Nervous System

Chapter 34

34.1

• Neurons and Glia

Nervous System – 2 types of cells

1. Neurons – Nerve Cells2. Glia – Glial cells

Neurons

• 4 main parts1.Cell Body2.Dendrite3.Axon4.Axon terminal

Cell body

• Contains the nucleus and the organelles• Has dendrites extending from it

Dendrites

• Extends from the cell body• Shrublike• Bring information from other neurons or

sensory cells to the cell body• Different degree of branching depending on

the type of neuron

Axon

• Long projection off cell body– Can extend for example from your spinal cord to

your toe!

• “telephone lines” of the nervous system• Act on information received by the dendrites• Generates action potentials (nerve impulse)

down the axon toward a target cell• A bundle of axons is called a nerve

Axon Terminal

• Swelling of nerve endings• Is very close to the membrane of the target

cell to form a synapse» Tiny gap across which 2 neurons communicate either

with electical signals or with chemical signals» Neuron sending the information is the presynaptic

neuron» Neuron receiving the information is the postsynaptic

neuron

Glial Cells

• More numerous than neurons• They release neurotransmitters

» Chemicals associated with nerve impulses

• Provide homeostasis for neurons by clearing the synapse of neurotransmitters

• Repair neurons and remove dead neurons• 3 different kinds of glial cells– Astrocytes, microglia, and Shwann cells

Astrocytes

• Surround the smallest, most permeable blood vessels in the brain

• Contributes to the blood-brain barrier that prevents toxic chemicals from reaching the brain (it’s not perfect)

• This barrier usually prevents antibodies from entering the brain – so the brain gets its immune defenses by another glial cell, called microglia

Microglia

• Glial cell that acts as macrophages and mediators of inflammatory responses

Schwann Cells• Forms a multilayered wrap on axons, which forms

a lipid-rich sheath called myelin.• Myelinated axons have a white appearance,

giving rise to white matter– Areas of the brain that do not appear white (areas rich

in cell bodies) area gray and are called gray matter

• Nodes of Ranvier are between the schwann cells• ALD (Lorenzo) and Multiple Sclerosis are diseases

where the myelin is affected

3 Functional Categories of Neurons

1. Afferent neurons – Carry sensory information into the nervous system (coming

from sensory cells) that transduce sensory stimuli into action potentials (nerve impulses)

2. Efferent neurons– Carry commands to physiological and behavioral effectors such

as muscles and glands (example – motor neurons carry commands to muscle cells)

3. Interneurons– Integrate and store information and communicate between

afferent and efferent neurons (most neurons in brain are interneurons)

Neural Networks

Neural Networks

Neural Networks

34.2 Electrical Signals

Action Potentials

• Nerve impulses• Carry information along neurons• Sudden and large changes in membrane

potential (difference in electrical charge across the plasma membrane) that travel along axons and cause the release of chemical signals at the axon terminal

Voltage

• Measure of the difference in electrical charge between 2 points

• Represents potential energy because opposite charges will move together if given a chance

• In wires, electrical current is carried by electrons

• In solutions and across cell membranes, electric current is carried by ions

Ions• Major ions involved are– Sodium (Na+)– Potassium (K+)– Calcium (Ca(2+)– Chloride (Cl-)

• In cells these ions are kept at different concentrations inside and outside the cell

• The result of differing these concentrations is the voltage across the cell membrane, known as a membrane potential

Resting Membrane Potential

• Occurs in an inactive neuron (not sending or receiving a signal)

• Typically between -60 and -70 millivolts (mV)– The minus sign refers to a electrically negative cell

compared to the outside of the cell

• Action Potentials (nerve impulses) are generated when there is sudden change in this voltage to where it is more positive inside than outside

• How is this done?• Sodium Potassium Pump!

Sodium Potassium Pump Review

• Form of active transport, uses ATP• Na+ ions are pumped out of the cell and exchanged for

potassium ions from the outside of the cell• Remember this exchange is uneven. The sodium

potassium pump is constantly pumping Na+ out and K+, but the concentration of Na+ is higher outside than inside and the concentration of K+ is higher inside than outside.– These concentration gradients will be used to generate the

resting potential and changes in the resting potential– How does the resting potential change?

A stimulus occurs (light, pinch, etc.)

• That triggers a voltage gated Na+ channel to open which brings Na+ ions into the cell– Sodium is going into the cell because it moves from H

L and there is more sodium outside the cell than inside the cell

• So now the inside of the cell becomes less negative (70mV 50mV) – this is called depolarization– When the inside of the neuron becomes less negative

(more positive)

What happens next?

• Additional voltage gated Na+ channels open, causing a rapid spike of depolarization – an action potential.– The action potential is traveling down the axon.

• The depolarization triggers voltage gated K+ ions to open, which allows K+ to flow out of the cell (from H L) – this is called hyperpolarized – Membrane is becoming even more negative

Chapter 49

Click below to watch the Visual Concept.

Visual Concept

Action PotentialSection 1 Neurons and Nerve Impulses

Action Potentials• Signal strength does not change during travel• All or nothing

– Positive feedback mechanism to ensure that action potentials always rise to their maximum value

• Self regenerating – 1 action potential stimulates another etc.

• Cannot go in reverse due to the refractory period (time during which membrane is returning to resting potential)

• Travel faster in myelinated axons and in larger-diameter axons– Squid axons are big, so their response time is rapid!

Communication between neurons• Once an action potential reaches the axon terminal, it releases

neurotransmitters into the synaptic cleft. These neurotransmitters bind to receptors proteins and open the ion channels of the new neuron cell.

• If enough ion channels are opened, the action potential will continue through the new neuron. If not, the nervous signal will be terminated.

• After the neurotransmitters have opened the ion channels, they will be cleared out of the synaptic cleft by being reabsorbed by the neuron that released them or broken down by enzymes.

Chapter 49

Click below to watch the Visual Concept.

Visual Concept

Release of NeurotransmitterSection 1 Neurons and Nerve Impulses

Chemical Synapse

• Neurotransmitters released from a presynaptic cell bind to receptors in the membrane of a postsynaptic cell

• The neurotransmitter used by all vertebrate neuromuscular synapses is acetylcholine (ACh)

• 7 steps

7 steps to a chemical synapse

1. Action potential arrives at axon terminal.2. Na+ channels open; depolarization causes voltage gated Ca2+

channels to open3. Ca2+ enters the cell and triggers fusion of acetylcholine vesicles

with the presynaptic membrane4. Acetylcholine molecules diffuse across the synaptic cleft and bind

to receptors on the postsynaptic membrane5. When binding occurs, they open up their channels and

depolarize the postsynaptic membrane6. The spreading depolarization fires an action potential in the

postsynaptic membrane7. Acetylcholine is broken down

Neurotransmitter Action – how does it stop?

• They must be cleared from synapse• Enzymes may destroy the neurotransmitters• Or neurotransmitters might simply diffuse

away from the cleft

Types of Neurotransmitters

• More than 50 recognized• Acetylcholine is used in the brain with motor

neurons• Others are GABA, dopamine, norepinephrine,

serotonin, endorphins

Drugs interfere

• Drugs can interfere with neurotransmitter release

• Toxins from Clostridium destroy proteins necessary for the binding of vesicles to the presynaptic membrane.

• These toxins cause botulism and tetanus – fatal diseases that involve muscle impairment due to loss of neurotransmitter release