Component 3: Nerves Flashcards
How are are responses in mammals processed?
In mammals, responses to many external and internal stimuli involve the reception of of information and its transfer from a receptor to an effector via the nervous system or as hormones via the blood
What does the structure of the human central nervous system include?
The brain, spinal cord and the peripheral nervous system
Label the parts of a transverse section through a spinal cord
- central canal
- grey matter
- white matter
- dorsal root
- ventral root
- sensory neurone
- dorsal root ganglion
- relay neurone
- motor neurone
What are the steps in the reflex arc?
Stimulus -> Receptor -> Sensory Neurone -> Relay Neurone -> Motor Neurone -> Effector -> Response
Describe the nervous system in a hydra
- Nerve Net
- Ganglion cells is the only type of nerve cell
- cell process lengths are short
- axons are unmyelinated
- conduction speed of 5m/s^-1 (slow)
- impulse from stimulation point can travel in both directions
- limited number of stimuli that can be detected by sensory receptors
- limited number of effectors
Give ways in which the nerve net in Hydra differs from that of that of the nervous system of a vertebrate
- axons have no myelin insulation
- limited number of effectors
- no central nervous system
- action potential can be carried in more than one direction along neurone
What are the parts of the motor neurone?
- Dendrite(s)
- Cell Body
- Nucleus
- Axon
- Myelin Sheath
- Schwann Cell
- Node of Ranvier
- Pre-synaptic knob
What is the function of the dendrite?
thin extensions which carry the impulses towards the cell body
What is the function of the axon?
long cytoplasmic extension which transmits away from the cell body
What is the function of the Schwann cells?
cells which surround axon and insulate them
What is the function of the myelin sheath?
schwann cells grow around the axon to form thin multi-layered fatty sheath, acts as an electrical insulator that speeds up transmission along the axon
What is the function of the Nodes of Ranvier?
areas along the axon where the myelin sheath is missing
What is the function of the cell body?
part of the neurone which contains the nucleus, RER, numerous mitochondria and other cell organelles
What is a reflex action?
Rapid reaction to stimulus which is involuntary
What is the reflex arc?
- pathway taken by a nerve impulse during a reflex action
- the co-ordinator is the spinal cord
- it is for protection (survival) of the organism from dangerous situations
Why are ‘giant’ squid axons important?
- giant axons are big enough to connect electrodes too
- microelectrodes are used to stimulate the axon and a cathode ray oscilloscope is used to measure the potential difference across the membrane
What is the nerve impulse?
- a nerve impulse is the movement of ions in and out the axon membrane
- based on the movement of Na+ and K+ ions across the membrane of the axon
What are the 4 types of transport proteins in the axon membrane?
- a Na+/K+ pump which requires ATP
- voltage-gated Na+ channels (sometimes open/shut)
- voltage-gated K+ channels (sometimes open/shut)
- K+ channels (always open and make membrane more permeable)
Describe what is happening in the neurone at resting potential
- At rest the Na+/K+ pump actively transports 3 Na+ ions out axon for every 2 K+ ions it actively transports in
- Makes concentration gradients for the ions
- Some K+ ions may diffuse out down a concentration gradient through the K+ channels (won’t be many)
- Over time this will make the inside of the axon negatively charged compared to the outside
- The potential difference (difference between the inside and outside of axon) is -70mV (resting potential)
- The membrane is said to be polarised (outside axon has more positive ions than inside the axon, negative)
What is action potential?
rapid, fleeting change in potential difference across the membrane is called an action potential
Describe what is happening in the neurone at depolaristation?
- Stimulation of axon (brief reversal in pd from -70mV to +40mV) causes voltage gated Na+ channels to open
- When channels open Na+ rapidly diffuses into axon down an electrochemical gradient
- This depolarises the axon (makes the inside more positive)
- If it reaches -55mV it creates a threshold change, soall gated Na+ channels will open, Na+ rushes in (down conc gradient) and the potential difference rises to +40mV
- axon membrane is said to be depolarised
Describe what is happening in the neurone at repolarisation?
- A fraction of a second later, voltage gated K+ channels open (voltage gated sodium channels close) and the membrane becomes more permeable to K+ which diffuse out of the axon down their concentration gradient
- This removal of positive charge from the inside of the axon starts to repolarise the membrane
Describe what is happening in the neurone at hyperpolarisation?
- Due to so many K+ ions leaving the axon, the inside of the membrane briefly becomes even more negative than at normal resting potential
- This overshoot is called hyperpolarisation
- Then restores to resting potential
What is the all-or-nothing law?
- the action potential will only be generated by a stimulus if the depolarisation of axon membrane exceeds threshold value
- threshold stimulus causes depolarisation of axon membrane and starts off an action potential in neurone
- stimulus weaker than threshold value = no action potential
- if stimulus intensity exceeds threshold and causes depolarisation, the action potential generated is always the same size and strength (!) no matter what size/strength of stimulus
- however the greater the stimulus above the threshold value, the greater the frequency of the action potentials along axon (more often)
