Chapter 2- Nerves

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Specification

(a) outline the roles of sensory receptors in mammals in converting different forms of energy into nerve impulses;
(b) describe, with the aid of diagrams, the structure and functions of sensory and motor neurones;
(c) describe and explain how the resting potential is established and maintained;
(d) describe and explain how an action potential is generated;
(e) describe and explain how an action potential is transmitted in a myelinated neurone, with reference to the roles of voltage-gated sodium ion and potassium ion channels;
(f) interpret graphs of the voltage changes taking place during the generation and transmission of an action potential;
(g) outline the significance of the frequency of impulse transmission;
(h) compare and contrast the structure and function of myelinated and non-myelinated neurones; (i) describe, with the aid of diagrams, the structure of a cholinergic synapse;
(j) outline the role of neurotransmitters in the transmission of action potentials;
(k) outline the roles of synapses in the nervous system. 

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The reflex arc

Nerve impulses do not reach the brain, instead stay in the spinal cord. Some reflex arcs do involve the brain though.

Receptor -> effector without any concious thought. 

Impulse travels along sensory neurone and passes through dorsal root ganglion. Can be passed directly to motor neurone or to a relay neurone. The impulse travels down motor neurone axon and arrives at an effector, often less than  one second after being detected by the stimulous. 

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Relay (intermediate) Neurones

Cell bodies insde the spinal cord

Function to carry impulses between other neurones. 

The axon and dendrites are had to distinguish between.

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Motor Neurones

Transmits action potentials from the CNS to an effector (eg muscle or gland)

Cell body located in spinal cord or brain

Dendrites extend from cell body, and conduct impulses towards the cell body.

Axon also extends from the cell body but conducts impulses away from the cell body.

Motor Neurones are sometimes myelinated.

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Sensory Neurones

Carry impulses via dendron from sense rgans to the brain or spinal cord. 

Cell bodies located in the dorsal root ganglia. Axon then relays impulse from cell body to CNS. 

Can be myelinated

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Resting Potentials

-70mV is the resting potential of the cell membrane in the majority of neurones. 

There are Sodium- potassium pumps in the plasma membrane which pump out 3 sodium ions for every 2 potassium ions pumped in.

Some sodium and potassium ions leak out of the cell through the plasma membrane, which is more permeable to potassium ions than it is to sodium ions. The outside of the cell has more positive ions, meaning that there's a positive charge on the outside compared to the inside. 

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The role of sensory receptors

Sensory receptors can also be known as transducers, as they convert energy from one form to another. The role of sensory receptors is to detect a change in stimulous.

Types of receptors:

Rods and Cones (eyes): Detect light intensity and the wavelength of light.
Taste buds (Tongue): Detects the presence of soluable chemicals.
Pacinian Coruscles (Skin): Detects change of pressure on the skin.
Cochlea (Ear): Detects vibrations in the air.

The bigger the stimulous the more impulses are created.
Action potentials don't change in size but messages are conveyed by the frequencey not the strength of action potentials

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Action Potentials

There are sodium/potassium pumps on a neurones membrane, and when a neurone is at rest, they are kept closed.

If some Na/K pumps are opened, then Na+ will quickly diffuse down their concentration gradient, causing depolarisation of the membrane. In pacinian corpuscles, the gates are initially opened by deformation of the neurone.

Gates further down the neurone open allowing a large influx of Na+ due to the change in potential difference across the membrane. They are called voltage gated channels.

If the change in stimulous is very small, then the voltage gated channels don't open. If the change in stimulous is large enough to reach threshold potential (difference across membrane of -50mV), then voltage gated channels are opened. This causes Na+ ions to flood in, and cause depolarisation that reaches +40mV, which is classed as an action potential. Once an action potential starts, it will continue, even with no added stimulous. Action potentials don't differ in strength, but in the number instead, which is how the intensity of a change in stimulous is.

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Action Potentials and Ionic movements

  • Membrane starts in resting state (polarised- inside -60mV compared to outside)
  • Sodium ion channel opens, and Na+ diffuses into cell
  • Causes the membrane to deploarise- reaches threshold value of -50mV
  • Voltage gated Na+ ion channels open and a larger influx of Na+ floods into the cell.
  • The more Na+ ions that enter, the more positively charged the inside becomes compared to the outside, changing the potential difference to around +40mV
  • Na+ channels close and K+ channels open.
  • K+ ions diffuse out of the cell and bring the potential difference down. This is called repolarisation
  • The potential difference overshoots slightly making the membrane hyperpolarised
  • Original potential is restored after Na+/K+ pumps restore resting potential of -60mV.

After an action potential, Na+ and K+ are in the 'wrong' place, so ionic concentrations at either side of the cell membrane muse be restored by the Na+/K+ pumps.
There is a refactory period at the end of every action potential, allowing the cell to 'recover', but also means that action potentials can only be transmitted in one direction.

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Action Potential and Ionic movement diagram

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Myelination

The axon also has Schwann cells wrapped around parts of it. Schwann cells are often referred to as a myelin sheath, and is made of lipids and proteins.

There are some spaces in between the myelin sheath called the nodes of ranvier, and occur once every 1-3 mm (Myelin sheath can also be found on sensory neurones).

Action potentials travel faster in myelinated neurones, as depolarisation only occurs at the nodes of ranvier, as local currents are elongated, so action potentials 'jump' from one node to another, known as saltatory conduction.

The cytoplasm of the neurone conducts an electrical charge so depolarises the next node, so the impulse 'jumps' from node to node.

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Synapses

An action potential arrives at the synaptic knob, causing voltage gated calcium channels to open. Calcium diffuses into the synaptic knob and cause synaptic vesicles to move towards and fuse with the pre synaptic membrane. 

Acetylcholine is released by exocytosis and diffuses across the synaptic cleft, and bind to the receptor sites on the sodium ion channels in the post synaptic membrane, causing the sodium channels to open. 

Sodium ions diffuse across the postsynaptic membrane into the postsynaptic neurone. 

This causes a generator potential (or excitatory potential) to be created. If enough generator potentials are made then threshold potential is reached and an action potential is generated. 

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The role of synapses

Synapses are extremely useful in the nervous system asmany presynaptic neurones can converge to one post synaptic neurones- this allows for action potentials from different parts of the nervous system create the same response. 

Synapses also mean that action potentials can only be transmitted in one direction, as only synaptic knobs contain vesicles of acetylcholine.

Low level action potentials can be filtered out by the synapses, as several vesicles containing acetylcholine will be released. 

Some action potentials can be amplified by a process called summation. This occurs when a low level but persistent stimulous is generating successive action potentials. This will release many vesicles of acetylcholine in a short space of time and create an action potential.

Acclimatisation: After a repeated stimulous for a long length of time, a synapse may run out of vesicles and be 'fatigued' and unable to respond to the stimuli any longer.

Specific pathways are created that are said to be the basis of concious thought and memories. 

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Neurotransmitters

'a chemical that diffuses across the cleft of the synapse too transmit a signal to the postsynaptic neurone'. They are responsible for the generation of new action potentials in the post synaptic neurone. 

Acetylcholinesterase

Acetylcholinesterase is an enzyme found in the synaptic cleft, whose role is to hydrolyses acetylcholine into ethanoic acid and choline, which stops the transmission of signals to the post synaptic neurone. The ethanoic acid and choline are recycled and re-enter the synaptic knob by diffusion. They are recombined into acetylcholine with ATP and is stored in the synaptic vessels.

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