C3 - The Nervous System Flashcards
Describe synapses
Small gap junctions between 2 nerve cells where impulses pass by diffusion of a neurotransmitter.
Define nervous impulse
Transmission of a change in pot along a nerve fibre associated w/ the movement of Na+ ions.
Define the dendrite
Thin extensions that carry impulses towards the cell body.
Define the axon
Long membrane-covered cytoplasmic extension, that transmits impulses away from cell body.
What’s the function of the 3 types of neurones?
S = Carries impulse from receptor to CNS
M = Carries impulse from CNS to effector
R = Carries impulse along CNS
Define voltage gated channel
One in which opens or closes in response to a particular voltage across the membrane
Define the Schwann cells
Cells which surround + support peripheral neurones
Define the myelin sheath + its function
== Schwann cells grown around the axons to form a multi-layered fatty sheath.
Acts as an electrical insulator that speeds up transmission along the axon because impulse jumps from 1 node of ranvier to the next
What only has a myelin sheath?
Vertebrates
Define the Nodes of Ranvier
Areas along the axon where the myelin sheath thins
Define the cell body
Part of the neurone that contains nucleus, RER, mitochondria and other cell organelles
What are the 2 main parts of the nervous system?
PNS - peripheral nervous system
CNS - central nervous system
Explain the CNS
Brain + spinal cord
Processes info provided by a stimulus
Explain the PNS
- Somatic NS (so pairs of nerves that originated in the brain or spinal cord + their branches)
Sensory + motor neurones
- Autonomic NS which unconsciously controls the functions of internal organs
Where is the Hydra’s nerve net?
In its ectoderm
What does the nerve net in Hydra do?
Connects sensory photoreceptors + touch sensitive nerve cells in body wall + tentacles.
What is the nerve net made of?
Simple nerve cells w/ short extensions joined together + branching w/ multiple synapses.
Why is the transmission of impulse slow in a nerve net?
Lack of myelin sheath
What directions do the impulses travel in?
Hydra - Both
Human - One
What can’t the Hydra’s nerve net do?
Detect the direction of a stimulus.
What are the 2 types of cells in a nerve net?
Ganglion cells = providing connections in many directions.
Sensory cells = detecting stimuli
How many stimuli can be detected by sensory receptors ?
Hydra - Limited
Human - Many
How many effectors do Hydra + humans have?
Hydra - 1 = tentacles
Humans - lots = muscles + glands
Define reflex arc
Pathway travelled by the nerve impulses during a reflex action.
What are the 2 types of reflexes?
Spinal
Cranial
Define cranial reflex
Pathways travelled by the nerve impulses during a reflex action through cranial nerves and the brainstem.
Define the spinal reflex
Info may be transmitted to brain, but it’s the spinal cord, not brain, that’s responsible for the integration of sensory information and a response transmitted to motor neurones.
Most reflexes
Define resting potential
p.d between inside + outside of membrane, when a nerve impulse is NOT being conducted
Typically -70mV, the inside is -ive due to accumulation of more Na+ outside cell than K+ inside.
Membrane is said to be POLARISED
How long are the axons in the 3 types of neurones
Sensory - Short
Motor - Long
Relay - Short
Where is the location of the cell body in the 3 types of neurones
Sensory - Dorsal root ganglion
Motor - Grey matter
Relay - Grey matter
How is the myelin sheath formed?
Schwann cells become associated w/ axons + dendrons during embryonic development.
These surround axon to form myelin sheath made of many layers of myelin or phospholipid bilayers.
Why is the white matter white and the grey matter grey?
White = Myolinated due to phospholipids in myelin sheath
Grey = Many nuclei + cell bodies found there
What is the name given for the cells that partially cover neurones?
Schwann cells
What is the name given to the material which the cells produce to cover the axon?
Myelin
What fluid does the spinal canal contain?
Cerebral spinal fluid
What does the white matter contain?
Ascending fibres that carry action potential to the brain.
Descending fibres which carry action potential from the brain.
What does a thicker neurone mean in regards to flow of impulse?
Thicker = less resistance
How is the resting potential achieved?
Na/K pumps maintain conc of an uneven distribution of Na+ + K+ ions across the axolemma (membrane).
— 3Na+ out + 2 K+ in = Requires ATP but NOT simultaenous
Membrane is more permeable to k+ ions than Na+ due to protein channels for K+ ions. = They can diff out of axon due to conc grad created by Na+/K+ pump ⬆️ the +ive charge outside.
Anion (large protein molecules COO-) + organic phosphates (ATP4-) concentrations are higher inside neurone.
How can you reach threshold potential?
Any stimulation of an axon causes some Na+ channels to open temporarily.
Some Na+ ions diff down electrochemical + conc grad back into axon.
== ⬇️ p.d across membrane
If reduction is great enough it will reach threshold potential in which voltage gated channels open.
Why is AT needed in the maintenance of a resting potential
Because the neurone membrane is impermeable to the entry of sodium ions
When must an action potential be generated?
Before an impulse can be established + transmitted
How is the action potential generated?
Energy of stimulus causes some VOLTAGE-GATED Na+ channels in axon membrane to OPEN.
(BIGGER stimulus = MORE channels open)
This ⬆️ in permeability of membrane to Na+ ions allow them to RAPIDLY DIFF INTO axon down their electrochemical + conc gradients.
So then, the -ive charge of -70mV inside axon becomes charge of +40mV.
== Cell membrane becomes DEPOLARISED.
Once pot inside cell is +40mV, Na+ channels CLOSE.
GENERATION OF ACTION POTENTIAL
What must the stimulus be?
Above a certain minimum value = threshold stimulus
AND produced a maximum of all or no response in the neurone.
GENERATION OF ACTION POTENTIAL
Where might the stimulus come from?
External environment i.e light or pressure changes
Internal environment i.e chaining blood temp or pH
Describe what happens in REPOLARISATION
Voltage gated K+ channels open + K+ ions quickly diff out of axon down electrochemical + conc gradients.
Cell becomes less +ive inside as more diffuse out + membrane is REPOLARISED.
Briefly describe the propagation of an impulse in a MYELINATED neurone
Electrons of the local currents jump from one Node of Ranvier to the next.
What is the refractory period?
Time after a stimulus has established an action potential before a second stimulus can set up another action potential.
Define reflex action
Rapid, involuntary response to a stimulus
What must have occurred before a second action potential can arise?
Enough potassium outflow from the neurone
In how many directions can synapses conduct impulses?
1
What are the consequences to electrons of the local currents jumping from one Node of Ranvier to the next in a myelinated neurone
Gain or loss of ions is restricted to nodes and so Na/K pumps have to expend less energy in recovery.
Saltatory conduction greatly ⬆️ speed at which impulses travel.
What is the jumping conduction called?
Saltatory conduction
What do inhibitory synapses do?
Won’t transmit signal so stimulation of inhibitory neurone will lead to the stopping of activity under control.
Is any energy lost in transmission of an impulse?
NO
What are the tough protective membranes called that surround the brain + spinal cord?
meninges
Describe HYPERPOLARISATION
More K+ ions diff out than NA+ in, so p.d across membrane = more -ive than resting pot = membrane hyper polarised. (-90mV)
Refractory period occurs.
What is an excitable cell?
One in which the potential across the membrane can be altered.
How is the p.d across the membrane restored after hyper polarisation?
Na/K pump pump:
k+ ions IN
Na+ ions OUT
== restores ion balance of resting pot.
What does the reversal of pot after Hyperpolarisation set up?
Local currents as Na+ ions move laterally through axon.
How is an impulse carried along the neurone after an action potential has been generated?
Once impulse is made, local current is set up.
The lateral flow of some Na+ towards -ive area causes Na+ channels to open + depolarisation occur. = AP moves down neurone.
Where is local current set up?
Between area of AP + resting area next to it.
What is the absolute refractory period?
Time in which another stimulus given to the neurone (no matter how strong) will not lead to a second action potential.
Lasts around 1ms.
When might you get a stimulus but no action potential generated?
When intensity of stimulus is below certain threshold value.
Does an ⬆️ in the intensity of a stimulus give a greater action potential?
NO.
AP is always +40mV
Instead, frequency of AP ⬆️.
What is the ‘All or nothing law”?
Nervous impulse is either initiated or not + is always the same size.
Allows the AP to acts as a filter, preventing minor stimuli setting up nervous impulses so that the brain isn’t overloaded with info.
What are the 3 major factors that affect the speed of conduction of a nerve impulse?
Temperature
Diameter of axon
Myelination
FACTORS AFFECTING SPEED OF CONDUCTION
Explain temperature
⬆️ temp = ⬆️ KE = ⬆️ Ion movement.
⬆️ temp = ⬆️ rate of respect = ⬆️ production of ATP - needed for NA/K pump
Why do birds + mammals transmit nervous impulses more quickly and have faster responses than all other groups of animals??
Because they are homeothermic taxa.
FACTORS AFFECTING SPEED OF CONDUCTION
Explain diameter of axon
Greater diameter = ⬆️ Na+ ions flow through axon = ⬆️ impulse speed.
Less resistance from cytoplasms to the flow of ions.
How big are the diameters of human non-myelinated axons?
0.2-1.5 micrometers.
How does the squid compensate for very low temperature in regards to speed of impulse conduction?
Giant axons w/ diameter up to 1mm.
Compare the speed of impulses along myelinated neurones + non-myelinated
M = 100m/sec
Um=1-3m/sec
How does saltatory conduction work?
AP at NofR causes Na+ ions to move into axon.
Na+ ions diff to areas of -ive charge further down con towards insulated axon.
VG Na+ channels ONLY present at NofR so a new AP only occurs at the next Nof R….
What are the 2 classes of synapse?
Electrical - 3nm across
Chemical - 20nm gap (too big for nervous impulse to jump). so branches of axons lie close to dendrites of other neurone but don’t touch. Impulse is transmitted by neurotransmitter.
Describe the process of synaptic transmission
AP arrives at axon terminal - synaptic knob which ⬆️ membranes permeability to Ca+ ions.
So, Ca+ ions diff in down conc grad.
Causes synaptic vesicles to move towards presynaptic membrane until they fuse w/ it + release acetylcholine into synaptic cleft by exocytosis.
Acetylcholine mol. diff across cleft to post-synaptic membrane to then bind to receptors on postsynaptic membrane.
Ligand-gated Na+ protein channels OPEN = Na+ diff into dendrite of postsynaptic neurone.
AP is generated in postsynaptic memb. - Depolarisation.
Cholinesterase breaks down acetylcholine in protein receptor = choline + acetate.
Na+ channels in postsynaptic membrane close - no more AP.
Choline + acetate diff back to presynaptic mem.
Energy from mitochondria is used to resynthesise acetylcholine + package into vesicles.
What are the neurotransmitters in synaptic transmission?
Acetylcholine molecules
What’s the difference between voltage gated channels + ligand gated channels?
VG = open in response to voltage
LG = open in response to a ligand (some chemical signal) binding to them.
What would happen if acetylcholine remained in the synaptic cleft?
Would constantly initiate new impulse in post-synaptic membrane + so impulses wouldn’t be distinct.
How is it ensured that acetylcholine doesn’t remain in synaptic cleft? (3)
- Direct uptake of acetylcholine into pre-synaptic neurone so none left in cleft to bind to post synaptic receptor.
- AT of Ca+ ions out of synaptic knob so no more exocytosis of acetylcholine occurs.
- Hydrolysis of acetylcholine
How do neurones transmit impulses in only 1 direction?
- Depolarisation happens behind AP + so depolarisation could not happen at that point.
- Synaptic vesicles only occur at s. knob of the pre-synaptic neurone.
- Neurotransmitter receptors only occur on post-synaptic membrane.
What are the functions of synapses? (7)
Transmit info between neurones across the junctions.
Flow of impulses in 1 direction = precision in nervous system.
Protects response system from overstimulation.
Filter out low-level stimuli
Allows acclimatisation
Convergence
Integration + divergence
FUNCTIONS OF SYNAPSES
Explain Integration + divergence
Impulse travelling down a neurone may reach a synapse which has several post synaptic neurones all going to different locations.
i.e sensory neurone detects a ⬇️ in temp so diverges impulse to:
- neurone — shiver
- neurone — hairs
- neurone — vasoconstriction
FUNCTIONS OF SYNAPSES
Explain convergence
Several presynaptic neurone might converge to 1 postsynaptic neurone.
Allowing signals from diff parts of body to create same response.
FUNCTIONS OF SYNAPSES
Explain flow of impulses in 1 direction = precision in nervous system.
Because vesicles containing transmitter are only in presynaptic neurone + the receptor molecules are only on the post synaptic membrane.
FUNCTIONS OF SYNAPSES
Explain acclimatisation
After repeated stimulation, a synapse might run out of the neurotransmitter.
i.e getting used to a smell or background noise.
FUNCTIONS OF SYNAPSES
Explain Filter out low-level stimuli
Post-synaptic neurone doesn’t reach threshold stimulus + no AP is generated due to low levels of transmitter.
How does the Action potential travel along an UNMYELINATED axon?
Influx of Na+ ions + depolarisation of membrane causes a local flow of ions (circuit). - This is due to the lateral flow of Na+ in the axon towards -ively charged regions.
Local current trigger the adjacent region of the axolemma to become more permeable to Na+ by opening the VG Na+ channels so causing a depolarisation in the next region.
The region of the membrane that has just depolarised is then non-responsive due to the inactivation of the Na+ channels so the region of depolarisation only moves in 1 direction.
Explain Acetylcholinesterase
Enzyme that hydrolyses acetylcholine in the synaptic cleft into choline + ethanoic acid.
These diff back across synaptic cleft + ATP is used to reform neurotransmitter molecules + store them in vesicles.
Where is Acetylcholinesterase released from
Cholinergic synapses - junction between a nerve and muscle
What drugs and how can drugs INCREASE synaptic transmission?
Agonists / excitatory drugs
- By mimicking the action of NT
- By inhibiting the breakdown of NT in synaptic cleft
- ⬆️ secretion of NT into synaptic cleft by exocytosis.
AGONIST DRUGS
Explain how they mimic the action of neurotransmitters
By having the same shape + bind to LG receptors on post-synaptic membrane = initiating an AP in the post synaptic membrane.
What drugs and how can drugs DECREASE synaptic transmission?
Antagonists / Inhibitory drugs
- By mimicking the action of the NT, blocking the LG receptors on post synaptic membrane-no AP in the post synaptic membrane.
- Preventing exocytosis of NT
- Prevent Ca+ diff into synaptic knob
- Prevent recycling/synthesis of NT
Give 2 examples of psychoactive drugs
Cocaine
Cannabis
How do psychoactive drugs affect the nervous system
Acts on CNS by altering brain function, causing temporary changes in perception, mood, consciousness + behaviour
Physical dependence may develop w/ sustained use.
May be excitatory or inhibitory drugs.
Give an example of an excitatory/ agonist drug
Cocaine
Give an example of an inhibitory/ antagonist drug
Morphine
Define excitatory/ agonist drugs
Stimulate nervous system by creating more AP in post-synaptic membranes
Define Antagonists / Inhibitory drugs
Inhibit the nervous system by creating fewer action potentials in post-synaptic membranes
Explain Organophosphorus insecticides
Inhibits acetylcholinesterase
= Prolongs the effects of acetylcholine which remains in the synaptic cleft and causes repeated firing of the post-synaptic cell.
This results in repeated firing of the neurones at the neuromuscular junction resulting in permanent contraction of the muscle.
Results in paralysis and breathing stops.
Briefly describe the reflex pathway
Stimulus
Receptor
(Sensory pathway)
CNS
(motor pathway)
Effector
Response
Fb
Briefly describe what happened in the experiments w/ ‘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
FACTORS AFFECTING SPEED OF CONDUCTION
Explain myelination
Speeds up the rate of transmission by insulating the axon.
A myelinated nerve fibre only depolarises at nodes of Ranvier where resistance is low.
Voltage gated channels only occur here so this is where Na+ ions enter.
== AP jumps from node to node === SALTATORY CONDUCTION
What does it mean that sensory receptors are transducers?
They detect energy in one form + convert it into electrical energy.
What do excitatory synapses do?
Transmit signals from 1 cell to the next by establishing impulses in the post-synaptic membrane
What can synapses be?
Excitatory
Inhibitory
== have an important homeostatic role
