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.