Ch. 35.2

 Nearly all multicellular organisms have communication systems  Specialized cells carry messages from one cell to another.  NERVOUS SYSTEM CONTROLS AND COORDINATES FUNCTIONS THROUGHOUT THE BODY AND RESPONDS TO INTERNAL AND EXTERNAL STIMULI

Neurons  Electrical signals are called impulses  NEURONS  cells that transmit the electrical impulses  SENSORY: impulses from sense organs to spinal cord and brain  INTERNEURONS: connect sensory ad motor neurons

Neuron Structure  Largest part is the CELL BODY  contains the nucleus and cytoplasm  Metabolic activity takes place in the cell body  Short, branched extensions DENDRITES  carry impulses from the environment or other neurons TOWARD the cell body  Long fibers AXON  carry impulses AWAY from the cell body  Neurons may have many dendrites by only one axon  Form NERVES when axons and dendrites are clustered together

 Some neurons are surrounded by an insulated layer  MYELIN SHEATH

The Nerve Impulse  Nerve impulse is like electricity through a wire  Sodium/potassium pump  ions diffuse across the cell membrane  Builds up charges and creates a RESTING POTENTIAL

Moving Impulse  Nerve impulse begins when a neuron is stimulated by another neuron or by its environment  The protein channels help ions pass in and out of the cell  When positive ions flow in, gains a positive charge an then switches to a negative charge  ACTION POTENTIAL  One impulse causes another impulse at the next point on the membrane

Threshold  Strength of impulse is always the same  Minimum level of a stimulus that is required to activate a neuron is THRESHOLD

The Synapse  Neurons make contact with other cells at the axon terminal  Neuron transfer impulses to another cell is a SYNAPSE  NEUROTRASMITTERS  chemicals used by a neuron to transmit an impulse across a synapse to another cell