Lecture 15- Neurons and nervous system Flashcards
What are the two categories of cells that make up the nervous system?
Neurons and glia (also called glial cells)
What does it mean that neurons are ‘excitable’?
They can generate and propagate electrical signals
What is the name of the electrical signal that is generated and propagated by neurons?
Action potentials or nerve impulses
What is the function of glial cells with the nervous system?
To provide support and maintain the extracellular environment of the neurons physically, immunologically and metabolically.
What is a nerve?
A bundle of axons that come together from many different neurons
What is the purpose of long extensions on neurons?
Axons enable them to conduct action potential over long distances
How are nervous systems able to process information?
Their neurons are organised into networks of three functional categories.
Name the three functional categories of neuron cells.
Afferent neurons (information comes from sensory neurons)
Efferent neurons
Interneurons
What do afferent neurons do?
Carry sensory information into the nervous system from sensory neurons
What do sensory neurons do?
These transduce sensory information input into action potentials
(Afferent and sensory neurons are the same category)
What do efferent neurons do?
Carry commands to physiological and behavioral effectors such as muscles and glands
What do interneurons do?
Integrate and store information and facilitate communication between sensors and effectors
Simple animals such as cnidarians have simple networks of neurons that communicate directly between…
Sensory cells and effectors
Where is the cnidarian’s nerve net most developed?
Around the tentacles and oral openings to facilitate food detection or danger
Animals that move around for food and mates process and integrate larger amounts of information. How?
More neurons
Neurons organised into clusters called ganglia
How are ganglia distributed within different bodies?
- Different purposes are distributed around the body, such as in earthworms and squid
- Bilateral symmetry- ganglia come in pairs (1 per side)
- Large animals have a large pair of ganglia- brain
In vertebrates, where are most cells of the nervous system found?
The brain and spinal cord
What are the brain and spinal cord together called?
The central nervous system
What are neurons that extend or reside outside of the brain or spinal cord called?
The peripheral nervous system
How many neurons does the human nervous system contain?
10^11
What are synapses?
Structures where neurons are in close proximity and pass on information
What is the name of the neuron sending the message?
The presynaptic neuron
What is the postsynaptic neuron?
The neuron that receives a message from the presynaptic neuron
How many synapses does the human brain contain and why?
10^14
Because each neuron can receive information from 1000+ synapses
What do specialized ganglia in squid do?
Serve more complex behaviors
How fast is transmission along a neuron?
100m per second or more
What are the 4 main regions of a neuron?
The cell body, dendrites, axon, axon terminals
What does the axon hillock do?
Integrates information collected by dendrites and initiates action potentials
What does the cell body contain?
The nucleus and most cell organelles
What is the function of dendrites?
Receive information from other neurons or sensory cells to the cell body
What is the function of the axon?
Conducts action potentials away from the cell body
What structures are at the end of the axon, away from the cell body?
- Axon divides into a spray of fine nerve endings
- At the tip of these, there is an axon terminal that forms a synapse
What type of synapse is most abundant?
Chemical synapses
What distance separates the pre and post synaptic membranes?
25 nanometers
The arrival of an action potential at an axon terminal causes…
the release of chemical messenger molecules, neurotransmitters, from the axon terminal.
Nervous systems vary in size and complexity. Give an example of differences between the brains of vertebrate species.
- Region that processes olfactory information is more developed in pigs than humans
- Human brain has much of their brain given over to integration of complex behaviors, learning and memory
- Shark brains deal primarily with sensory and motor information
What do brain stems serve?
Basic physiological functions
Are neurons or glial cells more abundant in the nervous system?
Glia
What are some roles of different glia?
- Physical support and orientation of neurons during embryonic development
- Supply neurons with nutrients, maintain extracellular environment
- Insulation
- Removal of debris/foreign matter
What type of glial cells are seen in the CNS?
Glia called oligodendrocytes
What type of glial cells are seen in the PNS?
Schwann cells
What do oligodendrocytes and schwann cells do?
Wrap around axons of neurons and cover them with concentric layers of insulating plasma membrane, called myelin
How does myelination effect axons?
Myelinated axons conduct action potentials faster than those not myelinated
What is another type of glial cell?
Astrocytes
What is the function of astrocytes?
Contribute to the blood-brain barrier
What does the blood brain barrier do?
Protects the brain from toxic chemicals in the blood
How do astrocytes protect the blood brain barrier?
They surround the smallest, most permeable blood vessels in the brain
What does the blood-brain barrier consist of and is hence permeable to?
Plasma membranes- permeable to fat-soluble substances such as anesthetics and alcohol
What is the charge of the inside of a neuron at rest compared to the outside?
Negative
What is the membrane potential?
Any difference in the electric potential across the plasma membrane
What is membrane potential measured in?
Milivolts
What is resting potential?
The membrane potential when a neuron is resting and not firing action potentials.
What does voltage cause?
Electric current as ions to move across cell membranes
In solutions and across cell membranes, what carries electric current?
Ions
What are the major ions that carry electric charges across the plasma membranes of neurons?
Na+, K+, Ca2+, Cl-
What is voltage?
A force that causes electrically charged particles to move between 2 points
How is membrane potential measured?
Electrodes
Made from a glass pipette and sharp tip filled with a conducting solution- one electrode inside the neuron and one outside- difference is measured.
What is the resting potential of a typical axon?
-60 to -70 milivolts
How does resting potential provide a means for neurons to respond to stimulus?
Chemical or physical processes that change the permeability of the plasma membrane to ions produce a change in the cell’s membrane potential
What is an action potential?
A sudden and rapid reversal in the voltage across a portion of the plasma membrane.
The plasma membrane is impermeable to ions. How do they cross the plasma membrane?
Ion pumps and channels
How are ions moved against their concentration gradient?
Ion pumps which use energy
What is a major ion pump in the plasma membranes of neurons?
Sodium-potassium pump (sodium-potassium ATPase)
What does the sodium potassium pump do?
Actively expels Na+ from inside the cell and exchanges it for K+ outside the cell
Ion channels are generally…
selective
What is an electrochemical gradient?
The voltage difference and concentration difference across a plasma gradient that determines net movement of ions
What channels are most commonly open in the plasma membranes of resting neurons?
Potassium channels
Describe the movement of potassium ions in a resting neuron.
Potassium is pumped in
Concentration of potassium is lower outside of cell- potassium diffuses out
Charge inside the neuron becomes more negative
Potassium diffuses back in
What is the membrane potential at which there is no net movement of potassium ions?
The potassium equilibrium potential
How is the potassium equilibrium potential calculated?
Using the Nernst equation
What is the Nernst equation?
Eion=2.3*RT/zF log([ion o]/[ion i])
How can ion channels and their properties be studied?
Patch clamping
Describe how patch clamping works.
- Recording pipette filled with conducting solution is placed on the neurons membrane
- Slight suction clamps the membrane to tip
- Retracting pipette removes the membrane patch and ion channels
- Record as electric currents
What type of ion channel can open and close in response to voltage changes across the plasma membrane?
Voltage-gated channels
What do chemically gated channels open or close in response to?
Presence of absence of specific molecules that bind to the channel protein or receptor that in turn alters the channel protein
Other than chemical and voltage gated channels, what type of channel is there and what does it respond to?
Mechanically gated channels open or close in response to mechanical force applied to the plasma membrane
What happens when sodium channels in the plasma membrane of a neuron open?
Na+ diffuses in
The plasma membrane is depolarised
When does the membrane become hyperpolarised?
When gated K+ channels open and K+ leaves
What does it mean when a membrane is hyperpolarised?
The cell becomes more negative inside
What happens when a cell is depolarised?
Voltage gated Na+ channels open and Na+ rushes into the axon- influx of + ions causes more depolarization.
When is a threshold reached?
5-10mV above resting potential, when influx of Na+ cannot be offset by outward movement of K+
What happens when the threshold is reached?
Voltage-gated Na+ channels open- the membrane becomes positive and the action potential occurs
What is the name given to the period of time when Na+ channels cannot open?
Refractory period
What are the two Na+ channel gates?
Activation gate, inactivation gate
When is a sodium channel activation gate open and closed?
Closed at rest, opens quickly at threshold
When is an sodium channel inactivation gate open and closed?
Open at rest and closes at threshold but responds more slowly, the gate reopens 1-milliseconds later than the activation gate closes
How do voltage gated K+ channels contribute to the refractory period?
By remaining open
What does the movement of K+ ions back in to the neuron do?
Makes the membrane potential less negative than resting potential for a brief period- hyperpolarisation or undershoot
What does hyperpolarisation (undershoot) do?
Prevents the action potential moving backwards along the axon
Why can an action potential be described as an all-or-none event?
Voltage gated sodium channels have a positive feedback mechanism- ensures maximum value of action potential
Why can an action potential be described as self-regenerating?
It spreads to adjacent membrane regions
What effect does myelination by glial cells have?
Increases conduction velocity of axons
What are nodes of Ranvier?
Regularly spaced gaps where the axon is not covered with myelin
Where are action potentials generated?
At nodes- the positive current flows down the inside of the axon
What happens when the action potential reaches the next node?
The membrane is depolarised and another axon potential is generated
The movement of action potentials ‘jumping’ from node to node is called…
saltatory conduction
What is the name of the neurotransmitter used by vertebrate motor neurons?
Acetylcholine
