1. Neurons and Nervous System
Nervous Tissue

2. Nervous Tissue
Nervous tissue is composed of two principal types of cells:
1. Neurons (nerve cells) – functional units of the nervous system. Receive sensory information, convey the information to a center (such as the brain) and conduct signals
2. Neuroglia – supporting cells; support and nourishment to the neurons

3. Neuron Cell body – contains the nucleus and other organelles
Dendrites – branched, receive signals
Axon – conveys the information
(They are bundled together to form nerves and are often called nerve fibers)
Myelin Sheath – insulating layer that covers the axon (acts as electrical insulator)
Schwann Cells – form the myelin sheath, are separated by gaps of the unsheathed axon over which the impulse travels as the signal runs along the neuron.
Nodes of Ranvier – where the gated ion channels that produce an action potential are concentrated

5. Types of Neurons
Motor (efferent) neurons – take nerve impulses from the Central Nervous System (CNS) to muscles or glands
- multipolar shape: many dendrites, single axon
Sensory (afferent) neurons – take nerve impulses from sensory receptors to CNS
- unipolar shape: cell body divides into a branch that extends to the periphery and another that extends to the CNS
Interneurons – occur within the CNS, convey nerve impusles between various parts of the CNS, some lie between sensory neurons and motor neurons
-multipolar shape

6. Transmission of the Nerve Impulses
Julius Bernstein (early 1900s) suggested that the nerve impulse is an electrochemical phenomenon involving the movement of unequally distributed ions on either side of an axonal membrane
Resting Potential
Action Potential

7. Resting Potential – when the axon is not conducting an impulse
- There is a higher concentration of sodium ions (Na+) outside the axon and higher potassium ions (K+) inside the axon.

8. Resting Potential
- Unequal distribution of the ions  due to the activity of the sodium-potassium pump
- The pump is an active transport system in the plasma membrane that pumps thee sodium ions out of and two potassium ions into the axon:
* The pump is always working because the membrane is somewhat permeable to these ions and they tend to diffuse toward their lesser concentration.
* The membrane is more permeable to potassium ions than to sodium ions

9. Action Potential – rapid change in polarity across a portion of an axonal membrane as the nerve impulse occurs.
- A gated ion channel allows sodium ions to pass through the membrane and another allows potassium ions to pass through.
- Threshold: the minimum change in polarity across the axonal membrane that is required to generate an action potential.
- Depolarization: the inside of a neuron becomes positive because of the sudden entrance of sodium ions.
- If the threshold is reached, many more sodium channels open and the action potential begins.

11. Action Potential
- In nonmyelinated axons: the action potentials travels down an axon one small section at a time (speed of about 1m/sec)
- In myelinated axons, the gated ion channels that produce an action potential are concentrated at the Nodes of Ranvier
* Ion exchange only at the nodes makes the action potential travel faster:
Saltatory conduction

12. Action Potential
- As soon as the action potential has moved on, the previous section undergoes Refractory period in which the Sodium ions are unable to open
* The Action potential cannot move backwards

13. Review

14. Transmission across a Synapse