Chapter 2: Nerves

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Neurones Myelin Relex arc structure of a nerve Transmission of nerve impulse Types of receptors Synapes

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Human nervous system =  central nervous system + peripheral nervous system

Nerves and  central nervous system are made up of highly specialised cells called neurones.

Information is transferred along neurones by action potential (nerve impulses)These are fleeting changes in the eletricrical charge on either side of the plasma membrane

Motor Neurone

Function of motor neurone:-Transmit action potentials from the central nervous system to an effector such as a muscle or a glandLocation of motor neurones-Found in the brain or the spinal cordDendrite: (extended out from the cell body)  conducts impulses  towards the cell body (short processe)Axon: Conducts impulses away from the body, longer process too.An axon can be very long due to the cell being located in the spinal cord.

Organelles:-Usual organelles are presentE.g. Mitochondria, endoplasmic reticulum, golgi apparatusLarge amount of mitochondria are found at the tips of the terminal branches of the axon, together with many vesicles containing chemicals called neurotransmitter substances. These are involved in passing nerve impulses from the neurone to a muscle.

2 other  types of neurone:-Sensory neurone-Intermediate neurone

Sensory neurone: Carry impulses via a dendron from sense organs to brain or spinal cord. Their cell bodies are inside structures called dorsal root ganglia, just outside the spinal cord

Intermediate neurones (relay neurones):Cell bodies and their cytoplasmic processes inside the brain or spinal cord. They are adapted to carry impulses from and to numerous other neurones

Myelin SheathSchwann cells wrap themselves around the axon all along its length.How?The schwann cell spirals around. enclosing the axon in many layers if its plasma membraneThis is also known as the myelin sheath (made with large amount of lipids and some protein)

Nodes of RanvierKnown as the gaps founded along the axons where there are spaces between Schwanna cells.How? They occur every 1-3mm. One third of our motor & sensory neurons are myelinated. More sheath = higher speed of conduction for nerve impulses

What is a reflex arc and how does it work?A reflex arc is the pathway along which impulses are carried from a receptor to an effector without any conscious thought(s)Effector = part of the body that responds to a stimulusEg: Muslces and glands

The response by the effector can be extremely rapid. It's called the reflex actionIt's a really fast response that keeps us away from danger.E.g: Hand moving away from hot water after touching it via accident, moving the hand away from the stimulus

Structure of a nerveAxons of neurones are almost found in bundles. There maybe thousand of them lying side by side and surrounded by a protective covering called the perineurium.In short, it's like a cable with tons of electrical wires.

Types of nervesSome nerves contain only sensory neurones, some only motor neurones and some for a mixture of both.These nerves are known as :-Sensory nerves-Motor nerves-Mixed nervesIn each type of nerve, some of the axons are myelinated and some are not.

Transmission of nerve impulses:-Neurones transmits impulses as electrical signals. These signals travel very rapidly from one end of the neurone to the other. How then?THEY ARE NOT A FLOW OF ELECTRONS, LIKE AN ELECTRICAL CURRENTThe signals are very brief changes in the distribution of electrical charge across the plasma membranes and they are caused via very rapid movement of sodium ions and potassium ions into and out of the axon

Resting Potential:-The Na+ and K+ pump pumps out every 3 Sodium ions for 2 Potassium ions-Some of these sodium and potassium ions leaks back to where they come from through other parts of the plasma membrane.-The membrane is leakier for potassium ions than sodium ions-Hence... Inside the membrane = NEGATIVE ( - - - )                Outside the membrane = POSITIVE (+++)Difference of voltage between two sides = -70mV (millivolts)

Resting/Action potenial

Action potential:Example:Stimulus = TouchReceptor = Sensory neurone found in hand-Stimulus is seen by the receptor, some sodium channels open.-Sodium ions that had been pumped out now floods back into the cell due to electrical gradient for them

The reason behind this is because:-Electrical gradient-Chemical gradientChemical + Electrical = Electrochemical gradientElectrical gradient: The membrane has more positive charge on the outside than on the inside hence the ions move to equal out the charge on the two sidesChemical gradient:There are more sodium ions outside than inside so they diffuse inwards down their concentration gradient.

When an action potential at any point in an axon's membrane triggers the production of an action potential in the part of the membrane just next to it.

How does transmission of an action potential works?-Hence a 'local circuit' to be set up between the depolarised region and the resting regions on either sides. -This depolorises these adjoining regions and so causes voltage-gated sodium and potassium channels to open-Sodium ions flood in, later potassium ions flood out, causing an action potential.-In this way, the action potential sweeps along the membrane of the neurone.-In normal circumstances, nerve cell axons only transmit an action potenial in one direction. A 'new' action potential is created ahead of the action potential, not behind it. This is because the region behind is still recovering from the action potential it just had. The distribution of Na+ and K+ in this region is still not back in normal and therefore incapable of generating an action potentialThe time taken to recover is known as the refractory period

How action potential carry information?Action potentials are always the same size, a light touch on your hand will generate the same size of action potentials as a strong touchHowever, a strong stimulus is likely to stimulate more neurones than a weak stimulus.-Strong stimulus = stimulate more neurones  (more action potential)-Weak stimulus = stimulate less neurones   (less action potential)

(a) High frequency of impulses produced when receptor receives a strong stimulus(b) A lower frequency of impulses produced when a receptor is given a weak stimulus.Note: The size of each action potential remains the same, the only thing that change is their frequency

The brain can therefore interpret the frequencey of action potentials passing along the axon of a sensory neurone and the number of neurone carry action potentials to get information about the strength of the stimulus detected by the receptor

Speed of conduction:The speed at which an action potential sweeps along an axon is not the same for every neurone. It depends on the diameter of the axon and whether it's myelinated or not.E.g: Earthworms respond quickly due to having 'giant axons' However, humans use a different system for speeding up transmission of nerve impulses.

Speed of transmission in myelinated and non-myelinated axons of different diameters

The "jump":Myelin insulates axon, speeding up the rate of transmission of an action potential along them.Sodium and potassium ions cannot flow through the myelin sheath so it's not possible for depolarisation or action potential to occur in parts of the axon that are surrounded by it.It can only happen between the sheathe, at the nodes of Ranvier.Hence, the potential 'jumps' from one node to the next one. It's known as saltatory conduction which speeds up the transmission up to 50 times.

Saltatory conduction of an potential along a myelinated neurone

The receptors transform mechanical energy in whatever is causing the pressure to electrical energy in the neurone, acting as a transducer. Receptors all work by converting a particular form of energy into nerve impulses.

SynapsesWhen two neurones meet, they do not quite touch  each other. A small gap usually about 20nm wide is between them, known as the synaptic cleftThe parts of the neurones near to the cleft plus the cleft itself makes up a synapse

A Synapse

How impulses cross a synapse?Action potential cannot jump across synapses but the signal is passed across via a chemical known as a transmitter substance.Summary:-An action potential arriving along the plasma membrane of the presynaptic neurone causes it to release transmitter substance into the clef-The transmitter substance molecues diffuse across the cleft (takes within milliseconds due to distance)-This may set up an action potential in the plasma membrane of the post synaptic neurone

Synapses which use Acetylcholine as the transmitter substance = Cholinergic synapses

A neuromuscular junction is a synapse between the end of a motor neurone and a muscle

Recharging the synapseIf the Acetylcholine remained bound to the post-synaptic receptors , the sodium ion channels would remain open. Action potentials might fire continuously or it might be impossible to reinstate the rest potential across the membrane so that there could be no new action potential generatedWhat to do then?To avoid this from happening and also avoid wasting of acetylcholine, it is recycled.-The synapatic cleft contains an enzyme, acetylcholinesterase, which splits each acetylcholine into acetate and choline-Choline is taken back into the presynaptic neurone which is combined with acetyl CoA to for acetylcholine once more.This resynthesis requires ATP. supplied via mitochondria. The acetylcholine is then transported into the presynaptic vesicles, ready for the next action potential

Ensuring one-way transmissionSignals can only pass in one direction at synapses.Why?This ensures that signals can be directed along specific pathways rather than spreading at random through the nervous system

Interconnecting nerve pathwaysSynapses allow a wider range of behaviour than could be generated in a nervous system in which neurones were directly 'wired up' to each other. At most synapses, many different neurones converge so that different possible pathways for impulses are brought together. It may be necessary for action potentials to arrive along several neurones simultaneously before an action potential at certain synapses actually reduces the likelihood of an action potential starting up that neuroneThese are call 'inhibitory' synapses.

Effects of other chemicals at synapses:Many drugs are known for affecting events at synapses:-Nicotine-Botulinum toxin (Botox)-Organophosphorous insecticides

Nicotine:Has a molecue with a similar shape to acetylcholine which will fit into the acetylcholine receptors on post-synaptic membranes.Results:-Produces similar effects to acetylcholine, action potentials in post-synpatic neurone on muscle fibre. -Nicotine can't be broken down by enzymes and so remains in the receptors for longer than acetylcholine. Large dose of nicotine can be fatal

Botox:  Produced by anaerobic bacterium which breeds in contaminated canned foodResults-Acts at the presynaptic membrane, where it prevents the release of acetylcholine. Eating food that has this bacterium is often fatal.-However, Botox can help in medical useE.g. Someone who can't open their eyes due to muslce contraction, the toxin allows the muscles to relax

Entire sequence of events:Initial arrive of Action Potential                          to  reformation of acetylcholine                  takes    5-10ms

Insecticides:Results:Inhibit the action of acetylcholinesterase, thus allowing acetyl to cause continous production action potential in the post-synaptic membrane

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