Nervous System

The nervous system is broken down into two major parts:

1. Central nervous system (CNS) Brain and spinal cord

2. Peripheral nervous system (PNS) Every other neuron extending throughout the body

Nervous system is a network of cells called NEURONS All neurons are divided into three main groups: Sensory neurons Interneurons Motor neurons

Carries information about environmental changes (both inside and outside the body) to the brain and spinal cord. ( from senses) Sensory Neurons Changes are detected by receptor neurons

Interneurons Neurons that make up the brain and spinal cord. Integrates the sensory input with the motor output.

Motor Neuron Relays impulse received from interneurons Delivers impulse to effector cells (muscle cells or glands).

Sensory neuron Inter neuronMotor neuron Direction of nerve impulse

Neuron Structures Most neurons share some common features. Neuron is a living cell. It has a nucleus, organelles, and a membrane.

The nucleus and organelles of a neuron are held in the cell body.

Dendrites Short, branching projections extending away from the cell body. They conduct impulses (signals) toward the cell body

Dendrites

Axon Conducts impulses away from the cell body. Most neurons have only a single long axon.

Sciatic nerve in the leg has a single axon that can stretch to over a meter long Axon

A collection of axons that are packed together are called nerves

dendrites Cell body Axon

The axons of many neurons are surrounded by supporting cells called Schwann cells Schwann cells produce an insulating layer, myelin sheath.

Axon Schwann cell Myelin Sheath nucleus

Myelin sheathing causes the impulse to travel down the axon at a faster rate.

The impulse jump to the spaces between the Schwann cells

The spaces or gaps between the schwann cells are the Nodes of Ranvier The impulse jumps from node to node.

Axon Schwann cell Myelin Sheath nucleus Node of Ranvier

Neuron Impulse and the Action Potential

A resting neuron has an electrical charge. Charge is established due to ions located both inside and outside the axon. These ions are separated by the plasma membrane

Sodium (Na + ) and Potassium (K + ) are the main ions involved. Sodium is located outside the membrane, and potassium is located inside

The action potential is an all or nothing event. Unless enough stimulus is provided to meet the threshold level, no action potential results

Summary of Action Potential (see ½ sheet) 1) A resting neuron has Na+ outside and K+ inside. It is polarized. (due to other molecules inside – inside is negative, outside is positive) Na + K+K+ K+K+ + -

2) A nerve impulse cause Na+ gates to open so Na+ rushes inside. It is depolarized. Gates open like dominoes falling. One opens then that triggers the next gate to open. Na + K+K+ K+K+

3) This causes K+ gates to open so K+ flows out. It is repolarized but K+ and Na+ ions are in wrong places. 4) So K+ is pumped in and Na+ is pumped out. It becomes polarized and is back at resting state. Na + K+K+ K+K+ K+K+ K+K+ K+K+ K+K

Synaptic Terminals

When the impulse has reached the end of the axon, it must be passed to the next neuron. The impulse must jump the space or synapse between the cells.

The end of the axon may be branched into hundreds of ends. These endings are called synaptic terminals

Synaptic Terminals

Synaptic terminals have vesicles containing thousands of neurotransmitters. Neurotransmitters are chemicals that are released to stimulate the next neuron

Neurotransmitters are released into the synapse and diffuse across the gap to the next neuron.

Presynaptic cell Postsynaptic cell synapse neurotransmitters

When the neurotransmitters reach the next neuron, they cause the impulse to start moving down the cell body and axon.

Some estimates suggest the average person uses only 10% of their brain. It is still unknown what all the parts of the brain do.