Chapter 7 – Part 2The Nervous System

Axons and Nerve ImpulsesAxons and Nerve Impulses Axons end in axonal terminals Axonal terminals contain vesicles that contain

chemicals called neurotransmitters When the impulses reach the axonal

terminals, they stimulate the release of neurotransmitters into the extracellular space.

Axonal terminals are separated from the next neuron by a gap Synaptic cleft – gap between adjacent

neurons Synapse – junction between nerves

Myelin Myelin

Most long nerve fibers are covered with a whitish, fatty material called myelin Has a waxy appearance Protects and insulates the

fibers Increases the transmission

rate of nerve impulses Myelin sheaths – a tight coil

of wrapped membranes that encloses the axon

Nerve Fiber CoveringsNerve Fiber Coverings

Schwann cells – produce myelin sheaths in jelly-roll like fashion in axons outside the CNS

Nodes of Ranvier – gaps in myelin sheath along the axon

Multiple Sclerosis (MS)Multiple Sclerosis (MS)

The myelin sheaths around the fibers are gradually destroyed and converted to hardened sheaths called sclerosis.

As this happens, the circuit is short-circuited.

The affected person loses the ability to control his or her muscles and becomes increasingly disabled.

Is an autoimmune disease in which a protein component of the sheath is attacked.

Neuron Cell Body LocationNeuron Cell Body Location

Most are found in the CNS Nuclei – clusters of cell bodies within the white

matter of the CNS Well-protected location within the bony skull or

vertebral column is essential Neurons do not routinely undergo cell division

after birth If it is damaged the cell dies and is not replaced

Some are found outside the CNS Ganglia – collections of cell bodies outside the

CNS

Nerve FibersNerve Fibers

Tracts – bundles of nerve fibers (neuron processes) running through the CNS

Nerves – bundles of nerve fibers (neuron processes) running through the PNS

White matter – consists of dense collections of myelinated fibers (tracts)

Gray matter – contains mostly unmyelinated fibers and cell bodies

Functional Classification of NeuronsFunctional Classification of Neurons

1. Sensory (afferent) neurons Carry impulses from the sensory receptors

to the CNS Keep us informed about what is happening

both inside and outside the body The dendrite endings of the sensory neuron

are usually associated with specialized receptors.1. Cutaneous sense organs – found in the skin

2. Proprioceptors – detect stretch or tension in the muscles and tendons

Functional Classification of NeuronsFunctional Classification of Neurons

2. Motor (efferent) neurons Carry impulses from the CNS to the

muscles and glands

3. Interneurons (association neurons) Found in neural pathways in the CNS Connect sensory and motor neurons

Neuron ClassificationNeuron Classification

Structural Classification of NeuronsStructural Classification of Neurons

Multipolar neurons – many extensions from the cell body

Most common type: all motor and association neurons are multipolar

Structural Classification of NeuronsStructural Classification of Neurons

Bipolar neurons – Neurons with two processes - one axon and one dendrite

Rare in adults

Found only in some special sense organs such as the ear and the eye, where they act as sensory receptor cells

Structural Classification of NeuronsStructural Classification of Neurons Unipolar neurons – have a short single

process leaving the cell body The single process is short and divides

almost immediately into central and peripheral fibers.

In this case, the axon conducts nerve impulses both toward and away from the cell body.

Functional Properties of NeuronsFunctional Properties of Neurons

Neurons have two major functional properties:

1. Irritability – ability to respond to stimuli

2. Conductivity – ability to transmit an impulse

Plasma Membrane of a Resting Plasma Membrane of a Resting NeuronNeuron

The plasma membrane at rest is polarized. Fewer positive ions are inside the cell than

outside the cell. The major positive ions inside the cell are

potassium (K+). The major positive ions outside the cell are

sodium (Na+). As long as the inside remains more negative

as compared to the outside, the neuron will stay inactive.

StimulusStimulus

Many different stimuli excite neurons to become active and generate an impulse. Light excites the eye receptors, sound excites

some of the ear receptors, and pressure excites some cutaneous receptors of the skin.

Most neurons in the body are excited by neurotransmitters released by other neurons.

Regardless of what the stimulus is, the result is always the same – the “sodium gates” in the membrane open allowing an inward rush of sodium ions.

Starting a Nerve ImpulseStarting a Nerve Impulse• A stimulus depolarizes the

neuron’s membrane.• Depolarization – the loss of

a negative charge inside the plasma membrane.

• A depolarized membrane allows sodium (Na+) to flow inside the membrane.• The inside becomes more

positive.

• The exchange of ions initiates an action potential in the neuron.

The Action PotentialThe Action Potential

If the action potential (nerve impulse) starts, it is propagated over the entire axon.

RepolarizationRepolarization Almost immediately after the Na+ rush into the

neuron, the membrane permeability changes again: It becomes impermeable to Na+, but permeable to K+.

K+ rush out of the neuron, which repolarizes the membrane

Repolarization - the outflow of positive ions, which restores the electrical conditions at the membrane to the resting state.

Until repolarization occurs, a neuron cannot conduct another impulse

RepolarizationRepolarization Until repolarization occurs, a neuron cannot

conduct another impulse Refractory Period – Period of repolarization of the

neuron during which it cannot respond to a second stimulus

The Sodium Potassium PumpThe Sodium Potassium Pump The sodium-potassium pump restores the

original configuration of sodium and potassium ions inside and outside the neuron. This action requires ATP.

Nerve Impulse PropagationNerve Impulse Propagation

The impulse continues to move toward the cell body

Impulses travel faster when fibers have a myelin sheath Nerve impulses literally

jumps or leaps from node to node along the fiber.

No current can flow across the axonal membrane where there is fatty myelin insulation.

Blocking Nerve Impulse ConductionBlocking Nerve Impulse Conduction It is possible to block nerve impulses by reducing

membrane permeability to sodium ions. No sodium entry = no action potential Alcohol, sedatives, and anesthetics all do this.

It is also possible to hinder impulse conduction by interrupting blood circulation (interrupt the delivery of oxygen and nutrients). Examples:

1. Cold – fingers get numb when you hold an ice cube.2. Continuous pressure – when you sit on your foot, it

“goes to sleep” When you warm the fingers or remove the pressure from

the foot, the impulses begin to be transmitted again, leading to an unpleasant prickly feeling.

Continuation of the Nerve Impulse Continuation of the Nerve Impulse between Neuronsbetween Neurons

Impulses are able to cross the synapse to another nerve

Neurotransmitter is released from a nerve’s axon terminal

The dendrite of the next neuron has receptors that are stimulated by the neurotransmitter

An action potential is started in the dendrite

How Neurons Communicate at How Neurons Communicate at SynapsesSynapses