Nervous System - Finished Flashcards
Why do we need a nervous system?
To detect stimuli, process the information and initiate responses
What is the structure of the nervous system?
Central Nervous System - Brain and Spinal cord
Peripheral Nervous System - Pairs of nerves - Contains sensory and motor neurones
What happens in a reflex arc?
Stimulus -> Receptor -> Sensory Neurone -> CNS -> Motor Neurone -> Effector -> Response
What is a stimulus?
A detectable change in the internal or external environment of an organism
What is a reflex action?
A reflex action is a rapid and involuntary response to a stimulus. It is frequency a response which protects you from harm.
Give examples of reflex actions
Withdrawl Reflex
Blinking
Sneezing
Decrease in pupil diameter
What type of nervous system is found in simple organisms
Nerve net
What is a nerve net?
Simple nerve cells with short extensions joined to each other and branching in a number of different directions
3 differences between a Hydra nervous system and Human nervous system
Hydra - Does not have CNS - Limited number of Stimuli - Small number of effectors Human - Has CNS - Wide range of stimuli - Large numbers of effectors
What are neurones?
Specialised cells adapted to rapidly carry nerve impulses from one part of the body to another
What are the 3 types of neurones?
Sensory Neurones - Bring impulses from the receptors into the CNS
Relay neurones - Receive impulses from sensory neurone and relay them to motor neurones
Motor Neurones - Carry impulses from the CNS to effector organs (muscles & glands)
What is the cell body?
Contains a nucleus and many ribosomes which are concerned with the formation of neurotransmitter substances
What are dendrites?
Thin extentions of the cell body which recieve impulses from other neurones
What is an axon?
A long membrane covered cytoplasmic extention of the cell body which transmits impulses away from the cell body
What do you call the branches at the end of the axon that form synapses with the other neurones?
Axon terminals
What are axon terminals called when they form a synapse with a muscle?
Neuromuscular junction
What acts as an electrical insulator and speeds up the transmittion of nerve impulses?
Myelin sheath
What is the name of the cells that wraps around the axn to form the Myelin Sheath?
Schwann Cell
What is a node of Ranvier?
Intervals between the myelin sheath which are important as they speed up nerve impulse transmission by causing impulses to jump from node to node (known as saltatory conduction)
How can you measure a nerve impulse?
Small electrical charges in neurones are measured using microelectrodes. Microelectrodes can be connected to a cathode ray oscilliscope and the signals are displayed as a trace on the screen
What is a resting potential?
The electrical difference found across the membrane of a resting neurone.
THe inside of the membrane has a negatie electrical charge compared to the outside (polarised) (-70mV)
How is the resting potential established?
Due to the combined effects of:
Na/K pumps - 3NA out for every 2 K in
Differential permeability of the membrane to Na and K ions - Most K ion channels are open allowing K out, most Na ion channels shut stopping Na coming in)
When are neurones prepared to transport a nerve impulse?
When it’s resting potential has been established
What causes depolarisation of the membrane?
The energy of the stimulus (if above threshold level) causes the sudden opening of some Na channels and there is a rapid influx of Na which diffuse into the axon. The membrane becomes depolarised.
What is the action potential in a depolarised membrane?
+40mV
What is the action potential in a polarised membrane?
-70mV
How is the membrane repolarised?
Once the action potential of +40mV is established the Na ion channels close and no more Na can enter the axon. K ion channels open and K diffuses out of the axon.
What is hyperpolarisation?
THere is a slight overshoot when the potential difference drops down to -70mV and drops to -75mV due to the K leaving as the Na/K pump restores the balance. This means it cannot produce an action potential, Called the refractory period.
During the refractory period another action potential cannot be generated. What effects does this have?
The impulse is unidirectional
It limits the fequency of action
What is the all or nothing principle?
ALL - a stimulus with an intensity which exeeds the threshold value, results in the generation of an action potential. Any further increase in the stimulus does not have any effect on the size of the action potential
NOTHING - If the intensity of a stimulus is below the threshold value, NO action potential is initiated.
How are nerve impulses transmitted?
The formation of the action potential seets up a lock electrical current which actsas a stimulus to the next part of the membrane causing that to depolarise. THis process repeats itself along the length of the axon
What factors affect the speed of a nerve impulse?
- Myelination - Increases the distance over which a local current/circuit can bring about depolarisation of the next section of axon.
- Larger Diameter - There is less resistance to the current. (Found in giant squid- useful in rapid escape responses)
What is Saltatory Conduction?
When myelin insulates a section of the axon preventing depolarisation. Depolarisation is only possible in the nodes of Ranvier where there is no myelin present. the impulse jumps from one node to the next/
What is a synapse?
a synapse is a tiny gap between to neurones
What is the function of a synapse?
The main role of a synapse is to transmit nerve impulses between neurones in one direction only.
What happens at a synapse?
- The action potential arrives at the axon terminal depolarising the pre-synaptic membrane
- Ca2+ channels to open and Ca2+ ions rush into the synaptic knob
- This causes synaptic vesicles to fuse with the pre-synaptic membrane
- The neurotransmitter is released by exocytosis into the synaptic cleft.
- The neurotransmitter diffuses across the synaptic cleft and binds to receptors on the post synaptic membrane
- The causes the Na channels in the post synaptic membrane to open and Na ions to enter depolarising the membrane
- Sufficient depolarisation results in the generation of an action potential in the post synaptic membrane. a nerve impulse the then transmitted across the axon.
- The neurotransmitter (Acetylcholine) is broken down by the enzyme Acetyl Cholinesterase in the synaptic cleft. The products diffuse back into the axon terminal. ATP is used for the re-synthesis and packaging of acetylcholine into synaptic vesicles.
in what form is the transmission across the synapse?
Chemical not electrical.
What are the most common neurotransmitters?
Acetylcholine, Noroadrenaline
What are drugs and how do they affect the body?
Psychoactive drugs are substances that can alter the consciousness, mood and thoughts of those who use them.
They include cocaine and cannabis
THEY AFFECT SYNAPSES IN THE BRAIN
What are organophosphorus insecticides?
Chemicals used to kill pests
What affect do organophosphorus insecticides have on synapses in humans?
In human they act as cholinesterase inhibitors
THe insecticide combines with the enzyme cholinesterase which inhibits the enzyme so it cannot breakdown acetylcholine
Acetylcholine remains in the synaptic cleft and on the post-synaptic membrane, causing repeated stimulation of the post-synaptic neurone. If this happens at a neuromuscular junction, repeated stimulation and contractions of the muscle occur.
What are Excitatory drugs?
drugs that stimulate the nervous system by creating more action potential in post synaptic membrane.
what are inhibitory drugs?
Drugs that inhibit the nervous system by creating fewer action potential in post synaptic membranes
What actions can excitatory drugs take?
- Mimics the normal transmitter
- Blocks the uptake of the neurotransmitter into the presynaptic knob
- inhibits the enzyme involved in the breakdown of the neurotransmitter
What actins can inhibitory drugs take?
- Prevents the release of the neurotransmitter from the presynaptic knob
- binds with receptors on the post synaptic membrane blocking the binding of the normal transmitter (DOES NOT MIMIC TRANSMITTER)
- Prevents entry of calcium ions into the pre-synaptic knob.
Do plants respond to their environment?
Yes. However their responses are slow because they are co-ordinated by hormones
What is the definition of tropism?
Growth response towards or away from a unidirectional stimulus
What is the definition of photoperiodism?
Flowering in response to the relative periods of daylight and darkness
What different categories can flowering plants be divided into?
- Day neutral plants - Flowering not affected by day length
- Long day plants - Flowering is induced by exposure to dark periods SHORTER than a critical length (lots of light)
- Short day plants - Flowering is induced by exposure to dark periods LONGER than a critical length (small light)
What is the site of detection of the photoperiodic stimulus (light)?
Leaves - Contain a photoreceptor pigment called PHYTOCHROME. this exists in 2 forms which are interconvertible. Sunlight - Pr- 660. Darkness - Pfr - 730
What happens to the phytochromes in the leaf?
In the daylight Pr rapidly converts to the Pfr form so Pfr accumulates.
In darkness Pfr is unstable and slowly reverts back to the Pr form which accumulates.
High levels of Pfr promotes flowering in long day plants and inhibits flowering in short day plants.
The exact mechanism involved in the flowering is not understood but it may involve a hormone named florigen.
