Nervous System FUNCTION: Senses, processes, interprets, and determines the response to stimuli from...

Nervous System

FUNCTION: Senses, processes, interprets, and determines the response to stimuli from the environment

• Central Nervous System (CNS) - made of the brain and spinal chord

• Peripheral Nervous System (PNS) - all nerve cells outside of the CNS

Divisions of the Nervous System (see pages 388-89 for more detail on each)

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“Fight or flight” “rest & digest”

voluntaryinvoluntary

Motor division - relaysinformation to organs/glands

Sensory division - receivesInformation (senses)

Cells of the Nervous System

• Neurons - cells that conduct nerve impulses (action potentials) to communicate with organs and glands

• Neuroglia (glial cells) - support, protect and nourish neurons (do not send nerve impulses





Structure of a Neuron

• Axon - transmits nerve impulses to communicate with other cells and organs

• Dendrites - receive signals from other neurons

• Myelin sheath - fatty coating on axon that speeds up action potential

• Nodes of Ranvier - gaps in the myelin sheath where the axon is exposed

• Cell body - part of neuron from which dendrites arise (also contains nucleus of cell)

• Axon terminals - end of axon/part that releases neurotransmitters to communicate with other neurons



Functional Regions of the Neuron



Receptive zone-Receivesinput from other neurons

Conducting zone-generates action potential(nerve impulse)

Secretory zone-Releasesneurotransmitters

Functional Classification of Neurons

Sensory (afferent) neurons• Receive information from the

environment (senses)Motor (efferent) neurons• Send signals to

muscles/glands/organs to carry out response

Interneurons• Relay signals between sensory

and motor neurons



Nerves vs. Neurons

Nerves are bundles

of neurons



Synapse - area where two or more neurons communicate with each other



Action Potentials

• A brief reversal in charge across a membrane

• Happens in the axon membrane at Nodes of Ranvier (Saltatory conduction)

• Voltage gated ion channels for Na+ and K+ open and close in response to changes in membrane potential (charge)



Action potentialis initiated by the axon hillock

Stages of an Action Potential

1) More Na+ outside cell/more K+ inside

2) Na+ enters the axon (charge becomes more positive - depolarization)

3) K+ leaves axon (charge becomes more negative)

4) Too much K+ has left (hyperpolarization - more negative than resting)

5) Na/K pump restores original conditions



View action potential stages in action

Neurotransmitters

• Chemical messengers that cross the synapse allowing one neuron to communicate with another

• Can be excitatory (cause post-synaptic neuron to depolarize (become more positive))

• Can be inhibitory (cause post synaptic membrane to hyperpolarize (become more negative))

Neurotransmitters (cont.)

• Stored in vesicles in axon terminals

• Ca2+ rushes into the terminal in response to arriving action potentials

• Ca2+ causes vesicles to release neurotransmitters into synaptic cleft (example)

• Neurotransmitters bind to their receptors on the post-synaptic membrane

• Neurotransmitters are broken down by enzymes in the synaptic cleft or are taken back up by the pre-synaptic neuron via transporter proteins

Neurotransmitters (cont)

Examples:

• GABA (inhibitory)

• Glutamate (excitatory)

• Dopamine, serotonin, norepinephrine (excitatory or inhibitory depending on the nature of the synapse)

• Over 50 identified

IPSP vs. EPSP

• Inhibitory post-synaptic potentials (IPSP) decrease the likelihood of the post-synaptic neuron sending an action potential (hyperpolarizes post-synaptic neuron: lets Cl- in or lets K+ out)

• Excitatory post-synaptic potentials (EPSP) increase the likelihood of the post-synaptic neuron sending an action potential (depolarizes post-synaptic neuron: lets Na+ in)



Summation

• Additive effect of the inputs of all pre-synaptic neurons

• If there are more excitatory than inhibitory signals, then depolarization may occur and an action potential may be sent by the post-synaptic neuron



Long-term Potentiation (LTP)

• LTP is the long-lasting strengthening of synapses between two neurons

• Post-synaptic neurons become more sensitive to neurotransmitters coming from the pre-synaptic neuron(s) by:

1) making more receptors

2) increased sensitivity of existing receptors

• Involved in learning and memory formation





BEFORE:Glutamate (excitatory neurotransmitter) stimulates NMDA receptors (in green) at a high frequency

AFTER:Because of the frequency of stimulation, there is an increase in the number and sensitivityof receptors on the post-synaptic neuron (increasing the strength of the synapse)

What causes this to happen?

Nervous System FUNCTION: Senses, processes, interprets, and determines the response to stimuli from...

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