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The Nervous system

Dr. Paromita Das

217 Biomedical Research Facility

Tallahassee FL

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•The human brain is a network of more than 100billion individual nerve cells interconnected in systemsthat construct our perceptions of the external world

• These nerve cells or neurons are the basic units of the brain-they are the functional units

• Nerve cells share the same basic architecture and yet produce complex human behavior

• This is possible due to formation of precise anatomicalcircuits

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The nervous system has two classes of cells:1)Neurons2)Glia

• Glial cells far outnumber the neurons, they are 10-50times more in number than neurons• Glial cells form the supporting network for neurons providing the brain with structure

Types of Glia:

1)Microglia- phagocytic cells which respond to injuryinfections and disease2) Macroglia- i) oligodendrocytes ii) Schwann cellsiii) astrocytes

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i) Oligodendrocytes : tightly wind around the neuron in form of a sheath also called the myelin sheath to form insulation- in the central nervous system

ii) Schwann cells: same function in the peripheral nervous system

iii) Astrocytes: Star like structure-maintain K+

concentration in extracellular fluid, essential for the function of neurons

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Functions of Glia1)Support network for neurons2)The two types of glia, oligodendrocytes and Schwann cells produce myelin sheath which insulate the neurons3)They are scavengers-remove dead cells and debris from the environment4) Release nourishing factors or growth factors which promote neuronal survival

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Neurons: A typical neuron has 4 morphologically defined

regions:1)Cell body (Soma)- contains nucleus and

endoplasmic reticulum2) Dendrites3) The axon4) The presynaptic terminals

The soma or cell body gives rise to dendrites which are short branching structures and a single axon

which is a long tubular structure

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1)Dendrites – receive electrical signals2)Axon- is the conducting unit and carries electrical signals away from the cell body to other neurons

Structure of the neuron:Know the following terminology and function :1)Dendrites2)Axon3)Synapse- site of contact of neuron with another neuron or an effector organ4) Myelin sheath5) Nodes of Ranvier6) Axon hillock 7) Presynaptic terminal8) Postsynaptic terminal

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Neurons are functionally classified into three major catergories

1)Sensory neurons2)Motor neurons3)Interneurons

1) Sensory neurons: carry information from the periphery to the brain for purpose of perception

2) Motor neurons: carry information/commands from the brain to muscle or glands (effector cells) to

respond to sensory perception3) interneurons: are defined as all nerve cells that

are not specifically sensory or motor

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Simplex reflex arc- knee jerk response

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Communication in neurons:The shape of nerve cell is specialized for reception andtransmission of information

Remember: dendrites-receive electrical signals axon-propagates information

When a neuron is activated, an electrical impulse isgenerated at the axon hillock or the initial segmentThe signal is then conducted along the length of theaxon

This electrical signal is called the action potential

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The action potential is propagated to the nerve terminalalso called the presynaptic nerve terminal

This then causes the release of certain chemicals calledNeurotransmitters. The neurotransmitters are releasedinto the synapse.

The neurotransmitters bind to proteins onpostsynaptic nerve terminals, which further propagatethe electrical signal

At the synapse, neurons communicate with chemical signals

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In order to understand how action potential is initiated/generated, we need to understand the concept of resting membrane potential

Remember: The cell membrane is selectivelypermeable to ionsIons can flow across cell membrane through 3 typesOf ion channels/proteins1) non-gated- always open at rest2) Ligand-gated- open in response to binding of Neurotransmitter3) Voltage-gated –open in response to changes in Voltage across the membrane

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At rest (when there is no electrical and chemical signaling) , non-gated channels are always open

The cell’s exterior has a high concentration of sodiumwhile the cytosol has a high concentration of potassium

Schematic on white board

So, at rest, the inside of the neuron is more negativecompared to the extracellular environment. The potentialdifference that exists due to the uneven distribution ofcharge is called the “resting membrane potential”

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K+ Na+

High K+

High Na+

Cytoplasm

Extracellular

At rest, non-gated Na+ and K+ channels always open

Lot more K+ channels than Na+ channels are open. Hence more efflux of positive ions take place. Thus net efflux of positive charge to the outside leads to a more negative charge in the intracellular side of cell membrane.

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If this process were to continue for an infinite period of time, the membrane potential would collapse. To prevent this, the Na+-K+ ATP pump actively transports 2 K+ into the cell and pumps out 3Na+ out of the cell

K+ Na+

2K+

3Na+

Na+-K+

ATPase

The Na+-K+ ATPase and the non-gated K+ and Na+

channels contribute to the resting membrane potential

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Mem

bran

e po

tent

ial (

Em)

0 mV

-68 mV

+ 40 mV

Threshold

1)

Dep

ola

riza

tion

Vo

ltage

-ga

ted

Na+

cha

nne

ls o

pen

Resting membrane potential

2) Repolarization- closing of Na+ channels and opening Of voltage-gated K+ channels

resting

3) After hyperpolarization

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1) An action potential is a “all or none event”2) The action potential is propagated without decrease in amplitude of response3) Generally lasts for about 1 millisecond (1/1000th

of a second) after which the membrane comes to rest

4) A reduction in membrane potential to more positive is called depolarization. Depolarization enhances a cell’s ability to generate action potential and hence

is excitatory in nature5) An increase in membrane potential to more negative

is called hyperpolarization which diminishes the cell’s ability to generate action potential. Hyperpolarization is inhibitory.

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+ + +- - -+ + +

Local currents

Local signals are propagated to the 1st node of Ranvier where, if the signal is strong enough, it will generate an action potential. Local signals can travel only 1-2 mm distance after which they become weak.

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How does action potential conduct quickly down theaxon?

1)invertebrates- increase in diameter= increase in conduction2) mammals- myelin sheath and nodes of Ranvier, “saltatory conduction” (Latin- Saltus= to jump) Evolutionary advantage

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Pre-synaptic nerve terminal

Ca2+

Voltage-gated calcium channel

NT

NT

NT

NT

Post-synaptic nerve terminal

current

Na+ K+

Neuron 1 Neuron 2

synapse

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How will the next neuron respond?• Depends on which neurotransmitter activates which receptor on the recipient neuron

A) Excitatory neurotransmitter receptors i) glutamate ii) aspartate B) Inhibitory neurotransmitter receptors i) GABA (valium, alcohol) ii) glycine