1. 1 The Nervous system Dr. Paromita Das 217 Biomedical Research Facility Tallahassee FL

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3. 3 The human brain is a network of more than 100 billion individual nerve cells interconnected in systems that 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 anatomical circuits

4. 4 The nervous system has two classes of cells: 1)Neurons 2)Glia Glial cells far outnumber the neurons, they are 10-50 times 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 injury infections and disease 2) Macroglia- i) oligodendrocytes ii) Schwann cells iii) astrocytes

5. 5 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

6. 6 Functions of Glia 1)Support network for neurons 2)The two types of glia, oligodendrocytes and Schwann cells produce myelin sheath which insulate the neurons 3)They are scavengers-remove dead cells and debris from the environment 4) Release nourishing factors or growth factors which promote neuronal survival

7. 7 Neurons: A typical neuron has 4 morphologically defined regions: 1)Cell body (Soma)- contains nucleus and endoplasmic reticulum 2) Dendrites 3) The axon 4) 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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10. 10 1)Dendrites – receive electrical signals 2)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)Dendrites 2)Axon 3)Synapse- site of contact of neuron with another neuron or an effector organ 4) Myelin sheath 5) Nodes of Ranvier 6) Axon hillock 7) Presynaptic terminal 8) Postsynaptic terminal

11. 11 Neurons are functionally classified into three major catergories 1)Sensory neurons 2)Motor neurons 3)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 perception 3) interneurons: are defined as all nerve cells that are not specifically sensory or motor

12. 12 Simplex reflex arc- knee jerk response

13. 13 Communication in neurons: The shape of nerve cell is specialized for reception and transmission of information Remember: dendrites-receive electrical signals axon-propagates information When a neuron is activated, an electrical impulse is generated at the axon hillock or the initial segment The signal is then conducted along the length of the axon This electrical signal is called the action potential

14. 14 The action potential is propagated to the nerve terminal also called the presynaptic nerve terminal This then causes the release of certain chemicals called Neurotransmitters. The neurotransmitters are released into the synapse. The neurotransmitters bind to proteins on postsynaptic nerve terminals, which further propagate the electrical signal At the synapse, neurons communicate with chemical signals

15. 15 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 selectively permeable to ions Ions can flow across cell membrane through 3 types Of ion channels/proteins 1) non-gated- always open at rest 2) Ligand-gated- open in response to binding of Neurotransmitter 3) Voltage-gated –open in response to changes in Voltage across the membrane

16. 16 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 sodium while the cytosol has a high concentration of potassium Schematic on white board So, at rest, the inside of the neuron is more negative compared to the extracellular environment. The potential difference that exists due to the uneven distribution of charge is called the “resting membrane potential”

17. 17 K+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.

18. 18 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+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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20. 20 Membrane potential (E m ) 0 mV -68 mV + 40 mV Threshold 1) Depolarization Voltage-gated Na+ channels open Resting membrane potential 2) Repolarization- closing of Na+ channels and opening Of voltage-gated K+ channels resting 3) After hyperpolarization

21. 21 1) An action potential is a “all or none event” 2) The action potential is propagated without decrease in amplitude of response 3) Generally lasts for about 1 millisecond (1/1000 th 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 nature 5) An increase in membrane potential to more negative is called hyperpolarization which diminishes the cell’s ability to generate action potential. Hyperpolarization is inhibitory.

22. 22 + + + - - - + + + Local currents Local signals are propagated to the 1 st 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.

23. 23 How does action potential conduct quickly down the axon? 1)invertebrates- increase in diameter= increase in conduction 2) mammals- myelin sheath and nodes of Ranvier, “saltatory conduction” (Latin- Saltus= to jump) Evolutionary advantage

24. 24 Pre-synaptic nerve terminal Ca 2+ Voltage-gated calcium channel NT Post-synaptic nerve terminal current Na + K+K+ Neuron 1Neuron 2 synapse

25. 25 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