Spinal motor neurons send nerve impulses to synapses on muscle cells. Axon of each motor neuron makes one or more synapses with target muscle. End plate Part of the muscle membrane that is contacted by the axon terminal

Role of Ions in Summation

The influx of sodium ions accompanying one EPSP is added to the influx of sodium ions accompanying a second EPSP if the two occur close together in time and space. If the two influxes are remote in time or in space or in both, no summation is possible. The same is true regarding effluxes of potassium ions. When they are close in time and space, they sum; when they are far apart in either or both ways, there is no summation.

Role of Voltage-Sensitive Ion Channels:9

The potassium channels open more slowly than the sodium channels, and they remain open longer. The efflux of K+ reverses the depolarization produced by Na+ influx and even hyperpolarizes the membrane.

Negative pole

The source of electrons; higher charge

Relative refractory period

The state of an axon in the later phase of an action potential, during which stronger electrical current is required to produce another action potential Potassium channels are still open.

Absolute refractory period

The state of an axon in the repolarizing period, during which a new action potential cannot (usually) be elicited because gate 2 of sodium channels, which is not voltage-sensitive, is closed

Role of Voltage-Sensitive Ion Channels:7

The voltage-sensitive sodium channels are more sensitive than the potassium channels and so open first. As a result, the voltage change due to Na+ influx takes place slightly before the voltage change due to K+ efflux can begin.

Threshold potential

Voltage on a neural membrane at which an action potential is triggered Opening of Na+ and K+ voltage-sensitive channels Approximately −40 mV relative to extracellular surround

Domino Analogy

Voltage-sensitive channels along the axon resemble a series of dominoes. When one domino falls, it knocks over its neighbor, and so on down the line. There is no decrement in the size of the fall.

How Sensory Stimuli Produce Action Potentials

We receive information about the world through bodily sensations (touch and balance). auditory sensations (hearing). visual sensations (sight). chemical sensations (taste and olfaction). Neurons related to these diverse receptors all have ion channels on their cell membranes. These ion channels initiate the chain of events that produces a nerve impulse.

Depolarization can be produced by

an influx of sodium ions produced by the opening of normally closed gated sodium channels.

stretch-senstive channel

ion channel on a tactile sensory neuron that activates in response to stretching of the membrane, initiating a nerve impulse.

For the membrane to become hyperpolarized,

its extracellular side must become more positive, which can be accomplished with an efflux of K+ ions. The membrane can also be hyperpolarized by an influx of Cl− ions.

transmitter-senstiive channel

receptor complex that has both a receptor site for a chemical and a pore through which ions can flow.

IPSPs are associated with

the opening of potassium channels (allows an efflux of K+) or with the opening of chloride channels (allows an influx of Cl−).

EPSPs are associated with

the opening of sodium channels: allows influx of Na+.