3.8 Nervous System Flashcards
Define stimuli, receptors, reflex arc, effectors and response
- changes that can be detected by the body
- cells that detect changes
- pathway of neurones leading to a rapid automatic response
- muscles or glands that carry out the response
- how the body reacts to a stimulus
State the main roles of the nervous system
- detects changes in body and environment
- process and store information
- initiates responses
Describe the CNS
- brain and spinal cord
- processes info from stimulus
Structure of spine
- surrounded by meninges
—> three membranes: dura mater, arachnoid, pia mater)
—> line brain and spinal cord - white matter contains nerve fibres surrounded by myelin
- grey matter contains nerve fibres of relay neurones and cell bodies of relay and motor neurones
Describe the PNS
- somatic nervous system - pairs of nerves that originate in brain or spinal cord, and their branches
—> fibres of sensory neurones and motor neurones - autonomic nervous system - unconscious control of the functions of internal organs
Draw and label a sensory, relay and motor neurone
Draw and label spinal cord
Describe a nerve net
- simplest type of nervous system
- diffuse network of cells that group into ganglia but don’t form a brain
—> ganglion cells provide connections in several connections
—> sensory cells detect stimuli
Compare nervous system of hydra v human
- nerve net vs CNS
- 2 cell types vs many
- rapid regeneration vs very slow if at all
- absent myelin sheath vs present myelin sheath
- slow conduction vs fast conduction
Role of sensory, motor and relay neurones
Sensory: carry impulses from receptors to CNS
Motor: carry impulses from CNS to effectors
Relay: carry impulses from sensory to either relay or motor in CNS
Functions of the cell body, cytoplasm, nucleus, nissl granules, dendrite, and axon in nerve cells
- cell body: contains a nucleus and granular cytoplasm
- cytoplasm: contains many ribosomes
- nucleus: holds DNA
- nissl granules: cytoplasmic granules comprising ribosomes grouped on RER
- dendrite: thin fibre carrying impulses toward cell body
- axon: thin fibre carrying impulses away from cell body
Functions of Schwann cells & myelin sheath
- glial cells that surround and support nerve fibres
- wrap around developing axons many times and withdraw their cytoplasm, leaving a multi layered phospholipid myelin sheath
—> myelin sheath is an electrical insulator that speeds up transmission of impulses
Function of nodes of ranvier
- gaps in myelin sheath where adjacent Schwann cells meet and axon membrane is exposed
- allows impulses to be transmitted rapidly by saltatory propagation
Function of synaptic end bulb and axon terminal
- synaptic end bulb: swelling at end of axon where NTs are synthesised
- axon terminal: secretes NT which transmits impulse to adjacent neurone
What is meant by a nerve?
A bundle of neurones
Define saltatory propagation/conduction
- transmission of a nervous impulse along a myelinated axon, in which the action potential jumps from one node of ranvier to the adjacent node
Define resting potential
The potential difference across the membrane of a cell when no nervous impulse is being conducted
Why can neurones change their resting potential?
They are excitable cells
Describe how a resting potential is generated
- inside of cell has higher conc of K+ ions and lower conc of Na+ than outside
- K+ diffuse out and Na+ in
- some channels that allow K+ ions to diffuse out are open, whilst most of channels allowing Na+ to move in are closed
- axon membrane much more permeable to K+ ions which diffuse out faster than Na+ diffuse back in
- sodium-potassium exchange pumps pump K+ ions back into cell and Na+ ions back out
- transmembrane proteins with ATPase that transport K+ and Na+ ions against conc gradient by active transport
- maintain conc and uneven distribution of ions across membrane
- Na+ ions out faster than K+ in so inside membrane more -ve than outside
Define action potential
The rapid rise and fall of electrical potential across a nerve cell membrane as a nervous impulse passes
Define a nervous impulse
Transmission of a change in potential along a nerve fibre associated with the movement of sodium ions
Describe how action potential is generated
- energy of stimulus causes voltage gated sodium channels in axon membrane to open
- sudden increase in permeability of membrane to Na+ ions allows them to rapidly diffuse into axon down conc gradient
- charge from -70mV to 40mV
—> cell membrane is depolarised - sodium channels close to prevent further influx of sodium ions
- potassium channels open and K+ ions diffuse out down conc gradient
- cell becomes less positive inside as more diffuse out and membrane is repolarised
- more K+ ions diffuse out than Na+ in so potential difference across membrane becomes even more -ve than resting potential
—> membrane is hyperpolarised - sodium-potassium pumps pump K+ ions back in and Na+ ions out, restoring ion balance of resting potential
Define depolarisation
A temporary reversal of potential across the membrane of a neurone such that the inside becomes less negative than the outside as an action potential is transmitted
Draw and annotate action potential graph
Describe how action potential travels along an axon
- at the site of an action potential, Na+ ions diffuse into axon and K+ diffuse out
- reversal of potential sets up local current as Na+ ions diffuse along axon
- they depolarise the adjacent section of the membrane
- this opens more voltage gated sodium channels in those regions and more Na+ flood in, depolarising axon
- Na+ diffuse further down axon and a self-perpetuating wave of depolarisation spreads along axon
- at site of initial action potential, sodium channels are inactivated and cannot open again until resting potential reestablished so new action potential cannot be initiated there
- this is the absolute refractory period
- ensures action potential is not protracted back in direction from which it came, and nervous potential travels in one direction only
- relative refractory period then occurs where if an impulse is strong enough, a new action potential may pass
- occurs while sodium-potassium pumps are restoring resting potential
What is meant by the all or nothing law?
- a nervous impulse is either initiated or it is not, and it is always the same size
- increase in intensity of stimulus does not give greater action potential - frequency of action potential increases instead
- allows action potential to act as a filter, preventing minor stimuli from setting up nervous impulses so brain is not overloaded
How does temperature impact speed of conduction of nervous impulse?
- ions move faster at higher temp as have more KE
- warm blooded animals transmit nervous impulses quicker than all animals
How does diameter of the axon impact speed of conduction of nerve impulse?
- greater diameter of axon = greater volume in relation to area of membrane
- more Na+ ions can flow through axon so impulses travel faster
How does myelination impact speed of conduction of nervous impulses?
- speeds up rate of transmission by insulating axon
- sodium ions flow through axon but a myelinated nerve fibre only depolarises at the nodes of ranvier
- voltage gated ions only occur at nodes of ranvier so this is where sodium ions enter
- action potential therefore jumps from node to node by saltatory propagation
Differentiate between an electrical and chemical synapse
- electrical: 3nm across, small enough that electrical impulse is transmitted directly from one neurone to the next
- chemical: 20nm across, impulse transmitted by NT which diffuses across synaptic cleft
Define a neurotransmitter
A chemical secreted in response to an action potential which carries a chemical signal across a synapse from one neurone to the next, where a new action potential is limited
Define a neurotransmitter
A chemical secreted in response to an action potential which carries a chemical signal across a synapse from one neurone to the next, where a new action potential is limited
Describe synaptic transmission
- arrival of impulse at synaptic end bulb alters its membrane permeability, opening voltage-dependant calcium channels, so calcium ions diffuse to end bulb down conc gradient
- influx of calcium ions causes synaptic vesicles to fuse with the presynaptic membrane which releases the NT acetylcholine into the synaptic cleft by exocytosis
- NT diffuse across synaptic cleft and binds to receptor protein
—> 2 subunits, 2 molecules of acetylcholine bind with cooperative binding - when acetylcholine binds, the receptor protein changes shape, opening a channel for Na+ ions to diffuse in
- post synaptic neurone is depolarised, and an action potential is initiated
3 ways to prevent acetylcholine remaining in the synaptic cleft
- direct uptake of acetylcholine into pre synaptic neurone so none remains in synaptic cleft
- active transport of calcium ions out of synaptic end bulb, so no more exocytosis of acetylcholine occurs
- hydrolysis of acetylcholine - destroyed by acetylcholinestarase in cleft to produce chlorine and ethanoic acid which diffuse back to pre synaptic neurone, reforming acetylcholine
Why are there mitochondria in axon end bulbs?
Energy is required to reform acetylcholine and for exocytosis
Why do neurones only transmit an impulse in one direction?
- hyper-polarisation happens behind action potential so depolarisation could not happen at that point
- synaptic vesicles only occur at end bulb of presynaptic neurone
- neurotransmitter receptors are only found on post-synaptic neurones
Describe the properties of synapses
- transmit information between neurones
- lass impulses in one direction, generating precision in nervous system
- act as junctions
- protect the response system from overstimulation because the impulse is always the same size
- filter out low-level stimuli
—> action potential only initiated when depolarisation is large enough to reach a threshold value built by either temporal summation or spatial summation
Briefly describe temporal summation and spatial summation
Temporal: depolarisation builds up over time to reach the threshold at which action potential is initiated
Spatial: several presynaptic neurones synapse with the same post-synaptic neurone and all contribute to the growing depolarisation which generates a large enough action potential
Define a drug and brief overview of their impact
Drug: molecule that has a physiological effect on the body when ingested, inhaled, absorbed, or injected
—> act at synapses and disrupt normal functions of NT, producing abnormal patterns of nervous impulse
Differentiate between sedatives and stimulants
Sedative: inhibit nervous system creating fewer action potentials in post-synaptic neurones
Stimulants: stimulate nervous system by allowing more action potentials in post-synaptic neurones Stimulants
Describe how drugs that mimic action of NTs work
- similar shape and bind to post synaptic neurone in same way a NT would, increasing the frequency of action potential
—> ie nitrogen atoms in nicotine and acetylcholine allow them both to bind to the same receptor, but nicotine cannot be removed by hydrolysis so continues to initiate impulses
—> body becomes habituated to nicotine so nervous system can’t function without
—> nicotine leads to increase in dopamine levels
Describe how drugs that prevent the breakdown of NTs work
- organophosphate inhibit acetylcholinestarase meaning acetylcholine remains in the synaptic cleft causing repeated firing of the post synaptic neurone
—> phosphates of esters found in insecticides, herbicides, and nerve gases
—> nerve gases inhibit acetylcholinestarase at neuromuscular junctions generating repeated, uncontrollable contractions of muscles that can break bones
Explain how lidocaine acts as a local anaesthetic
- sodium ions o longer enter axon
- threshold not reached
- no action potential align sensory neurone to brain
Describe how a chemical that blocks calcium channels on the presynaptic neurone could work as an anaesthetic
- calcium ions don’t diffuse into synaptic end bulb
- prevents synaptic vesicles fusing w membrane
- prevents NT release into synapse
Name some drugs that mimic NTs by activating receptors
- morphine
- nicotine
- marjuana
Name some drugs that block dopamine pumps
- cocaine
- amphetamine
Name some drugs that activate or inhibit channels
- alcohol
- ketamine
How do psychoactive drugs affect the brain?
- can increase or decrease release of NTs
- breakdown reuptake proteins which are responsible for returning used components of NT to presynaptic
- block reuptake proteins
- mimic or block NTs binding to the receptors on post synaptic membranes
- inhibit production of new NTs
Explain action of inhibitory drugs
- mimic inhibitory NT
—> increase hyperpolarisation of post-synaptic neuron leading to fewer action potentials - block excitatory NTs
—> prevent propagation of action potentials
Explain action of excitatory drugs
- block reuptake pumps for NTs
—> NT remains in synapse - mimic excitatory NTs
—> increase depolarisation of post-synaptic neurons
—> more action potentials - block inhibitory NTs
—> prevent inhibition of action potentials
How is a resting potential maintained?
- imbalance of Na and K ions
- more Na out and more K in
- Na-K pump actively transports 3Na out and 2K in which creates electrochemical gradient
- sodium build up outside and potassium build up inside
- K+ move out and Na+ move in
- membrane is more permeable to K+ so moves out quicker than Na+ moves in
How is resting potential maintained?
- Na-K uses ATP to actively transport three Na+ out of the neurone and 2 K+ into the neurone
- K+ channels are leaky and so the ions pass out of the cell by facilitated diffusion
- Na+ channels are closed so Na+ ions remain outside the neurone
- overall effect of the ion movement is the inside of the neurone being more negative than the outside
