6.5 Neurons and Synapses
How does the inherent design of neural connections imply vulnerability to the effect of chemical drugs?
How does your body effect an appropriate response to a stimulus?
To what extent can memory, identity, and learning be explained in terms of physical neurology?
Lesson One: A nerve impulse
Objectives: today we are going to learn what a nerve impulse is, how it travels through the body, and how it helps you to formulate a response.
Stimulus. Anything that is detected by the body and triggers a response eg. a loud sound.
Response. A reaction of the body to a stimulus
Stimulus-Response pathway. The way that a nerve impulse travels through the body when a stimulus – response combination occurs.
Nerve impulse. An electrical signal, which travels along an axon.
Neuron. A nerve cell. The axon is the elongated part of the cell, which carries the nerve impulse.
Myelin sheath: The insulation which surrounds the axon, made from special cells called Schwann cells wrapped around the sheath.
Nodes of ranvier: Gaps where there is no myelin.
Introduction: Watch this video on electrocution. Be ready to formulate a response to this question, what does electricity do to our bodies? How is this related to the nervous system?
Information travels through the nervous system in the form of nerve impulses. A nerve impulse is an electrical signal that travels along an axon. Nerve impulses travel through neurons, or nerve cells.
Neurons have elongated nerve fibres called axons, which are insulated with a fatty tissue, and may stretch for example – from your toes to your spinal cord. Shorter nerve fibres called dendrites, transmit nerve impulses from near by eg. in your brain or spinal cord.
The insulation is made from a fatty tissue called myelin, which forms part of the a specialised cell which wraps around the axon called a schwann cell.. It is not continuous, but has gaps called nodes of ranvier. The impulse in a myelated axon can literally jump between the nodes of ravier, speeding up nervous transmission greatly. This is called saltatory conduction (like the spanish word saltar – to jump)
Synapses represent the junctions between neurons. For a typical stimulus – response pathway, nerve impulses have to pass through three neurons. This means that two synapses are involved.
A stimulus response pathway
Illustration: in the image below, showing what happens if you prick your hand on a pin (stimulus) and withdraw it (response). Note the gaps between neurons, these are synapses.
Nerve impulse transmission – how does it work?
First of all, a simple introduction:
Impulse transmission: Detail
Step one: Normal state. This is called the resting potential. This is achieved:
- because of a sodium / potassium pump, which pumps Na+ out and K+ in
- and because the membrane of a neuron is much more permeable to K+ than Na+, accumulated K+ flows back out through the membrane
- this provides a potential difference (-70mV) where the inside of the axon is negative compared to the outside.
Step two (and three): Excitation. A nerve impulse is generated. This is called the action potential, and the nerve impulse is generated – called depolarization, and then the membrane recovers – this called repolarization.
Step two, Depolarization:
- Gated sodium channels open, allowing Na+ to flood across the membrane, causing a temporary positive charge.
Step three, Repolarization:
- Gated sodium channels close, and potassium channels open, allowing K+ to flow out across the membrane, restoring the negative charge.
- For a moment the negative charge is more than normal (hyperpolarization).
- The sodium/potassium pump restores the normal balance of ions.
How the impulse transmits:
- The influx of sodium ions allows some of the sodium ions to diffuse along the axon, disrupting the resting potential in the adjacent part of the neuron
- If enough sodium ions diffuse, this will trigger the opening of the gated sodium channels – generating an action potential in the adjacent part of the axon
- In order for this to occur, a certain voltage must be generated by the sodium, called the ‘threshold potential’.
Discussion: how does the axon achieve a resting potential?
Skill: Analysing the change in potential difference across the membrane of a neuron. You should be able to identify the different stages of impulse transmission from a graph:
- Resting potential
Notes on Reaction times mini-lab
Lesson two: Synaptic transmission
Objectives: Link synapses to memory, learning, and poison!
- Synapse: a junction between nerve cells
- Neurotransmitter: a chemical which carries a signal across a synapse
- Synaptic cleft: the space between the pre-synaptic, and the post-synaptic membrane
A nerve impulse crosses a synapse with the aid of a neurotransmitter eg. acetyl choline. That is a chemical agent that carries a signal across a synapse.
How synaptic transmission occurs:
- A nerve impulse reaches the end of the pre-synaptic membrane
- Depolarisation causes an influx of Ca2+ ions into the membrane
- Ca2+ causes the vesicles containing the neurotransmitter to fuse with the membrane (exocytosis)
- The neurotransmitter diffuses across the space (the space is called the synaptic cleft)
- The neurotrasnmitter binds to the post-synaptic membrane, triggering sodium channels to open
- Na+ floods in, triggering a new action potential in the post-synaptic membrane
- The neurotransmitter is broken down and removed.
Overview: these pesticides kill insects by blocking the neurotransmitter acetyl choline. This paralyses and effectually kills insects, but not mammals. Some people think they are harming natural honeybee populations.
Acetyl Choline is a neurotransmitter which transmits information between motor neurones and muscles. A higher percentage of synapses are cholinergic in insects (contain acetyl choline).
For acetyl choline to transmit an impulse across a synapse, it has to bond to the post-synaptic membrane receptor. It binds for a short time, and then breaks down into choline and acetate because of an enzyme called acetycholineesterase.
Neonicotinoid pesticided bind permanently to the acetyl choline receptor on the post-synaptic membrane. This blocks the binding of acetyl choline, and effectively blocks the transmission.
The result is paralysis and death for insects. Mammals are less affected as less synapses are cholinergic. A worry of using neonicotinoids pesticides is that they will also kill useful insects, like wild honeybee populations.
6.5 Synapses Checkpoint questions
Q2) Explain the effects of cocaine in terms of its action at synapses in the brain and its social consequences.
Scroll down for answer
excitatory (psychoactive) drug;
cocaine attaches to dopamine pumps/transporters (on presynaptic membrane);
blocks uptake/recycling / causes dopamine to persist in the synaptic cleft;
amplifies synaptic transmission / causes constant stimulation of postsynaptic neuron;
causes euphoria/feelings of happiness/pleasurable effects;
causes feelings of great energy/alertness/talkativeness;
addictive / causes addiction;
changes in personality / problems with family/friends/work;
crimes to pay for cost of drug/crime associated with the production/distribution;