neurones Flashcards
neurones
The nervous system is made up of billions of neurones, and these can be categorised into three main groups: sensory neurones, relay neurones and motor neurones.
All three neurones have these common features:
• Cell body: Contains the organelles found in a typical animal cell, including the nucleus. Proteins and neurotransmitter chemicals are made here.
• Dendrons: Carry the action potentials to surrounding cells.
• Axon: Conductive, long fibre that carries the nervous impulse along the motor neurone.
myelinated neurones
• Myelinated neurones have Schwann cells which wrap around the axon to form the myelin sheath, whichtis a lipid and therefore does not allow charged ions to pass through it.
• There are gaps between the myelin sheath, called nodes of Ranvier.
• The action potential jumps from node to node (saltatory conduction), which means the action potential travels along the axon faster as it doesn’t have to generate an action potential along the entire length, just at the nodes of Ranvier.
sensory neurones
• Carry electrical impulses from the sensory receptor cell to the relay neurone (sometimes also to the motor neurone and the brain).
• They have a long dendron which carries the impulse from the sensory receptor cell to the cell body of the neurone, and then an axon to carry the impulse from the cell body to the next neurone.
relay neurones
• These neurones carry impulses between the sensory and motor neurones. They have multiple short axons and dendrons.
motor neurone
• These carry the impulse from a relay or sensory neurones to the effector (a muscle or a gland). The have one long axon and multiple short dendrons.
sensory receptors
Sensory receptors detect a stimulus.
These cells are transducer, converting different types of stimuli into electrical nerve impulses.
There are different types of sensory receptors, and each type can detect a different stimulus.
Type of receptor
Photoreceptors (rods and cone cells)-light
Thermoreceptors (skin)-heat
Mechanoreceptor (Pacinian corpuscle in skin)-pressure
pacinian corpuscle
Pressure receptor located deep in skin, mainly in fingers and feet.
The sensory neurone in the Pacinian corpuscle has special channel proteins in its plasma membrane.
The membranes of the Pacinian corpuscle have stretch-mediated sodium channels.
These open and allow Nat to enter the sensory neurone only when they are stretched and deformed.
When pressure is applied it deforms the neurone plasma membrane, stretches and widens the Na+ channels so Na+ diffuses in which leads to the establishment of a generator potential.
resting potential
When a neurone is not conducting an impulse, there is a difference between the electrical charge inside & outside of the neurone, this is known as the resting potential.
There are more positive ions, Na+ and K+, outside compared to inside, therefore the inside of the neurone is comparatively more negative at -70m V.
establishing a resting potential
• The resting potential is maintained by a sodium-potassium pump, involving active transport and ATP.
• The pump moves 2 K+ions in & 3 Na+ions out.
• This creates an electrochemical gradient causing K+ to diffuse out & Na+ to diffuse in.
• The membrane is more permeable to K+ so more are moved out resulting in the - 70mV.
action potential
An action potential is when the neurone’s voltage increases beyond a set point from the resting potential. This generates a nervous impulse
An increase in voltage, or depolarisation, is due to the neurone membrane becoming more permeable to Nat.
Once an action potential is generated, it moves along the axon like a Mexican wave.
changes in graph
-until there is stimulus we are at resting potential, stimulus provides energy that can cause voltage gated ion channels (sodium) in axon membrane to open, causes sodium ions to diffuse in, which increases positivity of axon, this causes more voltage gated ion channels to open, so even more sodium ions diffuse in.
-When a threshold of -55mV is reached/exceeded you will always reach this maximum of +40mV, when you reach maximum, voltage gated sodium ion channels close, but potassium ion channels remain open. That means no sodium ions move in, but potassium ions move out-repolarisation occurs
-More potassium ion channels open so even more leave, this is why there is an overshoot past -70mV which is called hyperpolarisation, meaning voltage becomes even more negative than resting potential- enters refractory period where you wouldn’t be able to generate another action potential because voltage is too low.
-This happens at all nodes of ranvier.
changes in graph
-until there is stimulus we are at resting potential, stimulus provides energy that can cause voltage gated ion channels (sodium) in axon membrane to open, causes sodium ions to diffuse in, which increases positivity of axon, this causes more voltage gated ion channels to open, so even more sodium ions diffuse in.
-When a threshold of -55mV is reached/exceeded you will always reach this maximum of +40mV, when you reach maximum, voltage gated sodium ion channels close, but potassium ion channels remain open. That means no sodium ions move in, but potassium ions move out-repolarisation occurs
-More potassium ion channels open so even more leave, this is why there is an overshoot past -70mV which is called hyperpolarisation, meaning voltage becomes even more negative than resting potential- enters refractory period where you wouldn’t be able to generate another action potential because voltage is too low.
-This happens at all nodes of ranvier.
all or nothing principle
If the depolarisation does not exceed -55 mV, an action potential & impulse are not produced (Nothing).
Any stimulus that does trigger depolarisation to
-55mV will always peak at the same maximum voltage (All). Bigger stimuli increase the frequency of action potentials.
This is the All-or-Nothing Principle.
This is important, as it makes sure that animals only respond to large enough stimuli, rather than responding to every slight change in the environment.
refractory period
After an action potential has been generated, the membrane enters a refractory period when it can’t be stimulated, because sodium channels are recovering and can’t be opened.
This is important for three reasons:
1. It ensures that discrete impulses are produced. An action potential cannot be generated immediately after another & this makes sure that each is separate.
2. It ensures that action potentials travel in one direction. This stops the action potential from spreading out in two directions which would prevent a response. *
3. It limits the number of impulse transmission. This is important to prevent over reaction to a stimulus.
synapses
Synapses are the gaps between the end of the axon of one neuron and the dendrite of another one.
Here the action potential is transmitted as neurotransmitters that diffuse across the synapse.
function of a synapse
-An action potential arrives at synaptic knob. Depolarisation of synaptic knob leads to opening of Ca2+ channels and Ca2+ diffuses into synaptic knob.
-influx of calcium ions causes Vesicles containing neurotransmitter to move towards and fuse with the presynaptic membrane. Neurotransmitter is released to the synaptic cleft.
-Neurotransmitter diffuses, down concentration gradient, across synaptic cleft, to post-synaptic membrane; neurotransmitter binds by complementarity of shape to receptors on the surface of the post-synaptic membrane.
-Na+ ion channels on the post-synaptic membrane open and Na* diffuse in; if enough neurotransmitter, then enough Na* diffuse in, above threshold, and post-synaptic neuron becomes depolarised.
-Neurotransmitter is degraded and released from the receptor; the Na+ channel close and the post-synaptic neuron can re-establish resting potential; the neurotransmitter is transported back into the presynaptic neuron where it is recycled.
function of a synapse
-An action potential arrives at synaptic knob. Depolarisation of synaptic knob leads to opening of Ca2+ channels and Ca2+ diffuses into synaptic knob.
-influx of calcium ions causes Vesicles containing neurotransmitter to move towards and fuse with the presynaptic membrane. Neurotransmitter is released to the synaptic cleft.
-Neurotransmitter diffuses, down concentration gradient, across synaptic cleft, to post-synaptic membrane; neurotransmitter binds by complementarity of shape to receptors on the surface of the post-synaptic membrane.
-Na+ ion channels on the post-synaptic membrane open and Na* diffuse in; if enough neurotransmitter, then enough Na* diffuse in, above threshold, and post-synaptic neuron becomes depolarised.
-Neurotransmitter is degraded and released from the receptor; the Na+ channel close and the post-synaptic neuron can re-establish resting potential; the neurotransmitter is transported back into the presynaptic neuron where it is recycled.
-Unidirectional
-cholinergic synapse- neurotransmitter is acetylcholine, enzyme which breaks down acetylcholine is acetylcholinesterase
summation
Summation is the rapid build-up of neurotransmitters in the synapse to help generate an action potential by two methods; spatial or temporal summation.
Spatial summation: many different neurones collectively trigger a new action potential by combining the neurotransmitter they release to exceed the threshold value.
Temporal summation: One neurone releases neurotransmitter repeatedly over a short period of time to add up to enough to exceed the threshold value.
inhibitory synapse
Inhibitory synapses cause chloride ions to move into the postsynaptic neurone & potassium ions to move out.
This makes the membrane potential decrease to -80mV, hyperpolarisation, and therefore an action potential is highly unlikely.
