Neuronal Communication, 5.3 Flashcards
Sensory receptors
Sensory nerve endings that detect a change in our surroundings, internal or external, and can create action potentials, they are tranducers
Tranducer
A cell that converts one form of energy to another in this case electrical energy
What are Pacinian Corpuscles?
A pressure sensor on the skin, detect changes not constantly applied pressure
Describe the structure of Pacinian Corpuscles
Oval shape, concentric rings of connective tissue wrapped around a nerve ending
How is a nerve impulse generated?
Sodium channel pump 3 out for every 2 potassium ions pumped in, sodium channels usually closed. Membrane is more permeable to potassium ions. Membrane is deformed by pressure and opens sodium channels, sodium ions enter (generator potential). Membrane depolarizes.
Function of a motor neurone
Carry AP from CNS to an effector
Function of a sensory neurone
Carry AP from sensory to the CNS
Function of a relay neurone
Connects sensory and motor neurones
Structure of neurones
Long - transmit over distance. Membrane has gated ion channels. Lots of mitochondria - produce ATP for active transport. Dendrites - connect to other neurons. Axon - carry impulse away from cell body. Fatty layer made of Schwann cells.
What is a myelin sheath?
A fatty layer made of tightly wrapped Schwann cells and several layers of membrane and thin cytoplasm
What are the nodes of Ranvier?
Gaps in the myelin sheath where movements of ions can occur.
How does myelination affect transmission?
Prevents movement of ions, currents can’t flow, means the impulse jumps from one node to the next, more rapid and better over long distances.
How do non-myelinated neurons work?
Schwann cells are loosely wrapped to insulate from electrical activity, impulses move in a wave, transmitted over short distances, takes longer.
What is a resting potential?
When the neuron is not transmitting an action potential
What happens during a resting potential?
Actively pumps ions - 3 sodium ions out for every 2 potassium ions in, membrane is more permeable to potassium ions
Why is the resting potential very negative? (-60mV)
More positive ions moving out and cytoplasm contains anions and proteins which are negatively charged
How is a generator potential generated?
Generator region - sodium ion channels open and sodium ions flood in. The cell membrane is depolarized - generator potential.
How is an action potential generated?
The more sodium ion channels open, these can be voltage gated, the more generator potentials created - these combine to reach the threshold value and produce an action potential. (positive feedback)
When is the action potential transmitted?
When depolarization reaches +40mV.
What is the All or Nothing Principle?
The size of the stimulus does mean a different size of the action potential - as long as it reaches the threshold
value.
What is the refractory period?
Ions out of place - Sodium inside, Potassium outside. For a short time generating an action potential is impossible - allows cells to recover.
What is hyperpolarisation?
Where potassium ions diffuse out to make the cell negative again but slightly overshoots.
Local currents
Movement of charged particles in the cytoplasm
Why does an action potential transmit?
Sodium ions move along the axon by diffusion - as their concentration is lower further along
Describe the process of transmitting an action potential
- Action potential is generated
- Sodium ions cause other sodium ion channels to open due to slight depolarization
- Sodium ions diffuse sideways where their concentration is lower
- Local currents causes slight depolarization further along opening more channels - causes full depolarization
What prevents an action potential from moving backwards?
Concentration of sodium ions is still high behind, they can also not cross back across the synapse
What is Saltatory Conduction?
Sodium and potassium ions cannot diffuse through the myelin sheath, can’t depolarize under the sheath, so ionic movement occurs at nodes of Ranvier.
Synapse
Junction between or more neurones
Synaptic cleft
Small gaps between 2 neurons about 20nm wide
Specialized features of the pre-synaptic bulb
Many mitochondria - active processes. Lots of SER - packaging vesicles. Vesicles containing acetylcholine. Voltage gated calcium ion channels.
Specialized features of the post-synaptic membrane
Specialized sodium ion channels which respond to acetylcholine. 5 polypeptide molecules: special receptor sites (2 acetylcholine molecules)
Describe how an action potential crosses a synapse
- AP arrives at pre synpatic bulb
- Voltage gated calcium ion channels open
- Calcium ions cause vesicles to move and fuse with the pre synaptic membrane
- Acetycholine released by exocytosis then diffuses
- Binds to receptor sites on sodium ion channels so they open
- A EPSP is created - if enough combine and the threshold is reach an action potential is created
What is an EPSP?
An excitatory post-synaptic potential (generator
potential)
What is the role of acetylcholinesterase?
Enzyme which hydrolyses acetlycholine to ethanoic acid (acetic acid) and choline. Stops the transmission of signals.
Summation
When the effects of several excitatory post-synaptic potentials are added together
Temporal summation
Several action potentials in the same pre synaptic membrane so more acetylcholine released
Spatial summation
Action potentials arriving from different pre synaptic neurones
What is an IPSP?
Inhibitory post-synaptic potential. Reduce the effect of summation and prevent an action potential.
Why is spatial summation useful?
Allow action potentials from different parts of the nervous system to combine, when warning of danger.
Why is temporal summation useful?
One action potential diverges to several post synaptic neurons useful in a reflex arc
Advantages of summation generally?
Can filter out low level stimuli, deal with sensory overload, persistent low level stimuli can be amplified.
What happens after repeated summation?
Over a long period of time a synapse may run out of vesicles, synapse is fatigued, we are said to become habituated to it.
Why is a synapse unidirectional?
Vesicles of neurotransmitter only found in the pre synaptic neuron. Receptors only found in the post
synaptic neuron.
What affects the speed of generating action potentials?
The diameter of the axon: Larger = faster, myelination and temperature.
How is heart controlled by the nervous system?
Sends an impulse to the SAN which controls frequency of contractions. Vagus nerve - decreases heart rate. Accelerator nerve - increases heart rate.
Why are synapses important in the nervous system?
- Ensures impulse moves in only one direction
- Neurones can release/receive impulses from/too many neurones
- Allows summation, filter out low level stimuli
Differences in structure between the motor and sensory neurone.
Sensory - short axon, long dendron, cell body outside CNS, cell body at the middle, dendrites not connected to cell body
Motor - long axon, no dendron, cell body in CNS, cell body at end of neurone, dendrites connected to cell body
How can the intensity of a stimulus be interpreted?
By the frequency of the action potential
What is the function of the Myelin Sheath?
- Electrical insulation
- Prevents movement of ions/depolarisation
- Speeds up conduction
- Action potentials only at nodes of Ranvier
Why is MS described as an auto-immune condition?
- Body’s immune system attack the nervous system
- Recognises neurones as foreign
What can happen if the neurone on a neuronal junction is damaged?
There will be a lower rate of action potentials reaching the Neurone Muscular Junction. Less acetylcholine being released and binding to the the receptors. Less depolarisation of the post synaptic membrane.
