Neurobiology

Question 1

Describe the division of the nervous system and name the functions of each part

Answer 1

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

Question 2

Describe and explain how a stimulus leads to a reaction

Answer 2

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.

Question 3

State the function of different parts of the neuron:

Answer 3

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

Question 4

 Draw fully labelled sketches of experimental setups used to record resting, action and postsynaptic potential and explain these setups:

Answer 4

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.

Question 5

Explain why ions cannot simply diffuse through biological membranes

Answer 5

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.

Question 6

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.

Answer 6

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.

Question 7

Describe the difference between the following ion channels: voltage gated channels and explain the role they play in the nervous system.

Answer 7

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.

Question 8

Describe the difference between the following ion channels: ligand-gated channels and explain the role they play in the nervous system.

Answer 8

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.

Question 9

Describe and explain how the resting potential is created and maintained.

Answer 9

• 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).

Question 10

Define the term voltage/potential difference and state the average value of the resting potential in mammals.

Answer 10

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)

Question 11

Define the erm equilibrium potential

Answer 11

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.

Question 12

Describe and explain how changes in the intra- and extracellular ion concentrations might influence the resting potential

Answer 12

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.

Question 13

Describe how the sodium-potassium pump works and explain its role.

Answer 13

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.

Question 14

Define the terms threshold, depolarization, repolarization, and hyperpolarization

Answer 14

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.

Question 15

Describe and explain how the typical shape of the action potential is brought about by Na+, K+, and voltage-gated ion channels.

Answer 15

1. The resting potential of -80.
2. 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.
3. Depolarization peak.
4. Repolarization – The voltage-gated Na+ channels close, the Ka+ channels open. The cell starts to become more negative again
5. Hyperpolarization
6. Hyperpolarization is overcome, and equilibrium is restored using leakage channels and the sodium-potassium pump.
7. The resting potential of -80 is restored, the cycle can begin again.

Question 16

Define the term refractory period and describe and explain its causes:

Answer 16

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.

Question 17

Describe and explain why AP is propagated in only one direction.

Answer 17

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.

Question 18

Describe and explain how an action potential is propagated in non-myelinated and myelinated axons and compare these two modes of conduction.

Answer 18

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.

Question 19

Compare the transmission of action potential in unmyelinated neurons and myelinated neurons

Answer 19

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).

Question 20

Describe and explain how the propagation speed of action potentials depends on the diameter in non-myelinated axons

Answer 20

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)).

Question 21

Describe the process taking place during the transmission of nerve impulses at a synapse including the creation of a postsynaptic potential

Answer 21

1. Action potential arrives at the axon terminal
2. Na+ channels open; depolarization causes voltage gated Ca2+ channels to open
3. Ca2+ enters the cell and triggers fusion of acetylcholine vesicles with the presynaptic membrane
4. Acetylcholine molecules diffuse across the synaptic cleft and bind to receptors on the postsynaptic membrane
5. Activated receptors open chemically gated cation (Na+, K+, Ca2+) channels and depolarize the postsynaptic membrane
6. The spreading depolarization fires an action potential in the postsynaptic membrane.
7. Acetylcholine is broken down and the components are taken back up by the presynaptic cell. Acetylcholine and vesicles are recycled.

Question 22

Describe and explain the function of enzymes that degrade neurotransmitters

Answer 22

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.

Question 23

SYNAPTIC INTEGRATION (EPSPs, IPSPs and summation)

Answer 23

EPSPs = excitatory postsynaptic potentials
IPSPs = inhibitory postsynaptic potentials

Question 24

Describe and explain spatial and temporal summation.

Answer 24

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.

Question 25

Describe and explain the difference(s) between EPSPs and IPSPs.

Answer 25

EPSPs are graded potentials that can initiate an AP in the axon; IPSPs produce a graded potential that lessens the chance of an AP in an axon.
EPSP - small depolarization is created; IPSP - small hyperpolarization is created.
EPSP - helps bring postsynaptic membrane closer to the threshold; IPSP - helps bring postsynaptic membrane further from the threshold.
EPSP - membrane becomes more excited; IPSP - membrane becomes less excited

Question 26

Define the term code-switching and name the location and the advantages of each

Answer 26

During the transmission of information from one neuron to the next, the information changes its code several times. Graded and all-or-none electrical signals alternate as a signal is carried along the neuronal circuit.
Within each neuron, the signal is carried electrically, whereas between neurons the signal is carried by chemicals.

Graded and all or none electrical signals alternate with one another as a signal is carried along the neuronal circuit. A stimulus applied to the receptor endings of a sensory neuron produces a graded receptor potential that reflects the amplitude (= intensity) and duration of the stimulus, although it is not identical to the stimulus in form. The potential spreads passively through the first part of the sensory neuron and, if the change in potential is sufficiently large at the spike-initiating zone (axon hillock), it elects all-or-none propagated action potentials in the axon. When they arrive at the terminals of the sensory neuron, the APS causes the release of a specific number of chemical neurotransmitters that induce a graded change in voltage in the next neuron. If the potential in the second neuron reaches the threshold, an AP or a train of APS will be produced. Thus, graded and all or none potentials alternate along the pathway, and the information is carried alternately be electrical and chemical signals. With each neuron, the signal is carried electrically, between neurons the signal is carried by chemicals.

Question 27

Describe and explain how neurotoxins might affect signal propagation in (axons and) signal transmission at synapses.

Answer 27

Neurotoxin: Sarin
Site of action: Cholinesterase
The effect at the synapse: By inhibiting the cholinesterase by forming covalent bonds at the active site, the synaptic cleft is filled with acetylcholine. This leads to a constant AP production at the postsynaptic cell, as the Na+ influx is unregulated
Symptoms:
-Runny nose
-Tightness of the chest
-Pupil constriction
-breathing difficulty
-nausea
-drooling
-> overstimulation leading to suffocation.
Neurotoxins: Muskarin
Site of action: Acetylcholine receptors The effect at the synpase: Initiates acetylcholine, stimulates the receptors, causing constant Na+ influx, causing constant muscle stimulation.
Symptoms:
-overstimulation causing cramp
-> cardiac /respiratory failure
Neutoxins: Atropis
Site of action: ACH receptors
The effect at the synpase: Mimics ACH, blocking the Na+ gates but does not allow passing through, no Na+ influx does not allow for a new AP to be transmitted
Symptoms:
Inhibition of secretions, relaxation of smooth muscle tissue, hyperactivity due to excitation of CNS, dilation of pupils
-> reversible, causes relaxation not to cramp.

Question 28

Describe and explain the function of substances that might work as antidotes again certain neurotoxins

Answer 28

Muscle relaxants are used during surgeries to prevent sudden muscle contractions of the patient. One substance that was commonly used for this purpose is curare, which is a mixture of naturally occurring alkaloids found in various south American plants and used as arrow poisons by South American indigenous people. Curare leads to death due to respiratory paralysis. The substance binds to postsynaptic Ach receptors of neuromuscular junctions without opening them. The binding is reversible (e.g., the substance bind and unbinds over and over again, as long as there are curare molecules in the synaptic cleft). The effect of curare can be shortened by adding AChE (=acetylcholinesterase) inhibitor.

Question 29

Compare sense organs regarding types of sensory receptors and adequate stimuli

Answer 29

Vision - Eye - Rods/cones - Photoreceptors;
Hearing - Ear - Sound waves - Hearing receptors inside the Organ of Corti;
Smell - Nose - Odour - Olfactory receptor ;
Touch - Skin - Heat / pressure -Thermoreceptors/mechanoreceptor;
Taste - Tongue - Chemical substances - Chemoreceptor

Question 30

Function of: Sclera, Choroid, Conjuctiva, Cornea, Iris, Pupil, Aqueous humour, Optic nerve

Answer 30

Sclera: A tough outer white layer of connective tissue;
Choroid: The intermediate layer of the eye, underlying the retina; rich in blood vessels, contains pigment;
Conjunctiva: Covers outer surface of the sclera helps keep the eyes moist;
Cornea: At the front of the eye, the sclera becomes transparent;
Cornea: lets light into the eye and acts as a fixed outer lens;
Iris: Doughnut shaped, formed by the anterior choroid, gives the eye its colour. By changing size, the iris regulates the amount of light entering the pupil;
Pupil: Opening at the centre of the Iris. It becomes smaller or larger under the action of the circular or radial smooth muscle fibres of the iris;
Aqueous humour: The watery fluid which fills the space between the back of the cornea and the lens. Functions as a liquid lens that helps focus light onto the retina;
Optic nerve: Runs from the retinal cells in one of the eyes to the optic chiasma.

Question 31

Function of: Lens, Retina, Optic Disk, Yellow Spot, Fovea, Ciliary body, Suspensory Ligament, Vitreous humour

Answer 31

Lens: Elastic, biconvex, transparent structure situated between the iris and the vitreous humour. Main tasks: refraction and accommodation;
Retina: A light-sensitive membrane that lines the inside of the back of the eye. Contains photoreceptors (rods and cones);
Optic disk: A small circular area in the retina at the back of the eye where all nerve fibres from the retina meet to leave the eyeball as the optic nerve. No photoreceptors are present -> light focused on this part of the retina is not detected;
Yellow spot: A small area of yellowish tissue in the centre of the retina with a central dip, the fovea. It is devoid of blood vessels. A high concentration of cones -> great visual acuity;
Fovea: Small central dip in the yellow spot in the eye. The concentration of cones is maximum, no rods. Here visual acuity is greatest;
Ciliary body: A circular band of muscular and blood vascular tissue. Contains the ciliary muscles (bring about changes in the shape of the lens). Produces aqueous humour;
Suspensory Ligament: Extends from the ciliary body to the lens, looking like a halo encircling the lens. Keeps the lens in place;
Vitreous humour: The transparent, jelly-like fluid which fills the greater part of the inner chamber of the eye. Constitutes most of the volume of the eye. Functions as a liquid lens that helps focus light onto the retina.

Question 32

Define the term sensory adaptation and give examples of sensory adaptation in everyday life.

Answer 32

Sensory adaptation is defined as the diminished sensitivity to a stimulus as a consequence of constant exposure to that stimulus. Brain cells begin to fire when they pick up on a new stimulus in your environment as signalled by your sensory organs (ears, eyes, nose, etc). However, if that stimulus remains unchanged in the environment (i.e., the sound of the running air conditioner), then the brain cells begin to fire significantly less in response to that stimulus and the result is a lack of attention to that particular stimulus.

Question 33

Compare phasic and tonic receptors.

Answer 33

A tonic receptor is a sensory receptor that adapts slowly to a stimulus and continues to produce action potentials throughout the stimulus. In this way, it conveys information about the duration of the stimulus.
A phasic receptor is a sensory receptor that adapts quickly to a stimulus. The response of the cell decreases very quickly and then stops. It does not provide information on the duration of the stimulus; instead, some of them convey information on rapid changed in stimulus intensity and rate.