C2.2 Neural signaling*

neurons definition

Specialised cells in the nervous system which carry electrical impulses through the body

nervous system function

converts sensory information into electrical impulses in order to rapidly

detect and respond to stimuli.

neuron structure

dendrite

soma (cell body) 

node of ranvier

schwann cells

axon terminals 

axon 

myelin sheath

dendrite

Shorter fibres which project from the cell body to communicate with other neurons

axon

Long single fibre; can be myelinated or unmyelinated

soma (cell body)

Cytoplasm and nucleus of the neuron.

nerve impulses simple process

Start at the dendrite on a neuron

Propagate along the axon

End at the axon terminal

Move over to the next neuron across a synapse

Only move in one direction

Before a nerve impulse is initiated in a neuron, it is at resting potential

resting potential

• when neuron is not conducting a nerve impulse but is reading to conduct one

• membrane potential due to an imbalance of positive and negative charges across membrane

• using sodium potassium pump

• pumps 3 sodium ions out and 2 potassium ions in neuron

• creates concentration gradient

• membrane also much more permeable to potassium so potassium diffuses back out neuron through leak channels

• hence sodium concentration is much greater outside neuron

• also negatively charged ions permentaly located in cutoplasm of neuron

• together create a resting membrane potential of -70mV inside neuron

Action Potential- Depolarization

• sudden change from a negative to positive internal charge.

• In response to a signal initiated at a dendrite, sodium channels open within the membrane of the axon 

• As Na+ ions are more concentrated outside of the neuron, the opening of sodium channels causes a passive influx of sodium

• The influx of sodium causes the membrane potential to become more positive (depolarization).

action potential definition 

rapid changes in charge across the membrane that occur when a neuron is firing.

includes depolarization repolarization hyperpolarization

During repolarization, the membrane cannot depolarize again, to ensure what

ensuring that action potentials cannot be propagated backwards along the axon.

Action Potential-Repolarization

• Refers to the restoration of a membrane potential following depolarization, restoring a negative internal charge

• Following an influx of sodium, potassium channels open within the membrane of the axon

• As potassium ions are more concentrated inside of the neuron, the opening of sodium channels causes a passive efflux of potassium

• efflux of potassium causes the membrane potential to return to a more negative internal differential (repolarization).

Action Potential-Refractory Period hyperpolarisation 

• Refers to the period of time following a nerve impulse before the neuron is able to fire again.

• In a normal resting state, sodium ions are predominantly outside the neuron and potassium ions mainly inside (resting potential)

• Following depolarization (sodium influx) and repolarization (potassium efflux), this ionic distribution is largely reversed.

• Before a neuron can fire again, the resting potential must be restored via the antiport action of the sodium- potassium pump.

speed of nerve impulse dependent on

Degree of myelination

Diameter of axon

myelin increase speed because

Myelin acts as insulation, preventing ions from leaking

Permits saltatory conduction, which enables impulse to “jump” along axon

saltatory conduction 6 marker

Nerve fibers conduct electrical impulses along the length of their axons. Some of these axons such as interneurons are unmyelinated, and therefore the impulse travels much slower.

• The greater the diameter, the greater the speed of the nerve impulse.

• Some axons are surrounded by a mixture of protein and phospholipids called myelin that collectively form a myelin sheath.

• Many layers of myelin are deposited around the axon by special cells called schwann cells

• The myelin sheath insulates the axon and greatly increases the speed of the nerve impulse.

• In between the myelin are gaps called the node of ranvier 

• In myelinated neurons, the impulse can jump from one node to the next. This is called saltatory conduction

• This allows myelinated neurons to conduct impulses up to 100x faster than unmyelinated axons.

if diameter great speed is

less rebounce = greater the speed

synapses

junctions between cells in the nervous system

They are tiny, fluid-filled gaps approximately 20 nm wide

where can sypases be found 

Sensory receptor cells and neurons

Neurons in the CNS

Neurons and effector cells (muscles or glands)

A signal can only pass across a synapse in one direction - from

from the presynaptic neuron to the postsynaptic neuron

Neurotransmitters where it exist 

exists in many types of synapse, including neuromuscular junctions

neurotransmitters definition

chemicals diffused across a synapse from pre-synaptic membrane to post-synaptic membrane to send a signal to the next cell.

synaptic transfer happens after

an action potential reaches the axon terminal?

synaptic transfer process

• As the nerve impulse reaches the axon terminal of the pre-synaptic neuron, the positive charge from the depolarization causes voltage-gated channels permeable to Ca2+ to open.

• Ca2+ flows into the pre-synaptic neuron, increasing the amount of Ca2+ in the pre-synaptic neuron.

• This Ca2+ causes vesicles containing neurotransmitters to bind to the membrane and release their neurotransmitters into the synaptic cleft

• These neurotransmitters diffuse across the synaptic cleft and bind to receptor sites on the membrane of the post synaptic neuron.

• The binding of these neurotransmitters open ion channels allowing ions such as Na+ to diffuse into the post synaptic neuron.

• This influx of positive charge possibly leads to an action potential and a depolarization in the post synaptic neuron.

• The neurotransmitter is reabsorbed by the pre-synaptic neuron or broken down in the synapse by enzymes.

The binding of neurotransmitters to transmembrane receptors to create

excitatory postsynaptic potential

binding sites on postsynaptic membranes are 

specific 

neurotransmitter can do what 2

They maintain signals in the nervous system by binding to receptors on post-synaptic neurons and triggering electrical impulses.

• They also activate responses by effector organs (such as contraction in muscles or hormone release from endocrine glands).

A stimulus – known as the threshold (-55mV) – is the

level required to open voltage gated ion channel 

• If the threshold potential is not reached,

an  action potential cannot be generated and hence the neuron will not fire.

PNS composed

peripheral nerves

links CNS to receptors and effectors

neurotransmitters may be either what in effect

excitatory -trigger depolarisation - increasing likelihood of response eg ach

inhibitory - trigger hyperpolarization - decreasing

detail example of neurotransmitter

acetycholine

use by both cns pns

release at neuromuscular junctions and binds to receptors on muscle fibres to trigger muscle contraction

release within automatic nervous system to promote parasympathetic response (rest and digest)

created in axon terminal by combining cholin with an acetyle group

stored in vesicles within axon terminal until releases via exocytosis in reponse to a nerve impulse

neonicotinoids (pesticides) example

bind to acetylcholine receptors in cholinergic synapses in CNS of insects

Acetylcholinesterase does not break down neonicotinoids—binding irreversibly.

Acetylcholine now can't bind, and neural transmission is stopped

insects paralysis and die

benefit—effective and not highly toxic to human and mammals

• also affects beneficial insects such as honeybees.

all or none principle

• This minimum stimulus – known as the threshold (-55 mV)—is the level required to open voltage-gated ion channels.

• If the threshold potential is not reached, an action potential cannot be generated and hence the neuron will not fire.

Threshold potentials are triggered when the combined stimulation from the dendrites exceeds a minimum level of depolarization.

• If the overall depolarization from the dendrites is sufficient to activate voltage-gated ion channels in one section of the axon, the resulting displacement of ions should be sufficient to trigger the voltage-gated ion channels in the next axon section.