Neurons and Nervous System

Nervous Tissue

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

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

http://sandiegooillovers.com/Neurons.html

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

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

• Resting Potential – when the axon is not conducting an impulse

http://www.google.com/imgres?imgurl=http://media.tumblr.com/tumblr_lks1yl1d331qc9f5v.jpg&imgrefurl=http://captain-nitrogen.tumblr.com/post/5244013529/pick-your-poison&h=395&w=500&sz=29&tbnid=VHJ3ofHjqbN6gM:&tbnh=89&tbnw=113&prev=/search%3Fq%3Ddiagram%2Bof%2Bresting%2Bpotential%2Bneuron%26tbm%3Disch%26tbo%3Du&zoom=1&q=diagram+of+resting+potential+neuron&usg=__2InKREgV8dFJV4A9vxt908gvUE4=&docid=ZDV-RySyCSQ25M&hl=en&sa=X&ei=HFxGUZCIFIri4AOmrYDoBA&ved=0CEIQ9QEwBg&dur=448

- There is a higher concentration of sodium ions (Na+) outside the axon and higher potassium ions (K+) inside the axon.

• 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

• 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.

http://bioserv.fiu.edu/~walterm/GenBio2004/new_chap45_nervous/sp07_nervous_system.htm

• 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

• 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

Review

http://www.brighthubeducation.com/science-homework-help/62839-function-and-structure-of-neurons-in-the-brain/

Transmission across a Synapse