Neurons and Synapses

purpose of nervous system

Coordinates actions of complex organisms via transmission of electrochemical signals that are transmitted by neurons. Neurons convert sensory information into electrical impulses in order to rapidly detect and respond to stimuli

basic structure of neurons (label picture)

• dendrites

• axon

• soma

• some have myelin sheath covering axon

dendrites

Short-branched fibres that convert chemical information from other neurons or receptor cells into electrical signals

axon

An elongated fibre that transmits electrical signals to terminal regions for communication with other neurons or effectors

soma

A cell body containing the nucleus and organelles, where essential metabolic processes occur to maintain cell survival

myelin sheath

Insulating layer around axon that improves the conduction speed of electrical impulses along axon but require additional space and energy

types of neurons

• sensory neurons - transmit information from sensory receptors to the central nervous system

• relay (interneurons) - transmit info within central nervous system as part of decision-making process

• motor neurons - transmit info from the central nervous system to effectors (muscles or glands) in order to initiate response

central nervous system

brain, spine and relay neurons

peripheral nervous system

cranial nerves, spinal nerves, peripheral nerves and motor and sensory neurons

nerve impulses

neurons generate and conduct electrical signals by pumping positively charged ions (K+ and Na+) across their membrane. Pumps out 3 K+ ions while pumps in 2 Na+ ions = creates unequal distribution on each side of membrane = creates charge difference called membrane potential

resting potential

When a neuron is at rest, the difference in electrical charge is the resting potential. Normally at resting potential inside is more negative, approx -70mmv Sodium-potassium pumps pump out 3 Ka+ ions and pumps in 2 Na+ ions maintain the difference

action potentials

rapid localised change in membrane that occur in order to generate and propagate an electrical impulse along a neuron. 3 main stages:

• depolarisation

• repolarisation

• refractory period

Action potential will occur if initial depolarisation exceeds a threshold potential of -55mv

depolarisation

as a stimulus reaches the membrane, sodium channels open allowing the Na+ to flow in, depolarising the membrane and making the inside less negative

repolarisation

Occurs quickly after depolarisation. Sodium channels close and potassium channels open allowing K+ ions to flow out which causes the membrane potential to return to a more negative potential

refractory period

the resting ionic distribution is largely reversed. the resting potential must be restored by the sodium-potassium pump. Nerve can’t fire again until resting potential is restored

propagation of action potentials

the nerve impulse (action potential) moves along axon as a wave of depolarisation. the ion-channels open due to changes in membrane potential, When one opens, it causes the channel in the next section of the axon to open and so on

oscilloscopes

measures and shows the membrane potential changes at rest and during an action potential

myelination

Some neurons are covered in myelin (a fatty, insulating substance) that increases the speed of nerve impulse transmissions by saltatory conduction.

• In unmyelinated neurons action potentials are propagated sequentially along the axon in a continuous wave of depolarisation.

• Myelinated neurons, action potentials hop between gaps in myelin sheath called nodes of Ranvier = an increase in the speed of electrical conduction by a factor of up to 100-fold

synapse

junctions between two neurons. Where action potentials are transmitted via release of neurotransmitters. When neurotransmitters diffuse across synapse to other neuron causes depolarisation in receiving neuron

synapse transmission

1. arrival of action potential at end of neuron = causes influx of calcium ions = vesicles stored near membrane releases neurotransmitters into synaptic cleft

2. neurotransmitters diffuse across synaptic cleft and bind to receptors on dendrites of postsynaptic neuron

3. sodium channels in postsynaptic membrane open, causing an influx of Na+. This response may or may not generate a new action potential and nerve impulse

4. neurotransmitter is deactivated by an enzyme located on membrane. components of neurotransmitter are actively reabsorbed back into synaptic knob and are recycled and repackaged

neurotransmitters

transmits signals between neurons by binding to receptors on post-synaptic neurons and trigger electrical impulses. Also activate responses by effector organs. May either excitatory or inhibitory in their effect

excitatory = trigger depolarisation, increasing likelihood of response

inhibitory = trigger hyperpolarisation decreasing likelihood of a response

Acetylcholine

Neurotransmitter that activates a post-synaptic cell by binding to 1 of 2 specific receptors (nicotinic or muscarinic).

why must acetylcholine be broken down

Must be continually removed from synapse as overstimulation can cause fatal convulsions and paralysis. Broken down by enzyme acetylcholinesterase (AChE) into 2 components. AChE is either released into synapse by pre-synaptic or in membrane of post-synaptic. Liberated choline is returned to the presynaptic where it reforms acetylcholine

insecticides

neurotransmitters can be used by scientists for insecticides. Most insecticides are based on excitatory neurotransmitters e.g. neonicotinoids. Neonicotinoids mimic the action of acetylcholine and bind irreversibly to the postsynaptic nicotinic acetylcholine receptor causing overstimulation of the neuron, resulting in death of insect