Summer PBP Program: Structure and Functions – Physiology of the Nerve Detron M. Brown, MPH

Cells of the Nervous System There are two types of cells: There are two types of cells: Neurons Neurons Glia Glia

Neuron These are known as “excitable” cells These are known as “excitable” cells They are responsible for conducting the “signal” or impulse that make the nervous system function. They are responsible for conducting the “signal” or impulse that make the nervous system function. The human brain contains approximately about 100 billion or 10 % of all nerve cells in the body. The human brain contains approximately about 100 billion or 10 % of all nerve cells in the body.

Structure of Neuron

Cell Body The largest part of the nerve. The largest part of the nerve. Its has the majority of the of the major organelles. Its has the majority of the of the major organelles. The ROUGH ER is responsible for making the ribosomes that form the nissl bodies. These provide the protein molecules needed for the transmission of nerve signals and are useful in maintaining and regenerating nerve fibers. The ROUGH ER is responsible for making the ribosomes that form the nissl bodies. These provide the protein molecules needed for the transmission of nerve signals and are useful in maintaining and regenerating nerve fibers.

Dendrites The branched part of the cell. The branched part of the cell. Receive the stimuli that initiate the nerve signals. Receive the stimuli that initiate the nerve signals. The electrical signal received is conducted toward the cell body/and or axon of the neuron. The electrical signal received is conducted toward the cell body/and or axon of the neuron.

Axon Single process Single process Stems from the tapered portion of the cell body at location called the axon hillock Stems from the tapered portion of the cell body at location called the axon hillock Axons conduct impulses AWAY from the cell body Axons conduct impulses AWAY from the cell body

Glial cells 100 billion neurons 100 billion neurons 10x more glial cells 10x more glial cells Glial cells Glial cells Support neurons (literally, provide physical support, as well as nutrients) Support neurons (literally, provide physical support, as well as nutrients) Cover neurons with myelin Cover neurons with myelin Clean up debris Clean up debris “ Housewives ” “ Housewives ”

Size of Axon and Its effect on Function Some can be a meter long and some can be as short as a couple millimeters. Some can be a meter long and some can be as short as a couple millimeters. The larger the diameter the faster the impulse. The larger the diameter the faster the impulse. Axons can be myelinated or not. Only axons can have myelin sheath. Axons can be myelinated or not. Only axons can have myelin sheath.

Neuron Classification Structural Structural Functional Functional

Functional organization of neurons Conductile component/action potential/ : Conductile component/action potential/ : Signal is electrical Signal is electrical action potentials are all or none action potentials are all or none All action potentials have same amplitude and duration All action potentials have same amplitude and duration information in the signal is represented by frequency and duration information in the signal is represented by frequency and duration Out put component : Out put component : signal chemical transmitter signal chemical transmitter total number of action potential determine how much neurotransmitter should be released total number of action potential determine how much neurotransmitter should be released

Myelin Insulates axons and facilitates speed of action potential transmission Insulates axons and facilitates speed of action potential transmission Arranged in concentric bimolecular layers Arranged in concentric bimolecular layers Has a composition similar to plasma membranes Has a composition similar to plasma membranes Schwan cells form myelin of peripheral nerves and Oligodendrocyts that of central nerves Schwan cells form myelin of peripheral nerves and Oligodendrocyts that of central nerves Schwan cells express their myelin gene in response to contact with axon while Oligodendrocytes depend also on the presence of Astrocytes Schwan cells express their myelin gene in response to contact with axon while Oligodendrocytes depend also on the presence of Astrocytes

Myelin Sheath Fatty material made by glial cells Fatty material made by glial cells Insulates the axon Insulates the axon Allows for rapid movement of electrical impulses along axon Allows for rapid movement of electrical impulses along axon Nodes of Ranvier: gaps in myelin sheath where action potentials are transmitted Nodes of Ranvier: gaps in myelin sheath where action potentials are transmitted Multiple sclerosis is a breakdown of myelin sheath Multiple sclerosis is a breakdown of myelin sheath Speed of neural impulse Ranges from 2 – 200+ mph Speed of neural impulse Ranges from 2 – 200+ mph

Ion channels Conduct ions at fast rate Conduct ions at fast rate Selective for specific ions Selective for specific ions Ion channels are proteins that span the cell membrane Ion channels are proteins that span the cell membrane Flux of ions through the ion channel is passive Flux of ions through the ion channel is passive Opening and closing of ion channel involves conformational change Opening and closing of ion channel involves conformational change Open and close in response to specific stimulus Open and close in response to specific stimulus Voltage –gated Voltage –gated Ligand –gated Ligand –gated Mechanically –gated Mechanically –gated Gap-junction channels Gap-junction channels The binding of exogenous ligands /toxins, poisons and drugs/can make channels open or close The binding of exogenous ligands /toxins, poisons and drugs/can make channels open or close Ion channels are composed of several subunits Ion channels are composed of several subunits Channels are also important targets of diseases Channels are also important targets of diseases myasthenia gravis myasthenia gravis hyperkalemic periodic paralysis hyperkalemic periodic paralysis

Figure 1: Ion Channels

Synaptic transmission The average neuron makes connections The average neuron makes connections Two basic forms of transmission Two basic forms of transmission Chemical Chemical Electrical Electrical Electrical transmissions are Electrical transmissions are Short lasting Short lasting Only excitatory Only excitatory Do not induce long lasting postsynaptic changes Do not induce long lasting postsynaptic changes Gap junction channels Gap junction channels Bidirectional transmission Bidirectional transmission

Synaptic Transmission Chemical transmissions are Chemical transmissions are Variable signaling: inhibitory or excitatory Variable signaling: inhibitory or excitatory Produce complex behavior Produce complex behavior Longer lasting / delay in transmission Longer lasting / delay in transmission Amplify signals Amplify signals Modify post synaptic receptors both functionally and anatomically Modify post synaptic receptors both functionally and anatomically Ionotropic receptors :conformational change that opens the channels on binding transmitter Ionotropic receptors :conformational change that opens the channels on binding transmitter Metabotropic receptors: act by altering intracellular metabolic reaction Metabotropic receptors: act by altering intracellular metabolic reaction

Chemical transmitters Classical Classical Acetylcholine Acetylcholine Cathecolamines Cathecolamines Glutamates Glutamates GABA GABA Serotonine Serotonine histamine histamine Peptides Peptides Substance p Substance p Enkephaline Enkephaline Endorphine Endorphine Prolactin, oxytocine, vasopresin Prolactin, oxytocine, vasopresin Soluble gases Soluble gases NO NO

Cellular basis of connectionist approach Principle of dynamic polarization : electrical signals within a nerve flow only in one direction Principle of dynamic polarization : electrical signals within a nerve flow only in one direction Principle of connectional specificity : nerve cells do not connect indiscriminately with one another to from a network Principle of connectional specificity : nerve cells do not connect indiscriminately with one another to from a network

Cont. Chemical transmitter Chemical transmitter The type of neurotransmitter they use The type of neurotransmitter they use The type of Receptors they have The type of Receptors they have Myelin content Myelin content Location in the nervous system –central/peripheral Location in the nervous system –central/peripheral These differences and others may account for different patterns of disease These differences and others may account for different patterns of disease

Neurotransmitters  chemical messengers that traverse the synaptic gaps between neurons  when released by the sending neuron, neurotransmitters travel across the synapse and bind to receptor sites on the receiving neuron, thereby influencing whether it will generate a neural impulse neurotransmitters travel across the synapse and bind to receptor sites on the receiving neuron, thereby influencing whether it will generate a neural impulse

Neurotransmitters (>60) Acetylcholine (ACh) Acetylcholine (ACh) 1 st substance identified as NT 1 st substance identified as NT Links motor neurons and muscles (contract or relax) Links motor neurons and muscles (contract or relax) e.g. curare vs black widow spider e.g. curare vs black widow spider Also involved in memory, learning, sleep, dreaming Also involved in memory, learning, sleep, dreaming (acetylcholine movie) Endorphins (the brain ’ s own morphine) Endorphins (the brain ’ s own morphine) 1973 injected rats with morphine 1973 injected rats with morphine Bound like NTs Bound like NTs Brain had receptors for exogenous substance? Brain had receptors for exogenous substance? Brain must produce its own morphine Brain must produce its own morphine Released during pain and discomfort Released during pain and discomfort

Reference 1. /bs/dent/MGDC1205/012009/data/Physiology_I _Week_4.ppt /bs/dent/MGDC1205/012009/data/Physiology_I _Week_4.ppthttp://up.edu.ps/ocw/repositories/academic/up /bs/dent/MGDC1205/012009/data/Physiology_I _Week_4.ppt 2. Human Anatomy and Physiology. Marib and Hoehn 2. Human Anatomy and Physiology. Marib and Hoehn