The nervous system Flashcards
How does nervous changes differ to hormonal changes
- hormonal changes are more slow, long term response and chemicals being carried by the blood
- nervous is more rapidly involve short term reponse and information carried by neurones
What is the nervous system responsible for
detecting changes within the internal and external environment processing that information and initiating a response
What is the stimulus response model
stimulus -> receptor -> coordinator -> effector -> response
What is a stimulus
change in the environment
What is a receptor / detector
contains cell which can detect stimuli converting energy from one form into an electrical impulse
What is a coordinator
central nervouse system consisting of brain and spinal cord coordinates a response
What is an effector
- brings about response
- either mucle of gland
what is the response
change in the organism
What does the peripheral nervous system made up of
- somatic nervous system
- autonomic nervous system
What is the somatic nervous system made up of
- pair of nerves originate from the brain and spinal cord containing both sensory and motor neurones
What is the autonomic nervous system
- controls involutnary actions
- digestion and control of heartbeat
What are the three types of neurones and what do they do
- sensory neurones - carry impules from receptors to the CNS
- relay neurones - within CNS recieve impulses from sensory or other relay neyornes and transmit them onto motor neurones
- motor neurones - transmit impulses from the CNS to effectors (muscles or glands)
What is the function of the cell body
- contains granular cytoplasm with ribosomes for protein synthesis
- DNA is present within a nucleus and acts as the site for transcription
What is the function of the axon
- carries the impulse away from the cell body
What is the function of the myelin sheath
- surrounds the axon (and dendron in sensory neuornes)
- providing electrical insulation resulting in faster impulse transmission
What is the function of the Schwann cell
- surround the axon (and dendrons in sensory neurones) and forms the myelin sheath
What is the function of the nodes of ranvier
- gaps in the myelin sheath between schwann cells where the axon membrane is exposed
- allow faster nerve impulse conduction (saltatory conduction)
What is the function of the axon endings
secrete a neurotransmitter whcih results in depolarisation of the adjacent neurone
What is the function of the synpatic end bulbs
swelling found at the end of an axon where the neurotransmitter is synthesised
Describe the process of a reflex arc
- a stimulus is detected by a receptor passed to the CNS along a sensory neurone
- the impulse is then relayed directly to a motor neurone and its effector by a relay neurone
What is a reflex
- rapid automatic responses to stimuli that could prove harmful to the body and therefore protective in nature
What is the response of a reflex usuallly
- contraction of a muscle
- release of a hormone
What are nerve nets and where are they found
- simple animals eg cnidarians like hydra do not possess a nervous system
- it consists of sensory photoreceptors and touch receptors in the wall fo the body and tentacles
- ganglion cells provide conncenctions between the neurones in several directions but they do not form a brain
What is the difference between a cnidarian nerve net and mammalian nervous systems
Cnidarian nerve net
* 1 type of simple neurone
* unmyelinated
* short branched neurones
* impulse transmitted in both directions
* slow impule transmission
Mammalian nervous system
* 3 types of neurones
* myelinated
* long unbranched neurones
* impulse transmitted in one direction
* fast impulse transmission
resting potential
of a neurone is the potential difference between the inside and outside when a nerve impulse is not being transmitted it is around -70mV the membrane is polarised
What happens when neurone is at rest
- no impulses are being transmitted - resting potential
- at rest charge across membrane is -70mv respect to inside (outside is more positive)
Describe the movement of Na+ and K+ into the phospholipid bilayer
- phospholipid bilayer is impermeable to Na+ /K+ ions
- ions are oly able to move across the membrane through intrinsic proteins and sodium potassium poump (active transport)
- some intrinsic proteins have gates which can be opened or closed to allow/inhibit ion movement
- Na+ gates allow Na+ ions to pass in K+ gates all K+ ions to pass out
- most K+ gates are open whereas most Na+ gates are closed making the mebrane more permeable to K+ ions than Na+ ions
- resting potential is negative as fewer positive inside than outside
action potential
the rapid rise and fall of electrical potential across a neurone membrane as a nerve impulse passes
depolarisation
a temporary reversal of the potential difference across the membrane of a neurone such that the inside becomes less negative relative to the outside as an action potential is transmitted
Describe how action potential is bought about
- at resting potential the Na+ gates are closed and some K+ gates are open which together the sodium potassium pump result in a potential difference across membrane of -70mV
- energy of stimulus causes Na+ voltage gated gates to open and Na+ ions flood in down their concentration gradient depolarising the neurone
- now the charge across membrane becomes more positive due to more positive charges inside
- as more Na+ ions enter more gates open so even more Na+ ions rush in
- when potential reaches +40mV the neurone is depolarised Na+ gates close preventing further influx of Na+ ions K+ gates then begin to open
- K+ ions flood out of the neurone down their concentration gradient lowering the positive gradient across the membrane
- further K+ chaannels open resulting in even more K+ ions leaving the neurone
- the neurone is repolarised
- too many K+ ions leave the neurone so the electrical gradient overshoots -70 reaching around -80 - hyperpolarisation
- restablish the resting potential -70 K+ gates now close and sodium potassium pump restablisshes the resting potential
What is the ration for the sodium potassium pump
- 3 Na+ out
- 2K+ in
Describe impulse propagation in unmyelinated neurones
- neuorne membrane is polarised charge across membrane is now -70mV
- Na+ channels open so Na+ ions rush into the axon cytoplasm. A local circuit is established where Na+ ion are pumped out of the adjacen sodium potassium pump
- Na+ channels in the adjacent part of the membrane open causin depolarisation
- meanwhile Na+ channels close K+ channels open causing repolarisation behind it
- depolarisation continues along neurone membrane the membrane at start is now polarised again
Describe how ions travel across myelinated neurones
- ion can only move across at the node of raanvier where there is no myelin present therefore local circuits are etablishes over greater distances (between each node)
Where does depolarisation and action potential occur in myelinated neurones
- only occurs at the nodes and the action potential effectivley jumps from node to node increasing the speed of impule transmission
What is the total refractory period
- represent the period during which it is not normally possible to send another impulse
What is the abolute refractory period
i the period during which it is not posible to send another impulse irrespective of how big the stimulus is
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What is the relative refractory period
- period during which it is possible to send another impulse if th stimulus is big enough to overcome the threshold
What is the all or nothing rule
- impulses will pass if a threshold value is exceeded -55mV a large stiulus will result in more impulses passing per second (increased frequency of ction potentials) rather than a greater evel of depolariation
- impulses either pass or do not and they are always the same size
How does myelination effect speed of impulse transmission
- saltatory conduction is faster than impulse trasmission in unmyleinated neurones
- depolarisation only occur at the nodes of ranvirer
- so the action potential jumps form node to node
- rate of tranmission is faster in myelinated neurones
How does diameter of the axon affect speed of impulse transmission
- impulse transmission speed increases with axon diameter
- due to less leakage of ions from larger axons (due to a larger volume to surface area)
What are the factors affecting speed of impulse transmission
- myelintion
- diameter of axon
- temperature
How does temperature affect the speed of impulse transmission
- impulse transmission speed increases with temperatre as the rate of diffusion increases due to increased kinetic energy of ions involved
- only in organisms which do not control their internal body temperature
What is a synapse and it’s function
- gap between two neurones
- impulse is transmitted from one to other by a neurotransmitted which diffuses acros the synaptic cleft from the pre synaptic membrane to receptors on post synaptic neurone triggering depolarisation in the post synaptic neurone
What is the name of the neurotransmitter
acetylcholine
Outline the functions of synapses
- tranmit information between neurones
- transmit information in one direction only
- act as junctions
- filter out low level stimuli
- prevent over stimulatio of neurone and fatigue
Describe the event of a synaptic transmission
- impulse arrives at the pre synaptic knob
- calcium channels open causing calcium ions to diffuse rapidly into pre synaptic knob
- vesicles containing acetylcholine migrate to and fuse with pre synaptic membrane
- contents of vesicles are released into the synaptic cleft by exocytosis
- acetylcholine molecules diffuse acros the cleft and bind to receptors on the post synaptic membrane causing sodium ion channels to open
- sodium ions rush into the post syntaptic neurone resulting in depolarisation of the post synaptic membrane an action potential is initiated
- acetylcholinesterase splits acetylcholine into ethanoic acid and choline releasing them form the recepetor and sodium channels close the products diffuse back across the cleft
- products are reabsrobed into the pre synaptic knob
- ATP is used to reform acetylcholine in the pre synaptic knob
Describe how repeated depolrision of the post synaptic neurone is prevented
- hydrolysis of acetylcholine
- reabsorption of ethanoic acid and choline back into pre synaptic knob
- active transport of calcium ions out of the pre synaptic knob preventing further exocytosis of neurotransmitter
What happens if insufficient acetylcholine is released
- not enough sodium ion channels open on posrt synaptic membrane to exceed threshold potential of -55mV so action potential is not initiated
Wht are the two main types of drugs that effect chemicals on the synapse
- excitatory (stimulant of agonists) like caffiene and cocaine whcih result in more action potentials
- sedatives (inhibitory) like cannabis result in fewer action potentials
Describe the effect of organophosphorus insecticides
- are agonists by inhibiting cholinesterase so acetylcholine lingers at the synapse causing repeated depolarisation of the post synaptic membrane
Describe the effect of nicotine on the effects of chemicals on synapses
- mimic the action of nerotransmittters unlike acetylcholine nictotine is not removed by cholinesterase
- over time body produces less acetlycholine and person is more reliant on nicotine for the normal functioning of the synapes
- increased release of dopamine from the brain resulting in addiction
