3.8 the nervous system Flashcards
what is the nervous system responsible for?
- detecting changes within the internal or external environment (a stimulus)
- processing that information and initiating a repsonse
what is the order of stimulus response?
- stimulus -> detector -> coordinator -> effector -> response
what is a stimulus?
- a change in the environment
what is a detector?
- contains cells which can detect stimuli
- e.g visible light by the retina, sound by the inner ear, temperature by the dermis of the skin
- it converts energy from one form e.g light, into an electrical impulse
what is the coordinator?
- the central nervous system consisting of the brain and spinal cord
- it coordinates the response
what is an effector?
- an effector brings about a response
- it is either a muscle or a gland
what is the response?
- the response is the change in the organism
in humans, what does the nervous system consist of?
- the central nervous system (CNS)
- the peripheral nervous system
what is the peripheral nervous system made up of?
- somatic nervous system
- autonomic nervous system
pairs of nerves that originate from the CNS and carry nerve impulses into and out of the CNS
what does the somatic nervous system consist of?
- pairs of nerves that originate from the brain and spinal cord
- containing both sensory and motor neurones
- voluntary actions
what does the autonomic nervous system do?
- controls involuntary actions
- e.g digestion and control of heartbeat
what are the 3 types of neurones in humans?
- sensory neurones
- relay (or connector) neurones
- motor neurones
what do sensory neurones do?
- carry impulses from receptors to the CNS
- via the dorsal root
what do relay neurones do?
- are within the CNS (spinal cord)
- receive impulses from sensory or other relay neurones and transmit them onto motor neurones
what do motor neurones do?
- transmit impulses from the CNS to effectors (muscles or glands)
- via the ventral root
what does a motor neurone contain?
- cell body (centron)
- dendrites
- axon
- myelin sheath
- schwann cell
- nodes of Ranvier
- axon endings/terminals
- synaptic end bulbs
what is the function of the cell body of a motor neurone?
- contains a 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 of a motor neurone?
- carries the impulse away from the cell body
what is the function of the myelin sheath of a motor neurone?
- surrounds the axon (and dendron in sensory neurones)
- providing electrical insulation resulting in faster impulse transmission
what is the function of the schwann cell of a motor neurone?
- surround the axon (and dendron in sensory neurones)
- and forms the myelin sheath
what is the function of the nodes of ranvier of a motor neurone?
- gaps in the myelin sheath between schwann cells are approx 1µm wide where the axon membrane is exposed
- they allow faster nerve impulse conduction
what is the function of the axon terminals/endings of a motor neurone?
- secrete a neurotransmitter which results in depolarisation of the adjacent neurones
(are branched endings of an axon that approach the muscle fibre)
what is the function of the synaptic end bulbs of a motor neurone?
- swelling found at the end of an axon where the neurotransmitter is synthesised
- contains synaptic vesicles filled with neurotransmitters
what are reflexes?
- rapid, automatic responses to stimuli that could prove harmful to the body
- so are protective in nature
- in a reflex arc, a stimulus is detected by the _____ and passed to the ______ along a sensory neurone
- the impulse is then relayed directly to a motor neurone and its effector by a relay neurone
- receptor
- CNS
the response is rapid and involves the contraction of a muscle or release of a hormone (in most cases a reflex involves the spinal cord, but some reflexes e.g pupil reflex, will involve the brain as it is the closest part of the CNS)
do simple animals e.g Cnidarians like Hydra possess a nervous system?
no
- but they have a simplified nervous system called a nerve net
what do nerve nets consist of?
- sensory photoreceptors and touch receptors in the wall of the body and tentacles
- Ganglion cells provide connections between the neurones in several directions but they dont form a brain
- (consists of interconnected nerve cells with short extensions allowing a response to a limited number of stimuli)
compare a Cnidarian nerve net to a mammalian nervous system:
CNIDARIAN NERVE NET:
- 1 type of simple neurone
- unmyelinated
- short, branched neurones
- impulses transmitted in all directions
- slow impulse transmission
- respond to a limited number of stimuli
MAMMALIAN NERVOUS SYSTEM:
- 3 types of neurones (sensory, relay and motor)
- myelinated
- long, unbranched neurones
- impulses transmitted in one direction
- fast impulse transmission
resting potential of a neurone definition
- potential difference across a neurone membrane when a nerve impulse is not being transmitted
- is around -70mV
- the membrane is polarised
when a neurone is at rest i.e no impulses are being transmitted, it is said to be at ____ ___
resting potential
what is the charge across the axon membrane when at rest?
around -70 mV
action potential definition
- the temporary change in electrical potential across the membrane of an axon as a nerve impulse passes
action potential steps:
- at resting potential the charge across the membrane is -70mV
- energy of stimulus arriving causing Na+ voltage-gated channels to open and Na+ ions flood in down their concentration gradient, depolarising the neurone. now the charge across the 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 (positive feedback)
- 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. as a result, further K+ channels open, resulting in even more K+ ions leaving the neurone. the neurone becomes repolarised
- too many K+ ions leave the neurone so the electrical gradient overshoots -70mV reaching around -80mV (which is called hyperpolarisation). to re-establish the resting potential (-70mV) K+ gates now close and the Na+/K+ pump re-establishes the resting potential
Saltatory propagation:
- in myelinated neurons, ions can only move across the membrane at the nodes of Ranvier where there is no myelin present, therefore local circuits are established over greater distances (between each node)
- depolatisation only occurs at the nodes and the action potential effectively ‘jumps’ from node to node, increasing the speed of impulse transmission
is it the impulse that ‘jumps’ from node to node?
NO
- it’s the action potential
what is the total refractory period?
- represents the period during which it is not normally possible to send another impulse
- lasts for approx 6ms
what is the absolute refractory period?
- the period during which it is NOT possible to send another impulse, irrespective of how big the stimulus is
what is the relative refractory period?
- the period during which it is possible to send another impulse, if the stimulus is big enough to overcome the threshold
what is the all or nothing law/rule?
- impulses either pass or do not, and they are always the same size
(- impulses will pass if a threshold value is exceeded (usually -55mV).
- a large stimulus will result in more impulses passing per second (increased frequency of action potentials) rather than a greater level of depolarisation)
will a large stimulus result in more impulses passing per second (increased frequency of action potentials) or a greater level of depolarisation?
more impulses passing per second (increased frequency of action potential)
what are some factors affecting the speed of impulse transmission?
- myelination
- diameter of axon
- temperature
how does myelination affect the speed of impulse transmission?
- saltatory conduction is faster than impulse transmission in unmyelinated neurones, as depolarisation only occurs at the nodes of Ranvier so the action potential effectively ‘jumps’ from node to node
- the rate of transmission varies from 1m/s in unmyelinated neurones to 100m/s in myelinated ones
how does diameter of axon affect the 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)
- (the wider the diameter of an axon the less resistance there is so the action potential travels faster if axon diameter is increased)
how does temperature affect the speed of impulse transmission?
- impulse transmission speed increases with temperature because the rate of diffusion increases due to the increased kinetic energy of ions involved, but only in organisms which do not control their internal body temperature (some ectotherms)
- (diffusion happens faster)
what is the synapse?
- a 20nm gap between two nerve cells or a nerve cell and an effector
how is an impulse transmitted from one neurone to the other?
- by a neurotransmitter
- which diffuses across the synaptic cleft from the pre-synaptic membrane to receptors on the post-synaptic neurone, triggering depolarisation in the post-synaptic neurone