5.1.3: Neuronal communication Flashcards
Why do multicellular organisms require a communication system?
- Distance: cells monitoring change far away from those responding to it.
- Specialisation: cells are specialised to do a particular job (i.e. either detect or respond to stimuli)
- Seasonality: need to produce a coordinated response e.g. detect increased day length to flower at right time.
Homeostasis
the maintenance of a constant internal environment despite external changes; communication between cells allows multicellular organisms to achieve this.
Nerve impulse
Action potentials propagated along the axon of a neurone.
Nerve impulses can be described as “__-or-______”
All-or-nothing i.e. action potentials are always the same size
How is a response to a stimulus coordinated?
Coordination relies on communication between cells known as cell signalling, either between adjacent cells or cells that are far apart.
Action potential
The reversal and restoration of the electrical potential across the plasma membrane as an electrical impulse passes along the axon.
Membrane potential
voltage inside the axon compared to that outside the axon.
Sequence of events in an action potential
1) Resting potential
2) Depolarisation
3) Repolarisation
4) Hyperpolarisation
5) Return to resting potential
Describe resting potential
- Na⁺ ions pumped out of neurone and K⁺ pumped in by sodium-potassium pump
- K⁺ diffuse out more than Na⁺ diffuse in
- An electrochemical gradient of sodium-potassium ions is built up across the membrane
- The inside of the axon has a negative electrical charge compared to the outside
- Membrane potential is -70mV
Describe depolarisation
- Voltage-gated Na⁺ channels open when voltage reaches threshold potential (-50mV)
- Na⁺ rapidly diffuse into the neurone down their electrochemical gradient
- The inside of the neurone becomes more +vely charged w.r.t. the outside
Describe repolarisation
- Na⁺ channels close and voltage-gated K⁺ channels open
- The inside of the neurone become less +vely charged
- Membrane potential becomes -ve
Describe hyperpolarisation
- K⁺ ions diffuse into the neurone down their electrochemical gradient
- The electrical potential across the plasma membrane becomes more -ve than resting potential
- A new action potential cannot be generated immediately
Describe the return to resting potential
- Resting potential is re-established by the sodium-potassium pump and outward diffusion of K+ ions
- The potential difference across the membrane returns to -70mV
Propagation of an action potential along a non-myelinated neurone
1) At resting potential, outside of the neurone is more positive w.r.t. the inside. Membrane potential is -70mV.
2) A stimulus causes a sudden influx of Na⁺ ions (known as the action potential). The membrane is depolarised.
3) The influx of Na⁺ ions causes the formation of a localised electrical current, and thus the opening of voltage-gated Na⁺ channels a little further along the axon. The resulting influx of Na⁺ ions causes depolarisation.
4) Behind this new region of depolarisation, voltage gated Na⁺ channels close and voltage gated K⁺ channels open. K⁺ ions leave the neurone.
5) The action potential is propagated in the same way further along the neurone. The outward movement of K⁺ ions leads to depolarisation of the area of the axon behind the action potential.
6) Following repolarisation, the axon membrane returns to resting potential, allowing a new action potential to be generated.
Propagation of an action potential along a myelinated neurone
- In a myelinated neurone, there are gaps between each section of myelin sheath. These are known as the nodes of Ranvier.
- An action potential forms a localised circuit as it is propagated along a neurone.
- In a myelinated neurone, the localised circuit is longer than in a non-myelinated neurone, because this circuit must stretch between the adjacent nodes of Ranvier.
- This means that the action potential effectively ‘jumps’ from node to node along the neurone in a process known as saltatory conduction.
- Saltatory conduction is faster than the continuous movement of an action potential along a non-myelinated neurone (where the lack of myelin sheath and nodes of Ranvier means the action potential cannot ‘jump’).
Dendron
Carries an impulse towards the cell body; structurally the same as an axon.
Axon
Carries an impulse away from the cell body; structurally the same as a dendron.
Role of the axons/dendrons
Transmit nerve impulses away from/towards the cell body.
Role of the cell body
Contains the endoplasmic reticulum and mitochondria necessary for production of neurotransmitters.
Sensory neurones transmit impulses…
from sensory receptor cells to CNS
Relay neurones transmit impulses…
between neurones within the CNS
Motor neurones transmit impulses…
from the CNS to an effector
An effector can be a…
muscle or a gland
Sensory neurone myelination
Axons have myelin sheath
Relay neurone myelination
No myelin sheath
Motor neurone myelination
Axons have myelin sheath
Speed of impulse transmission (Sensory neurone)
Up to 100m/s
Speed of impulse transmission (Relay neurone)
Approx. 1 m/s
Speed of impulse transmission (Motor neurone)
Up to 100 m/s
Position of the cell body (Sensory neurone)
Outside the CNS
Position of the cell body (Motor neurone)
Inside the CNS
What is meant by myelination?
- Creation of a myelin sheath around an axon
- Sheath consists of many layers of plasma membrane, produced by Schwann cells, which grows around the axon
- Sheath acts as a layer of insulation and enables impulses to be transmitted at a faster speed
Autoimmune disease
a disease in which the immune system mistakenly attacks healthy body tissue.
Role of myelin
Forms an electrically insulating sheath around axons of certain neurones in the body, speeding up the transmission of nerve impulses.
Why might a damaged myelin sheath be a problem/lead to e.g. blindness?
Impulse received is too slow/does not reach from sensory receptor cells in eye to brain
Factors that affect the speed of nerve impulse transmission
- Myelination
- Axon diameter
- Temperature (after 40°C, proteins denatured)
Positive feedback
the enhancing or amplification of an effect by its own influence on the process which gives rise to it.
Example of positive feedback
- As soon as Na⁺ ions enter the axon, the potential difference across the membrane becomes less -ve
- This prompts the opening of more sodium channels, so more Na⁺ ions enter the axon
What type of sensory receptor is in the skin?
Mechanoreceptor
What is the name given to the receptor cells in the skin?
Pacinian corpuscles
How does a Pacinian corpuscle convert mechanical pressure into a nervous impulse?
- At resting potential, stretch mediated Na⁺ channels in neurone membrane are too narrow to let Na⁺ through
- When mechanical pressure applied to the Pacinian corpuscle, the lamellae are deformed and press on tip of neurone
- This deforms the neurone’s plasma membrane and widens the stretch-mediated Na⁺ channels
- Na⁺ diffuse into the neurone, depolarising it –> produces generator potential
- Threshold reached: action potential generated at first node of Ranvier
- Action potential propagated along sensory neurone to CNS
Transduction
Changing stimulus into a nerve impulse (called a generator potential).
What term describes specialised cells which convert one type of energy into an electrical nerve impulse?
Transducer
Transducer
Specialised cells which are capable of converting one type of energy into an electrical nerve impulse.
Process at a synapse
1) Action potential arrives at the end of presynaptic neurone.
2) Depolarisation of the membrane of the presynaptic neurone causes Ca2+ channels to open.
3) Ca2+ ions diffuse into the synaptic knob.
4) Vesicles containing neurotransmitter fuse with the plasma membrane and release neurotransmitter into the synaptic cleft by exocytosis.
5) Neurotransmitter diffuses across synaptic cleft.
6) Neurotransmitter binds with receptor protein on the post-synaptic membrane.
7) Binding of neurotransmitter results in Na+ channels opening and Na+ ions diffusing into the postsynaptic neurone.
8) This causes depolarisation of the postsynaptic membrane. If threshold potential reached, action potential generated and propagated along postsynaptic neurone.
9) Neurotransmitter is broken down by specific enzymes to prevent continuous synaptic transmission.
10) Neurotransmitter fragments diffuse back across the synaptic cleft to be reabsorbed, reassembled, and repackaged into vesicles in the synaptic knob.
11) Ca2+ ions pumped out of synaptic knob.
