Nerves - B16 Flashcards
What is the nervous system?
-the nervous system involves detection of stimuli by receptors, transmission of nerve impulses by neurones and response by effectors
-the nervous system gives a very rapid response to stimuli and involves the transmission of electrical impulses along neurones, and chemical transmission(neurotransmission) across synapses
Describe the structure of the nervous system.
-two systems: central nervous system(CNS) and the peripheral nervous system.
-CNS:brain and spinal cord
-peripheral: all the neurones that connect the CnS to the rest of the body
What nervous systems are involved in the peripheral nervous system?
-sensory nervous: carries nerve impulses from receptors toward the CNS
-motor nervous: carry impulses away from the CNS to effector
What is the motor nervous system split into?
-somatic(voluntary): controls conscious activity
-autonomic: controls unconscious activity
What does the autonomic nervous system split into?
-sympathetic: “fight or flight” that gears the body for action
-parasympathetic: “rest and digest” system that calms the body down
What are neurones?
Specialised cells which are adapted to their function# to cary electrical impulses from one part of the body to another
What are the 3 main types of neurone?
-sensory: transmit impulses from receptors toward the CNS
-motor: transmit electrical impulses from the CNS to effectors
-relay: transmit electrical impulses between sensory and motor neurones
What is the structure of a neurone?
-soma: nucleus and cytoplasm, produces proteins and neurotransmitters
-dendrites: carry impulses towards the soma
-axon: carry impulses away from the soma
-Schwann cells: produce myelin
-myelin sheath: myelin(lipid) insulate axon, increasing speed of transmission of impulse
-node of ranvier: gap between Schwann cells where myelin sheath is absent
-axon terminal/motor end plate: connect neurone to effector
What is the spinal cord?
-a column of nervous tissue that runs along the back and lies inside the vertebral column for protection
-pairs of nerves emerge at intervals along the spinal cord
What is the nervous response?
-when an electrical impulse reaches the end of a neurone, neurotransmitters (chemical messengers) are secreted directly onto cells- so the response is localised
-neurotransmitters are quickly removed once they’ve done their job, so the response is short-lived
-electrical impulses are really fast so the response is usually rapid-allowing animals to react quickly to stimuli
What is a reflex?
A reflex is a rapid, short-lived, localised and involuntary response, in response to a stimulus, to prevent further injury
-the pathway involved in a reflex is known as the reflex arc
Describe the reflex arc
1)stimulus
2)receptor
3)sensory neurone passes impulse to the spinal cord
4)relay neurone-in the spinal cord, links the sensory neurone to the motor neurone
5)motor neurone- carries impulses from the spinal cord to the effector
6)effector-causes a response - contraction
7)response
Why is the reflex arc important?
-involuntary- control of actions that do not need to be considered. This leaves the brain free to carry out more complex actions, as the brain won’t be overloaded with information that always requires the same response
-they protect the body from harm- they do not have to be learnt
-fast- neurone pathway is short, with very few synapses-important in withdrawal reflexes
-the absence of decision making means the action is rapid.
What is a nerve impulse?
-not an electrical current
-a temporary reversal of the electrical potential differences across the axon membrane
What is the resting membrane potential?
-in anuerones resting state(when it’s not being stimulated) the outside id the membrane is positively charged compared to the inside
-so the membrane is polarised- there’s a difference in charge(potential differences across or voltage) across it
-at rest voltage across the membrane is around -70mV
What is meant by the term polarised?
When the neurone is at rest and the charge is more negative inside the axon than outside
What is an electrochemical gradient?
A difference in electrical charge and distribution of ions
Explain the movement of sodium and potassium ions.
-the resting potential is created and maintained by sodium-potassium pumps and potassium ion channels in a neurones membrane
-sodium-potassium pumps use active transport to move 3 sodium ions(Na+) out of the neurone for every 2 potassium ions(K+) moved in. ATP is needed.
-K+ channels are open and allow facilitated diffusion of K+ out of the neurone, down their conc. gradient- making the axon membrane more permeable to K+.
-sodium ion channels are closed during a resting potential- axon membrane is impermeable to Na+.
How is an electrochemical gradient produced during resting potential?
The combination of the Na+/K+ pump; the K+ channels being open and the Na+ channels being closed produces an electrochemical gradient.
What is an action potential?
-when a stimulus is of sufficient size and is detected by a receptor, its energy causes a temporary reversal of these charges either side of the axon membrane.
-the negative charge inside the neurone of around -70mV becomes +40mV
-this is known as an action potential-in this condition this part of the axon is said to be depolarised
Why does depolarisation occur in an action potential?
Because channels in the membrane change shape because of channel voltage across the membrane- hence voltage-gated channels.
Explain the process of an action potential.
1)At resting potential some K+ voltage-gated channels are open(these are always open) but Na+ voltage-gates channels are closed.
2)energy from stimulus causes some Na+ voltage-gated channels to open, allowing Na+ to diffuse down the electrochemical gradient into the axon- triggers a reversal in voltage across the membrane.
3)If the P.D reaches threshold(around -55mV) more Na+ channels open, causing an even greater influx of Na+.
4)at around +30mV, the Na+ channels close- preventing further diffusion of Na+ and the voltage-gates K+ channels begin to open.
5)voltage-gated K+ channels open, allowing more K+ to diffuse down the conc. gradient starting the depolarisation of the axon.(remember before there was an electrical gradient, preventing more K+ from leaving the axon).
6)voltage-gated K+ channels are slow to close, so there’s a slight ‘overshoot’, where too many K+ diffuse out of the axon. The P.D. becomes more negative than -70mV. This is called hylerpolarisation.
7)Closable voltage-gated K+ channels now close and the activities of the Na+/K+ pump once again re-establishes a resting potential-this process is known as repolarisation.
What is the action of the sodium potassium pump at these stages of the action potential?
a)depolarisation
b)repolarisation
c)hyperpolarisation
a)Depolarisation
-Na+channels open so Na+ can diffuse into the axon
-causes more Na+ channels to open so more Na+ can diffuse into the axon- positive feedback
-P.D. from -70mV to +40mV
b)repolarisation
-Na+ channels close and k+ channels open so K+ diffuse out of the axon
-P.D. from +40mV to -70mV
c)hyperpolarisation
-K+ channels are slow to close so too many K+ diffuse out
-P.D. from -70mV to -90mV
-voltage-gate channels ultimately close
What is the all or nothing principle?
-there is either an action potential or there isn’t
-in order for an action potential to occur, threshold must be reached, where enough Na+ channels open in order for enough Na+ to diffuse into to make the ,enrage depolarised enough to cause an action potential
-if the stimulus is small and not enough Na+ channels open, not enough Na+ will diffuse into, causing an insufficient depolarisation to occur, threshold not reached=no action potential.
How does the body acknowledge a stronger stimulus?
Increased frequency of impulses
What is the refractory period?
-the time where no new action potentials can occur
-because Na+ channels are closed and unable to open
-this is an advantage because it keeps action potentials travelling unidirectionally down an axon
What is saltatory conduction?
-myelin sheath(lipid) acts as an insulator and prevents Na+ and K+ from passing through the axon membrane
-allows saltatory conduction to occur, where depolarisation will only occur at nodes of ranvier.
- action potentials jump from node to node
-if the axon is unmyelinated- action potentials pass slower along the axon, as the whole length of the axon has to be depolarised
How can temperature increase speed of an action potential?
-when hotter - ions have more kinetic energy
-(facilitated) diffusion of Na+ and K+ happens faster
-faster depolarisation/action potential/neurotransmission
How can axon diameter increase the speed of an action potential?
-the thicker the axon, the faster the rate of transmission
-greater surface area
-greater diameter=more ion channels over one area of the axon=less resistance to the flow of Na+ into the axon=faster depolarisation
-an action potentials jump in one part of the axon pushes the next section to threshold more quickly.
What is a synaptic knob?
The swollen end of the terminal branches of another neurone
What is a cholinergic synapse?
Those that use the neurotransmitter acetylcholine(Ach)
What is acetylcholinesterase?
-an enzyme that hydrolyses acetylcholine into separate acetyl (ethanoic acid) and choline.
-sodium ion channels close
-the acetyl and choline diffuse back across the cleft into the presynaptic neurone and are reuptaken
-this allows the neurotransmitter to be recycled.
Why do we break down acetylcholine?
-if the neurotransmitter is not broken down this could allow it to continuously generate new action potentials
-breaking down acetylcholine prevents this.
Describe the process of synaptic transmission.
1)action potential from the axon causes depolarisation of the pre-synaptic neurones membrane
2)this causes Ca ion channels to open so calcium ions diffuse in.
3)this causes vesicles containing neurotransmitter(acetylcholine) to move towards and fuse with the pre-synaptic membrane
3)neurotransmitters are released into the synapse
4)neurotransmitters bind to receptors on the pot-synaptic membrane
-causes depolarisation
-if depolarisation is large enough to reach threshold, an action potentials pass slower will be produced.
Why can a nerve impulse only travel in one direction?
-vesicles containing neurotransmitters are only released from the presynaptic membrane
-receptors are only present on the postsynaptic membrane
What is summation?
-if the stimulus is weak, a small amount of neurotransmitter will be released from the neurone into the synaptic cleft
-may not be enough to cause enough Na+ channels to open and not enough Na+ to diffuse into the postsynaptic neurone, so only a small depolarisation occurs so threshold is not reached so no AP.
-summation is where the effect of neurotransmitters released from many neurones(or one neurone that’s stimulated more frequently in a small amount of time) is added together.
What is spatial summation?
-two or more presynaptic neurones release their neurotransmitters at the same time onto the same post synaptic neurone
-more neurotransmitter is released->enough Na+ channels open, enough Na+ will diffuse in->cause a big enough depolarisation to reach threshold->action potential produced
What is temporal summation?
-two or more action potentials arrive in quick succession from the same presynaptic neurone
-more neurotransmitter released->enough Na+ channels open, enough Na+ will diffuse in->cause a big enough depolarisation to reach threshold ->action potential produced
What does an excitatory neurotransmitter do?
Causes action potentials
What does an inhibitory neurotransmitter do?
-reduces the resting potential by opening Cl- channels or stimulating K+ channels to open
-less likely to reach threshold
-more Na+ needed to cause an action potential
What happens if a cell is affected by both excitatory and inhibitory synapses?
The effect of Na+ entering may be cancelled out. Threshold level isn’t reached so action potential not produced.
Why is inhibition important?
-allows specific pathways to be stimulated while preventing random impulses all over the body
-allows you to learn specific skills and control movements. Pathways become refined by inhibitory circuits.
What can happen if some neurones release inhibitory neurotransmitters?
The total effect may be no action potential
What is a neuromuscular junction?
Between a motor neurone and a muscle cell(where the terminal branches ‘connect’ with a muscle cell).
Similar to cholinergic synapses.
What neurotransmitter is usually used by neuromuscular junctions?
-uses acetylcholine as a neurotransmitter
-this binds to receptors called(nicotinic) cholinergic receptors
What is the same between a neuromuscular junction and synapse?
-use acetylcholine
-use acetylcholinesterase
-neurotransmitters move by diffusion
-binding of neurotransmitters cause an influx of Na+
What are the differences between neuromuscular junctions and synapses?
-a synapse is a junction between two neurones, NMJ is a junction between a motor neurone and a muscle cell
-NMJ always use ACh, synapses can also use adrenaline and noradrenaline and other neurotransmitters
-NMJ are always excitatory, synapses can be inhibitory or excitatory
-when an action potential reaches muscle, it always triggers a response
-lots of folds/clefts. These store acetylcholinesterase.
-NMJ post synaptic membrane has more receptors
