Nerve Impulses About 200 a ago Luigi Galvani ( ) (Galvanometer) observed that an electrical impulse sent along the nerve of a pithed frog’s leg made it twitch. Since that time it has been known that nerve conduction is associated with electrical phenomena.

Neurons have an potential between the inside and outside of the cell. This comes about due to the differences between the composition of the inside of the cell (where, are in higher concentration) and outside the cell (where, are in higher concentration) When a is applied to these neurons, Na+ joins flow into the cell, voltage. To recover, K+ ions flow out, voltage decreases. How Do Nerve Impulses Work?

When the fibre is transmitting a signal, it is said to have a  Resting Potential is about A sodium-potassium pump maintains the resting potential that involves keeping sodium ions out and potassium ions inside the fibre. Resting Potential

5 Action Potential When a nerve cell is (by an electrical shock, heat or by just touching it) an impulse travels along its length causing a response in its connection.

Brief but rapid change in electric potential that travels along a nerve fibre As an passes by, the fibre changes from to to polarized again For a signal to be passed along a fire, there must be a stimulus that is enough to exceed a stimulation Action Potential

1.Resting: no impulse, cell is (+ on outside and – on inside) Sodium/ Potassium pump in axon using maintains this polarity (  requires energy) 2. Impulse: stimulus neuron Na + gates open Na+ goes inside, therefore inside becomes  Goes from to Depolarization occurs  Impulse moves in 3. K+ protein channels open and K+ ions move out of the fibre  Fibre now repolarized and negative charge is restored 4. Sodium-Potassium pump then moves Na+ out of the fibre and K+ into the fibre  (-70mV) How an Impulse is Transmitted

When A Stimulus Arrives and the Neuron Fires

Action Potential = maximum depolarization

Repolarization

Refractory Period = time

At rest, the inside of the neuron is slightly negative due to a higher concentration of positively charged sodium ions outside the neuron. When stimulated past threshold (about –30mV in humans), sodium channels open and sodium rushes into the axon, causing a region of positive charge within the axon. This is called depolarisation The region of positive charge causes nearby voltage gated sodium channels to close. Just after the sodium channels close, the potassium channels open wide, and potassium exits the axon, so the charge across the membrane is brought back to its resting potential. This is called repolarisation. This process continues as a chain-reaction along the axon. The influx of sodium depolarises the axon, and the outflow of potassium repolarises the axon. The sodium/potassium pump restores the resting concentrations of sodium and potassium ions

Saltatory Conduction--‐Impulse Leaps In Myelinated Neurons In vertebrates fibres (all large nerve fibres of vertabrates.) result in an impulse jumping from node to node rather than creeping along like a flame on a fuse.

Threshold Response- All or Nothing A signal be strong or weak – it either exists or it doesn’t  The response is on the number of fibres carrying a signal and the of the signals in the fibres.

When a stimulus opens Na+ gates when the neuron was at rest, then the resting potential becomes.

Firing frequencies between 100 to 700 per second can occur

Synaptic Transmission Neuron to neuron communication

23 Passage of Action Potential between cells Signals are transmitted between neurons (  ) across a specialized junction known as the by chemicals called Two neurons never touch; the synaptic cleft (synapse gap) is about 1/ mm

Two Types of Junctions: 1.Neuron – Muscle (neuromuscular) (**not called a synapse**) – Transmission from to fibres – The arrival of a nerve impulse the muscle fibre  A. P. spreads over the muscle surface resulting in the of the fibre – A neuromuscular junction is structurally similar to a synapse but functions somewhat differently

25 Two Types of Junctions: 2. Neuron - Neuron Synapse (synaptic transmission) – A motor neuron typically has about synapses – A neuron of the CNS receives many synaptic messages on its dendrites and cell body (up to 1000's) – Synaptic transmission is always direction (presynaptic neuron -> postsynaptic neuron)

26 Synaptic Transmission An impulse is transmitted from one neuron to the next through a synapse.

1.Signal arrives at the of an axon 2.Calcium ions enter the fibre (not sodium!!) 3.Calcium stimulates vesicles containing chemicals called neurotransmitters (stimulates receptor on cell) to be released by the neuron into 4. Neurotransmitter across synapse and with sites on the membrane on adjacent fibres Let’s Summarize:

Transmissions To Other Neurons

29 Neurotransmitters When the A. P. reaches the tip of an axon it causes the of tiny sacs which contain neurotransmitters Neurotransmitters are similar to, in that they are chemical messages that stimulate a receptor on a cell These vesicles dump their chemicals into the, the transmitter diffuses across the gap & combines with receptor molecules on the membrane of the post synaptic cell

1.Excitatory: stimulates of a signal Cause on the postsynaptic neuron to open  sodium enters and depolarizes the postsynaptic neuron   Signal Transmitted along neuron 2. Inhibitory: of a signal Cause on postsynaptic neuron to open K+ flows out of the neuron, creating a greater negative charge  The simultaneous release of excitatory and inhibitory neurotransmitters in a synapse is called Types of Neurotransmitters

Acetylcholine (Ach) Of the bodies 30 neurotransmitters acetylcholine is the most common Ach is an transmitter found on the end plates of many nerve cells Ach the of the postsynaptic cell -> a depolarization capable of producing an A.P. in the post synaptic neuron

Opens the sodium ion channels (postsynaptic neuron) Sodium enters and depolarizes the postsynaptic neuron  action potential created and signal is transmitted along the neuron Depolarization Acetylcholine (Ach)

33 Cholinesterase Prevents from causing repeated A. P. in the receiving cell (neurotransmitter) Released from the membrane This molecule occupies the synaptic cleft and is an enzyme that splits Ach molecules and destroys their usefulness One cholinesterase molecule can destroy 20 million Ach molecules/ minute

Some insecticides work by interfering with neurotransmitters in insects Exposure to the insecticide the release of cholinesterase, which prevents the destruction of acetylcholine. Causing the insect’s heart, which is entirely controlled by nerves, to contract and never relax

The venom of the black widow spider is called “latrotoxin”. This toxin results in a massive release of the neurotransmitter acetylcholine from the neuromuscular junctions of victims and may cause muscle spasms, pain, increased blood pressure, nausea and vomiting.

Excitatory neurotransmitters cause sodium gates on the postsynaptic neuron to open  cause depolarization and creates and action potential transmitting a signal Inhibitory neurotransmitters cause potassium gates on the presynaptic neuron to open  potassium flows out of the neuron creating a greater negative charge (hyperpolarized) Neurotransmitters can be both inhibitory and excitatory depending on the location and situation The balance of influence by excitatory and inhibitory neurotransmitters is called summation Summary

Norepinephrine Excitatory or Inhibitory neurotransmitter Stimulates sympathetic nervous system  wakefulness Secreted from the and

Excitatory neurotransmitter Effects movement and emotions Secreted from the CNS and PNS Dopamine

Inhibitory neurotransmitter Effects sleep Secreted from the CNS Serotonin

Inhibitory neurotransmitter Effects behavior Secreted from the CNS GABA (gamma-aminobutyric acid) setohome/animation/gaba- anim-main.html

Inhibitory Neurotransmitters Decrease the response of the post synaptic membrane Molecules increase the polarization of the cell making it hyperpolarized and thus unable to respond

45 Information Transmission How does your brain know that you have just driven a nail through your foot and not just scratched it?

Information Transmission The brain receives information regarding strength and type of stimulus in two ways: 1. Number of neurons depolarized The brain receives information from many sources and interprets the multiplicity of sources as a very strong stimulus 2. Repeated depolarization  The brain uses the of signals as the primary index of the strength of the stimulus (Example: # of nerve impulses arriving at a given place / unit time) Somehow the brain is able to these patterns and to convey its commands to the muscles and glands via a similar code.

Drugs & Neurotoxins Many substances, such as drugs, painkillers, chemicals, and neurotoxins, can with the functions of synapses and neurotransmitters.

Stimulant – neurotransmitter, decreases rate of of neurotransmitter or increases release of neurotransmitter Depressant – receptor site, decreases production of neurotransmitter, or increases the breakdown of neurotransmitter

The synapse is affected by certain drugs: Example: Curare - a plant extract that was used by S. A. Indians to poison the tips of arrows – Drug binds to the Ach receptors resulting in a blockage of impulse transmission => paralysis – It is now used medicinally as a muscle relaxant during surgery

50 Synapse is affected by certain drugs: Bacterial botulism - produces the most poisonous substance known – Prevents nerve endings from releasing Ach therefore paralysing the impulse – Kills by paralysis of the muscles responsible for breathing

Nerve conduct electrochemical impulses from the dendrites along the axon to the end plates of the neuron Active transport and diffusion of sodium and potassium ions establish a polarized membrane An action potential is caused by the inflow of sodium ions Nerve cells exhibit an all-or-none response Neurotransmitters allow the nerve message to move across the synapse Summary

Questions #5-7 page 420 Copy and be able to explain Table 1 on page 422 Read and answer the questions for the case study “Drugs and the Synapse” on pages Questions #2,3,4,5,6,7a, Your Task