neurons

• Neurons communicate with each other via synaptic transmission. Each individual neuron is separated from the next at a synapse. The synapse is made up of the space between neurons (synaptic cleft), the end of one neuron (presynaptic neuron) and the start of another neuron (postsynaptic neuron). Signals within neurons are sent electrically, but signals between neurons are sent chemically (by neurotransmitters).
• Release of neurotransmitters

  electrical signal reaches the end of a neuron, it arrives at the terminal button. Vesicles which contain neurotransmitters. electrical signal causes the vesicles to release neurotransmitter molecules. These travel across the synaptic cleft to the next neuron in the chain.
• Reuptake of neurotransmitter

  Once the transmitter crosses the synaptic cleft, it attaches to the next neuron at the postsynaptic receptor sites which are located on the dendrites of the next neuron. chemical message is turned back into an electrical impulse.

  The chemical neurotransmitter left in the gap is broken down + reabsorbed by the presynaptic neuron
• Excitation and inhibition

  Neurotransmitters can have an excitatory or inhibitory effect on the next neuron in the chain. For example, adrenaline as a neurotransmitter generally causes excitation of the postsynaptic neuron. This increases the neuron's positive charge and makes it more likely to fire.

  Other neurotransmitters, such as serotonin, generally increase the negative charge of the postsynaptic neuron. This is inhibition which makes the neuron less likely to fire.
• The synapse is the area at the end of two neurons. The actual gap between two neurons is called the synaptic cleft.

  One neuron is called the presynaptic neuron. This is the neuron that is transmitting the message. The end of the neuron is called the terminal button.

  The other neuron is called the postsynaptic neuron. This is the

  neuron that is receiving the message. There are postsynaptic receptor sites on the postsynaptic neuron that receive the neurotransmitters.

  
• Neurons are nerve cells that process and respond to stimuli in our environment. They communicate using electrical and chemical signals
• Sensory neurons - carry messages from PNS to CNS; messages to the brain about the environment by processing information from the senses
• Relay neurons - connect sensory neurons to motor and other realy neurons; carry messages from one part o f the CNS to another
• Motor neurons - connect the CNS to effectors such as muscles/glands; helps organs and muscles function and move
• What do neurons look like?

  Sensory neurons - cell body on axon (middle), myelin sheath

  Relay neurons - no dendrite on cell body, no myelin sheath, receptor cell

  Motor neurons - cell body on head, myelin sheath, myelin sheath, motor end plate,

  All 3 have axon terminal
• All neurons have

  • A cell body which contains a nucleus
  • Dendrites which protrude from the cell body. These carry nerve impulses from neighbouring neurons towards the cell body (receptor sites)
  • Axons which carry the impulse away from the cell body down the neuron
  • An axon terminal which communicates with the next neuron in the chain across the synapse 
• Sensory neurons found in the hand recognise that there is pain being felt in the finger caused by the heat of the candle. This message is sent on to relay neurons in the spinal cord CNS

  Relay neurons in the CNS spinal cord receive the information from the sensory neurons and pass these messages on to the motor neurons PNS with the instruction that the hand should move

  Motor neurons in the PNS receive the message from the relay neurons and cue the arm the effector to move in order to stop the feeling of pain
• The neuron originally sending a message is called a presynaptic neuron. The one that then receives the message is called a postsynaptic neuron. It is the axon terminal of a presynaptic neuron that meets the dendrites of a postsynaptic neuron at a synapse.

  However, because neurons connect in chains, neurons that are postsynaptic because they receive a message then pass this message on, making them presynaptic at the next synapse.
• Each time a neurotransmitter is picked up at a receptor site, it affects whether the postsynaptic neuron sends the message on. The effect depends on whether the neurotransmitter is excitatory or inhibitory.

  Often more than one neurotransmitter is sent at once, some that might be excitatory and some that might be inhibitory (E.g. sending serotonin and adrenaline). In this case, the influence is summed (a process called summation).
• If there is more inhibitory neurotransmitters, it will be less likely to fire (it will not pass on another electrical impulse) – known as inhibitory post synaptic potential (IPSP)

  If there is more excitatory neurotransmitters it will be more likely to fire (it will pass on another electrical impulse) – known as excitatory post synaptic potential (EPSP)
• For a neuron to generate another electrical impulse it must reach a -55mv threshold. Resting potential of neurons is usually -70mv.

  Inhibitory neurotransmitters will make the neuron more negative (IPSP) and therefore move it further away from the -55mv threshold and less likely to fire an impulse. Excitatory neurotransmitters will make the neuron less negative (EPSP) and therefore move it closer to the -55mv threshold and more likely to fire an impulse.
• Acetylcholine (ACh)

  Activates motor neurons; makes muscles contract; associated with attention, arousal, learning, and memory; people with Alzheimer's disease are usually found to have a substantially low level of this.
• Serotonin

  Effect on emotion, mood, and anxiety; involved in regulating sleep, wakefulness, and eating; significantly low level of this is believed to be associated with conditions like depression, suicidal thoughts, and obsessive compulsive disorder.
• Dopamine

  Controls voluntary movements; associated with the reward mechanism of the brain (regulates pleasurable emotions); low level is associated with Parkinson's disease; high level associated with hallucinations + delusions ~of schizophrenia.
• The sympathomedullary pathway (our response to a threat)

  • Hypothalamus recognises that there’s a threat and activates the sympathetic division of the ANS
  • The sympathetic division activates adrenal glands especially the adrenal medulla and causes them to release adrenaline.
  • Adrenaline travels through the endocrine system and noradrenaline travels to the brain
  • Sympathetic activation symptoms occur such as increased breathing, increased heart rate and sweating
• Neurons can only transmit information in one direction for the following reasons

  • The synaptic vesicles containing the neurotransmitter are only present on/released from the presynaptic membrane
  • The receptors for the neurotransmitters are only present on the postsynaptic membrane
  • It is the binding of the neurotransmitter to the receptor which enables the signal/information to be passed/transmitted on (to the next neuron).
• Zap acts like an inhibitory neurotransmitter

  Explain how Zap might affect the process of synaptic transmission through inhibition

  when an inhibitory neurotransmitter binds to the post-synaptic receptors it makes the post-synaptic cell less likely to fire (IPSP) Summation – if inhibitory inputs are higher than excitatory they can cancel out excitation and inhibit an action potential occurring

  Zap would decrease the overall activity

  Zap would make the post-synaptic cell less likely to fire, reducing brain activity may lead to reduced pain.
• Synaptic transmission:

  Nervous impulse reaches presynaptic neurone.

  This stimulates synaptic vesicles, containing neurotransmitters, in the presynaptic neurone, to migrate to the end of the presynaptic neurone, and fuse, causing neurotransmitter to be released into the synaptic cleft. Neurotransmitters diffuse across the synapse and bind to specific receptors on the post-synaptic neurone. This causes either an inhibitory or excitatory effect on the post-synaptic neurone. After a certain amount of time, the neurotransmitters get broken down by specific enzymes, and the products are reabsorbed into the pre-synaptic neurone to be re-synthesised into new neurotransmitters.