Nerves Flashcards
Describe the structure of a neuron.
Neurons are specialised cells that allow communication between different parts of the body as nervous impulses.
Compare the hormonal and nervous system (8 points).
- Hormonal uses chemicals to communicate; nerves use impulses as communication.
- Hormones are transported in the blood; impulses are transmitted by neurons.
- Hormonal: transmission is relatively slow; transmission is very rpaid in the nervous system.
- Hormones travel to all areas of the body but only target cells respond; nerve impulses travel to a specific part of the body.
- Hormonal: response is wide-spread; reponse is localised in the nervous system.
- Hormonal: response is slow; response is rapid in the nervous system.
- Hormones are long-lasting; nerve impulses are short-lived.
- Hormone effect may be permanant and irreversible; nerve impulse effects are usually temporary and reversible.
Definition of excitable
Can detect and respond to stimuli.
Definition of conductive.
Can transmit impulses or action potentials.
What are the three types of neurons?
- Sensory- impulse from receptor to central nervous system.
- Intermediate- Impulse between neurons.
- Motor- Impulse from central nervous system to effector.
Describe the function of Schwann cells.
Surrounds the axon and forms the myelin sheath.
Describe the myelin sheath.
Covers the axon and is rich in lipid, known as myelin.
Definition of electrical impulses.
A brief change in the distribution of electrical charges across the axon membrane of neurons.
Definition of resting potential.
The difference in electrical charge maintained across the membrane of an axon of a neuron hen not stimulated.
Describe how to produce a resting potential (7 points).
- Sodium-potassium pump actively pumps 3 Na+ ions out of the cell.
- 2 K+ ions into the cell at the same time requires ATP.
- Some K+ ions diffuse back out the neuron through open K+ channels. The membrane is more permeable to K+ / less permable to Na+ ions.
- More positive ions are outside the cell than inside the cell.
- This sets up an electrochemical gradient.
- The potential difference outside the cell compared to inside the cell is -65mV.
- The membrane is polarised.
Definition of action potential.
A brief reversal of the resting potential across the axon membrane.
Definition of threshold potential.
Minimum intensity that a stimulus must reach in order to trigger an action potential in a neuron.
Describe the resting potential of a neuron briefly.
Polarised at -65mV.
Na+ / K+ ions are actively transported; 3 Na+ ions out, 2 K+ in the cell.
Few K+ channels are open, few K+ ions diffuse out the cell.
Na+ gated channels are closed.
Describe deplarisation (7 points).
- Stimulus causes some Na+ channels to open.
- Some Na+ diffuse out the cell.
- The membrane depolarises and the inside becomes less negative compared with the outside and reaches a threshold value of -50mV.
- If threshold is reached, more Na+ channels open and Na+ flood into the cell.
- The inside of the cell becomes positive compared with the outside of the cell.
- The potential difference across the membrane reaches +40mV.
- The Na+ channels close, the K+ channels open.
Describe repolarisation (2 points).
- The membrane become repolarised and positive inside compared with the outside of the cell.
- More K+ channels open, causing more K+ ions to diffuse out the cell.
Describe the refractory period (4 points).
- The membrane is hyper-polarised.
- The potential difference becomes more negative than -65mV due to the flood of K+ ions out the cell.
- The K+ channels close and the Na+/K+ pumps to restore the resting potential of of -65mV.
- This re-establishes the ion concentration across the membrane.
Definition of the all or nothing response.
If threshold is reached, an action potential will fire with the same change in voltage, no matter how big the stimuli is.
How does the brain interpret the size of the stimulus?
By having different neurons with different threshold potentials allows the brain to interpret the number and type of neurons that are transmitting an action potential to determine the size of the stimulus.
Explain how the refractory period ensures the action potential only goes in one direction (3 points).
- Action potential passes from active to resting region.
- Action potential cannot be produced in refractory period.
- Prevents action potential from spreading in both directions along the axon.
Explain how the refractpry period produces discrete impulses (3 points).
- New action potential cannot be formed immediately behind first action potential.
- Allows action potential to be separated.
- Helps show strength / frequency of stimulus.
Explain how the refractory period limits the number of action potentials (3 points).
- Action potentials are discrete.
- Limits the strength of the stimulus that can be detected.
- Body / brain not overloaded by stimulus.
Describe non-myelinated transmission of an action potential (6 points).
- During an action potential, gated Na+ channels are open.
- There is a high concentration of Na+ ions in the depolarised region of the axon and low concentration of Na+ in the resting regions to either side.
- This sets up a concentration gradient.
- Na+ diffuses sideways from a high to a low concentration along the axon.
- Diffusion of Na+ depolarises the next region causing Na+ channels to open (when it reaches -50mV threshold potential).
- This causes the generation of a new action potential in this region. The action potential is therefore a travelling wave of depolarisation.
Describe myelinated transmission of an action potential (6 points|).
- Myelin electrically insulates axons which increases the speed of transmission of conduction.
- Sodium ions and potassium ions cannot flow through the myelin sheath.
- Ion exchange can only occur across the axon membrane at the Nodes of Rnvier.
- Action potentials appear to jump from one Node to the next which is called saltatory conduction.
- This can increase the speed of transmission by up to 50 times.
- Local currents are elongated and Na+ diffuse along the inside of the neuron from one Node of Ranvier to the next.
Describe and explain how temperature affects the speed at which action potentials can travel.
A lower temperature causes a slower rate of diffusion of ions, so a slower rate of conduction of ions. Less kinetic energy, so a slower response.
A higher tmeperature causes a faster rate of diffusion of ions, so a faster rate of conduction of ions. More kinetic energy, so a faster response.
Describe and explain how diameter of the axon affects the speed at which action potentials can travel.
The smaller the diameter, the slower the the diffusion of ions. More resistance to the flow of ions in cytoplasm. Therefore, a slower response.
The larger the diameter, the quicker the diffusion of ions. Less resistance to flow of ions in cytoplasm, so quicker response.
Describe and explain how the presence of a myelin sheath affects the speed at which action potentials can travel.
A myelinated neuron has a quicker diffusion of ions as there is a steeper gradient. Depolarisation only at Node of Rnvier. A quciker response.
A non-myelinated neuron has a slower diffusion of ions so a shallower concentration gradient. Depolarisation along length of membrane. A slower respsonse.
Definition of a synapse.
The junction between 2 neurons. There is a very narrow gap of about 20nm between neurons called the synaptic cleft.
Definition of a cholingeric synapse.
Synapses that use acetylcholine.
Describe sequences of events at a synapse (10 points).
- Action potential arrives at synaptic knob causing depolarisation of membrane.
- Ca^2+ channels open and Ca^2+ ions enter the axon by facilitated diffusion.
- Causes vesicles containing neurotransmitter to move and fuse.
- With pre-synaptic membrane.
- Neurotransmitter released into the synaptic cleft and diffuses across cleft.
- Neurotransmitter binds with specific receptors on the post-synaptic membrane.
- Na+ channels open and Na+ flood in.
- Post-synaptic membrane is depolarised.
- Acetylcholinesterase hydroylses acetylcholine into acetyl and choline.
- Product diffuses back to pre-synaptic membrane to be re-synthesised.
State and explain a unidirectional synapse.
State: Impulses are only transmitted in one direction.
Explain: Neurotransmitter is only produced in the pre-synaptic membrane in vesicles. Action potential causes movement of vesicles. Neurotransmitter binds to receptor only found on post-synaptic membrane.
State and explain a temporal summation synapse.
State: Synaptic transmission of an impulse can use temporal summation.
Explain: Pre-synaptic neuron stimulated to release neurotransmitter. Low frequency of impulses doesn’t exceed threshold potential, so no action potential. High frequency of impulses does exceed threshold potential, so action potential is generated.
State and explain a spatial summation synapse.
State: Synaptic transmission of an impulse can use spatial summation.
Explain: Many pre-sysnaptic neurons connected to one post-synaptic neuron. If both pre-synaptic neurons release neurotransmitters and threshold potential is reached, action potential is fired.
State and explain an inhibitory nsynapse.
State: synapses can be inhibitory.
Explain: Less likely for an action potential to be fired in the post-synaptic neuron.
Describe how neurons can be inhibitory (6 points).
- Pre-synaptic neurons release neurotransmitter which binds to Cl- channels on post-synaptic membrane.
- Neurotransmitter alos binds to K+ channels.
- Cl- channels open, Cl- ions diffuse into the post-synpatic neuron into the cleft.
- The movement of ions causes hyperpolarisation of post-synaptic membrane.
- More Na+ channels must be opened to allow more Na+ to diffuse in.
- To reach threshold.
Explain how a drug that creates more action potentials effects synapses.
More action potentials are created.
Stimulates the release of more neurotransmitters or inhibits enzymes which break down the neurotransmitters. The body’s repsonse to impulse is enhanced.
Explain how a drug that creates fewer action potentials effects synapses.
Fewer action potentials are created.
Some drugs inhibit the release of neurotransmitters / blocks receptors on the post-synaptic membrane. The body’s response to impulse is reduced.
Describe endorphins and provide two examples of drugs that mimic enorphines’ effects.
Used to block pain pathways.
Morphine and codeine bind to specific receptors in the brain where endorphins normally work.
