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
steps of a transmission of an impulse across a synpase
- nerve impulses arrive and depolarises the pre-synaptic membrane.
- voltage gated ca+ channels open and calcium enters the presynaptic knob down a concentration gradient (via diffusion)
- influx of calcium causes vesicles of neurotransmitter to move towards the presynaptic membrane
- vesicles fuse with presynaptic membrane and releases its contents by exocytosis into the presynaptic cleft
- neurotransmitter molecules diffuse across the cleft
- and then binds to complementary receptors on the post-synaptic membrane
- sodium gate channels open.
what are the roles of synapses
makes sure the flow of impulses are one direction only. vesicles containing transmitter in presynpatic membrane, receptors on postsynaptic membrane
allows integration - synapses has many post synaptic neurones going to different locations
several pre-synaptic neurones may converge to go to one post-synaptic neurone which will then create the same response
acclimatisation - neurone may run out of neurotransmitter due to repeated stimulation.
what does organophosphorus insecticides do
inhibits acetylcholinesterase
prolongs the effect of acetylcholine which remains in the synaptic cleft and causes repeated firing of the post-synaptic neurones
resulting to a repeated firing of neurones at neuromuscular junction resulting in repeated contractions of muscle which can cause paralysis and breathing will stop
how does psychoactive drugs such as cocaine work
they attach to the dopamine transporter and blocks the recycling process causing a build up of dompamine in synapse which is the cause of the pleasurable effects of cocaine.
what is the motor neurone disease
it is the degeneration of motor nerve cells in spinal cord and brain that controls voluntary muscles e.g skeletal muscles.
usually death within 5 years after diagnosis
develops at different speeds and ways for different people
what are the symptoms of the motor neurone disease
gradual impairment of legs and arms
muscle twitch, pain, stiffness around joints
difficulty in swallowing and speech if throat muscles affected
difficulty in breathing if thoracic muscles affected.
what is the diagnosis for motor neuron disease
difficult to find
a needle is injected to various muscles to measure electrical activity.
what is the cause and treatment for motor neurone disease
cause is unknown
there is no treatment but people can use the pill (riluzole) to slow down progression.
what is parkinsons disease
progressive disease caused by death of brain cells that produce dopamine
dopamine are important neurotransmitters that carries signals between neurones and brain
dopamine enables smooth-co-ordinated movements
what are the symptoms of parkinsons
temors, slowness of movement, affects talking, walking, swallowing
cure/treatments for dopamine
no cure
treatments
early treatments = levadopa (replaces missing dopamine
long term treatments = physiotherapy
what is a stroke
blood clots in the blood vessels in the brain where neurones surrounding blood vessels becomes starved of oxygen and die.
stroke on right side affects movement on left side
stroke on left side affects speech
memory is often impaired
symptoms of stroke
numbness weakness/paralysis on one side of body slurred speech blurred vision confusion unsteadiness severe headache
treatment of stroke
immediate - clotbusting drugs e.g streptokinase
longer term- aspirin, reduces recurrence
rehabilitation therapy
risk factors of stroke
hypertension
obesity
smoking
lack of exercise
