The Nervous System

Questions to consider in this section:- What is the function of the nervous system?- What is the function of neurons?

Terms to be aware of in this section:- Central Nervous System (CNS)- Peripheral Nervous System (PNS)- Cell Body- Dendrite- Axon

Terms continued

• Myelin Sheath• Resting potential• Action potential• Threshold• Synapse• Neurotransmitter

What is the function of the Nervous System?

Our nervous system is our window on the world. Our nervous system collects information about the internal and external environment, processes the information, and then responds to it.These functions are carried out by the central nervous system (CNS) and by the peripheral nervous system (PNS).

The Central Nervous System (the pink bits) &

The Peripheral Nervous System (the yellow bits)

The PNS input CNS output PNS

How does your nervous system do what it does?

Your PNS detects a stimulus from the environment and you either have:a) A voluntary response – one you have control

over – like finding $20 in an old jacket or,b) An involuntary response – like the response

your body has when it gets cold.Either of those responses is handled by your CNS. The CNS will then the response to the stimulus back out to the PNS. In the case of a) you would be very happy, and in the case of b) you would shiver.

How do the PNS and CNS ‘talk’ to each other?

Impulses, both incoming and outgoing, are handled by Neurons There are different types of neurons, and they are located throughout the brain and body.1) Sensory Neurons – carry impulses from sense organs to the CNS and the brain.2) Motor Neurons – carry impulses from the brain and CNS out to muscles and glands.

3) Interneurons – do the high-level work of processing information from sensory neurons and either passing it on to other interneurons, or passing it on to motor neurons.

A typical neuron

Parts of the neuron

Axon ending – the terminal end of the axon. Myelin sheath – an insulating membrane found around the axons of some neurons.Node (not labeled) – the node is the gap between each segment of myelin sheath along the axon membrane. Nerve impulses jump these gaps and this allows the impulses to travel much faster than through an axon lacking the sheath.

Parts of a neuron continued

Nucleus – the nucleus is like any other cell nucleus .Cell body – the largest part of the neuron and it contains the bulk of the cytoplasm.Dendrites – dendrites receive impulses from other neurons and carry those impulses into the cell body. Impulses travel from the dendrites into the cell body and then down the axon to the axon ends where they are picked up by other dendrites.

The Nerve Impulse

Think of a nerve impulse as an electrical current. The neuron has 2 different states:1) The resting state – the inside of a neuron has a

voltage difference of -70 millivolts (mV) from the outside. The neuron is said to be ‘at rest’. Active transport moves Na+ ions out of the cell and K+ ions into the cell. K+ ions naturally diffuse back out the cell and this creates a charge difference, with the interior of the cell becoming more negative.

The nerve impulse continued

A neuron will remain in a resting state until it receives a strong enough ‘push’ to enter:The moving impulse state – this state begins when a neuron receives a stimulus from either another neuron, or from the environment.The impulse will travel down the axon, away from the cell body, toward the axon ends. During a moving impulse, the Na+ ions suddenly rush back into the cell through open channels. The interior of the cell becomes more positive than the exterior.

NOTE: A nerve impulse is self-propagating.

Not all stimuli are sufficiently ‘strong’ to invoke a response. Neurons have a threshold that must be crossed for the neuron to ‘fire’. Once it is initiated, a nerve impulse is an all-or-nothing situation. Either an impulse is produced, or it isn’t. Our brains determine if a stimulus is weak (only 3 or 4 neurons fire, for example) or strong (many, many neurons fire at once). Think of smacking your fingers with a hammer as a strong stimulus.

Nerve synapses

Nerve synapses are those gaps between neighboring neurons. They are also called synaptic clefts. The synaptic cleft is where neurotransmitters are released. The following diagram illustrates this:

The Synaptic Cleft