17-1 Part I - The Nervous System Function: To coordinate the actions of your body To ensure effective behavior To maintain the internal environment within safe limits (homeostasis)

17-2 Workings of the Nervous System Messages are relayed throughout the body via electrochemical messages from the brain or through chemical messengers – hormones There are more nerve cells in the body than there are visible stars in the Milky Way! 1 cm 3 of brain tissue houses several million neurons with each connecting with several thousand others

17-3 Nervous Tissue The nervous system is divided into a - Central nervous system =brain & spinal cord and a Peripheral nervous system = nerves carrying sensory & motor information between the CNS & muscles & glands.

Central Nervous System 17-4

Peripheral Nervous System 17-5

17-6

17-7 Neuron Structure -A nerve cell is called a neuron -Neurons are composed of dendrites that receive signals, a cell body with a nucleus, and an axon that conducts a nerve impulse away. Sensory neurons take information to the CNS. Motor neurons take information from the CNS to muscles

Neurons 17-8

17-9 Parts of a Neuron dendrites – receive information conducting towards the cell body (~200 dendrites/cell body) cell body – location of the nucleus, high metabolic rate (so contains mitochondria) axon– may be 1m long, very thin, conducts the impulse towards other neurons

Sensory and Motor Neurons 17-10

17-11 Myelin Sheath nodes of Ranvier– the unmyelinated sections of a myelinated neuron, impulses “jump” between the nodes of Ranvier

17-12 The Nerve Impulse- Action Potential The nervous system uses the nerve impulse to convey information. Voltage (in millivolts, mV) measures the electrical potential difference between the inside and outside of the axon.

The Action Potential 17-13

17-14 Membrane Polarization (Resting Potential)Resting Potential When an axon is not conducting a nerve impulse, the inside of an axon is negative (-70mV) compared to the outside (+40mV); -This is the resting potential. To establish the –70mV potential in the cell: Na+ is actively pumped out of the cell K+ is actively pumped into the cell Sodium pump

The Action Potential 17-15

17-16 Membrane Depolarization When the nerve cell is excited, the membrane DEPOLARIZES (Action Potential)Action Potential The membrane’s polarity changes: – Na+ channels open, Na+ rushes in, K+ gates close The positive ions flowing in causes a charge reversal to +40 mV inside the neuron

Depolarization- Action Potential 17-17

17-18 Membrane Repolarization Once the charge becomes positive, the Na+ gates close, K+ gates open, eventually restoring the charge inside the neuron to –70 mV (but the Na+ excess is inside and K+ excess is outside!) The Na/K PumpThe Na/K Pump restores the ion concentrations inside and outside the cell

17-19 Membrane Repolarization During the repolarization, the nerve cannot be reactivated – this is called the refractory period (1 to 10 ms) and is a recovery time for the neuron The pump requires ATP in order to operate

Refractory Period 17-20

17-21 Fig Cell body Schwann cell Depolarized region (node of Ranvier) Myelin sheath Axon

17-22 Transmission Across a Synapse The junction between neurons or neurons & effectors is called the synapse. Transmission of a nerve impulse takes place when a neurotransmitter molecule stored in synaptic vesicles in the axon bulb is released into a synaptic cleft between the axon and the receiving neuron.

17-23 Synapse structure and function

17-24 Neurotransmitter Molecules Acetylcholine (ACh) Norepinephrine (NE) Neurotransmitters are removed from the synapse by the enzyme acetylcholinesterase (AChE) that breaks down acetylcholine. This prevents continuous stimulation or inhibition

Other Neurotransmitters Serotonin Dopamine GABA Glutamate *see table of neurotransmitters and their functions 17-25

Reflex Arc Diagram 17-26