Nervous system Flashcards
Two systems structure
CNS
PNS
CNS
Consists of your brain and spinal cord
PNS
Consists of all the neurones that connect the CNS to the rest of the body - these are sensory neurones which carry nerve impulses from the receptors to the CNS, and the motor neurones which carry nerve impulses away from the CNS to the effectors
Within the PNS
Broken down into somatic and autonomic nervous systems (functional)
Somatic nervous system
Under conscious control - it is used when you voluntarily decide to do something ; when you decide to move a muscle to move your arm, the somatic nervous system carries impulses to the body’s muscles
Autonomic nervous system
Works constantly and under subconscious control ; used when the body does something automatically without deciding to do it - causing heart to beat/digest food
Autonomic divisions
Sympathetic and parasympathetic nervous system
Sympathetic nervous system
If the outcome increases activity - increase in heart rate for example
Parasympathetic nervous system
If the outcome decreases activity ; a decrease in heart rate or breathing rate
Why is coordination needed?
Cells within organisms have become specialised to perform specific functions : must work together and require different needs thus coordination is needed - coordinate the function of different cells and systems to operate effectively
Homeostasis
Functions of organs must be coordinated in order to maintain a relatively constant internal environment
Cell signalling
Nervous and hormonal systems coordinate the activities of whole organisms - relies on communication at a cellular level through cell signalling
How are signals transferred?
Locally - between neurones at synapses (neurotransmitter used)
Across large distances - hormones are used which secrete a hormone that acts on cells in the target organ
Stimulus
Changes in the internal or external environment - response is then triggered
Neurones
Transmit impulses rapidly around the body so that the organism can respond to changes in its internal and external environment ; work together to carry information detected by a sensory receptor to the effector
Cell body neurone
Cell body - contains the nucleus surrounded by cytoplasm ; large amounts of ER and mitochondria which are involved in the production of neurotransmitters
Dendrons neurone
Short extensions which come from the cell body
Sensory neurone
Receptor to relay neurone or CNS
Have one dendron till cell body and then an axon
Relay neurones
Cell body looks like spider with dendrons sending impulses into neurone and axon sending it out
Transmit impulses between neurones - many short axons and dendrons
Motor neurone
Cell body at end so just an axon (many dendrites) - transmit impulses from a relay or sensory neurone to an effector
Myelinated neurones?
Schwann cells produce these many layers of plasma membrane by growing around the axon - acts as an insulating layer and allows electrical impulses to be transmitted at a much faster rate
Between each Schwann cell?
Nodes of ranvier which creates gaps in the myelin sheath to allow action potentials to jump from one node to the next - allows them to be transmitted much faster
Much slower if transmitted along nerve fibre of non-myelinated nerve fibre
Features of sensory receptor?
Specific to a single type of stimulus
Act as a transducer - convert stimulus into a nerve impulse
Mechanoreceptor
Detects pressure and movement - example is Pacinian corpuscle (skin)
Chemoreceptor
Chemical stimulus - olfactory receptor (detects smell) and in the nose
Thermoreceptor
Detects heat and end bulbs of Krause - tongue is sense organ
Photoreceptors
Light is stimulus - cone cells in the eye
Role as transducer
Converts any stimulus into a generator potential
Pacinian corpuscle
Specific sensory receptor that detects mechanical pressure and deep within the skin (feet/hands)
Structure of Pacinian corpuscle
Capsule on outside
Then blood capillary
Layers of connective tissue with viscous gel between
Neurone ending
How do pack an corpuscle work?
I’m resting state the stretch mediated sodium ion channels are too narrow to allow ions to pass through them = RESTING POTENTIAL
When pressure is applied - the corpuscle changes shape and the membrane stretches
Causes stretch mediated sodium ion channels to widen and sodium ions can diffuse INTO neurone ending
Influx causes depolarisation - results in generator potential
This creates an action potential that passes along the sensory neurone
Where is action potential transmitted to?
Along neurones to the CNS
Resting potential
Potential difference across membrane is -70mV - membrane is polarised
Outside of membrane is more positively charged than the inside
3 sodium ions are actively transported out of the axon whereas 2 potassium ions are actively transported into the axon via pump
More sodium ions outside than inside and more potassium ions inside than outside
Sodium gated ion channels are closed so na+ ions cannot diffuse back in but potassium ions can diffuse out of the axon (channel is open) thus there are more positively charged ions outside the axon than inside
Sequence of events for action potential
Resting potential
Depolairsation
Repolarisation
Hyperpolarisation
Depolarised to resting potential
Depolarisation
Change in potential difference from negative to positive
Sequence of events
Neurone has resting potential and K+ channels are open but sodium voltage gated ion channels are closed
Stimulus causes sodium voltage gated ion channels to open making the membrane more permeable to sodium ions so they diffuse into the axon down electrochemical gradient thus neurone is less negative
POSITIVE FEEDBACK which causes more sodium ion channels to open
When potential difference reaches +40 the voltage gated sodium ion channels close and K+ open ; sodium can no longer enter the axon but membrane mow more permeable to potassium ions
K+ ions diffuse out of the axon down electrochemical gradient reducing charge again
Initially A LOT OF K+ ions move out resulting in hyperpolarisation and axon becoming less negative but sodium potassium pump restores this abck to -70 (and potassium ion channels close)
Saltatory conduction
Depolarisation of axon can only occur at nodes of ranvier where no myelin is present - sodium ions can pass through the protein channels in the membrane
Action potential then jumps from one node to another in a process called saltatory conduction - much faster than a wave of depolarisation
Why is saltatory conduction important?
Every time channels open and ions move in takes time so reducing the number of places this happens speeds up transmission
Long term also more energy efficient as repolarisation uses ATP in the sodium pump so by reducing the amount of repolarisation needed, saltatory conduction makes the conduction of impulses more efficient
What factors affect speed at which an axon potential travels?
Axon diameter - bigger the axon diameter the faster the impulse is transmitted, less resistance to the flow of ions in the cytoplasm
Temperature - higher the temperature, faster the nerve impulse because ions diffuse faster at higher temperatures ; only up to 40 degrees as higher than that proteins get denatured
All or nothing principle
A certain level of stimulus, threshold value, always triggers a response… if this threshold is reached an action potential will always be created no matter how large the stimulus is - SAME SIZED ACTION POTENTIAL WILL ALWAYS BE TRIGGERED
If threshold value not reached
No action potential will be triggered
Larger the stimulus?
Does affect the frequency of action potentials generated in a given time
Role of synapses 1
Ensure impulses are unidirectional - neurotransmitter receptors are only on postsynaptic neurone and impulses can only travel from the pre to post synaptic neurone
Role of synapses 2 and 3
Allow an impulse from one neurone to be transmitted to a number of neurones at multiple synapses (single stimulus creating a number of simultaneous responses)
Number of neurones May feed in to the same synapse with a single postsynaptic neurone - stimuli from different receptors interacting to produce a single result
Summation
Amount of neurotransmitter from a single impulse is not enough to reach threshold value often - if the amount builds up the. This will trigger an action potential
Spatial summation
Number of presynaptic neurones connect to one postsynaptic neurone - each releases neurotransmitter which builds up to a high enough level in the synapse to trigger an action potential
Temporal summation
Occurs when a single presynaptic neurone releases neurotransmitter as a result of an action potential several times over a short period … builds up in the synapse until quantity sufficient enough to trigger an action potential
Synaptic cleft
Gap which separates the axon of one neurone from the dendrite of the next neurone
Synaptic knob
Swollen end of the presynaptic neurone - many mitochondria and ER to enable it to manufacture neurotransmitters
Synaptic vesicles
Vesicles containing neurotransmitters - fuse with presynaptic membrane and release their contents into synaptic cleft
Neurotransmitter receptors
Receptor molecules which the neurotransmitter binds to in the postsynaptic membrane
Types of neurotransmitter
Excitatory - neurotransmitters result in the depolarisation of the postsynaptic neurone - if threshold is reached an action potential is triggered (Acetylcholine is an example)
Inhibitory - result in hyperpolarisation of the postsynaptic neurone which prevents an action potential from being triggered like GABA which is found in the brain
Transmission of action potential across synapse?
Arrival of action potential at end of presynaptic neurone causes calcium ion channels to open and ca2+ to enter the synaptic knob
Influx causes synaptic vesicles to fuse with the presynaptic membrane so releasing acetylcholine into the synaptic cleft
Acetylcholine molecules fuse with receptors sites on the sodium ion channels in the membrane of the postsynaptic neurone ; causes sodium ion channels to open allowing sodium ions to diffuse in rapidly along a concentration gradient
Influx of sodium ions generates a new action potential in the postsynaptic neurone
Acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid which diffuses back across the synaptic cleft into the presynaptic neurone (recycling) ; breakdown of acetylcholine also prevents it from continuously generating a new action potential in the postsynaptic neurone
ATP released by mitochondria is used to recombine choline and ethanoic acid into acetylcholine which is stored in synaptic vesicles for future use - sodium ion channels CLOSE in the absence of acetylcholine in the receptor sites
