Define Sensory Receptor
Specialised cells which can detect changes in our surroundings. They are energy transducers.
Name 3 types of sensory receptors
- Light sensitive cells in the eye (rods and cones)
- Olfactory cells in the nasal cavity
- Taste buds in the tongue
- Pressure receptors in the skin (Pacinian Corpuscles)
- Sound receptors in the ear (cochlea)
- Muscle spindles (proprioceptors)
How many Na and K ions are transported in/out of the sodium/potassium pump
2K and 3Na
What is a polarised membrane
A membrane with a potential difference across it
What is depolarisation
The loss of polarisation across a membrane, when gated sodium channels open.
What is an action potential
An impulse initiated by a large change in p.d.
Describe the structure and function of a sensory neurone
Relays messages from receptors to the brain or spinal cord
- Long dendrites and short axon
- Cell body and dendrite are outside of the spinal cord; the cell body is located in a dorsal root ganglion
- Synapse
- Node of ranvier
- Schwann cell
Describe the structure and function of a motor neurone
Relays messages from the brain or spinal cord to the muscles and organs
- Short dendrites and long axon
- Entirely within the spinal cord or CNS
- Synapse
- Node of ranvier
- Schwann cell
What is a resting potential
The p.d. across the neurone when an action potential is not being generated
Describe the ionic movements generating an action potential
- The membrane is at resting state: -70mV inside compared to outside
- Na+ ion channels open and some Na+ ions diffuse into the cell.
- The membrane depolarises- it become less negative with respect to the outside and reaches the threshold potential of -40mV.
- Voltage-gated sodium ion channels open and many Na+ ions enter. As more Na+ ions enter, the more positively changed the cell becomes, compared to outside.
- The potential difference across the membrane reaches +40mV. The inside is now positive compared to the outside.
- The Na+ ion channels shut and the K+ ion channels open.
- K + ions diffuse out of the cell, bringing the potential difference back to negative compared with the outside repolarisation.
- The potential difference overshoots slightly, making the cell hyperpolarised -90mV.
- The original potential difference is restored by the Na/K pump, so the cell returns to its resting state -70mV.
What is a local current
The movement of ions along the neurone
Describe the creation of localised currents
- V-G Sodium ion channels open allowing sodium ions to diffuse into the neurone
- Sodium ions diffuse along the axon
- Sodium Gate opens allowing the AP to move along the neurone
What is a myelin sheath
An insulating layer of fatty material
Why is myelination important
Na and K cannot diffuse through the myelin sheath
Causes saltatory conduction across nodes of ranvier. This speeds up AP transmission.
Define synapse
A junction between two or more neurones
What is released so the AP can pass across the synapse
Neurotransmitter
Give an example of a neurotransmitter
Acetylcholine (ACh)
Describe the transmission of a signal across a synapse
- Action potential arrives at the synaptic knob
- V-G Ca ion channels open
- Calcium ions diffuse into the synaptic knob
- The presence of Ca causes the synaptic vesicles to move and to fuse with the pre-synaptic membrane
- ACh is released by exocytosis
- ACh molecules diffuse across the cleft
- ACh molecules bind to receptors on the sodium ion channels in the post-synaptic membrane
- Sodium ion channels open
- Sodium ions diffuse to the post-synaptic membrane
- An EPSP is generated and if sufficient generator potentials combine, an AP is created
What enzyme breaks down ACh
Acetylcholinesterase
Why does ACh need to be broken down
To stop the transmission of signals so the synapse doesnt continue to produce AP’s
What does an all-or-nothing response mean
If the threshold is not surpassed, an AP is not generated
Define summation
Several small p.d. changes can combine to produce a larger change in p.d. causing an AP
Give the two types of summation
Temporal
Spatial
Define spatial summation
multiple neurones can fire impulses to one receiving neurone, where one of those action potentials alone will not be sufficient to produce an action potential
Define temporal summation
where one neurone fires multiple impulses from the same place, which with pauses in between will not be sufficient to generate an impulse on the receiving neurone
Give two factors which can affect synaptic transmission
Inhibition of calcium voltage-gated channels on the pre-synaptic membrane
Inhibition of acetylcholine receptor sites on sodium ion channels
Inhibition of the enzyme acetylcholinesterase (snake venom)
Inhibition via substance binding to receptors (nicotine and caffeine)
Inhibition via hyperpolarisation of the post-synaptic neurone (alcohol)
Describe convergence
several pre-synaptic neurones converge to one post-synaptic neurone so that signals from different parts of the central nervous system can trigger the same response
Describe divergence
one pre-synaptic neurone might diverge to multiple post-synaptic neurones which allows a signal to be transmitted from one neurone to several areas of the nervous system simultaneously
Define stimulus
Any change in the environment that causes a response.
Define a response
A change in behaviour or physiology as a result of a change in environment.
What are the requirements of a good communication system?
Must be able to send and receive messages and signals and effectively distinguish between them. It must extend over the whole body.
What is a generator potential
Small depolarisation caused by Na ions entering the cell.
Suggest why neurones have to carry a large number of mitochondria.
Mitochondria produce ATP by aerobic respiration to maintain the resting potential and for exocytosis of neurotransmitter chemicals at synapses
What are Voltage-gated channels
Channels in membrane that allow passage of ions. Gates open and close the channel responding to potential difference across membrane.
Describe saltatory conduction
This means ‘jumping conduction’. It refers to the way that action potential appears to jump from one node of Ranvier to the next
