Unit 3.8 - The nervous system Flashcards
What kind of animals have nervous coordination?
Only higher animals (e.g - mammals) have developed it
Purpose of the nervous system
To respond to changes in the external or internal environment, and do something to respond to these changes
Name for the things we respond to with the nervous system
Stimuli
(Singular = stimulus)
How would the nervous system act when the sun comes through the window?
Receptor cells in the retina in the eyes convert the signal into an electrical signal that’s transmitted to the brain = react
What does the nervous system allow us to do?
Respond to changes in our environment
Stimulus
Any detectable change in the internal or external environment of the organism
What do receptor cells act as and how?
Transducers
They detect energy in one form and convert it into electrical energy
what act as transducers in the nervous system?
Specialised receptor cells
How does electrical energy travel along neurones?
As a nerve impulse
What is a nerve impulse?
Electrical energy travelling along neurones
Neurones
Nerve cells
What do nerve impulses do?
Initiate a response in an effector
What is an effector?
A muscle or a gland
What do muscles and glands do as part of the nervous system?
Act as effectors
What are the 2 main parts of the nervous system?
The central nervous system (CNS)
The peripheral nervous system (PNS)
What is the central nervous system composed of?
The brain and spinal cord
What does the central nervous system do?
Processes information provided by a stimulus and coordinates a response
What is the peripheral nervous system made up of?
Neurones (nerve cells)
Which part of the nervous system then has two further parts?
The peripheral nervous system
2 parts of the peripheral nervous system
The somatic nervous system
The autonomic nervous system
The somatic nervous system:
What is it made up of?
What do these do?
Made up of pairs of nerves branching from the brain and spinal cord
These neurones carry impulses from receptor cells to the CNS and then from the CNS to the effectors
What does the autonomic nervous system do?
Provides unconscious control of the internal organs (e.g -heartbeat and breathing)
Which part of the peripheral nervous system is made up of pairs of nerves branching from the brain and spinal cord?
The somatic nervous system
Which part of the nervous system is composed of the brain and spinal cord?
The central nervous system
Which part of the peripheral nervous system provides unconscious control of the internal organ?
The autonomic nervous system
What is the nervous system made up of?
Neurones
What type of neurone is the one we have a detailed labelled diagram of?
A motor neurone
Which part of neurones contains the nucleus and most organelles?
The cell body
What does the cell body of a neurone contain?
The nucleus and most organelles
What come from the cell body of a neurone?
Many extensions
Names for the extensions from the cell body of a neurone
Many extensions - dendrites
1 long extension - axon
What do the dendrites of a neurone do?
Conduct nerve impulses towards the cell body
What does the axon of a neurone do?
Conducts nerve impulses away from the cell body towards the axonal terminals
Name for the ends of the axonal terminals
Synaptic end bulbs
What happens at the synaptic end bulbs of the neurone?
Where the neurone connects to another neurone or a muscle or a gland (effector)
What do Schwann cells form?
Form the myelin sheath
What form the myelin sheath?
Schwann cells
Describe Schwann cells
Wrapped around along the axon many times in several layers
What are Schwann cells wrapped around many times in several layers?
The axon
Purpose of Schwann cells wrapping around the axon many times in several layers
Provide a form of electrical insulation for the axon
Why do Shwann cells need to provide a form of electrical insulation for the axon of a neurone?
The electrical insulation increases the speed of nerve impulses along the axon
Do Shwann cells form a complete sheath?
No
Gaps between Shwann cells
Nodes of Ranvier
Why are nodes of Ranvier important?
Important in the way in which the nerve impulse propagates along the axon
Describe neurones
Highly specialised cells which carry nerve impulses in one direction
How are nerve impulses carried?
In one direction by neurones
Three main types of neurones in a vertebrate
Sensory
Relay
Motor
What do sensory neurones do?
Carry nerve impulses from the receptor cells in the sense organ to the CNS
What do relay neurones do?
Connect the sensory and motor neurones
Where are relay neurones found?
In the CNS
What do motor neurones do?
Transport nerve impulses from the CNS to the effectors (muscles and glands)
Where are sensory, relay and motor neurones found?
In both the somatic and central nervous systems
Another word for Shwann cells
Myelin sheath
What does the plasma membrane of a Shwann cell do?
Provides electrical insulation to the axon
Describe the plasma membrane of Shwann cels
Layers of membrane
Where is axoplasm in Shwann cells and what does it do?
Inside the axon
Contains organelles (e.g - mitochondria)
What is the plasma membrane of Shwann cells made up of?
Phospholipids
Why is the plasma membrane of Shwann cells a good electrical insulator?
Phospholipids contain non-polar fatty acid tails, making the membrane a good electrical insulator (non-polar)
Between the membranes of Shwann cells
Myelin
What does myelin do in Shwann cells?
Is a fatty substance that also acts as an electrical insulator
Reflex arc
A mechanism that controls a reflex
Describe the features of reflex responses
Unconscious (doesn’t go through the brain)
Innate (present in an individual from birth + not learnt)
Protects us from injury
Rapid, automatic, beneficial
How come reflex responses are unconscious?
Don’t go through the brain
What does it mean that reflex responses are innate?
Present in an individual from birth and not learnt
Give 2 examples of reflex reponses
Hand moving away from a hot surface
Blink response when something approaches your eye
What’s the purpose of the gaps between the vertebrae of the spinal cord?
Allow spinal nerves to branch off from the spinal cord and branch into the various parts of the body
How many neurones make up the branches from the spinal nerves?
Thousands
What do reflex responses rely on?
The 3 types of neurones previously mentioned
What’s the name for the region at the back of the spinal cord?
Dorsal region
What’s the name for the region at the front of the spinal cord?
Ventral region
Dorsal region of the spinal cord
Back of the spinal cord
Ventral region of the spinal cord
Front of the spinal cord
What are all of the different features that the transmission of a nerve impulse along a three-neurone reflex arc involves?
Stimulus
Receptor
Sensory neurone
Relay neurone (in CNS)
Motor neurone
Effector
Response
Explain exactly how a nerve impulse is transmitted along a three-neurone reflex arc
Stimulus
Receptor that initiates a nerve impulse to travel along the…
Sensory neurone. The axon of the sensory neurone enters the spinal cord via the dorsal root. It first enters the dorsal root ganglion, which is where the cell body of the sensory neurone is.
The axon of the sensory neurone then continues to the grey matter, where it forms a synapse with the…
Relay neurone (in CNS). The replay neurone then forms a synapse with the…
Motor neurone, which carries the nerve impulse to the…
Effector to give a response
Example of a receptor involves in a reflex response
Temperature receptors in the skin
What initiates the nerve impulse that travels along the sensory neurone?
Receptors
How does the axon of the sensory neurone enter the spinal cord?
Via the dorsal root
Where is the cell body of the sensory neurone?
The dorsal root ganglion
What is in the dorsal root ganglion?
The cell body of the sensory neurone
Where does the sensory neurone form a synapse with the relay neurone?
In the grey matter
Where does the relay neurone form a synapse with the motor neurone?
In the grey matter in the spinal cord
Which neurone carries the nerve impulse to the effector to give a response?
The motor neurone
Why is the great matter in the spinal cord grey?
Lots of relay neurones and cell bodies (cell bodies of both relay and motor neurones)
The cel bodies of which neurones are in the grey matter of the spinal cord?
Relay and motor neurones
Explain why white matter in the spinal cord is white
Full mostly of axons
Axons contain Shwann cells and since these make the axon myelinated, this is a fatty substance so the area appears white
Word for Shwann cells contained myelin
Myelinated
Explain what the spinal cord is
A flattened cylinder of nervous tissue
Where does the spinal cord run from and to?
Form the base of the brain to the lumbar region
What is the spinal cord protected by?
Vertebrae
Describe the central canal of the spinal cord
Runs all the way through
Is filled with cerebrospinal fluid, which also surrounds the spinal cord
Where is cerebrospinal fluid found?
Fills the central canal of the spinal cord
Surrounds the spinal cord
What is cerebrospinal fluid responsible for?
Carrying nutrients
Absorbing shocks to avoid damage to the spinal cord
What is responsible for absorbing shocks to avoid damage to the spinal cord?
Cerebrospinal fluid
Number of effectors in simple invertebrate organisms + explanation
Small
Respond to a limited number of stimuli
What type of organisms only respond to a limited number of stimuli?
Simple invertebrate organisms
Example of a simple invertebrate organism
Hydra
What do simple invertebrate organisms such as Hydra have as opposed to a nervous system?
Nerve net
What does a nerve net not have that a nervous system does?
No brain
No spinal cord
No central nervous system
No myelinated axons
Briefly explain what a nerve net is
Simple nerve cells with short extensions joined to each other and branching in a number of different directions
What can a simple nerve net only respond to?
A limited number of stimuli
What does a simple nerve net have as a result of only being able to respond to a limited number of stimuli?
A small number of effectors
Compare the nerve net in hydra to the nervous system of mammals
(First is hydra, second is mammals)
Nerves branch in all directions, nerves branch in one direction (along the axon)
Impulses travel slowly, impulses travel at high velocity
Only one type of nerve cell, three types of neurone: sensory, relay and motor
Short branches, axons can be very long e.g - from the spinal cord to the tips of fingers in humans
Can we detect the source of a stimulus in nerve nets? Why?
No, since the nerves branch in all directions
What is maintained across the axon membrane when no nerve impulse is being transported along the axon?
A resting potential
Describe the axon membrane at the resting potential
Charged (outside positive, inside negative)
Potential difference across the axon membrane during a resting potential
-70mV
What does the axon have to do to set up the resting potential and how is this one?
Has to separate charges across the membrane
Uses active transport proteins; sodium-potassium pumps in the axon membrane
What type of active transport proteins are used in order to set up the resting potential of the axon?
Sodium-potassium pumps
Explain in detail how the resting potential is set up in the axon membrane?
1.) sodium-potassium pump actively transports sodium ions out of the axon into tissue fluid and pumps potassium ions into the axon
2.) even though both of these ions are positively charges, there is a potential difference formed since the sodium-potassium pump pumps 3 Na+ out of the axoplasm and only 2 K+ in
3.) now there is an unequal distribution of sodium and potassium across the membrane, with more positive charges outside of the axon, so the membrane is polarised
4.) also, since there is now a concentration gradient and the axon membrane is highly permeable to K+, they leak out by facilitated diffusion through open channels, which contributes to the charge distribution
What is required to maintain the resting potential of the axon and why?
Energy in the form of ATP
Active transport is occurring
How is ATP provided for setting up the resting potential of the axon?
Produced by the numerous mitochondria present in the axoplasm of the axon
What does the use of ATP to maintain the resting potential of the axon prove?
We require ATP even when resting
What happens when a nerve impulse is generated in the axon?
The action potential
What are nerve impulses caused by?
A rapid change in the permeability of the neurone membrane to K+ and Na+
Describe the action potential in an axon in detail
- The membrane of the axon also has specific channels for Na+ and K+
- Since these channels are voltage-gated channels, they can close to prevent them from allowing ions through
- When an impulse arrives, the sodium ion cancels on the membrane open first
- Na+ flood into the axon down their concentration gradient
- Potential across the membrane changes from -70mV (resting potential) to +40mV (action potential) —> the membrane is said to be depolarised
- About a millisecond after the sodium ion channels close, the potassium ion channels are open
- K+ diffuse out of the axon down their concentration gradient
- Polarity is restored —> repolarises the membrane
- An excess of K+ leave the son before the Na+/K+ pump restored the resting potential. This is known as the refractory period during which no further action potentials can occur
Which ion channels open first when a nerve impulse arrives in the axon?
Sodium
Potential of the membrane at the action potential
+40mV
What is the membrane said to be when at +40mV?
Depolarised
What happens dung the refractory period of the action potential?
An excess of K+ leave the axon before the Na+/K+ pump restored the resting potential
No further action potentials can occur
When during the action potential can no further action potentials occur?
The refractory period
How is the polarity restored in the axon during a nerve impulse?
When K+ diffuse out of the axon down their concentration gradient
List the stages of the action potential of a nerve impulse
Resting potential
Threshold potential
Depolarisation
Action potential
Repolarisation
Hyperpolarisation - refractory period
What type of response is action potential?
All or nothing
What do we mean when saying that action potential is an “all or nothing” response?
Same size of polarisation and depolarisation each time
Explain when action potential would be an “all” and when it would be a “nothing” response
If the stimulus isn’t strong enough for enough sodium ions to cross the membrane and we don’t reach the threshold, the impulse drops to the resting potential
If enough sodium ions diffuse into the axon and we pass the threshold, we get the whole action potential
Under which circumstance would the whole action potential not occur? What would happen instead?
If the stimulus isn’t strong enough for enough sodium ions to cross the membrane and we don’t reach the threshold, the impulse drops to the resting potential
When do we get the whole action potential?
If enough sodium ions diffuse into the axon and we pass the threshold we get the whole action potential
Explain what is happening when the line shoots up on the oscilloscope graph of action potential
This is where the membrane is depolarised
Sodium ions are diffusing rapidly into the membrane (channels are open)
what causes the axon to depolarise?
Sodium ions diffusing in
How do we generate an action potential graph?
Using an oscilloscope to measure the potential difference
What causes repolarisation of the membrane of the axon?
Potassium diffusing out of the axon out of their channels down a concentration gradient
What is the hyper polarisation of the membrane of the axon?
When the line falls below the resting potential when repolarised
What do we need for the refractory period of the axon potential?
Hyperpolrisation of the membrane
What happens during the refractory period of the action potential?
The sodium-potassium pumps restore the resting potential by pumping the ions back
A new action potential can’t be formed until the resting potential has been restored
During which point can’t a new action potential by formed?
The refractory period
What does the refractory period ensure?
The separation between action potentials, no matter how strong the stimulus
What would happen if the refractory period didn’t occur?
One big action potential would form and the central nervous system wouldn’t be able to tell the different between strong and weak stimuli
How does the central nervous system know the difference between strong and weak stimuli?
It’s due to the frequency of the impulse in the axon
Example of a situation where the central nervous system would need to differentiate between a strong and weak stimulus
Reaping to a warm v.s boiling surface
Describe the frequency of a strong stimulus
High frequency of action potentials
Describe the frequency of a weak stimulus
Low frequency of action potentials (less impulses produces per second)
With the nerve impulse process described, what kind of axon was this occurring on?
Non-myelinated
What type of axons are in our cells?
Myelinated
What do the Schwann cells do and what does this act as?
Wrap around the axon and secrete a fatty myelin sheath which is an electrical insulator
Where in a myelinated axon can depolarisation not occur and why?
Wherever Schwann cells are
Schwann cells secrete a fatty myelin sheath which is an electrical insulator
What cannot occur on the axon where a Shwann cell is present?
Depolarisation of the axon
What are the only parts of a myelinated axon that can become depolarised?
The nodes of Ranvier
What do the nodes of Ranvier contain in a myelinated axon to allow them to become depolarised?
Voltage gated Na+ channels
What happens in a myelinated axon in terms of the cation potential?
Cation potential of the nerve impulse jumps from one node to the next, which speeds up the rate of transmission
What type of axons do nerve impulses travel fastest along?
Myelinated
Name for depolarisation only occuring in the nodes of ranvier of a myelinated axon
Saltatory conduction
Saltatory conduction
Depolarisation only occuring in the nodes of ranvier in a myelinated axon
What increases the rate of transmission in a myelinated axon?
Greater distance between the nodes
What type of species are the only ones to have myelinated axons?
Vertebrates
Symptoms of someone with demyelinated neurones
Feel tired
Slower reaction times
Paralysis
Advantages of having myelinated axons
Increased rate of conduction
Less ATP required (to restore resting potential) = aerobic respiration can occur at a lower rate
What type of respiration can occur at a lower rate with myelinated axons?
Aerobic
Another way of speeding up the impulse along the axon that isn’t myelination
Have a bigger axon
Effect of having a bigger axon on the speed of the impulse along the axon +explanation
The greater the diameter of the axon, the lower the resistance to the movement of ions
Correlation between the axon diameter and conduction velocity?
Positive
Example of creatures with giant axons
Cephalopod molluscs (e.g - squids)
Why do squids need giant axons?
Have unmyelinateed axons
Are very active and move very fast and so they need a very efficient nervous system
Size of squid axons + importance of this
1mm in diameter
Important in our understanding of the nervous system
Allow them to react quickly in low temperatures = conduct nerve impulse quickly
Synapse
The junction between 1 neurone and a cell
Could be an effector cell (e.g - muscle or gland) or another neurone
Functions of the synapse
Transmit information from neurone to neurone
Pass impulses in one direction
Act as junctions
Prevents overstimulation
Filter out low level stimuli
In which direction does synaptic transmission occur?
In 1 direction only
Example of a neurotransmitter substance involved in synaptic transmission
Acetylcholine
What is acetylcholine?
A neurotransmitter substance
Do neurotransmitters vary?
Yes - there are different neurotransmitters in different parts of these nervous system
3 examples of neurotransmitters
Oxytocin
Serotonin
Acetylcholine
Explain synaptic transmission in detail
1.) when a nerve impulse (actionpotential) arrives at a synaptic end bulb, it causes voltage-dependent calcium channels in the membrane to open
2.) calcium ions diffuse rapidly into the pre-synaptic end bulb down their concentration gradient
3.) in the pre-synaptic neurone end bulb, there are vesicles that contain a neurotransmitter ( a chemical produced by the cell) substance. Ca2+ stimulates here synaptic vesicles that contain neurotransmitters (e.g - acetylcholine) to move towards and fuse with the pre-synaptic membrane
4.) the contents of the vesicles are released into the synaptic left by exocytosis
5.) the neurotransmitter diffuses along the synaptic cleft and binds to a receptor (a transmembrane protein) in the post-synaptic membrane
6.) sodium ion channels in the post synaptic membrane open and sodium ions diffuse into the post-synaptic neurone
7.) this causes depolarisation
8.) if the threshold potential is reached, an action potential is initiated in the post synaptic neurone and starts a nerve impulse
What do neurotransmitters do?
Start a response in the effector cell
Why can’t neurotransmitters stay in the synaptic cleft?
Would keep stimulating the post synaptic cell
Enzyme that removes neurotransmitters in the synaptic cleft
Acetylcholine esterase
Acetylcholine esterase function
Removes neurotransmitters from the synaptic cleft
What happens when acetylcholine esterase is present in the synaptic cleft?
Without the neurotransmitter, there’s no longer an impulse set up in the post synaptic nerve
The products are absorbed back into the pre-synaptic cell to be reused
How are the products of the reaction between acetylcholine esterase and neurotransmitters in the synaptic cleft absorbed back into the post-synaptic cell to be reused?
Because of reuptake pumps - proteins in the membrane of the pre synaptic nerve that actively transport neurotransmitters back in
2 main types of drugs
Inhibitory drugs (antagonists)
Excitatory drugs (agonists)
Types of inhibitory drugs
Mimic inhibitory neurotransmitters
Block excitatory neurotransmitter receptors
Example of a mimic inhibitory neurotransmitter drug + how it works
Alcohol
Fewer action potentials set up
Slows down the nervous response
Example and explanation of how they work - block excitatory neurotransmitter receptors
Opiates (painkillers)
-prevent propagation of action potentials
-enter the synaptic cleft + block the receptors on the post synaptic membrane —> prevents neurotransmitters from attaching to receptors and opening the channels
3 examples of excitatory drugs
1.) block the re uptake pumps for neurotransmitters
2.) mimic excitatory neurotransmitters
3.) block inhibitory neurotransmitters
Example of an excitatory drugs that blocks the reuptake pumps for neurotransmitters + the neurotransmitter blocked
Cocaine blocks the reuptake of the neurotransmitter dopamine
Example of a mimic excitatory neurotransmitters drug + explanation
Cause increase in action potential formation in the post synaptic membrane
E.g - nicotine
Example of block inhibitory neurotransmitters + explanation
Prevent inhibition of action potentials in post synaptic neurone
e.g - caffeine (works in the opposite way to alcohol - but it doesn’t sober you up, just reverses the symptoms since alcohol is still in the body)
What’s the problem with organophosphate?
Destroyed the myelin sheath and effects the synapse
What is organophosphate commonly used in?
Farming as an insecticide
Explain how organophosphate poisoning occurs
Organophosphate acts as an inhibitor of the acetylcholine-esterase enzyme
Attaches to the enzymes’ active sites and inhibits them from breaking down the acetylcholine in the synaptic cleft
As a result, more of the neurotransmitters remain in the synaptic cleft
This will keep firing off the post-synaptic neurone, leading to cramps, spasms or even paralysis
Which enzyme does organophosphate inhibit?
Acetylcholine esterase
What inhibits acetylcholine esterase?
Organophosphate
Symptoms of organophosphate poisoning + explanation
Cramps, spasms, paralysis
Neurotransmitters remain in the synaptic cleft so they keep firing off the post-synaptic neurone
The impulse along which neurone is not occurring if no pain is felt by an individual?
The impulse along the sensory neurone to the brain isn’t occurring
which part of the brain produces nerve impulses?
hypothalamus
Word that has to be used when describing synaptic transmission
The fact that calcium ions diffuse rapidly into the pre-synaptic end bulb down their concentration gradient
what do we use to measure the potential difference across a membrane?
oscilloscope
what can happen is the distance between nodes on an axon is too far and why?
the impulse can actually slow down since it takes too long for ions to diffuse
how do we differentiate between a pre synaptic and a post synaptic neurone on a photomicrograph?
the pre synaptic neurone will have more synaptic vesicles
what does acetylcholine esterase do to neurotransmitters?
hydrolyses and breaks them down
what type of inhibitor is organophosphate to acetylcholine esterase enzymes?
competitive
Where is the action potential not sent with anaesthetics?
Along the sensory neurone towards the brain
