ANATOMY AND PHYSIOLOGY OF

NEURONSAP BiologyChapter 48

Objectives

•Describe the different types of neurons•Describe the structure and function of dendrites, axons, a synapse, types of ion channels, and neurotransmitters.

•Describe resting potential and the sequence of events that occur during an action potential.

Overview

• The human brain contains an estimated 1011 (100 billion) neurons.

•Each neuron may communicate with thousands of other neurons in complex information-processing circuits.

•Results of brain imaging and other research methods show that groups of neurons function in specialized circuits dedicated to different tasks.

Structural Organization of the Nervous System•Central nervous system (CNS) – brain and spinal cord

• Responsible for integration of sensory input and associating stimuli with appropriate motor output

•Peripheral nervous system (PNS) –network of nerves extending into different parts of the body that carry sensory input to the CNS and motor output away from the CNA

Nervous systems consist of circuits of neurons and supporting cells

•All animals except sponges (Phylum Porifera) have a nervous system

•What distinguishes nervous systems of different animal groups is how neurons are organized into circuits

Organization of Nervous Systems: Cnidarians•The simplest animals with nervous systems, the cnidarians, have neurons arranged in nerve nets

Echinoderms

•Sea stars have a nerve net in each arm connected by radial nerves to a central nerve ring

•What is the difference?

Platyhelmenthyes

•Relatively simple cephalized animals, such as flatworms, have a central nervous system (CNS)

•What changed?

Annelids and Arthropods•Annelids and arthropods have segmentally arranged clusters of neurons called ganglia

• These ganglia connect to the CNS and make up a peripheral nervous system (PNS)

•Change?

Mollusks

•Nervous systems in mollusks correlate with lifestyles

•Sessile mollusks have simple systems, whereas more complex mollusks have more sophisticated systems

•Why?

Vertebrates

• In vertebrates, the central nervous system consists of a brain and dorsal spinal cord

•The PNS connects to the CNS

•What has been selected for?

Information Processing• Nervous systems process information in three stages:

Information Processing

•Sensory neurons pick up and transmit information from sensors that detect external stimuli (light, heat, touch) and internal conditions (blood pressure, muscle tension).

• Interneurons, in the CNS, integrate the sensory input

•Motor output leaves the CNS via motor neurons, which communicate with effector cells (muscle or endocrine cells).• Effector cells carry out the body’s response to a stimulus.

Reflexes

•The stages of sensory input, integration, and motor output are easy to study in the simple nerve circuits that produce reflexes, the body’s automatic responses to stimuli.

Neuron Structure

• Most of a neuron’s organelles are in the cell body

• Most neurons have dendrites, highly branched extensions that receive signals from other neurons

• The axon is typically a much longer extension that transmits signals to other cells at synapses

• Many axons are covered with a myelinsheath• Which speeds up transmission

•Neurons have a wide variety of shapes that reflect input and output interactions

LE 48-6

Dendrites

Cell body

Axon

InterneuronsSensory neuron Motor neuron

Neurons have a wide variety of shapes that reflect input and output interactions

NEURON STRUCTURE POGIL

Ion pumps and ion channels maintain the resting potential of a neuron•Across its plasma membrane, every cell has a voltage difference called a membrane potential

•The cell’s inside is negative relative to the outside

The Resting Potential

•Resting potential is the membrane potential of a neuron that is not transmitting signals

•Resting potential depends on ionic gradients across the plasma membrane

•Concentration of Na+ is higher in the extracellular fluid than in the cytosol

• The opposite is true for K+

Animation: Resting Potential

LE 48-10

CYTOSOL EXTRACELLULARFLUID

Plasmamembrane

NEURON FUNCTION POGIL

#s 1-6

Gated Ion Channels• Gated ion channels open or close in response to one of three stimuli:• Stretch-gated ion channels

open when the membrane is mechanically deformed

• Ligand-gated ion channelsopen or close when a specific chemical binds to the channel

• Voltage-gated ion channels respond to a change in membrane potential

NEURON FUNCTION POGIL

#s 7-9

Action potentials are the signals conducted by axons• If a cell has gated ion channels, its membrane potential may change in response to stimuli that open or close those channels

• Action potential: rapid change in the membrane potential of an excitable cell, caused by stimulus-triggered selective opening and closing of gated ion channels.

• Once generated, the impulse travels rapidly down the axon away from the cell body and toward the axon terminals.

Production of Action Potentials•Depolarizations are usually graded only up to a certain membrane voltage, called the threshold

•A stimulus strong enough to produce depolarization that reaches the threshold triggers a response called an action potential

Action Potential

•An action potential is a brief all-or-none depolarization of a neuron’s plasma membrane

• It carries information along axons

Action Potential

•Voltage-gated Na+ and K+ channels are involved in producing an action potential

•When a stimulus depolarizes the membrane, Na+ channels open, allowing Na+ to diffuse into the cell

•As the action potential subsides, K+

channels open, and K+ flows out of the cell

Conduction of Action Potentials•An action potential can travel long distances by regenerating itself along the axon

•At the site where the action potential is generated, an electrical current depolarizes the neighboring region of the axon membrane

LE 48-14C

An action potential is generated as Na+ flows inward across the membrane at one location.

Na+

Action potential

Axon

Na+

Action potentialK+

The depolarization of the action potential spreads to the neighboring region of the membrane, re-initiating the action potential there. To the left of this region, the membrane is repolarizing as K+ flows outward.

K+

Na+

Action potentialK+

The depolarization-repolarization process is repeated in the next region of the membrane. In this way, local currents of ions across the plasma membrane cause the action potential to be propagated along the length of the axon.

K+

Conduction Speed•The speed of an action potential increases with the axon’s diameter

• In vertebrates, axons are myelinated, also causing an action potential’s speed to increase

LE 48-15

Cell body

Schwann cell

Depolarized region(node of Ranvier)

Myelinsheath

Axon

NEURON FUNCTION POGIL

#s 10-15

Four Phases of an Action Potential

•Resting state: no channels are open•Depolarizing phase: membrane briefly reverses polarity

• Cell interior becomes positive to the exterior

•Repolarizing phase: returns membrane to its resting level

•Hyperpolarized phase: refractory period

Depolarization phase

•Na+ activation gates open allowing an influx of Na+

•Potassium gates remain closed• Interior of the cell becomes more positive charged than the exterior

Repolarization phase

•Returns membrane to its resting level•Gates close sodium channels and opens potassium channels

•The inside of the cell becomes more negative compared to the outside of the cell

Hyperpolarization phase

•Membrane potential is temporarily more negative than the resting state

•Sodium channels remain closed by potassium channels remain open

•Refractory period occurs during this phase

• Neuron is insensitive to depolarizing stimuli• This limits the maximum rate at which action

potentials can be stimulated in a neuron.

LE 48-13_5

Resting potential

Threshold

Mem

bran

e po

tent

ial

(mV)

Actionpotential

Time–100

–50

+50

0

Potassiumchannel

Extracellular fluid

Plasma membrane

Na+

Resting state

Inactivationgate

Activationgates

Sodiumchannel K+

Cytosol

Na+

Depolarization

K+

Na+

Na+

Rising phase of the action potential

K+

Na+

Na+

Falling phase of the action potential

K+

Na+

Na+

Undershoot

K+

Na+

NEURON FUNCTION POGIL

#s 16-20

Neurons communicate with other cells at synapses• In an electrical synapse, current flows directly from one cell to another via a gap junction

• The vast majority of synapses are chemical synapses

• In a chemical synapse, a presynaptic neuron releases chemical neurotransmitters stored in the synaptic terminal

LE 48-17

Postsynaptic cellPresynaptic cell

Synaptic vesiclescontainingneurotransmitter

Presynaptic membrane

Voltage-gatedCa2+ channel

Ca2+Postsynaptic membrane

Postsynaptic membrane

Neuro-transmitter

Ligand-gatedion channel

Na+

K+

Ligand-gatedion channels

Synaptic cleft

When an action potential reaches a terminal, the final result is release of neurotransmitters into the synaptic cleft

Synaptic Transmission•Synaptic transmission involves binding of neurotransmitters to ligand-gated ion channels

•Neurotransmitter binding causes ion channels to open, generating a new postsynaptic potential

•After release, the neurotransmitter diffuses out of the synaptic cleft

• It may be taken up by surrounding cells (reuptake) and/or degraded by enzymes

Neurotransmitters

•The same neurotransmitter can produce different effects in different types of cells