Neurobiology Flashcards
Describe the division of the nervous system and name the functions of each part
Central nervous system (CNS)
-> Brain
-> Protected by the skull and three membranes or meninges (=Hirnhäute) that surround the brain.
-> Spinal Cord: central connecting element between the brain and the PNS; protected by the Spine
The function of the CNS: Uptake, forwarding and processing of information
Note: A colourless liquid, the cerebrospinal fluid (also called liquor), surrounds the brain and spinal cord. The fluid acts as a lubricant and shock absorber. It also provides CNS with nutrients and removes waste products.
Peripheral nervous system (PNS)
-> Compromises all the nerves and sensory receptors outside the central nervous system
-> Sensory neurons carry messages to the central nervous system for processing
-> Motor nerves carry impulses from the CNS to effectors: muscles and glands
Effectors -> Somatic Nervous System and Autonomic nervous system -> Sympathetic and Parasympathetic nervous system
Describe and explain how a stimulus leads to a reaction
The stimulus is picked up by a receptor of the peripheral nervous system, e.g., the light sensory cell in the eye being stimulated by the light of an incoming object. The stimulation causes the receptor to become stimulated and it creates an impulse. The afferent (=forwards impulses to the CNS) sensory neurons forwards this impulse generated by the stimulated receptor to the CNS, where the brain acts as a computing centre. After having computed the correct response, the efferent (forwarding impulses from the CNS to the muscles) motor neurons transmit the impulse to the effector, in this case, a muscle. When the muscles get stimulated by this impulse it causes us to execute a response to the specific stimuli, in this case making us duck away from the object or intercepting the object.
State the function of different parts of the neuron:
Synaptic knob: Functional connections to muscle fibres / to other neurons;
Soma (cell body): Housing for the nucleus/organelles;
Myelin sheath: Support, electrical insulation, and nutrition for neurons;
Axon branches: The signal transmission connects to the synaptic knob, spread the impulses;
Axon: Signal transmission from the soma to the synaptic knobs;
Nucleus: Cell regulation / cell information;
Dendrites: Transmits impulses along the soma to other neurons;
Nodes of Ranvier: Conducting electrical impulses along the axon;
Axon hillock: The origin point of the axon. Initiates action potentials. Maintains axon structure;
Synapse: Connection point that goes together with the synaptic knobs for signal transmission between neurons;
Schwann (Glial) cell: Compose the myelin sheath
Draw fully labelled sketches of experimental setups used to record resting, action and postsynaptic potential and explain these setups:
Membrane potentials can be recorded using isolated neurons placed in a saline solution, i.e., a solution that mimics the natural environment of the cells. The test itself is comprised of an oscilloscope to measure the electrical differentiation, an amplifier to amplify the incoming electrical current, two electrodes that can be attached to the neuron and/or its surroundings, a petri dish filled with saline solution to mimic the natural environment of the neuron and finally a neuron itself. In the experiment, the reference and recording electrode are put in different places on/around the neuron. In 1 both are outside the experiment which his, why one cannot see a potential differentiation, in 3 they are both in the saline solution which is why one can see a potential difference of 0, because both electrodes are measuring the same electrical current, and in 3 the recording electrode, is in the neuron and the reference neuron is still in the saline solution. One can only see a potential difference in 3 because the innards of the neuron have a different potential to that of the saline solution.
Explain why ions cannot simply diffuse through biological membranes
A biological membrane is also a semipermeable membrane, meaning it can only allow a certain type of molecule through whilst keeping another type of molecule out. In the example illustrated in fig.3 and fig.4 one can see a semipermeable membrane in action. In fig 3 there is a KCI solution in chamber 1 and nothing in Chamber 2. The semipermeable biological membrane can only allow K+ ions through. Later in the experiment, one can see that there is now K+ ions in Chamber 2 whilst the remaining components of the KCI solution in chamber 1 are still there. This is because the direction and magnitude of the movement of ions through a channel depend on the concentration gradient of the ion type across the cell membrane, as well as the voltage difference across that membrane. The two acting motive forces acting on an Ion are termed its electrochemical gradient. The electrochemical gradient of k+ ions, in this case, made it go from chamber 1 to chamber 2.
Describe the difference between the following ion channels: non-gated leak/voltage gated / ligand-gated channels and explain the role they play in the nervous system.
Non-gated leak
Explanation: An ion channel in a cell membrane that is always open, making the membrane permeable to ions.
Role in the nervous System:
The leak channels allow Na+ and K+ to move across the cell membrane down their gradients (from a high concentration toward a lower concentration). With the combined ion pumping the leakage of ions, the cell can maintain a stable resting potential.
Describe the difference between the following ion channels: voltage gated channels and explain the role they play in the nervous system.
Voltage-gated
Explanation: Voltage-gated ion channels open and close in response to membrane potential (difference of electrical potential inside and outside a cell)
Role in the nervous System:
They control the sodium exchange between the extracellular and intracellular spaces and are essential for the initiation and firing of action potentials.
Describe the difference between the following ion channels: ligand-gated channels and explain the role they play in the nervous system.
Ligand-gated
Explanation: Open in response to specific ligand molecules binding to the extracellular domain of the receptor protein, causing a conformational change in the structure of the channel protein, causing it to open.
Role in the nervous System:
Ligand-gated ion channels are transmembrane protein complexes that conduction flow through a channel pore in response to the binding of a neurotransmitter. Essential for neuron-to-neuron communication.
Describe and explain how the resting potential is created and maintained.
- Membrane selectively permeable for K+ (potassium) ions, diffusing out of the cell along their concentration gradient
- They leave behind unbalanced negative charges, generating an electrical potential across the membrane that pulls K+ ions back into the cell -> electrochemical gradient
- Resting membrane potential is generated with a consistent voltage difference across the membrane
- Resting membrane potential is mainly determined by K+ ions.
- Resting membrane potential is negative (intercellular is negative compared to extracellular).
Define the term voltage/potential difference and state the average value of the resting potential in mammals.
Voltage, electric potential difference, electric pressure or electric tension is the difference in electric potential between two points, which (in a static electric field) is defined as the work needed per unit of charge to move a test charge between the two points.
Cells are excitable in a range of -60 to -95 millivolts. (in most mammals -70)
Define the erm equilibrium potential
Equilibrium potential is the saturation of the momentary directional flow of charged ions at the cell membrane level. This phase typically features a zero-charge inhibiting the flow of ions between either side of the membrane. However, the phase is independent of the ion flow on both sides of the membrane.
Describe and explain how changes in the intra- and extracellular ion concentrations might influence the resting potential
Each ion has a different potential, with some being negative in concentration and some being positive. If any of these ions were to go into the cell, its potential would become positive or negative, respectively. Because the concentration of ions on the exterior and interior of the cell determines the concentration gradient (going from high concentration to low concentration), it can cause an influx of a certain positively charged ion that can alter the potential of the cell, the resting potential.
Describe how the sodium-potassium pump works and explain its role.
The process of moving sodium and potassium ions across the cell membrane is an active transport process involving the hydrolysis of ATP to provide the necessary energy, creating ADP and Pi. It involves an enzyme referred to as Na+/K+-ATPase. This process is responsible for maintaining the large excess of Na+ outside the cell and the large excess of K+ ions on the inside. It accomplishes the transport of three Na+ to the outside of the cell and the transport of two K+ ions to the inside. This unbalanced charge transfer contributes to the separation of charge across the membrane. The sodium-potassium pump is an important contributor to action potential produced by nerve cells.
Define the terms threshold, depolarization, repolarization, and hyperpolarization
Threshold: The threshold value controls whether or not the incoming stimuli are sufficient to generate an action potential. It relies on a balance of incoming inhibitory and excitatory stimuli. The potentials generated by the stimuli are additive, and they may reach threshold depending on their frequency and amplitude.
Depolarization: During an action potential, the depolarization is so large that the potential difference across the cell membrane briefly reverses polarity, with the inside of the cell becoming positively charged.
Repolarization is a stage of an action potential in which the cell experiences a decrease in voltage due to the efflux of potassium (K+) ions along its electrochemical gradient. This phase occurs after the cell reaches its highest voltage from depolarization.
Hyperpolarization: Hyperpolarization is a change in a cell’s membrane potential that makes it more negative. It is the opposite of depolarization. It inhibits action potentials by increasing the stimulus required to move the membrane potential to the action potential threshold.
Describe and explain how the typical shape of the action potential is brought about by Na+, K+, and voltage-gated ion channels.
- The resting potential of -80.
- A stimulus is introduced, causing a depolarization, allowing the Voltage to reach a max. of +30mv. The voltage-gated Na+ channels open, allowing Na+ to the innards of the cell, making the cell more positive.
- Depolarization peak.
- Repolarization – The voltage-gated Na+ channels close, the Ka+ channels open. The cell starts to become more negative again
- Hyperpolarization
- Hyperpolarization is overcome, and equilibrium is restored using leakage channels and the sodium-potassium pump.
- The resting potential of -80 is restored, the cycle can begin again.
Define the term refractory period and describe and explain its causes:
The refractory period is the phase of the Ap where the voltage becomes more negative than the resting potential, a state reached by hyperpolarization. There is now a surplus of K+ ions in the cell, making the cells potential negative. In this phase, an action potential is more unlikely to happen again as the stimuli required would have to be greater to surpass the threshold potential.
Describe and explain why AP is propagated in only one direction.
Biologically, action potentials are propagated in one direction due to how neurons are connected. Signals are transmitted across synapses to eventually the soma of a neuron. These potentials are summated in either positive or negative ways and transmitted to the axon hillock. The hillock is where the signal begins to propagate down the axon. There is no comparable system of signal transmission in the opposite direction.
Describe and explain how an action potential is propagated in non-myelinated and myelinated axons and compare these two modes of conduction.
A: Continuous conduction: This conduction mode is found in neurons that do not have a myelin sheath (i.e., mostly in invertebrates). Because of the influx of Na+ ions at the site of an AP, the intracellular charge is positive. In adjacent areas, the charge is negative (like during a resting potential). Because of this, the positively charged Na+ ions flow into the negatively charged adjacent regions and depolarize them. The threshold potential is exceeded and another Ap is generated. Thereby APs are propagated in unmyelinated neurons.
B: Saltatory conduction (Saltus – jump): The saltatory conduction mode is found in myelinated neurons (typically found in vertebrates). Voltage-gated Na+ channels only exist at the Nodes of Ranvier. Consequently, APS only occur here. They “jump” from Node of Ranvier to Node of Ranvier, skipping the myelinated areas. The Schwann cells/myelin sheath has an insulating function: Because of it, the distance between intr- and extracellular space is increased. This is why there are almost no attractive forces between the intra- and extracellular space, i.e., the ions can move more easily.
Compare the transmission of action potential in unmyelinated neurons and myelinated neurons
Unmyelinated neurons: Mode of conduction: Continuous; Velocity: Slow, 1-20 m/s; Diameter: 1 – 500 micrometres; Energy demand: High; Material required: Much;
Myelinated neurons:
Mode of conduction: Saltatory;
Velocity: Faster, 3-120 m/s;
Diameter: 1-20 micrometres;
Energy demand: Lower (fewer Na+/K+ ion pumps needed -> only at the Nodes of Ranvier );
Material required: Little (because smaller in diameter).
Describe and explain how the propagation speed of action potentials depends on the diameter in non-myelinated axons
A larger diameter: - provides less resistance to the ions entering the axon
- Allows Na+ ions to travel down the axon easier (otherwise, there would be more ways and obstacles (e.g., microfilaments and tubules)).
Describe the process taking place during the transmission of nerve impulses at a synapse including the creation of a postsynaptic potential
- Action potential arrives at the axon terminal
- Na+ channels open; depolarization causes voltage gated Ca2+ channels to open
- Ca2+ enters the cell and triggers fusion of acetylcholine vesicles with the presynaptic membrane
- Acetylcholine molecules diffuse across the synaptic cleft and bind to receptors on the postsynaptic membrane
- Activated receptors open chemically gated cation (Na+, K+, Ca2+) channels and depolarize the postsynaptic membrane
- The spreading depolarization fires an action potential in the postsynaptic membrane.
- Acetylcholine is broken down and the components are taken back up by the presynaptic cell. Acetylcholine and vesicles are recycled.
Describe and explain the function of enzymes that degrade neurotransmitters
They have the function of ensuring that after the recycling process is completed no more acetylcholine molecules are in the synaptic cleft, as that could lead to permanent stimulation.
SYNAPTIC INTEGRATION (EPSPs, IPSPs and summation)
EPSPs = excitatory postsynaptic potentials IPSPs = inhibitory postsynaptic potentials
Describe and explain spatial and temporal summation.
In general:
The sum of EPSPs and IPSPs creates a graded membrane potential in the postsynaptic cell body. Each neuron may receive 1000 or more synaptic inputs, but it has only one output: an action potential in a single axon.
For most neurons, summation takes place in the axon hillock at the base of the axon.
A postsynaptic neuron integrates information by summing EPSPs and IPSPs in both space (spatial summation) and time (temporal summation).
Spatial Summation: EPSPs produced almost simultaneously by different synapses on the same postsynaptic nerve can add together, an effect called spatial summation.
Temporal Summation: If two EPSPs occur in rapid succession at a single synapse, the second EPSP may begin before the postsynaptic neuron’s membrane potential has returned to the resting potential after the first EPSP. When that happens, the EPSP’s add together, an effect called temporal summation.
