nervous system Flashcards
what does the sensory neurone do
brings impulses from sense organs/receptors into the central nervous system
what does relay neurone do
receives impulse from sensory neurone and relays it to motor neurone.
also known as a connector neurone
is unmyelinated so it appears grey
What does the motor neurone do
it carries impulses from the central nervous system to the effector (e.g muscles/organs)
what are dendrites
thin extensions that carries the impulses towards the cell body
what are axons
long membrane-covered cytoplasmic extensions that transmits impulses away from cell body.
what are schwann cells
cells which surround and support peripheral neurones
what is the myelin sheath
multi-layered fatty sheath surrounding axons made by schwann cells. acts as an electrical insulator, speeds up transmission along axon.
what is a node of ranvier
areas along the axon where there is no schwann cells meaning it is not electrically insulated
what is the cell body
the area of a neurone which contains the nucleus, rer, numerous mitochondria and other organelles
what is a reflex
an automatic, rapid response to an adverse stimulus
describe the pathway of the electrical impulse from receptor to effector
- stimulus is detected by receptors in the skin
- a nerve impulse is passed along the sensory neurone
- sensory neurone enters the spinal cord via dorsal branch of the spinal nerve
- the sensory neurone synapses with a relay neurone in the grey matter.
- then synapses to a motor neurone
- the motor neurone leaves via the ventral root
- it carries the impulse to an effector (muscle/organ)
- action (muscle contracts etc)
- action is a response.
experiment with ‘giant squid’ axons
microelectrodes are used to stimulate the axon and the cathode ray oscilloscopes are used to measure the potential difference across the membrane
what is a resting potential
the potential difference between the inside and outside of a membrane when a nerve impulse is NOT being conducted
what is the typical voltage for a resting potential and what is it said to be
-70mV (inside is negative with respect to outside) and it is said to be polarised.
how are resting potentials achieved?
Sodium-potassium pump is a transmembrane protein, they maintain the concentration of an uneven distribution of Na+ and K+ across the membrane
the membrane is more permeable to K+ than Na+.
K+ diffuses out of nuerone due to concentration gradient caused by the Na+K+ pump.
Anion concentrations are higher inside the nuerone.
what is the all or nothing law
where the size of the impulse is independent of the size of the stimulus.
if the intensity of stimulus is below a certain threshold intensity, an action potential will not be initiated
if the intensity of a stimulus is exceeds the threshold value, and action potential is initiated.
HOWEVER, any further increase in intensity does not give a greater action potential.
What is an action potential
- stimulation of an axon results in a change in the potential difference across the membrane from -70mV to +40mV
- this depolarization is due to a sudden increase in the permeability of the membrane to Na+ (Voltage-gated Na+ channels open)
- Na+ diffuse into axon down a concentration gradient
- A fraction of a second later, voltage-gated k+ channels open and the membrane becomes more permeable to K+ which diffuses out of the axon which repolarises the membrane.
- There is an overshoot (hyperpolarisation) of k+ leaving, but the Na/K pump restores the ionic balance. This is called the refractory period.
what does an action potential cause
they cause a small electric current across the membrane and as a portion of the membrane is depolarised, depolarisation of the next portion is initiated.
what allows another impulse to be initiated after one
resting potential
how to action potentials travel along an unmyelinated axon
- influx of Na+ ions and depolarisation of membrane causes a local flow of electrical current.
- these local currents trigger the adjacent region of the plasma membrane to become more permeable to Na+ ions (by opening voltage gated Na+ channels) causing a depolarisation in the next region.
How is myelination a factor that affects the speed of conduction of the nerve impulse?
Myelin is lipid rich, makes axons impermeable to ions (and acts as insulators) this makes ions unable to diffuse between tissue fluid and nuerone so action potentials cannot be generated in myelinated regions.
Action potentials are only generated at nodes of ranvier
Thus causing the action potentials to ‘jump’ from node to node (called a saltatory conduction)
how is the diameter of an axon a factor that affects the speed of conduction of the nerve impulse
the greater the diameter of axon, the greater the velocity of transmission. Less resistance from cytoplasm to the flow of ions
how is temperature of an axon a factor that affects the speed of conduction of the nerve impulse
increased temperature increases the rate of aerobic respiration therefore the production of ATP which is needed for the sodium-potassium pumps
there are many neurotransmitter substances - give exmaples
dopamine and serotonin - active in brain
acetylcholine released at synapses contacting with motor neurone - cholinergic synapses
noradrenaline released at synpases involved in the sympathetic Neurone systen - adrenergic synapses
