Neurons, Neurons, Neurons! Mrs. Hartley Anatomy and Physiology

Remember:Divisions of Nervous System

Neurons Primary structural and functional unit of nerve tissue Responsive cells that conduct impulses at great speeds Cell body, axons, dendrites, myelin sheaths, synaptic terminals Direction of impulse: dendrites to cell body to axon to axon terminal to synaptic end bulb

Cell Body: cytoplasm enveloped by plasma membrane Prominent nucleus/nucleolus Many functional organelles (mitochondria, Golgi, etc.)

Axon Conducts impulses away from the cell body Terminal end communicates with the next cell

Dendrites Thin, branching extensions of cell body Receive impulses from adjacent neurons Uni-directional: only towards cell body

Myelin Sheath (pink) White, fatty insulating barrier Aids in nerve impulse conduction

What Goes Wrong in MS? When myelin is damaged, dense, scar-like tissue forms around nerve fibers throughout the brain and spinal cord. These scars, sometimes referred to as sclerosis, plaques, or lesions, can slow down or completely prevent the transmission of signals between nerve cells. Messages from the brain and spinal cord cannot reach other parts of the body. Damage, or scarring, occurs in many places throughout the central nervous system, hence the term "Multiple Sclerosis."

Nodes of Ranvier Gaps in myelin sheath Aid in impulse conduction

Information from one neuron flows to another neuron across a synapse Information from one neuron flows to another neuron across a synapse. The synapse is a small gap separating neurons. The synapse consists of: 1. a presynaptic ending that contains neurotransmitters, mitochondria and other cell organelles, 2. a postsynaptic ending that contains receptor sites for neurotransmitters and, 3. a synaptic cleft or space between the presynaptic and postsynaptic endings. http://www.blackwellpublishing.com/matthews/nmj.html

So…How Are Impulses Created? Excitability: neurons have the ability to respond when a stimulus becomes great enough to alter the resting membrane potential of a particular region of a membrane Uneven distribution of ions (electrically charged particles) of sodium and potassium causing an electrical gradient

Depolarization and Repolarization Rapid change in membrane’s permeability to sodium ions causing the electrical charge inside the cell to become more +positive Soon after, resting membrane potential is restored Potassium flows out of the cell and sodium stops flowing in http://www.blackwellpublishing.com/matthews/channel.html

Action Potential Depolarization + rapid repolarization results in an action potential or nerve impulse Rapid at 1/1000s and in rapid succession

Myelinated vs. Unmyelinated Unmyelinated: impulse continues along the length of the neuron unimpeded 10 m/s Myelinated Nodes of Ranvier Myelin disrupts impulse…saltatory conduction….impulse skips, quickening the impulse 130 m/s http://www.brainviews.com/abFiles/AniSalt.htm http://www.blackwellpublishing.com/matthews/actionp.html

So Let’s Play a Game! “Saltatory Conduction” “All or None Game” All or None response: if stimulus is large enough, impulse will be conducted along the entire length of the neuron