The Nervous System Functions of the Nervous System

• Functions

• Sensation

• Motor function

• Behavior ! social interactions

• Integration

Organization of the Nervous System

• Central Nervous System (CNS)

• brain

• spinal cord

• Peripheral Nervous System (PNS)

• 12 cranial nerves

• 31 spinal nerves

The Neuron

• Dendrites

• Cell body

• Axon

• Myelin

• Nodes of Ranvier

Structural Classification

• Multipolar Neurons

• Found in neurons which have their cell bodies in the brain or spinal cord

• Unipolar

• Found in ganglionic neurons in the brain and spinal cord

• Bipolar Neurons

• found in the retina, some parts of nose and ear

Functional Classification

• Sensory neurons (Afferent)

• Receptors CNS

• Motor neurons (Efferent)

• CNS effector organs

• Interneurons (Association neurons)

• Lie entirely within the CNS

Neuroglia• Schwann Cells

• Form myelin sheaths around axons in the PNS

• Oligodendrocytes

• Form myelin sheaths around axons in the CNS

• Microglia

• Phagocytes of the CNS

• Astrocytes

• Regulate the external environment of the CNS - ions, nutrients

Membrane Proteins Membrane Proteins

Cell Membrane and Transmembrane Proteins The cell membrane is composed of a phospholipid bilayer and has many transmembrane proteins, including different types of channel proteins that serve as ion channels.

Mechanically-Gated Channels

Mechanically Gated Channels When a mechanical change occurs in the surrounding tissue, such as pressure or touch, the channel is physically opened. Thermoreceptors work on a similar principle. When the local tissue temperature changes, the protein reacts by physically opening the channel.

Ligand-Gated Channels

Ligand-Gated Channels When the ligand, in this case the neurotransmitter acetylcholine, binds to a specific location on the extracellular surface of the channel protein, the pore opens to allow select ions through. The ions, in this case, are cations of sodium, calcium, and potassium.

Voltage-Gated Channels

Voltage-Gated Channels Voltage-gated channels open when the transmembrane voltage changes around them. Amino acids in the structure of the protein are sensitive to charge and cause the pore to open to the selected ion.

Leak Channels

Leakage Channels In certain situations, ions need to move across the membrane randomly. The particular electrical properties of certain cells are modified by the presence of this type of channel.

The Language of the Nervous System

An Observation...

Cells are (-) inside and (+) outside!!!

Cells are (-) inside and (+) outside

1. 3Na+ / 2K+ pumps

2. inability of large (-) functional groups to escape from the cell

3. Slow leak of K+ out of the cell

3Na+ / 2K+ Pump How is information encoded by the nervous system?

Coding of Information

• ON = Depolarization

• ! (inside more +)

• OFF = Hyperpolarization

• ! (inside more -)

+

+

-

--

-

-

-

-+

+

+

++

+

OFF

Coding of Information

• ON = Depolarization

• ! (inside more +)

• OFF = Hyperpolarization

• ! (inside more -)

+

+

-

-

- -

-

-

-

+

+

+

++

+

ON

Input

Information comes in the form of depolarizing or hyperpolarizing signals from many other cells

Incoming signals lose strength as they move through the cytoplasm, due to current leak and cytoplasmic resistance

(it’s just like throwing a rock in a pond)

Threshold

• Inputs vary, but the trigger zone has a THRESHOLD VALUE that is either reached or not reached

• Other threshold examples: light switch, letter grades, orgasm

Integration - Making the Decision

• Input from all the axons are summed together at the trigger zone

• If threshold is reached, the neuron fires

• If threshold is not reached, it doesn’t

The Action Potential• Depolarization causes Na+ gates to open

• Na+ floods into the cell, further depolarizing it

• At the peak, Na+ gates close and K+ gates open

• K+ floods out of the cell, re-polarizing it

Action Potentials are "all-or-none" events

• Either threshold is reached at the trigger zone (and the neuron fires) or it doesn’t

• So how do neurons code for stimulus intensity? (LIFE isn’t on/off).

Spreading the Love: Axonal Transmission

• The AP at the trigger zone initiates the spread of depolarization all the way down the axon

The End of the Line: The Synapse

Electrical Synapses Chemical Synapses

Chemical Synapses• AP arrives

• Voltage-gated Ca++ channels open and Ca++ diffuses in

• Ca++ ! calmodulin

• Calmodulin causes synapsin ! synapsin-P

• exocytosis of NT into the synapse

• NT ! receptor protein in the post synaptic cell

• An excitatory or an inhibitory signal is sent to the post-synaptic cell

How doesit end?

Message Termination

Many drugs act on chemical synapses…