Module 5 Section 1: Communication and Homeostasis Flashcards
What is the nervous system made up of
What is the CNS composed of and what neurons does it contain
Brain and spinal cord
Contains relay neurons
What is the PNS composed of and what neurons does it contain
Composed of cranial nerves, spinal nerves, peripheral nerves
Contains sensory neurons and motor neurons
Acts as interface between CNS and environment and transmits electrical impulses to and from the CNS
What is the peripheral nervous system
Made up of the neurons that connect the CNS to the rest of the body
Has two different functional systems: somatic nervous system and autonomic nervous system
What is the somatic nervous system
Controls conscious conscious activities
E.g. running and playing videos games
What is the autonomic nervous system
Controls unconscious activities
E.g. digestion
Got two subdivisions that have opposite effects on the body
What is the sympathetic nervous system
Gets the body ready for fight or flight
Sympathetic neurons release the neurotransmitter noradrenaline from the adrenal glands
Causes heart rate to increase and allows rapid blood and glucose supply to respiring muscles
Allows high intensity activities like running from a predator to be an immediate response
What is the parasympathetic nervous system
Calms the body down in rest and digest system
Parasympathetic neurons release the neurotransmitter acetylcholine
What are neurons
Neurons are specialised cells that conduct electrical impulses within the nervous system
What is a nerve
A nerve is a bundle of many neuron fibres enclosed within a protective sheath
What are nerve fibres
These are the long axons of neurons together with any associated tissues
Different components of a nervous system reaction
Stimulus: change in internal or external environment (e.g. touching hot dish)
Receptor: detect stimulus (e.g. sense organ)
Coordinator: formulates a suitable response to a stimulus
Effector: produces a response
What do sensory receptors act as
Sensory receptors act as transducers
Convert energy of a stimulus into electrical energy
Pathway of a nerve impulse (conscious action)
All the receptors and what they are sensitive to
Receptors in eyes are sensitive to light
Receptors in ear are sensitive to sound
Receptors in tongue are sensitive to chemicals
Receptors in nose are sensitive to chemicals
Receptors in skin are sensitive to touch, pressure, pain and temperature
What features are found in all neurons
Neurons have a long fibre know as an axon
They have a cell body that contains the nucleus and other cellular structures
End of the axon (axon terminal) contains nerve endings
The axon terminal allows neurons to connect to many other neurons and receive impulses from them, forming a network for easy communication
3 types of neurons
Sensory neurons
Relay neurons
Motor neurons
What are sensory neurons
Transmits the electrical impulses from receptors to CNS
Have one dendron which carries impulses to the cell body and one axon which carries the impulse away from the cell body
What are motor neurons
Transmits electrical impulses from the CNS to the effector (muscle/gland)
What relay neurons
(Intermediate) transmits electrical impulses within CNS, connect sensory and motor neurons
Adaptations of neurons
High branched: thin dendrites extend from the cell body and communicate with other neurons to allow electrical impulses to pass from one to the other
Myelin sheath: insulates the axons to ensure the impulses travel rapidly along the axon
Myelin sheath
Axons can be myelinated and electrically insulated by a myelin sheath (fatty substance)
Has small uninsulated sections along the length (nodes of ranvier)
Myelin sheath made up of specialised cells called Schwann cells
Means that electrical impulses can jump from one node to the other so impulses can travel much faster
Each time they grow around the axon, a double layer of phospholipid bilayer is laid down
How does action potential travel faster down myelinated sheath
Between Schwann cell are patches of bare membrane called the nodes of ranvier
Sodium ion channels are concentrated at the nodes
In a myelinated neuron, depolarisation only happens at the nodes of ranvier (where Na+ can get through the membrane)
Neurons cytoplasm conducts enough electrical charge to depolarise the next node, so the impulse ‘jumps’ from node to node
Called saltatory conduction (100x faster than non-myelinated)
Where are many unmyelinated neurons
Many nerves in the CNS are myelinated
They make up the grey matter in the brain and spinal cord
How does action potential travel along unmyelinated neurons
Impulses travel as a wave along the whole length of the axon membrane
Slower than saltatory conduction
How many stimuli are receptors adapted to
Receptors are specific to only one type of stimulus
E.g. light, pressure, glucose concentration
Receptors can be cells or proteins
What is an example of receptors
Pacinian Corpuscles - pressure receptors in the skin
How do receptors work
When a receptor is in a resting state (not being stimulated), there’s a difference in charge between the inside and outside of the cell - generated by ion pumps and ion channels
Means that there’s a voltage across the membrane (potential difference)
Potentiation difference when a cell is at rest is called it’s resting potential
When a stimulus is detected, the cell membrane is excited and become more permeable, allowing more ions to move in and out of the cell which alters the potential difference
The change in potential different due to a stimulus is called the generator potential
Bigger stimuli excites membrane more, causing a bigger movement of ions and a bigger change in potential difference - so a bigger generator potential is produced
If a generator potential is big enough it will trigger an action potential (nerve impulse) along a neuron
An action potential is only triggered if the generator potential reaches a certain level called the threshold level
If stimulus is too weak the generator potential won’t reach the threshold, so there’s no action potential
What are pacinian corpuscles
These are mechanoreceptors that detect mechanical stimuli e.g. pressure and vibrations
Found in skin
Composed of a sensory neuron which is wrapped in layers of lamellae
When the pacinian corpuscle is stimulated the lamellae deform and press on the sensory nerve ending
This causes sodium ion channels to open and sodium ions diffuse into the cell, creating a generator potential
If the generator potential reaches the threshold - it triggers an action potential along the sensory neuron
What are dendrons
Short extensions which come from the cell body
These extensions divide into smaller and smaller branches known as dendrites
They are responsible for transmitting electrical impulses towards the cell body
Features of sensory receptors
Specific to a single type of stimulus
Act as transducers to convert a stimulus into a nerve impulse
How do sensory receptors act as transducers
Detect a range of different stimuli including light, heat, sound or pressure
The receptor converts the stimulus into a nervous impulse
This is a generator potential
What electrical potential is the inside of a resting axon
Resting axons (one not transmitting impulses) always have a negative electrical potential compared to the outside of the axon
This is called the resting potential
Around -70mV
What is the potential difference of an axon at resting potential
This potential difference is usually about -70mV
The inside of the axon has an electrical potential about 70mV lower than the outside
What factors contribute to maintaining the resting potential
Sodium-potassium pump active transports sodium ions and potassium ions (requires ATP)
Differential membrane permeability
Maintained by keeping more positive ions outside the cell than inside
This means that the inside is more negatively charged, it is polarised
How do the cell surface membrane of neurons allow ions to come in and out
Cell surface membrane has selective protein channels that allow sodium and potassium ions to move across the membrane by facilitated diffusion
Channels are less permeable to sodium ions than potassium ions
Means that potassium ions can diffuse back down their concentration gradient, out of the axon at a faster rate than sodium ions
What does the sodium-potassium pump do
Uses ATP to pump 3 sodium ions out of the cell and 2 potassium ions into the cell
SOPI (Sodium Out Potassium In)
What is the generator potential
When a stimulus is detected, the cell membrane is excited and becomes more permeable
This allows more ions to move in and out of the cell
This alters potential difference
Change in potential difference is due to a stimulus is called the generator potential.
The bigger the stimulus the bigger the generator potential that is produced
What is the all or nothing nature of action potentials
Once the threshold is reached, an action potential will always fire with the same change in voltage, no matter how big the stimulus is
If the threshold isn’t reached, an action potential will not fire
A bigger stimulus won’t cause a bigger action potential, but it will causes them to fire more frequently
So if the brain receives a high frequency of action potentials, it interprets this as a big stimulus and responds accordingly
What is an action potential
An action potential occurs via a brief change in the distribution of electrical charge across the cell surface membrane
How are action potentials caused and how does this work
Action potentials are caused by the rapid movement of sodium ions and potassium ions across the membrane of the axon
There are voltage-gated channel proteins which open and close depending on the electrical potential (or voltage) across the axon membrane and allow Na+ and K+ ions to pass through
They are closed when the axon membrane is at its resting potential
Steps of how action potential occurs
Stimulus
Depolarisation
Repolarisation
Hyperpolarisation
Resting potential
What happens when the stimulus is presented in the action potential process
Stimulus excites the neuron cell membrane
Causes sodium ion channels in the membrane to open
Membrane becomes more permeable to sodium
So more sodium diffuse into the neuron down the sodium ion electrochemical gradient
Inside of the neuron becomes less negative
What happens during depolarisation during action potential
If potential difference reaches threshold (around -55 mV) an action potential is stimulated
More voltage gated sodium ion channels in the membrane open
More sodium ions diffuse into the axon down the electrochemical gradient
The inside of the axon becomes less negative thereby reducing the potential difference across the axon
Depolarisation triggers more channels to open, allowing more sodium ions to enter and causing more depolarisation
This is positive feedback
The action potential that is generated will reach a potential of around +30mV
What happens during repolarisation during action potential
About 1ms after the potential difference has reached +30mV, all the sodium ion voltage-gated channel proteins in this section close, stopping any further sodium ions diffusing into the axon
Potassium ion voltage-gated channel proteins in this section of axon membrane now open, allowing the diffusion of potassium ions out of the axon, down their concentration gradient
This returns the potential difference to normal/resting potential (about -70mV) (repolarisation)
This is example of negative feedback
What happens during hyperpolarisation during action potential
Potassium ion channels are slow to close so there is a slight “overshoot” where too many potassium ions diffuse out of the neuron
This means that the potential difference across this section of axon membrane briefly becomes more negative than the normal resting potential (less than -70mV)
This refractory period acts as a time delay between one action potential and the next
It makes sure action potential don’t overlap but instead pass along discrete separate impulses
What happens during the return to resting potential during action potential
Once the potassium proteins are closed the sodium-potassium pump restores the resting potential
By pumping 3 sodium ions out and 2 potassium ions in (SOPI)
The sodium ion channel proteins in this section of membrane become responsive to depolarisation again
This maintains the resting potential until the membrane is excited by another stimulus
Each stage of action potential with concentrations of ions, membrane potential, whether sodium potassium pump is working, open or closed voltage gated sodium channels and open or closed voltage gated potassium channels
Why is there a refractory period
Acts as a time delay between one action potential and the next
Makes sure action potentials don’t overlap but instead pass along discrete separate impulses
Makes sure they are unidirectional
Means there is a limit to the frequency at which nerve impulses can be transmitted
How does action potential travel along an axon
An action potential triggered in the neuron causes depolarisation of that section of the axon
The current causes the opening of sodium ion channels a little further up the axon
This causes an influx of sodium ions in this section of the axon generating an action potential in this direction
The previous section of the axon is the repolarisation stage (the sodium channels are closed and potassium channels are open) and is unresponsive
This makes the action potentials discrete events and means the impulses can only travel in one direction
What is a synapse
This is a junction between a neuron and another neuron or between a neuron and an effector cell
E.g. a muscle or gland cell
What is a synaptic cleft
This is the small gap between the cells at a synapse
Process of synaptic transmission
Presynaptic neuron has a swelling called a synaptic knob
This contains synaptic vesicles filled with neurotransmitters
When an action potential reaches the end of a neuron it causes the neurotransmitters to be released into the synaptic cleft
They diffuse across to the postsynaptic membrane and bind to specific receptors
When neurotransmitters bind to receptors they might trigger an action potential (in neuron), cause muscle contraction, or cause a hormone to be secreted (from gland cell)
Neurotransmitters are removed from the cleft so the response doesn’t repeat
E.g. they’re taken back into presynaptic neuron or broken down by enzymes (and products taken back into the neuron)
About synapses that use Acetylcholine
These are called cholinergic synapses
They use acetylcholine (ACh)
These are broken down by an enzyme called acetylcholinesterase
Stages of how neurotransmitters transmit impulses between neuron
An action potential triggers calcium influx
Calcium influx causes neurotransmitter release
The neurotransmitter triggers an action potential in the postsynaptic neuron
What happens when an action potential triggers a calcium influx
Action potential arrives at the synaptic knob of the presynaptic neuron
Action potential stimulates voltage-gated calcium ion channels in the presynaptic neuron to open
Calcium ions diffuse into the synaptic knob (pumped out afterwards by active transport)
What happens when calcium influx causes neurotransmitter release
The influx of calcium ions into the presynaptic knob causes the synaptic vesicles to move to the presynaptic membrane
They must fuse with the presynaptic membrane
The vesicles release the neurotransmitter into the synaptic cleft by exocytosis
What happens when the neurotransmitter triggers an action potential in the postsynaptic neuron
The neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane
This causes sodium ion channels in the postsynaptic neuron to open
The influx of sodium ions into the postsynaptic membrane causes depolarisation
An action potential on the postsynaptic membrane is generated if the threshold is reached
The neurotransmitter is removed from the synaptic cleft so the response doesn’t keep happening
How can information be dispersed or amplified at a synapse
Synaptic divergence is where one neuron is connected to many neurons and information is dispersed to different parts of the body
Synaptic convergence is where many neurons connect to one neuron so information is amplified
What happens in spatial summation
When neurons converge, the small amount of neurotransmitter released from each neuron can be enough altogether to reach the threshold in the postsynaptic neuron and trigger an action potential
If some neurons release an inhibitory neurotransmitter then the total effect of all the neurotransmitters might be no action potential
Stimuli might arrive from different sources but spatial summation allows signals from multiple stimuli to be coordinated into a single response
What happens in temporal summation
Temporal summation is where two or more nerve impulses arrive in quick succession from the same presynaptic neuron
This makes an action potential more likely, because more neurotransmitter is released into the synaptic cleft
Label the synapse
Graph of how spatial summation works
Excitatory potentials from many neurons trigger threshold point
Graph of how temporal summation works
Many excitatory potentials from one neuron trigger action potential
Example of how adrenaline activates response from cells
Adrenaline is first messenger
Binds to specific receptors in cell membranes of many cells (e.g. liver cells)
When adrenaline binds it activates an enzyme in the membrane called adenylyl cyclase
Activated adenylyl cyclase catalyses the production of secondary messenger called cAMP from ATP
cAMP activates a cascade
E.g. cascade of enzyme reactions to make more glucose available to the cell by catalysing the breakdown of glycogen into glucose
Endocrine function and histology of the pancreas
Endocrine tissue in the pancreas are contained in the islets of langerhans
Found in clusters around blood capillaries
The islets of langerhans secrete hormones directly into the blood
Made up of alpha cells and beta cells
How is insulin secreted
Glucose transporter proteins (on the surface on the B- cells) absorb glucose by facilitated diffusion
Higher levels of glucose increase the rate of respiration and therefore the production of APT molecules
High ATP levels triggerthe potassium ion channels in the B cell membrane to close.
K+ ions build up inside the cell making it less negative.
B-cell membrane becomes depolarized
Depolarisation causes voltage gated calcium ion channels to open.
Calcium ions diffuse into the beta cell
Calcium ions bind to and move the vesicles to the plasma membrane of the B cell.
Vesicles bind to plasma membrane.
Insulin is released into the bloodstream via exocytosis
What does negative feedback do
Negative feedback keeps things around the normal level
e.g. body temperature is usually kept within 0.5°C above or below 37 °C
How does sweating reduce body temperature
More sweat is secreted from sweat glands when the body’s too hot.
The water in sweat evaporates from the surface of the skin and takes heat from the body.
The skin is cooled.
How do hairs lying flat reduce body temperature
Mammals have a layer of hair that provides insulation by trapping air (air is a poor conductor of heat).
When it’s hot, erector pili muscles relax so the hairs lie flat.
Less air is trapped, so the skin is less insulated and heat can be lost more easily.
How does vasodilation reduce body temperature
When it’s hot, arterioles near the surface of the skin dilate (this is called vasodilation).
More blood flows through the capillaries in the surface layers of the dermis.
This means more heat is lost from the skin by radiation and the temperature is lowered
How does shivering increase body temperature
When it’s cold, muscles contract in spasms.
This makes the body shiver and more heat is produced from increased respiration
How does having much less sweat increase body temperature
Less sweat is secreted from sweat glands when it’s cold, reducing the amount of heat loss.
How does hairs standing up increase body temperature
Erector pili muscles contract when it’s cold, which makes the hairs stand up.
This traps more air and so prevents heat loss.
How does vasoconstriction increase body temperature
When it’s cold, arterioles near the surface of the skin constrict (this is called vasoconstriction) so less blood flows through the capillaries in the surface layers of the dermis.
This reduces heat loss.
