3.8 The nervous system Flashcards
Why is the nervous system good? what does it do?
- Detects changes, or stimuli, in the body or environment
- Processes and stores information
- initiates responses
This increases an organisms chance of survival.
What are the two main parts of the nervous system?
- Central nervous system
- Peripheral nervous system
Describe the CNS:
no need to go in depth on the structure of the spinal cord here
The CNS comprises of the brain and the spinal cord. the CNS processes info provided by the stimulus. Both the brain and the spinal cord are surrounded by tough, protective membranes called collectively, the meninges.
Describe the PNS:
The PNS comprises two parts, the somatic nervous system and the autonomic nervous system.
- the somatic nervous system, i.e. pairs of nerves that originate in the brain or the spinal cord and their branches. These nerves contain the fibres of sensory neurons, which carry impulses away from the CNS to effectors.
- the autonomic nervous system provides unconscious control of functions of internal organs, e.g heartbeat, digestion.
In what order does a reflex arc occur?
Stimulus -> Receptor -> Sensory neurons -> Relay neurons in CNS -> Motor neurone -> effector -> Response
Describe the structure of the spinal cord:
The spinal cord runs down the middle of the vertebrae. It is surrounded by three membranes collectively called the meninges. The outside of the spinal cord is made of ‘white matter’- this contains myelinated neurons. Closer to the centre of the spinal cord is the ‘grey matter’- this contains many nuclei, cell bodies and non-myelinated neurons. Down the middle of the spinal cord is a central canal filled with cerebro-spinal fluid. On each side of the spinal cord the spinal nerves exit and enter. On the dorsal side (the back) is where the sensory neurons enter (this is called the dorsal root); it is characterized by a swelling called the dorsal root ganglion. The cell bodies of the sensory neurons are found in the dorsal root ganglion. On the ventral side (front) the motor neurons are found in the ventral root. Motor neurons have their cell bodies in the grey matter of the spinal cord. Sensory and motor neurons are connected by relay neurons in the grey matter. Relay neurons have synapses to sensory and motor neurons.
Draw out and label the spinal cord showing the sensory neuron and relay neuron and motor neuron:
What are the two types of cells in a nerve net?
- ganglion cells provide connections in several directions
- Sensory cells detect stimuli, e.g. light,sound,touch,temperature.
Draw out the structure of the motor neuron:
What is the function of the myelin sheath?
Electrical insulator; speeds up transmission of impulses
What does resting potential mean? and why is this relevant?
The potential difference across the membrane of a cell when no nervous impulse is being conducted.
A neuron is an excitable cell which means it can change its resting potential (most cells cant)
Explain why resting potential is at -70mV:
The cytoplasm of the axon has a higher concentration of potassium ions and a lower concentration of sodium ions than the outside. This means that potassium ions want to move out of the axons cytoplasm and sodium ions want to move in. But sodium and potassium ions can only move through the membrane via protein channels. The protein channel for sodium ions are always closed, meaning sodium ions cannot leave the outside and enter the axon. The channel for potassium ions is leaky, meaning some of the channels allow potassium to diffuse out. This means the membrane is 100 times more permeable to potassium ions. Also the membrane contains a sodium potassium pump, this pumps potassium ions back into the cell and sodium ions back out. This is against the gradient so it is active transport. The sodium ions are pumped out quicker than the potassium ions are pumped in, resulting in the axon being more negative than the outside (tissue fluid) And this value is that the axon is 70 mV more negative thus the action potential (difference between both sides) is -70mV.
What is the difference between the actual membrane of the axon in resting potential, action potential and repolarisation and what values do the thresholds need to hit for this change?
At -70mV the membranes sodium-potassium pump is working like normal, the channels are leaky to potassium ions and completely closed to sodium ions.
If a stimulus is significant enough to cause the sodium channel to open slightly and the value reaches -55mV then action potential kicks in.
During action potential the membranes potassium channel is completely shut, the sodium channel completely opens until around +40mV at which point depolarisation occurs to restore the value to -70mV.
The membrane in repolarisation is that the potassium channel fully opens and the sodium channel fully closes.
What does action potential mean?
The rapid rise and fall of the electrical potential across a nerve cell membrane as a nervous impulse passes.
What does depolarisation mean?
A temporary reversal of potential across the membrane of a neurone such that the inside becomes less negative than the outside as an action potential is transmitted.
What is a nervous impulse really?
The transmission of a change in potential along a nerve fibre associated with the movement of sodium ions.
How can the voltage change be measured?
A pair of microelectrodes and fed into an oscilloscope. the oscilloscope traces a graph showing how the voltage across the membrane changes with time.
What is a voltage gated channel?
A voltage gated channel is one that only opens or closes in response to a certain voltage across the membrane.
When a stimulant occurs, like a sharp pencil on the fingertip, what occurs in the membrane of an axon that causes the impulse?
The energy from the stimulant causes some of the voltage-gated sodium channels in the axon membrane to open. The sudden increase in the permeability of the membrane to Na+ ions allows them to rapidly diffuse into the axon, down their concentration gradient. As a result the negative charge of -70 mV rapidly rises to +40 mV. This causes the inside to become less negative than the outside and is thus DEPOARISED. Once the potential has risen to +40 mV the sodium channels shut to prevent further influx of sodium ions.
At what point and how does the potential become even more negative than the resting potential and what is this called?
After repolarisation occurs, the potassium channels are fully open and the sodium channels are fully shut, this causes the K+ ions to diffuse down their concentration gradient resulting in more positive ions leaving the cell. This means the potential is more negative than the resting potential and this is called hyperpolarised.
After hyperpolarisation how is the ion balance restored to -70 mV?
The sodium potassium pump pumps K+ back in and Na+ back out restoring the ion balance and thus resting potential.
What is the absolute refractory period and why is it useful?
It means that the site of the initial action potential, the sodium channels are inactivated and cannot open again until the resting potential has been re-established, so a new action potential cannot be initiated there. This ensures the action potential is not propagated back in the direction from which it came, and the nervous impulse only diffuses one way.
What is the ‘all or nothing’ theory?
if the intensity of the stimulus is large enough to to hit the threshold then an action potential will occur, but if it is too weak then no action potential will occur. The action potential is the same size (40mV) no matter the intensity of the stimulus. The only thing that changes when the intensity of the stimulus increases is the frequency of action potential which increases.
Why is the all or nothing aspect of nervous impulses important?
As it allows the action potential to act as a filter, stopping any minor stimuli from setting up a nervous impulse, so the brain is not overloaded with information.
