The Nervous System

Consists of circuits of neurons and supporting cells› A Neuron is a nerve cell; the fundamental unit

of the nervous system, having structure and properties that allow it to conduct signals by taking advantage of the electrical charge across its cell membrane

› In the simplest animals with a nervous system (ex. cnidarians), the neurons controlling the contraction and expansion of their gastrovascular cavity are arranged in diffuse nerve nets

More complex animals have nerve nets as well as nerves, which are bundles of fiber-like extensions of neurons› Ex. Sea stars have a nerve net in each arm,

connected by radial nerves to a central nerve ring. This organization is better suited than a diffuse nerve net for

› controlling more › complex movements.

http://www.cartage.org.lb/en/themes/Sciences/LifeScience/GeneralBiology/Physiology/NervousSystem/NervousSystems/nervsys_1.gif

Greater complexity of nervous systems and more complex behavior evolved with cephalization› Clustering of neurons in a brain near the front

end in animals with elongated, bilaterally symmetrical bodies

http://img.sparknotes.com/101s/biology/20-1.jpg

A small brain and longitudinal nerve cords constitute the simplest clearly defined central nervous system (CNS)› Simplest nervous system exhibited in

flatworms, such as the planarian› In more complex invertebrates, behavior is

regulated by more complicated brains and ventral nerve cords containing segmentally arranged clusters of neurons called ganglia

Nerves that connect the CNS with the rest of an animal’s body make up the peripheral nervous system (PNS).

Nervous system organization correlates with animal’s lifestyle

http://img.sparknotes.com/101s/biology/20-1.jpg

http://www.cartage.org.lb/en/themes/Sciences/LifeScience/GeneralBiology/Physiology/NervousSystem/NervousSystems/nervsys_1.gif

http://upload.wikimedia.org/wikibooks/en/e/e4/Horse_nervous_system_labelled.JPG

http://www.saburchill.com/images04/291007005.jpg

Three stages of processing of information by the nervous systems: sensory input, integration, and motor output› Sensory neurons transmit info from sensors

that detect external stimuli, internal conditions, or muscle tension

› This info is sent to the CNS, where interneurons integrate (analyze&interpret) the sensory input

› Motor output leaves the CNS via motor neurons, which communicate with the effector cells (muscle cells or endocrine cells) Example: Reflexes

http://www.youtube.com/watch?v=QmNQdLkkJHM&feature=related

Most of a neuron’s organelles are located in the cell body. › Two types of extensions from cell body:

Dendrites- highly branched extensions that receive signals from other neurons

Axon- a typically much longer extension that transmits signals to other cells

Axon hillcock- conical region of an axon where it joins the cell body, typically the region where the signals that travel down the axon are generated

http://science.kennesaw.edu/~jdirnber/Bio2108/Lecture/LecPhysio/48_05NeuronStructure_L.jpg

Many axons are enclosed by a layer called the myelin sheath Near its end, an axon usually divides into several branches,

each of which ends in a synaptic terminal› Snapse- the site of communication between a synaptic

terminal and another cell Information is passed from the transmitting neuron to the

receiving cell by means of chemical messengers called neurotransmitters

The complexity of a neuron’s shape is reflects the number of synapses it has with other neurons

http://science.kennesaw.edu/~jdirnber/Bio2108/Lecture/LecPhysio/48_05NeuronStructure_L.jpg

Gila are supporting cells that are essential for the structural integrity of the nervous system and for the normal functioning of the neurons› Astrocytes- provide structural support for neurons and

regulate the extracellular concentrations of ions and neurotransmitters Helps create the blood-brain barrier, which restricts

the passage of most substances into the CNS

http://porpax.bio.miami.edu/~cmallery/150/neuro/c7.48.7.astrocytes.jpg

Radial Glia- form tracks along which newly formed neurons migrate from the neural tube (the structure that gives rise to the CNS)› Both radial glia and astrocytes can act as stem cells,

generating neurons and other glia Oligodendrocytes- glia that form the myelin sheaths around

the axon in the CNS› Schwann cells- same thing, but in the PNS› Myelin sheath provides › electrical insulation of the axon

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All cells have an electrical potential difference (voltage) across their plasma membrane.› This voltage is called the membrane

potential In neurons, the membrane potential is

typically between -60 and -80 mv (millivolts) when the cell is not transmitting signals. The (-) indicates that the inside of the cell is negative relative to the outside.

The membrane potential of a neuron that is not transmitting signals is called the resting potential› Resting potential depends on the ionic

gradients that exist across the plasma membrane.

› Example: In mammals, the extracellular fluid has a sodium ion concentration of 150 mM (millimolar). In the cytosol, the Na concentration is 15 mM. Therefore, the Na concentration gradient is 150/15 = 10.

› (Outside concentration/Inside Concentration)

When the electrical gradient exactly balances the concentration gradient, an equilibrium is established.› The magnitude of the membrane voltage at

equilibrium is called the equilibrium potential (Eion) , and is given by the Nernst equation

› Eion = 62 mV [log ([ion] outside/ [ion] inside)] The Nernst equation applies to any

membrane that is permeable to a single type of ion.

The resting potential results from the diffusion of K and Na ions channels that are always open.

Neurons also have gated ion channels, which open or close in response to three kinds of stimuli..› Stretch-gated ion channels- are found in cells

that sense stretch and open when the membrane is mechanically deformed

› Ligand-gated ion channels are found at synapses and open or close when a specific channel when a specific chemical binds to the channel

› Voltage-gated ion channels- are found in axons and open or close when the membrane potential changes

Gated ions are responsible for generating the signals of the nervous system

Action Potential- the reversal and restoration across the plasma membrane of a cell, as an electrical inpulse passes along it (depolarization and repolarization).

A stimulus strong enough to produce a depolarization that reaches the threshold triggers the action potential

When an impulse passes along the neuron, sodium and potassium ions diffuse across the membrane through voltage-gated ion channels

The electrical potential is initially reversed and then restored. This is called an action potential.

Look in AP book for detail on pg. 1018 & Allot book pg. 53

Youtube video: http://www.youtube.com/watch?v=SCasruJT-DU