conduction along nerve synapse 2013

- Conduction of - Conduction of excitation excitation

along the nervealong the nerve - Neuromuscular - Neuromuscular synapsesynapse

Laws for stimulation of excitable Laws for stimulation of excitable tissuestissues

1.1. The force law (or “all or none”)The force law (or “all or none”)

The stimulus has to have a certain minimum strength to produce excitation - thresholdthreshold

- a sub-threshold stimulus will not elicit a response

- a supra-threshold will elicit the state of excitation

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2.2. The time law (the force-time law)The time law (the force-time law)The stimulus has to act over a minimum period of time

The strength-duration curve for stimulus of an excitable tissue

3.3. The law of force gradient (the accommodation law)The law of force gradient (the accommodation law)

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The stimulus has to act suddenly

Neural accommodation or neuronal accommodation occurs when a neuron or muscle cell is depolarized by slowly rising current (ramp depolarization) in vitro.

During neuronal accommodation the slowly rising depolarization drives the activation and inactivation, as well as the potassium gates simultaneously and never evokes action potential.

1. Depolarization of the membrane under the cathode.2. Hyperpolarization of the membrane under the anode

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The action of direct current on an excitable The action of direct current on an excitable tissues. Position of the polar lawtissues. Position of the polar law

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_ _ _ _ _ _ _ __ _ _ _ ____

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cathode anode

Types of changes in membrane potentialTypes of changes in membrane potential

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1 - Cell body2 - Dendrites3 - Axon4 - Axon hillock5 - Collateral axon6 - Myelinated region7 - Node of Ranvier 8 - Terminal branches9 - Synaptic knobs

The structure of neuronThe structure of neuron

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•myelinated fibersmyelinated fibers•unmyelinated fibers unmyelinated fibers

At the nodes of Ranvier the axon membrane is not covered by myelin and is in direct contact with the extracellular fluid.

The structure of myelinated fibers The structure of myelinated fibers and unmyelinated fibers and unmyelinated fibers

Schwann cell rotates around the axon many times, laying down multiple layers of Schwann cell membrane containing the lipid substance sphingomyelin. This substance is an excellent electrical insulator.

1. Schwann cells (in peryferal NS)2. Oligodendrocytes (in CNS)

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Factors that determine the Factors that determine the conduction velocity of conduction velocity of axonsaxons

1. the resistance of the axon to the flow of electrical current along its length (its internal electrical resistanceits internal electrical resistance)

2.2. the electrical resistance of the axon membranethe electrical resistance of the axon membrane3.3. the electrical capacitance of the axon membranethe electrical capacitance of the axon membrane.

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• the electrical the electrical resistanceresistance of the axon membrane is of the axon membrane is higherhigherthan for unmyelinated fibersthan for unmyelinated fibers

• the membrane the membrane capacitancecapacitance is is lowerlower than that of than that ofunmyelinated fibers.unmyelinated fibers.

Large axons have the thickest myelin and greatest internodal distance. As a result, large large myelinated axons myelinated axons have the highest have the highest conduction velocity.conduction velocity.

Propagation of Action Potentials in Nerve Propagation of Action Potentials in Nerve FiberFiber

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Propagation of Action Potentials in Nerve Propagation of Action Potentials in Nerve FiberFiber

myelinated fibers(saltatory)

unmyelinated fibers(continuous)

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Classification of nerve fibersClassification of nerve fibers

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In the general classification the fibers are divided into types A, B and C, and the type A fibers are further subdivided into α, β, γ, δ fibers.

Laws of conduction of stimulation Laws of conduction of stimulation along nerve:along nerve:

1.1. Two-way conduction of stimulation in nerve.Two-way conduction of stimulation in nerve. (During the brining on the threshold or suprathreshold stimulus on the nerve the stimulation will spread in both sides.)

2.2. Isolated conduction of stimulation along a nerve Isolated conduction of stimulation along a nerve trunk.trunk. Action potential spreads only along each fiber and doesn’t pass to another. This is accounted that, electrical resistance along fiber lower (axoplasm and intercellular fluid are good guides for electrical current), then between next situating fibers, which divide by cell membrane.

3.3. The law of physiological and anatomical continuity of The law of physiological and anatomical continuity of a nervea nerve.. Conduction of stimulation along the nerve impossible after it’s cutting - the anatomic continuity will disturbe. The disturbing at conduction by way of using of various chemical (local anaesthetic – novocain, hypoxia, inflammation, cooling)). 13

Two-way Two-way conduction of conduction of stimulation in stimulation in

nervenerve

During the brining on the threshold or suprathreshold stimulus on the nerve the stimulation will spread in both sides

A compound action potential recorded at A compound action potential recorded at different points along an intact nervedifferent points along an intact nerve

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Each wave reflects the activity of a group of fibers with a similar conduction velocity.

Neuromuscular transmission Neuromuscular transmission • The process of transmitting a signal from a motor nerve

(motoneuronmotoneuron) to a skeletal muscle to cause it to contract is called neuromuscular transmissionneuromuscular transmission.

• The motoneuronmotoneuron and the muscle fibers it controls form a motor unit as an action potential in the main axon of a motoneuron causes the contraction of all the muscle all the muscle fibers to which it is connectedfibers to which it is connected.

• The region of contact between a motor axon and a muscle fiber is called the neuromuscular junction the neuromuscular junction or motor end-plate (neuromuscular motor end-plate (neuromuscular synapssynaps)).

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THE NEURO-MUSCULAR JUNCTIONTHE NEURO-MUSCULAR JUNCTION

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• SynapsesSynapses connect nerve cells to other nerve cells (also applies for certain muscle cells) as well as to sensory and effector cells (muscle and glandular cells).

Synaptic TransmissionSynaptic Transmission

Structure of synapse:

•presynaptic membrane,presynaptic membrane,•postsynaptic membrane,postsynaptic membrane,•synaptic cleftsynaptic cleft

Electrical synapses are direct, ion-conducting cell–cell junctions through channels (connexons) in the region of gap junctions.

Chemical synapses utilize neurotransmitters for the transmission of information and provide not only simple 1:1 connections, but also serve as switching elements for the nervous system.

Chemical Chemical synapsessynapses

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The neuromuscular junctionThe neuromuscular junction1. First, an action potential in the motor axon invades the

nerve terminal and depolarizes the membranedepolarizes the membrane.

2. This depolarization opens voltage-gated calcium opens voltage-gated calcium channels channels and calcium ions enter the nerve terminal calcium ions enter the nerve terminal by diffusion down their electrochemical gradient.

3. This leads to a rise in intracellular free calcium rise in intracellular free calcium which triggers the fusion of docked synaptic vesicles with the fusion of docked synaptic vesicles with the plasma membranethe plasma membrane.

4. The AChACh contained within the synaptic vesicles is released into the synaptic cleftinto the synaptic cleft.

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The The neuromuscularneuromuscular junction junction5. ACh molecules diffuse across the cleft and bind to thebind to the

nicotinic receptorsnicotinic receptors on the postsynaptic membrane.

6. When the receptors bind ACh they cause nonselectivecation channels to open and this depolarizes the muscle membrane in the end-plate region. This depolarization is called the end-plate potentialend-plate potential, or epp.

7. Finally, when the epp has reached thresholdthe epp has reached threshold, the muscle membrane generates an action potential muscle membrane generates an action potential that propagates along the length of the fiber. This action potential triggers the contraction of the muscle the contraction of the muscle fiberfiber.

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Ca + ions

Ионы кальция

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ACETYCHOLINE

Ion channel

Potassium ion

Sodium ion

Cholinoreceptor

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The end-plate

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Cholinaesterase

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Axon transportAxon transport• Axonal transport (or Axoplasmic) is a cellular process

responsible for movement of mitochondria, lipids, synaptic vesicles, proteins, and other cell parts (i.e. organelles) to and from a neuron's cell body, through the cytoplasm of its axon (the axoplasm).

• Transport is bi-directional– anterograde

• fast (motor: kinesins complex proteins)• slow

– retrograde• fast (motor: dynactin complex proteins)• slow

– some neurotropic viruses such as poliomyelitis, herpes, and rabies and neurotoxins that enter peripheral nerve endings and ascend to infect the cell body via retrograde transport.

Axon transportAxon transport

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Vesicular cargoes move relatively fast (50–400 mm/day) whereas transport of soluble (cytosolic) and cytoskeletal proteins takes much longer (moving at less than 8 mm/day).

conduction along nerve synapse 2013

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