Unit 5- Topic 8 Flashcards
What is the nervous system made up of
Made up of interconnected neurones specialised for the rapid transmission of impulses throughout the organism
Where does an impulse travel from and to
Sensory neurones carry impulses from receptor cells into the central nervous system. Motor neurones then carry the impulse from the central nervous system to the effector cells in the effector organs.
Receptor cells give information about…
Give information about the internal and external environment
What is the basic function of effector cells
Provide the appropriate response to the internal and external information provided by the receptor cells.
What are nerves that carry only fibres from motor neurones called?
Motor nerves
What are nerves that carry only sensory fibres called?
Sensory nerves
What are nerves that carry a mixture of motor and sensory fibres called?
Mixed nerves
What are neurones?
Cells which are specialised in the transmission of electrical signals (nerve impulses)
What are the important organelles found within a neurone
Nucleus, mitochondria, the rough endoplasmic reticulum and ribosomes. All needed for the synthesis of the neurotransmitter molecules.
Special and unique characteristic of neurones
A long and thin nerve fibre which carries the nerve impulse.
Thin extensions from the cell body known as dendrites which connect to neighbouring nerve cells
What are the different names of the nerve fibre
Going towards the nerve cell body: dendron
Going away from the nerve cell body: axon (the whole of the motor neurone)
Where are relay neurones found
In the Central nervous system
Function of relay neurones
Connect motor and sensory neurones.
They are special since they have two fibres leaving the same cell body
Different types of neurones
Motor neurones
Sensory neurones
Relay neurones
Parts of the motor neurone cell
Dendrites (receive information)
Cell body with the nucleus
Axon
Myelin sheath
Schwann cell nucleus
Node of ranvier
Direction of impulse
Synaptic bulbs (pass on impulses)
Effector (muscle)
Parts of the sensory neurone cell
Receptor (pressure receptor in skin)
Dendrites (receive impulses)
Cell body with the nucleus
Dendron
Axon
Myelin sheath
Schwann cell nucleus
Node of ranvier
Direction of impulse
Synaptic bulbs (pass on impulses)
Parts of the relay neurone cell
Dendrites (receive information)
Cell body with the nucleus
Axon
Dendron
Direction of impulse
Synaptic bulbs (pass on impulses)
What is a nerve impulse
A minute electrical event produced by charge differences between the outside and inside of the neurone membrane. Based on ion movements through specialised protein pores and by an active pumping mechanism.
What forms the myelin sheath
Schwann cell wraping itself around the nerve fibre many times forming a fatty layer known as the myelin sheath
Why is the myelin sheath important
Protects nerves from damage
Speeds up the transmission of the nerve impulse
What is the role of the nerve cells
Carry electrical impulses rapidly from one area of the body to another
What does the speed at which the electrical impulses can be carried depend on
The diameter of nerve fibre- the thicker the fibre, the more rapidly impulses travel along it
Presence or absence of myelin sheath- myelinated nerve fibres can carry impulses much faster than unmyelinated ones
How have invertebrates evolved to react quickly
Invertebrates have mostly thin and unmyelinated nerve fibres causing nerve impulse to travel slowly. In order to be able to react quickly to avoid danger, invertebrates may have a number of giant axons which allow impulses to travel extremely fast allowing the animal to escape danger. Due to the large size of the axon, scientists use the giant axons to investigate the function.
Where are myelinated and unmyelinated nerves found in vertebrates
Voluntary motors neurones (transmit impulses to voluntary muscles to control movement) are all myelinated.
Autonomic neurones that control involuntary muscles (like in digestive system) have some unmyelinated fibres
Why are fibres myelinated in vertebrates rather than having a giant axons like in invertebrates
The myelin sheath speeds up the transmission of a nerve impulse without the need of a giant axon. Giants axons require a lot of space because of their size.
What ions are involved in the nerve impulse along an axon
Na+ (sodium) ions and K+ (potassium) ions
What occurs in the axon while it is resting (not conducting a nerve impulse)
The membrane is polarised. The concentration of ions outside the nerve fibre is greater than the concentration of ions in the cytoplasm of the axon. The gradient is created by the sodium/potassium pump by the active transport of Na+ ions out of the axon and K+ ions into. Due to sodium ions being pumped out of the axon, the concentration of sodium ions decreases, however, since the axon membrane is relatively impermeable to the ions, they can’t diffuse back into the axon. At the same time, K+ are pumped into the axon but the axon membrane is permeable to potassium ions so they diffuse along the concentration gradient through open K+ channels. Therefore making the inside of the cell slightly negatively charged relative to the outside.
How can the sodium/potassium ion pump do active transport?
The pump has an enzyme called Na+/K+ ATPase that uses ATP to allow the transport of ions in the resting state of the axon. This means that ATP is used in order to maintain the different charges at either side of the axon (the resting potential)
What is the potential difference while the axon is resting
Resting potential= -70mV
Overall what happens when an impulse travels along the axon
There is a change in the permeability of the cell membrane to sodium ions
What causes a change in the permeability of the axon membrane to sodium ions (what causes a nerve impulse to begin)
The change occurs
-in response to a stimulus ( light, sound, touch, taste or smell) in a sensory neurone
-the arrival of a neurotransmitter in a motor neurone
What happens when a neurone is stimulated
Sodium gates open allowing sodium ions to diffuse rapidly down their concentration and electrochemical gradient. This causes the inside of the cell to become positive with respect to the outside. This reverse of the charges is depolarisation.
What is the action potential
+40mV= action potential
What happens at the end of the action potential
Repolarisation occurs due to sodium ion channels closing and excess sodium ions being rapidly pumped out of by the active sodium/potassium pump. At the same time, the permeability of the membrane to potassium ions is temporarily increased due to the voltage-dependent potassium ion channels opening as a result of repolarisation. Therefore potassium ions diffuse out of the axon down a concentration and electrochemical gradient since they are attracted by the negative charge on the outside of the membrane. This causes the resting potential to be restored after a few milliseconds because the inside of the axon eventually becomes negatively charged
When does the action potential triggered
Once the threshold has been reached. Th size of the action potential is ALWAYS the same
What is the refractory period
The recovery time of an axon. Time it takes after the action potential to return to the resting potential
What is the length of the refractory period
The time it takes for ionic movements to repolarise the membrane and restore the resting potential
What affects the length of the refractory period
-the sodium/potassium pump
-the membrane permeability to potassium ions
What is the function of the refractory period
-Limit the rate at which impulses may flow along a fibre.
-ensures that impulses flow in only one direction along the nerves
Why can the refractory period ensure that impulses only flow in one direction in nerves
Because until the resting potential is restored, the part of the nerve fibre that the impulse has just left cannot conduct another impulse. So the impulse can only continue travelling in the same direction
How does an action potential travel along the nerve fibre
It occurs due to local currents created by the movement of ions in the action potential. When the nerve is stimulated, the membrane is depolarised causing a change in the charges. This potential difference in the membrane next to the first action potential changes, initiating a second action potential. In the first action potential then the voltage-dependent sodium ion channels close and voltage-dependent potassium ion channels open. K+ leaves the axon repolarising the membrane. The embrace then becomes hyperpolarised.
Why is the nerve impulse faster in myelinated neurones ?
Because of saltatory conduction. Ions can only pass in and out of the axon at the nodes of Ranvier so action potentials can only occur at the nodes. This speeds up the transmission as the ionic movements in the action potential occur much less frequently therefore needing less time. It appears to jump from node to node since the local currents created by the movements of the ions are elongated to start depolarisation at the next node.
How do neurones communicate with each other
They are linked by a synapse
where can you find synaptic knobs
In every cell in the central nervous system
How do the synapse allow the communication between neurones
The arrival of the impulse at the synaptic knob increases the permeability of the presynaptic membrane to calcium ions because calcium channels open up. Ca+ ions move into the synaptic knob down their concentration gradient causing thee synaptic vesicles containing neurotransmitters to move to the presynaptic membrane. Exocytosis of the vesicles occurs since they fuse with the presynaptic membrane and release the neurotransmitter into the synaptic cleft. These molecules diffuse across the gap and attach to specific receptors on the sodium channels of the post-synaptic membrane causing sodium ions to open allowing Na+ ions to flow into the nerve fibre. This causes a change in the potential difference and an. EPSP to be set up.
How can an action potential be triggered after a nerve impulse has travelled across a synapse
If enough ESPs, the positive charge in the post-synaptic cell becomes greater than the threshold
What are the two ways a neurotransmitter can affect the post-synaptic cell
EPSP and IPSP
How is an IPSP created in the post-synaptic cell
Different ion channels open int he membrane, allowing the movement of negative ions inwards, which makes the inside more negative than the normal resting potential. This makes it less likely that an action potential will occur in the post-synaptic fibre.
What happens to a transmitter after a nerve impulse has passed the synapse
It is destroyed by enzymes in the synaptic cleft so that the receptors on the post-synaptic membrane are emptied and can react to a subsequent impulse
Where is acetylcholine made
It is made in the synaptic knob using ATP (produced in mitochondria)
What enzyme breaks down acetylcholine
Acetylcholinesterase
What is the name of the nerves that use acetylcholine as their transmitter
Cholinergic nerves
Process of breaking down of acetylcholine by acetylchlorinesterase
The enzyme, embedded next the acetylcholine receptors in post-synaptic membrane, hydrolyses acetylcholine into acetate and choline.
Why is acetylcholine hydrolysed into acetate and choline after a post-synaptic potential
To ensure that the acetylcholine no longer affects the post-synaptic membrane and to release the components in order to recycle them.
How can components of a neurotransmitter be recycled after they have been broken down
The components diffuse across the synaptic cleft, down a concentration gradient and taken back into the synaptic knob where they are re-synthesised in their respective neurotransmitter
Where is acetylcholine found in
In all motor neurones, the parasympathetic nervous system and cholinergic synapses in the CNS
Where is acetylcholine found in
In all motor neurones, the parasympathetic nervous system and cholinergic synapses in the CNS
What is the overall effect of acetylcholine
Excitation in the post-synaptic membrane
What transmitter is used in the synapse of the sympathetic nervous system
Noradrenaline
What nerves use noradrenaline as their transmitter
Adrenergic nerves
What does the binding of noradrenaline to the receptors depend on
The concentration of noradrenaline in the synaptic cleft. Therefore after the release of noradrenaline stop, levels in the synaptic cleft fall. So noradrenaline is released from the post-synaptic receptors back into the synaptic cleft allowing 90% to be reabsorbed.
Effects of drugs increasing the nerve response
-increases the amount of neurotransmitter synthesised
-increases the release of neurotransmitter from the vesicles at the presynaptic membrane
-binds to post-synaptic receptors and activates them or increases the effect of the normal neurotransmitter
-prevents the degradation of neurotransmitter or prevents reuptake into presynaptic knob
Effects of drugs decreasing the nerve response
-blocks the synthesis of neurotransmitter
-causes neurotransmitter to leak from vesicles and be destroyed by enzymes
-prevents the release of neurotransmitter from vesicles
-blocks the receptors and prevents neurotransmitter binding
How does nicotine work
It mimics the effect of acetylcholine binding to specific acetylcholine receptors known as nicotinic receptors. This triggers an action potential but the receptor then remains unresponsive to more stimulation. This causes raised heart rate and high blood pressure as well as triggers the release of dopamine. At high doses it can block the acetylcholine receptors and cause death
How does lidocaine work
Lidocaine molecules block voltage-gated sodium channels preventing the production of the action potential in sensory nerves, so you can’t feel pain. It can be use to prevent heart arrhythmias as blocking the sodium channels raises the depolarisation threshold.
How does cobra venom work
It binds to acetylcholine receptors preventing the transmission of impulse across synapses. So muscles are not stimulated to contract and gradually the person becomes paralysed. When the toxin reaches the muscles involved in breathing, it causes death. In small doses cobra venom can be used to relax te muscles of the trachea and bronchi in severe asthma attacks.
What is a primary sensory receptor
Neurones with a dendrite that is sensitive to one particular stimulus
What is a secondary sensory receptor
One or more completely specialised cells (not neurones) that are sensitive to a particular type of stimulus and synapse with normal sensory neurones. Eg. Retinal cells
Different type of receptors
-proprioceptors
-chemoreceptors
-mechanoreceptors
-photoreceptors
-thermoreceptors
What are proprioceptors sensitive to
To the relative positions of the skeleton and degrees of muscle contraction. Used to maintain posture
What are chemoreceptors sensitive to
To chemical stimuli such as smell, taste and the pH levels of the blood
What are mechanoreceptors sensitive to
To mechanical stimuli such as pressure, tension, movement and gravity
What are photoreceptors sensitive to
To electromagnetic stimuli, like visible light
What are thermoreceptors sensitive to
To temperature changes
How do sensory receptors work
Receptor cells have a resting potential of a negative charge of the interior of the cell in relation to the outside. This is created by the sodium pumps. When the receptor receives a stimulus, Na ions moves into the cell along concentration and electrochemical gradients setting up a generator potential. If the generator potential produced reaches the threshold, an action potential will occur.
What is convergence
Several receptor cells synapsing with a single sensory neurone in order to increase sensitivity to low-level stimulus since the generator potential of all the receptor cells add up to trigger an action potential in the sensory cell.
What difference does a weak and strong stimulus have in the action potential of the sensory neurone
Weak stimulus results in low frequency of action potentials along sensory neurone. Strong stimulus results in rapid stream of action potential. The different levels allows information about the strength of the stimulus therefore we are aware of the difference between light an dark and the varying degrees of light and shade.
What wavelength can our eyes detect
Electromagnetic radiation of 400 and 700nm
What structures of the eye control the amount of light that enters the eye and focus that light in the retina
Cornea, iris, pupil, lens and aqueous and vitreous humour
what are the two main steps to seeing an image
- light focused on the retina thanks to different structures in the eye
2.retine perceives the light and provides the information to the brain to interpret the image
what types of photoreceptors can be found in the retina
rods and cones (they are both secondary receptors)
where are rods found in the retina
rods are found spread evenly across the retina except in the fovea where there are none
what is the function of rods
provide black and white vision only and are very sensitive to light, therefore they are used for seeing in low light intensities or at night.
what is the consequence of rods not being tightly packed together
several rods synapse to the same sensory neurone so due to them being spread out, they do not give a clear picture
why do rods not give a clear picture
because many rods need to be stimulated at the same time to cause an action potential in the sensory neurone but it also means that they are very sensitive to low light levels and to movements in the visual field.
why are rods sensitive to low light levels and to movements in the visual field
only a small stimulus are needed to produce several generator potentials and therefore trigger an action potential to the CNS by summation.
where are cones located in the retina
tightly packed together in the fovea
function of cones
vision in bright light and colour vision
why do cones have great visual acuity
because they are tightly packed in the fovea and a cone only synapses to one sensory neurone. however it is only when light is focused directly on the fovea that an image is clearly in focus
why is the structure of the retina surprising?
because it appears to be back to front. the neurones are found at the interior edge of the eyeball meaning that light has to pass throught the synapses and inner segments before reaching the outer segments containing visual pigments
what is the visual pigment found in rods
rhodopsin (visual purple)
what is rhodopsin made out of
opsin and retinal. retinal exists as two isomers cis-retinal and trans-retinal
what is the retinal isomer present in the dark
cis-retinal
what happens when a photon of light hits a molecule of rhodopsin
it converts cis-retinal into trans-retinal, changinf the shape of retinal therefore breaking the bond between opsin and retinal. so rhodopsin gets broken down. the breaking of the bond is known as bleaching.
what is different between cones and rods to the rest of neurones
most neurones are relatively impermeable to sodium ions whereas rods and cones are very permeable to them
how does bleaching create an action potential in the sensory neurone
it triggers a cascade reaction leading to sodium ion channels closing. this causes less sodium ions to move into the cell as it becomes less permeable. however, the sodium pump continues to work causing in the inside of the cell to become more negative. this hyperpolarisation is the generator potential in the rod. If the threshold is reached, neurotransmitters substances are released into the synapse with the bipolar cell. an action potential is triggered in the bipolar cell that passes to an action potential in the sensory neurone.
what affects the size of the generator potentials in rods
the amount of light hitting the rod therefore the amount of rhodospin bleached
what forms the optic nerve
All the sensory neurones which leave the eye at the same point leading to the brain.
what happens after rhodopsin is bleached
the rod cannot be stimulated again until rhodopsin is resynthesised. the reaction requires ATP since it is converting trans-retinal to cis-retinal and bonding it to opsin. the ATP is made in the many mitochondria foun in the cell. this reaction takes time so that is why our eyes need to adapt to darkness after being in a sunny street.
visual pigment of cones
iodopsin
how does iodopsin allow our brain to see colour
there are three types of iodopsin, each sensitive to one primary colour. the brain interpret the number of different types of cones stimulated as different colours.
what is habituation
a phenomenon where an organism becomes insensitive to repeated stimuli over time which does not threaten their survival or does not benefit them in any way. it is a type of learned behaviour
