15. Control and Coordination Flashcards
What is the purpose of communication systems in living organisms?
To pass information between different body parts and coordinate activities, or to regulate substances, or to change activity in response to internal/external stimuli.
Outline the structure and function of the nervous system.
Made of the CNS and PNS (cranial + spinal nerves, attached to brain + spinal cord).
Information transmitted as nerve impulses, along neurone CSMs, at high speeds, directly to target cells.
Sensory receptors, decision-making centres in the CNS and effectors are coordinated.
Outline the function of each type of neurone.
- Sensory: transmit impulses from receptors -> CNS.
- Intermediate/relay/connector: Transmit impulses from sensory -> motor neurones.
- Motor: Transmit impulses from CNS to effectors.
Outline the structure of a motor neurone.
- Cell body: cytoplasm (with rough ER), nucleus, dendrites. Lies within the spinal cord/brain.
- Axon (very long, mitochondria and organelles).
- Terminal branches (synaptic knobs have many mitochondria, transmitter substances and vesicles in cytoplasm).
Describe the structure of dendrites.
Branch out from thin cytoplasmic processes extending from the cell body. Dendrites are highly branched to provide a high SA for endings of other neurones.
Outline the structure of a sensory neurone.
One long axon with the cell body either near the source of the stimuli OR in a swelling of the spinal nerve (called a ganglion).
How is the myelin sheath synthesised?
Schwann cells spiral themselves around the axon (many layers of CSM).
This enclosing sheath = myelin sheath, made of many lipids and some proteins. Gaps in the myelin sheath = nodes of Ranvier.
How are axons arranged within nerves?
Groups of axons are enclosed by a perineurium, forming a nerve. Inside the nerve are also connective tissue, veins and arteries.
Define ‘reflex arc’.
Pathway along which impulses are transmitted from receptor to effector, without involving ‘conscious’ regions of the brain. Some may have no relay neurone, some may be in the brain.
How does a reflex arc work involuntarily?
Within the spinal cord, the impulse is passed to other neurones which take the impulse to the brain, occurs simultaneously with impulses travelling along the motor neurone. Effector responds before voluntary response from conscious regions of the brain. This is known as a reflex action.
Outline how nerve impulses are transmitted.
Not by a flow of electrons (current) but by brief changes in distribution of electric charge across CSM, from movement of Na+ in and K+ out of the axon (action potentials).
Describe the resting potential of an axon.
The inside of the axon has a slightly negative EP compared to the outside (difference = PD, usually -60 to -70 mV).
Produced and maintained by Na+-K+ pumps using ATP.
How is the resting potential of an axon maintained?
Along with the pumps, there are also protein channels which stay open. More of these are for K+ than Na+, so some K+ diffuses out faster, but there are large - molecules inside the cell which attract K+.
There is therefore an overall excess of - ions inside.
How can action potentials occur if the CSM is relatively impermeable to Na+?
Steep concentration gradient, inside of membrane negatively charged. Double gradient = electrochemical gradient.
What is an action potential?
A rapid, fleeting change in PD across the axon membrane (+30mV). Caused by changes in axon CSM permeability to Na+ and K+.
What do voltage-gated channels do?
Allow passage of Na+ and K+, depending on PD across membrane.
Outline the stages in an action potential.
1) Stimulus
2) Depolarisationm
3) Action potential
4) Repolarisation
5) Refractory period
1) How does depolarisation work?
Electric stimulus opens V channels and Na+ enter (down gradient). The PD across the CSM changes - now less negative on the inside.
2) How is the action potential generated, and by which mechanism?
At first only a few channels open, then depolarisation opens more - if the PD reaches -60 to -50mV (threshold potential), many more channels open, and the inside reaches a potential of +30mV compared to outside. Positive feedback mechanism.
Define ‘repolarisation’.
Removal of positive charge from inside the axon, to return the potential difference to -70mV.
3) How does repolarisation work?
After about 1ms, Na+ channels close and K+ channels open. K+ diffuse out, down an electrochemical gradient.
What happens during the absolute refractory period?
Another AP cannot be generated, as Na+ channels are closed. The axon is unresponsive to depolarisation.
What happens during the relative refractory period?
As Na+ channels activate again, a second AP can be generated but it requires a stronger stimulus, as the PD dips below -70mV. K+ are still flowing out of the axon, so they counteract depolarising stimuli.
How do action potentials help to transmit information along a neurone? (1)
The temporary depolarisation of the CSM at the site of an AP sets up local circuits between it and the adjacent resting regions. These local circuits depolarise the neighbouring regions and generate APs in them.
How do action potentials help to transmit information along a neurone? (2)
In the body, APs start at one end and only generate APs ahead of themselves, because the previous region is still recovering (Na+ channels shut tight, axon unresponsive).
Outline five characteristics of an action potential.
1) Discrete
2) Minimum time between APs at a point
3) Length of refractory period determines frequency of impulses.
4) Do not change in size
5) Do not change in speed
If all APs are the same size, how does the brain differentiate between stimuli?
Frequency, number of sensory neurones activated, position of sensory neurone (nature of stimulus).
An unusual stimulus will still send the same message (eg. pressure on eyeballs).
Describe two ways in which neurones are adapted for higher speeds of conduction.
Myelinated = much faster - insulates CSM so that Na+ and K+ cannot enter. APs and depolarisation can only occur at the nodes of Ranvier (pumps and channels are concentrated here). Local circuits exist from one node to the next - APs ‘jump’ (saltatory conduction).
Thicker axons = faster due to lower resistance.
Define ‘receptor cell’.
Cell that responds to a stimulus by generating an AP. Transducers; convert E in one form to E in an electrical impulse in a neurone.
How is the tongue structured to sense taste?
Covered in papillae, which are covered in taste buds, which have 50-100 receptor cells with chemoreceptor proteins. These detect chemicals from liquids / from solids (dissolved in saliva).
How do salt chemoreceptors work?
- Na+ diffuses through selective channels in microvilli, depolarising the membrane. Causes an increase in positive charge inside the cell (receptor potential).
- Ca2+ V channels open, Ca2+ enter cytoplasm and cause exocytosis of NTs from the basal membrane.
- AP stimulated in sensory neurone, impulses sent to taste centre in the cerebral cortex.
How do sweet chemoreceptors work?
Receptor protein shape change stimulates G protein, enzyme activated, cyclic AMP produced, signalling cascade -> K+ channels closed and membrane depolarised.
What happens when a stimulus is not very strong, in terms of receptors?
Only local depolarisation of the receptor cell occurs - sensory neurone isn’t activated to send impulses -> CNS.
