(5.3) Neuronal Communication Flashcards
What is the reflex arc?
Stimuli –> sensory receptor –> relay neurone –> motor neurone –> effector
What is the central nervous system?
Brain and spinal cord
Describe the structure of the sensory neurone
- Dendrites connecting to the dendron
- Shorter axon
- Cell body with nucleus outside of the CNS
- Axon Terminals connecting to the axon
- Myelinated sheath with nodes of Ranvier
Describe the structure of a relay neurone
- short dendrites
- several divisions of the axon
- cell body with nucleus
- non-myelinated
Why is the relay neuron non-myelinated?
It doesn’t need to transmit over long distances
What creates a myelinated sheath?
Schwann cell
Describe the structure of a motor neuron
- have cell body within the CNS
- long axon
- myelinated sheath
- axon terminals
- dendrites
What is the function of a sensory neurone?
Carry an action potential from the sensory receptor to the CNS
What is the function of a motor neurone?
Carry an action potential from CNS to an effector
What is an effector?
A muscle or a gland
What is the function of a relay neurone?
Connect a sensory and motor neurone and to conduct impulses within coordinated pathways
How is a myelinated sheath created?
- Schwann cells are wrapped tightly around the neurone
- The sheath consists of several layers of cell membrane and thin cytoplasm
- prevents movement of ions across membranes - can only take place at the nodes of ranvier
What is the structure of non-myelinated neurones?
- several neurones enshrouded in one schwann cell - loosely wrapped
- action potential moves along the neurone in waves rather than jumping over the nodes
What are the advantages of myelination?
- transmit quickly
- carry over long distances
- more rapid response to stimuli
How is a resting potential achieved and maintained?
- sodium and potassium ion pumps use ATP to pump 3 sodium ions out of the neurone and 2 potassium ions into the neurone
- sodium ion channels are kept closed but potassium ion channels are kept open
- potassium then tends to diffuse out
- large anions in the cell cytoplasm give the interior of the cell a negative potential
- cell membrane is then polarised
- potential difference is -60mV across the membrane
Describe the process of an action potential
- The sodium ion channels in the plasma membrane open and sodium ions diffuse into the neurone
- The threshold of -60mV is then reached
- positive feedback then causes sodium ion voltage gated channels to open and more sodium ions to diffuse in
- a value of +40mV is then reached causing sodium ion voltage gated channels to close and potassium ion voltage gated channels to open after the action potential
- Potassium ions diffuse out of the neurone
- Hyperpolarisation occurs
- The potassium ion voltage gated channels close and the sodium potassium ion pumps restore the resting potential
What is depolarisation during an action potential?
When the sodium ion channels in the plasma membrane open and sodium ions diffuse into the neurone
What is repolarisation?
When the potassium ion voltage gated channels open and potassium ions diffuse out of the neurone
What is hyperpolarisation?
When the potential difference overshoots
Define sensory receptors
Specialised cells that can detect changed in our surroundings. Most are energy transducers.
What is an energy transducer?
Convert one energy to another
Describe the structure of the pacinian corpuscles
- rings of connective tissue
- abrobias
- sensory neurone fibre
What are abrobias?
Cells that produce connective tissue
How does the pacinian corpucles work?
- when pressure is applied the rings of connective tissue are deformed
- this pushes on the nerve ending opening the sodium ion channels meaning sodium ions diffuse into the sensory neurone creating a generator potential
- this activates the generator potential as the diffusion of sodium ions has reached a threshold (if enough sodium)
How is a initiation failed?
the pressure is small that even though the sodium ion channels open the threshold is not reached meaning an action potential is not intiated
also called the all or nothing law
How are action potentials transmitted?
- sodium ion channels open (at point of action potential in neurone)
- open sodium ion channels allow sodium to diffuse into the neurone
- continues to diffuse sideways along the neurone called a local current (movement of charged particles)
- local current causes a slight depolarisation further along the neurone causing voltage gated channels to open
- open voltage gated channels causes further depolarisation along the neurone (action potential has been moved)
What is the refractory period?
- After an action potential has taken place the sodium and potassium ions are in the wrong place
- impossible to stimulate an action potential
- restored by sodium and potassium ion pumps
What is a saltatory conduction?
- sodium/potassium ions can’t diffuse through the myelin sheath
- diffusion occurs at the nodes of ranvier
- local currents are elongated in myelinated neurones
- sodium ions diffuse from one node to the next
- action potential appears to jump
- this is called a saltatory conduction
How can the intensity of a stimulus be detected by the brain?
- The frequency of transmission
- the more intense the stimulus is the more sodium channels open
- this what causes the production of more generator potentials
Define synapse
A junction between two or more neurones where the neurones can communicate
Describe the structure of the post-synaptic membrane
- contains specialised sodium ion channels that respond to neurotransmitters
- channels consist of 5 polypeptide molecules 2 of which have receptors for acetylcholine
- receptor sites are complementary
- when acetylcholine present in the synaptic cleft, binds to the 2 receptor sites and causes the sodium ion channels to open
Describe the transmission across a synapse
- action potential arrives at the synaptic bulb
- voltage-gated calcium ion channels open
- calcium ions diffuse into the synaptic bulb
- calcium ions cause the synaptic vesicles to move to and fuse with the pre-synaptic membrane
- acetylcholine is released by exocytosis
- acetylcholine molecules diffuse across the cleft and then bind to the receptor sites on the sodium ion channels in the post-synaptic membrane
- sodium ion channels open and sodium ions diffuse across the post-synaptic membrane into the post-synaptic neurone
- a generator potential or excitatory post-synaptic potential is created
- if sufficient generator potentials combine then the potential across the post-synaptic membrane reaches the threshold potential
- a new action potential is created in the post-synaptic neurones
What are converging neurones and spatial summation?
- several pre-synaptic membranes converge on one post-synaptic neurone
- allows action potentials from different parts of CNS to generate an action potential in post-synaptic membrane
- Useful when multiple stimuli are warning of danger
What are IPSPs?
- Inhibitory post-synaptic potential
- several EPSPs (excitatory post synaptic potential) could be be prevented from producing an action potential because of one IPSP
- reduce the effect of summation
What are the main two IPSPs?
- GABA
- Glycine
What are divergent neurones?
- one pre-synaptic neurone diverges to several post-synaptic neurones
- This allows one action potential to be transmitted to several parts of the CNS
- Useful in the reflex arc as one part of the post-synaptic neurone elicit a response whilst another informs the brain
What is one-way neurone transmission?
- synapses ensure that impulses are carried in the correct direction
- only the pre-synaptic bulb contains vesicles of acetylcholine
- if action potential starts half way along a neurone and ends at the post-synaptic membrane a response is not caused in the next cell
How do synapses filter?
- synapses filter out unwanted low-level signals
