3.8 The Nervous System Flashcards
Hormonal control
Endocrine response
Changes things more slowly
Long-term response
Relies upon chemicals being carried by the blood
Nervous control
Nervous response
Rapidly
Short term response
Relies upon information carried by neurones
What does the nervous system consist of
Central nervous system
Peripheral nervous system
What does the CNS consist of
Brain and spinal cord
What does the PNS consist of
Spinal nerves and cranial nerves
Order of how a stimulus gets a response
Stimulus
Detector
Coordinator
Effector
Response
What is a stimulus
Change in environment
What does a detector have and do
Contains cells that detect stimuli
Converts energy from one form to an electrical impulse
What is an effetor
Something that brings about a response
Muscle or gland
Order of neurones
Sensory
Relay
Motor
Describe a reflex arc
Rapid
Protective
Involuntary
How does a reflex arc occur
- Receptor detects a stimulus
- A nerve impulse is initiated and transmitted through a sensory neurone to the spinal cord
- The impulse crosses a synapse into a relay neurone in the grey matter
- The impulse is transmitted through the relay neurone to a motor neurone across a synapse
- The motor neurone transmits the impulse to an effector
- Effector makes the response
What is a hydra
A nerve net that consists of simple nerve cells of one type that have short extensions and connect with other nerve cells in a net like structure which branches in several different directions
Compare cnidarian nerve net and mammalian nervous system
• A cnidarian consist of 1 type of simple neurone whilst a mammalian has 3 neurones
• nerve nets are unmyelinated whilst nervous systems are myelinated so mammals have a faster impulse transmission
• nerve nets have short-unbranched neurones whilst nervous systems have long, unbranched neurones
• nerve nets have impulses transmitted in both directions whilst the nervous system is unidirectional
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
Function of the axon
Carries the impulse away from the cell body
Function of the myelin sheath
Surrounds the axon providing electrical insulation resulting in a faster impulse transmission
Function of Schwann cell
Surrounds the axon and forms the myelin sheath
Function of Nodes of Ranvier
Gaps in the myelin sheath between the Schwann cells that are approximately 1 um wide where the axon membrane is exposed. They allow faster nerve impulse conduction
Function of axon endings
Secrete a neurotransmitter which results in depolarisation of the adjacent neurone
Function of synaptic end bulbs
Swelling found at the end of an axon where a neurotransmitter is synthesised
What is the membrane at resting potential
Polarised
Resting potential value
-70 mV
The axoplasm is negative in relation to the outside of the cell
How is resting potential brought about
• a sodium potassium pump using ATP actively transports 3 Na+ out of the neurone and 2 K+ into the neurone
• K+ channels are leaky and K+ pass out of the axoplasm by facilitated diffusion
• Na+ channels are closed so Na+ remain outside of the neurone
• larger proteins and organic phosphates in the cytoplasm are negatively charged
What is happening to the nerve cell at resting potential
It is not transmitting an impulse
Potential difference of an action potential
+40mV
What happens during an action potential
When the threshold level is reached all the Na+ channels open and Na+ rapidly diffuse into the cell causing an influx of Na+.
What is the membrane described as during an action potential
Depolarised
What is the all or nothing law
• The threshold level of the stimulus must be reached in order for an action potential to be generated
• the action potential is always the same size if the threshold level is reached
• a large stimulus will result in more impulses passing per second rather than a greater level of depolarisation
Repolarisation
• return to resting potential from the action potential
• the Na+ channels close and K+ channels open
• the K+ leaves the axoplasm by facilitated diffusion and travel rapidly down their concentration gradient and potential difference falls quickly
Hyperpolarisation
• slight overshoot of K+ leaving the neurone so electrical gradient reaches -80mV
• the K+ gates close and Na+/K+ pump re-establishes resting potential
What is the refractory period
During an action potential and repolarisation further action potentials are not possible
How does the refractory period keep the transmission along the axon moving in 1 direction
Part of the membrane immediately behind an action potential is still repolarising and cannot be stimulated to respond
How does temperature affect the speed of conduction of a nervous impulse
A higher temperature increases the speed as the increased kinetic energy increases the rate of diffusion
How does axon diameter affect speed of conduction of a nervous impulse
Larger diameter increases the speed and there is less leakage of ions from large axons due to larger volume to surface area ratio
How does the myelin sheath effect the speed of conduction of a nervous impulse
It allows for saltatory propagation which increases the speed
What is saltatory propagation?
The voltage gated ion channels are found only at the nodes of Ranvier and there is a myelin sheath between the nodes and act as a good electrical insulation so that the action potential can jump large distances from node to node
Maximum speed of conduction for unmyelinated and myelinated neurones
Unmyelinated- 1m/s
Myelinated- 100m/s
What is the only kind of animals that have a myelin shesth
Vertebrates
What is a synapse
• a place where 2 neurones meet or where a motor neurone meets an effector
• is a gap between cells so transmission by ion transport across the membrane is not possible
Functions of a synapse
• transmits information between neurones
• transmits information in 1 direction only
• acts as junctions
• filter out low-level stimuli
• prevent over stimulation of neurone and fatigue
How does a synapse only transmit information in 1 direction
• vesicles are only found in the synaptic knob and the receptors are only found in the post-synaptic membrane
Process of synaptic transmission
- Impulse arrives at the pre-synaptic knob
- Calcium channels open so Ca2+ ions diffuse rapidly into the pre-synaptic knob
- Ca2+ stimulate the vesicles of neurotransmitter (eg acetylcholine) to migrate to and fuse to the pre-synaptic membrane
- The neurotransmitter is released into the synaptic cleft by exocytosis which requires ATP
- Acetylcholine diffuses across the cleft and binds to receptors on the post-synaptic membrane causing sodium channels to open
- Na+ ions rush into the post-synaptic neurone by diffusion resulting in the depolarisation of the post-synaptic membrane. An action potential is initiated
- Acetyl cholinesterase splits the acetylcholine into ethanoic acid and choline, releasing them from the receptor, and sodium channels close and the products diffuse back across the cleft
- Products are reabsorbed into the pre-synaptic knob
- ATP is used to reform acetylcholine in the pre-synaptic knob
How is the repeated depolarisation of the post synaptic neurone prevented
• hydrolysis of acetylcholine
• reabsorption of ethanoic acid and choline back into the pre-synaptic knob
• active transport of Ca2+ out of pre-synaptic knob which prevents the further exocytosis of neurotransmitter
What happens if not enough acetylcholine is released
Not enough sodium channels open so post-synaptic membrane does not exceed the threshold of -55 mV so no action potential initiated
What are the two main types of drugs
Antagonists and agonists
How do antagonists work
• affect by preventing post-synaptic depolarisation
Eg cannabis, alcohol
• it can decrease action potentials in the post-synaptic membrane
• it prevents the exocytosis and so the release of the neurotransmitter
• it binds with receptors on the post synaptic membrane and blocks it
• it prevents Ca2+ entry into the presynaptic knob
How do agonists work
• cause more post-synaptic depolarisation
Eg caffeine, cocaine, organophosphorous insecticide
• it acts as a cholinesterase inhibitor
• it mimics the neurotransmitter by linking to the receptors
• blocks the uptake back into the pre-synaptic knob which causes more exocytosis
• eventually it increases the number of receptors on post-synaptic membrane
