N E U R O N S , S E N S E O R G A N S , A N D N E R V O U S

S Y S T E M S

C H A P T E R 3 4

KEY CONCEPTS

• 34.1 Nervous Systems Are Composed of Neurons and Glial Cells

• 34.2 Neurons Generate Electric Signals by Controlling Ion Distributions

• 34.3 Neurons Communicate with Other Cells at Synapses

• 34.4 Sensory Processes Provide Information on an Animal’s External Environment and Internal Status

• 34.5 Neurons Are Organized into Nervous Systems

Quick intro video

NERVOUS SYSTEMS ARE COMPOSED OF NEURONS AND GLIAL CELLS• Animals need a way to transmit

signals at high speeds from place to place within their bodies (e.g., to avoid danger).

• Mammalian neurons transmit signals at 20–100 meters per second, similar to a banjo where each finger pluck is like a nerve impulse in brain and sent down the spinal cord.

NERVOUS SYSTEMS ARE COMPOSED OF NEURONS AND GLIAL CELLS• Neurons are excitable cells, which means its cell membranes can generate and conduct signals

called impulses

• This is a key specialization, only muscles are neurons are ‘excitable’ cells in the body

– they can generate and transmit electrical signals, called action potentials.

• Cell membranes ordinarily have electrical polarity: the outside is more positive than the inside.

• An impulse, or action potential, is a state of reversed polarity.

NERVOUS SYSTEMS ARE COMPOSED OF NEURONS AND GLIAL CELLS

• In an excitable cell, an action potential generated at one point propagates over the whole membrane.

• The region of depolarization moves along the cell membrane, and the membrane is said to “conduct” the impulse.

The impulse or action potential moves along the cell membrane, in a process called conduction or propagation

An impulse or action potential is a state in which the polarity across the cell membrane is reversed.

NEURONS ARE SPECIALIZED TO PRODUCE ELECTRIC SIGNALS• Neurons (nerve cells) are specially

adapted to generate electric signals, usually in the form of action potentials.

• Neurons are like ‘land lines’ and they must make contact with target cells and are very diverse in structure. (fast and addressed)

NEURONS ARE SPECIALIZED TO PRODUCE ELECTRIC SIGNALS• Synapse: cell-to-cell

contact point specialized for signal transmission

• A signal arrives at the synapse by way of the presynaptic cell and leaves by way of the postsynaptic cell.

• The postsynaptic cell can be another neuron or it can be a sensory cell or as discussed in chapter 33, it can be a muscle cell.

FOUR ANATOMICAL REGIONS

• Most neurons have four regions; dendrites, cell body, an axon, and a set of presynaptic axon terminals

• Dendrites (‘tree’)—carry signals to the cell body (incoming)

• They are short processes, or extensions that tend to branch from the cell body like twigs on a shrub

• Cell body—contains nucleus and organelles

• The main job is to combine and integrate the incoming signals quickly

FOUR ANATOMICAL REGIONS

• Axon—conducts action potentials away from the cell body; can be very long

• For example, an axon can run from your spinal cord all the way to your finger to coordinate a movement.

• Action potentials are generated at the cell body and move out the axon

• Presynaptic axon terminals—make contact with other cells

• These terminals make contact with other cells, neurons sensory cells or muscles.

Dendrites are the major site for synaptic input from other neurons

The cell body and, especially, the axon hillock – where the cell body transitions to the axon – are often major sites of integration of signals

The axon is the conduction component of the neuron propagating action potentials over a long distance to the axon terminal

At the presynaptic axon terminals. The output of the neuron can alter the activities of other cells

CONCEPT 34.1 NERVOUS SYSTEMS ARE COMPOSED OF NEURONS AND GLIAL CELLS• Presynaptic Axon Terminals

– A neuron is said to innervate the cells that the axon terminals contact.

– Axons of many neurons often travel together in bundles called nerves.

– Nerve refers only to axon bundles outside the brain and spinal cord.

– In the brain or spinal cord they are called tracts.

GLIAL CELLS• Glial cells support, nourish, and insulate neurons

• Glial cells, or glia or neuroglia is the second major type of cell in the nervous system

– 50% of the mammalian brain are glial cells– There are several distinct types of glial cells that have distinct

roles.

• They are not excitable or produce action potentials however, they have several functions:

• Help orient neurons toward their target cells during embryonic development

• Provide metabolic support for neurons• Help regulate composition of extracellular fluids and perform

immune functions • Assist signal transmission across synapses

GLIAL CELLS• In vertebrates typically not in invertebrates,

certain glial cells are insulated by a lipid-rich cell membrane, similar to the insulators of a power cord.

• In brain and spinal cord, this glial wrap around the axons is called Oligodendrocytes

• Schwann cells insulate axons in nerves outside of these areas.

• The glial membranes form a electrically nonconductive sheath called myelin.

• Myelin-coated axons are white matter and areas of cell bodies are gray matter.

Multiple layers of Schwann cell membrane insulate the axon

GLIAL CELLS

N E U R O N S G E N E R A T E E L E C T R I C S I G N A L S B Y

C O N T R O L L I N G I O N D I S T R I B U T I O N

3 4 . 2

INTRODUCTION• 1 min video

• One feature common to all nervous systems is that they encode and transmit information in the form of action potentials.

• Some basic electrical principles help us understand how neurons produce action potentials and other electrical signals.

ACTION POTENTIALS - TERMINOLOGY

• Current: flow of electric charges from place to place; in cells, current is based on flow of ions such as Na+

• Voltage, or electrical potential difference exists if positive charges are concentrated in one place and negative charges are concentrated in a different place.

• Voltages produce currents because opposite charges attract and will move toward one another if given a chance.

ACTION POTENTIALS

• No voltage differences exist within open solutions such as the intracellular fluids.

• Voltage differences exist only across membranes such as the cell membrane.

Within a few nanometers of the membrane on either side, net positive and negative charge concentrations may accumulate

Farther away, in the bulk solution on either side, the net charge is zero

ACTION POTENTIALS

• The voltage across a membrane is called membrane potential and is easily measured.

• Resting neuron: membrane potential is the resting potential, typically –60 to –70 millivolts (mV)

– Negative sign means the inside of the cell is electrically negative relative to the outside.

1. An electrode made from a glass tube (pulled to a sharp tip and open at the end) is filled with electrically conducting solution….

2…and connected with a wire to an amplifier

3. The voltage difference between the electrode placed inside the axon and a reference electrode outside the axon is detected…

4… and this small potential difference is displayed on a computer screen.

5, In an unstimulated neuron, a potential difference is about -65mV is observed between outside and inside. This is the resting potential

ACTION POTENTIALS

• Membrane potential can change rapidly, and only relatively small numbers of positive charges need to move through the membrane for this change of membrane potential to occur.

• Composition of the bulk solutions (the intra- and extracellular fluids) does not change.

• Animated tutorial 34.1 – good textbook link

SODIUM-POTASSIUM PUMP SETS UP CONCENTRATION GRADIENTS• Ion redistribution occurs through

membrane channel proteins and ion transporters in the membrane.

• Sodium–potassium pump—uses energy from ATP to move 3 Na+ ions to the outside and 2 K+ to the inside; establishes concentration gradients of these ions

SODIUM-POTASSIUM PUMP SETS UP CONCENTRATION GRADIENTS

• Potassium channels are open in the resting membrane.

• K+ ions diffuse out of the cell through leak channels and leave behind negative charges within the cell.

• K+ ions diffuse back into the cell because of the negative electrical potential.

• At this equilibrium point, there is no net movement of K+; called the equilibrium potential of K+.

SODIUM-POTASSIUM PUMP SETS UP CONCENTRATION GRADIENTS• Diffusion of ions is controlled by concentration effect and electrical effect. When they

are equal, electrochemical equilibrium is reached.

Crash Course –Nervous System 2 Video on Action Potentials and Sodium Potassium Pump

NO!!!!

• Electrochemical equilibrium is called the equilibrium potential of the ion, calculated by the Nernst equation:

Eion 2.3RTzFlog

ion outsideion inside

GATED ION CHANNELS CAN ALTER MEMBRANE POTENTIAL • Most ion channels are “gated”—they open and close under certain conditions. Most are

closed in a resting neuron, which is why K+ leak channels determine resting membrane potential.

• Voltage-gated channels open or close in response to changes in membrane potential

• Stretch-gated channels respond to tension applied to cell membrane

• Ligand-gated channels open or close when a specific chemical (ligand) binds to the channel protein.

GATED ION CHANNELS CAN ALTER MEMBRANE POTENTIAL • Opening and closing gated channels can alter membrane potential.

• If Na+ channels open, Na+ diffuses into the neuron because it is more concentrated outside the cell, and the cell membrane is more negative on the inside.

• When membrane becomes less negative on the inside, the membrane is depolarized.

• The membrane is hyperpolarized if the charge on the inside becomes more negative.

• Nerve Impulse explained

• Depolarization and hyperpolarization explained

CHANGES IN MEMBRANE POTENTIAL CAN BE GRADED OR ALL-OR-NONE• Two types of membrane potentials can occur: graded or all-or-none.

• Graded membrane potentials are changes from the resting potential that are less than the threshold of –50 mV.

– Graded means any value of the membrane potential is possible

– Caused by various ion channels opening or closing

CHANGES IN MEMBRANE POTENTIAL CAN BE GRADED OR ALL-OR-NONE• Graded membrane potentials spread only a short distance.

CHANGES IN MEMBRANE POTENTIAL CAN BE GRADED OR ALL-OR-NONE• If neuron depolarizes to the –50

mV threshold, an all-or-none event occurs: an action potential is generated.

• Action potentials are not graded (always the same size) and do not become smaller, they stay the same in size as they propagate along the cell membrane.

CHANGES IN MEMBRANE POTENTIAL CAN BE GRADED OR ALL-OR-NONE

• Graded changes can give rise to all-or-none changes by being summed together; provides a mechanism for integrating signals.

• A key area for this integration is the axon hillock, where action potentials are most often generated.

• Graded changes resulting from multiple signals reaching the dendrites, spread to the axon hillock, where all the depolarizations and hyperpolarizations sum.

CONCEPT 34.2 NEURONS GENERATE ELECTRIC SIGNALS BY CONTROLLING ION DISTRIBUTIONS• An action potential (nerve impulse) is a rapid,

large change in membrane potential that reverses membrane polarity.

• The membrane depolarizes from –65 mV at rest to about +40 mV (depolarization).

• It is localized and brief but is propagated with no loss of size—an action potential at one location causes currents to flow that depolarize neighboring regions.

CONCEPT 34.2 NEURONS GENERATE ELECTRIC SIGNALS BY CONTROLLING ION DISTRIBUTIONS• When membrane potential reaches threshold, many voltage-gated Na+ channels open quickly,

and Na+ rushes into the axon.

• The influx of positive ions causes more depolarization, and an action potential occurs.

PRODUCTION OF AN ACTION POTENTIAL (PART 1)

CONCEPT 34.2 NEURONS GENERATE ELECTRIC SIGNALS BY CONTROLLING ION DISTRIBUTIONS• The axon quickly returns to resting potential:

• Voltage-gated Na+ channels close

• Voltage-gated K+ channels open slowly and stay open longer—K+ moves out

FIGURE 34.7 PRODUCTION OF AN ACTION POTENTIAL (PART 2)

PRODUCTION OF AN ACTION POTENTIAL (PART 3)

POSITIVE FEEDBACK

• Positive feedback during depolarization:

– When the membrane is partially depolarized, some Na+ channels open; as Na+ starts to diffuse into the cell, more depolarization occurs, opening more channels.

– This continues until all voltage-gated Na+

channels open and maximum depolarization occurs.

ACTION POTENTIALS

• Action potential travels in only one direction:

– After the action potential, Na+ channels cannot open again for a brief period (refractory period) and cannot depolarize.

– Thus the action potential can only propagate in the direction of the axon terminals.

ACTION POTENTIALS

• Action potentials travel faster in larger diameter axons.

• Myelination by glial cells also increases speed of action potentials.

• The nodes of Ranvier are gaps where the axon is not covered by myelin.

• Action potentials are generated only at the nodes and jump from node to node (saltatory conduction).

N E U R O N S C O M M U N I C AT E W I T H

O T H E R C E L L S AT S Y N A P S E S

3 4 . 3

NEURONS COMMUNICATE WITH OTHER CELLS AT SYNAPSES

• Neurons communicate with other neurons or target cells at synapses.

• Chemical synapse: a very narrow space between cells (synaptic cleft) that an action potential cannot cross

– When an action potential arrives at the end of the presynaptic cell, a neurotransmitter is released that diffuses across the space.

CHEMICAL SYNAPSES ARE MOST COMMON, BUT ELECTRICAL SYNAPSES ALSO EXIST• Neurotransmitters diffuse across the

synaptic cleft very rapidly (short distance).

• They bind to receptors on the postsynaptic cell membrane, which generates another action potential or other change.

• Neurotransmitters are quickly removed from the cleft—to end signal transmission—by enzymatic breakdown, uptake by other neurons or glial cells, or reuptake by the presynaptic cell.

ELECTRICAL SYNAPSE • Electrical synapse: cells are joined by gap junctions where the cytoplasm is continuous;

signals cross with essentially no delay

– They occur where very fast, invariant signal transmission is needed, such as neurons that control escape swimming in some fish.

– Also occur where many cells must be stimulated to act together, such as fish electric organs.

VERTEBRATE NEUROMUSCULAR JUNCTION IS A MODEL CHEMICAL SYNAPSE • Neuromuscular junctions: chemical synapses between motor neurons and skeletal muscle

cells.

• The axon of the presynaptic cell branches close to the muscle cell, creating several axon terminals (boutons) that synapse with the muscle cell.

NEUROMUSCULAR JUNCTIONS• An action potential causes voltage-gated Ca+ channels to open in the presynaptic membrane,

allowing Ca+ to flow in.

• This induces release of the neurotransmitter acetylcholine (ACh):

– ACh is stored in vesicles that fuse with the cell membrane to release ACh into the cleft by exocytosis.

CONCEPT 34.3 NEURONS COMMUNICATE WITH OTHER CELLS AT SYNAPSES• ACh diffuses across the cleft and

binds to receptors on the postsynaptic cell.

• These receptors allow Na+ and K+

to flow through, and the increase in Na+ depolarizes the membrane.

• If it reaches threshold, more Na+

voltage-gated channels are activated and an action potential is generated.

• Synaptic Transmission

• Neurons and Synapses

• Put some Ach into it!

MANY NEUROTRANSMITTERS ARE KNOWN• Three categories of neurotransmitters:

• Amino acids—glutamate, glycine, andγ-aminobutyric acid (GABA)

– Biogenic amines include acetylcholine, dopamine, norepinephrine, and serotonin

– A variety of peptides (strings of amino acids)

MANY NEUROTRANSMITTERS ARE KNOWN

• In the brain, a postsynaptic neuron may have chemical synapses with hundreds or thousands of presynaptic neurons, which may use different neurotransmitters.

• Receptors for a given neurotransmitter on the postsynaptic cell may be of different types with different actions.

• This complexity in synapse function helps explain the complexity of brain function.

SYNAPSES CAN BE FAST OR SLOW DEPENDING ON THE NATURE OF RECEPTORS• Two broad classes of receptors are recognized, they are fast or slow• Neurotransmitter receptors:

• Ionotropic receptors are ligand-gated ion channels—cause changes in ion movement; response is fast and short-lived.

• Metabotropic receptors are G protein-linked receptors that produce second messengers that induce signaling cascades; responses are slower and longer-lived.

Khan Academy Video

FAST SYNAPSES PRODUCE POSTSYNAPTIC POTENTIALS THAT SUM TO DETERMINE ACTION POTENTIAL PRODUCTION

• Excitatory synapses produce graded membrane depolarizations called excitatory postsynaptic potentials (EPSPs); shift membrane potential towards threshold.

• Inhibitory synapses shift membrane potential away from threshold; produce graded membrane hyperpolarizations called inhibitory postsynaptic potentials (IPSPs).

FAST SYNAPSES PRODUCE POSTSYNAPTIC POTENTIALS THAT SUM TO DETERMINE ACTION POTENTIAL PRODUCTION• Each EPSP or IPSP is usually

less than 1 mV, and disappears in 10–20 milliseconds.

• They are graded potentials, typically produced at synapses on dendrites and the cell body.

• They affect membrane potential at the axon hillock, where action potentials are generated.

• Summation of the graded potentials is both temporal (must be present at the same time), and spatial.

FAST SYNAPSES PRODUCE POSTSYNAPTIC POTENTIALS THAT SUM TO DETERMINE ACTION POTENTIAL PRODUCTION• The postsynaptic cell sums the

excitatory and inhibitory input.

• Summation determines whether the postsynaptic cell produces action potentials.

• If the sum of EPSPs and IPSPs at the axon hillock is great enough to reach threshold, an action potential is produced.

SYNAPTIC PLASTICITY • Synaptic plasticity: synapses in an individual can undergo long-term changes in functional

properties and physical shape during the individual’s lifetime.• This may be one of the major mechanisms of learning.

– Experiences at one time in life produce long-term changes in synapses, so that future experiences are processed by the nervous system in altered ways.

SYNAPTIC PLASTICITY • Sea hares (mollusks) pull their gills inside when certain parts of the body are touched:

• They withdraw their gills more vigorously if they have previously been exposed to a noxious agent (sensitization).

• The synapses between the sensory neurons and the motor neurons for gill withdrawal are functionally strengthened—more neurotransmitter is released per impulse.

• The postsynaptic cell is thus excited to a greater degree.

SYNAPTIC PLASTICITY • In mammals, the hippocampus is associated

with spatial learning and memory formation.

• In studies of mice brains, when a circuit is repeatedly stimulated, the postsynaptic structures physically grow and the synapses strengthen functionally.

• The postsynaptic receptor molecules increase, increasing response.

• Synaptic plasticity has been shown to depend on second messengers, altered protein synthesis, and altered gene transcription.

SYNAPTIC PLASTICITY • In studies of mice brains, when a circuit is repeatedly stimulated, the postsynaptic structures

physically grow and the synapses strengthen functionally.

• The postsynaptic receptor molecules increase, increasing response.

• Synaptic plasticity has been shown to depend on second messengers, altered protein synthesis, and altered gene transcription.

Synaptic Plasticity 1Brain Repair - TedEd

S E N S O R Y P R O C E S S E S P R O V I D E I N F O R M A T I O N O N

A N I M A L S E X T E R N A L E N V I R O N M E N T A N D I N T E R N A L

S T A T U S

3 4 . 4

INTRODUCTION• Animals need information about their

external environments to move, locate food, find mates, and avoid danger.

– Echolocation

– Pheromones

– Key star

• They also need information regarding internal conditions, such as partial pressure of O2 and CO2 in the blood, and tension in contracting muscles.