Signalling with the nervous system

Question 1

What is interesting about squid when studying axons?

Answer 1

Squids have a giant axon, which could be isolated to then measure action potentials through it.

Question 2

How do neurons transmit electrical signals?
How can the resting potential be measured?
Describe an action potential?

Answer 2

They generate electrical signals based on the flow of ions across their plasma membrane through ion - channels. Under resting conditions, neurons also have a negative internal potential, the resting membrane potential.

The resting potential can be measured by recording the potential difference between the inside and the outside of the - cell, usually with a glass microelectrode.

The action potential, is a transient reversal of the resting membrane potential. Action potentials are propagated along the length of the axon and are the fundamental electrical signals that carry information from one place to another in the nervous system. Action potentials are ‘digital’ pieces of information that act as an all-or-nothing signal. These electrical signals depend upon ion fluxes across the nerve cell membrane. These ions fluxes are possible because of the non-uniform distribution of ions across the membrane that is the basis of the RESTING MEMBRANEPOTENTIAL.

Question 3

Why is membrane potential essential?

Answer 3

A negative membrane potential is essential for the transfer of ions, solutes and other molecules in and out of the cell, generation of energy for cell movement or division.

Question 4

What are the ion channels involved during action potentials

Answer 4

sodium and potassium ion channels

Question 5

Ion Channels and Ion Transporters​

Answer 5

Ions (e.g. Na+, K+) are charged, and can only pass through pores (channels) in the cell membrane or be carried by ion transporters to carry an ionic current or create a separation of ion charge​

The channels are formed by protein molecules that span the membrane to provide routes for ionic current flow that gives the basis of RESTING and ACTION POTENTIALS​

important channel is also Na/K ATPase (sodium pump)

Question 6

What is involved in generating and maintaining the resting potential?

Answer 6

Na/k ATPase =active transport of 3Na+ out and 2K in to make the cell more negative.
Tandem pore domain k+ channel = passive transport

Question 7

What is the membrane potential?

Answer 7

The membrane potential of any cell is the voltage across the cell membrane represented by the symbol Vm (sometimes as Em). Vm can be measured with a microelectrode that measures the potential difference between the inside of the cell and a reference (outside) The inside of the neuron is negatively charged with respect to outside. The resting potential in resting neurons is normally around -65 millivolts (b) (1mV = 0.001V)Vm=-65mV

Question 8

The resting potential

Answer 8

The resting membrane potential is the result of a charge separation across the cell membrane. The charge separation is produced by the interaction of two gradients
1) An ionic concentration gradient
2) An electrical potentialgradient

Question 9

The ionic concentration gradient

(accounts for 10% basal metabolic rate)

Answer 9

Ionic concentration gradients are established by the actions of ionic pumps. The sodium-potassium ATPase (sodium pump) breaks down ATP in the presence of internal Na+ releasing energy which drives the exchange of 3 internal Nations for 2 external K+ ions. The Na+/K+ pump ensures that K+ is concentrated inside the cell and that Na+ is concentrated outsidethecell

Question 10

How could we measure the exact value of the equilibrium potential for each permeant ion species?

Answer 10

Each ion ahs its own equilibrium potential.
We could use the NERNST equation. This takes into consideration the ion charge, the temperature and the ratio of internal and externalconcentrations

Question 11

The GOLDMAN-HODGKIN-KATZ equation

Answer 11

If the cell membrane were selectively permeable to only one ion species then Eion = Vm However Ex-80 mV, EN = +62 mV. In reality, the neuronal membrane is permeable to more than one species and the aggregate V is determined by the relative permeabilities of each species.
Under resting conditions the membrane has relatively HIGH PERMEABILITY TO K and LOW Na PERMEABILITY. The GOLDMAN-HODGKIN-KATZ equation gives the membrane potential from a given set of ionic concentrations inside and outside the cell PLUS the influence of the ionicpermeabilities

Question 12

Important Points to Remember​

Answer 12

​Active transport establishes Na+ and K+ gradients.​
Potassium is more concentrated inside. (100mM)​
Sodium is more concentrated outside. (150mM)​

Neuronal membrane potential depends on the ionic concentrations either side of the membrane. ​
(e.g. the higher the [K+]o , the lower the concentration gradient and the lower the Ek)​

At rest the membrane is not equally permeable to all ion species but has selective permeability.​

At rest the membrane has high permeability to K+, low permeability to Na+​

At rest the Em is dominated by the combined influence of:​
1. the outwardly directed K+ ionic [K+] gradient​
2. large relative K+ permeability (Pk)​

Question 13

The importance of regulating external potassium

Answer 13

The neuronal membrane has high permeability to K+ and is therefore sensitive to changes in the concentration of extracellular potassium A 10 fold increase in [K+]o, from 5 to 50 mM reduces Vm from -65 to -17mV. This change to a less negative value is termed a depolarisation.

Low EM =Depolarisation
High Em (mV) = Hyperpolarisation

Increasing extracellular potassium reduces the Em, and depolarises neurons. All cells, including neurons can only survive brief changes to theirrestingEm

Question 14

How could we buffer the changes in K+

Answer 14

The brain has evolved mechanisms to buffer alterations in [K+]o, The blood-brain barrier (bbb), limits movement of K into extracellular fluid of brain Within the CNS. Astrocytes possess K pumps that concentrate potassium intheircytosol.

Question 15

How are electrical potentials generated across the neuronal membrane?

Answer 15

1) There are differences in the concentrations of specific ichs across the membrane.
2) The resting membrane is selectively permeable to some of these ions which move across the membrane influenced by the electrical and chemicalgradients

Question 16

Alpha subunit on our sodium channel

Answer 16

The ion channel is formed by protein subunits that span the cell membrane and form a pore.​
The pore has selectivity for Na+ ions​
The pore is a voltage sensor, it opens and closes depending on the potential across the membrane​

The alpha subunit is a binding site for TTX (tetrodotoxin),
Neosaxitoxin (NSTX) and Saxitoxin (STX): Paralytic Shellfish Poisoning​

Local anesthesia also binds

Question 17

Describe the structure of potassium channels

Answer 17

Potassium channels have a tetrameric structure in which four identical protein subunits associate to form a fourfold symmetric (C4) complex arranged around a central ion conducting pore (i.e., ahomotetramer).

Question 18

What is spatial summation
What is temporal summation

Answer 18

*Spatial summation uses multiple synapses acting simultaneously
*Temporal summation occurs when one presynaptic neuron releases neurotransmitter many times over a period of time

Question 19

Describe the two types of synapses

Answer 19

chemical synapse=Neurotransmitter release: carries the ‘message’ from one nerve cell to another, or muscle or gland cell​ Pre and Post synapses​. Gap between cells is about 20 nanometres​, Speed of transmission is several milliseconds​
Can be excitatory or inhibitory​ and No loss of signal strength​

electrical synapse= Transmission of electrical signal​ Between 2 nerves​ pr between nerve and effector target​. Gap between cells is about 3.5 nanometres​, Speed of transmission is nearly instantaneous​, Excitatory only​ and Signal strength diminishes over time​.

Question 20

Describe signal transmission through a chemical synapse

Answer 20

The generation of an action potential in the presynaptic neuron. This electrical signal travels down the axon to the axon terminals, where it triggers the release of neurotransmitters.
Within the axon terminals, there are synaptic vesicles filled with neurotransmitters. These vesicles are located near the presynaptic membrane in specialized structures called active zones.
Calcium Influx: When the action potential reaches the axon terminal, voltage-gated calcium channels open, allowing calcium ions to enter the cell. The increase in intracellular calcium triggers the fusion of synaptic vesicles with the presynaptic membrane, leading to exocytosis.
The synaptic vesicles release neurotransmitters into the synaptic cleft through exocytosis. This process involves the fusion of vesicle membranes with the presynaptic membrane, causing the release of neurotransmitters into the synaptic cleft.

The synaptic cleft is a narrow extracellular space between the presynaptic and postsynaptic neurons. It acts as a barrier that neurotransmitters must cross to transmit the signal from the presynaptic to the postsynaptic neuron.
Neurotransmitters released into the synaptic cleft bind to specific receptors on the postsynaptic membrane. These receptors are typically ligand-gated ion channels, and their activation allows ions to flow across the membrane.
The binding of neurotransmitters to receptors generates postsynaptic potentials, which can be either excitatory (EPSP) or inhibitory (IPSP), depending on the type of ion channels activated.
The postsynaptic neuron integrates the excitatory and inhibitory signals received from multiple synapses. If the net effect is depolarization reaching the threshold, it may generate an action potential.
Neurotransmitter effects are terminated through various mechanisms, including reuptake into the presynaptic neuron, enzymatic degradation in the synaptic cleft, or diffusion away from the synapse.

Question 21

postsynaptic receptor gate ion channels either directly or indirectly

Answer 21

Direct gating=neurotransmitter binds directly to ion channel itself. Ion channel is composed of different subunits, these subunits cluster together or have pore in the middle allowing the ion to pass through.

Indirect gating= Neurotransmitters bind to receptors. Second messenger systems, such as cAMP or protein kinases, play a key role, leading to changes in ion channel conductance and cellular responses. This mechanism allows for signal amplification and precise control of ion permeability, contributing to diverse physiological processes.

Question 22

The Neuromuscular Junction​

Answer 22

Good example of directly gated synaptic transmission​

Using the frog NMJ, several scientists contributed fundamental discoveries to neuroscience​

Understanding the fundamental properties of acetylcholine (Ach) release at the NMJ was a major landmark in biology.​

Question 23

Describe the synaptic transmission at a neuromuscular junction

Answer 23

The synaptic transmission at a neuromuscular junction (NMJ) is a specialized form of chemical synapse where a motor neuron communicates with a skeletal muscle fiber. This process involves the release of neurotransmitters, typically acetylcholine (ACh), and the subsequent generation of a muscle action potential. Here is an in-depth description of the synaptic transmission at a neuromuscular junction:

Motor Neuron Action Potential:
The process begins with an action potential traveling down the motor neuron, specifically its axon terminals.

Depolarization of Axon Terminals:
The action potential depolarizes the axon terminals of the motor neuron, leading to the opening of voltage-gated calcium channels.

Calcium Influx:
Calcium ions (Ca2+) enter the axon terminals due to the opening of voltage-gated calcium channels. This influx of calcium is critical for the release of neurotransmitters.

Neurotransmitter Release:
The rise in calcium levels triggers the fusion of synaptic vesicles containing acetylcholine with the presynaptic membrane, resulting in exocytosis.
Acetylcholine is released into the synaptic cleft.

Acetylcholine Binding to Receptors:
Acetylcholine diffuses across the synaptic cleft and binds to nicotinic acetylcholine receptors (nAChRs) on the sarcolemma (cell membrane) of the muscle fiber.

Endplate Potential (EPP):
The binding of acetylcholine to nAChRs opens ion channels, allowing the influx of sodium ions (Na+) and efflux of potassium ions (K+).
This results in the depolarization of the muscle fiber membrane, generating an endplate potential (EPP).

Generation of Muscle Action Potential:
If the endplate potential is of sufficient magnitude, it triggers an action potential that propagates along the sarcolemma and into the T-tubules (transverse tubules) of the muscle fiber.

Release of Calcium from Sarcoplasmic Reticulum:
The action potential in the T-tubules activates voltage-gated calcium channels in the sarcoplasmic reticulum (a specialized endoplasmic reticulum in muscle cells), leading to the release of stored calcium ions into the cytoplasm of the muscle fiber.

Muscle Contraction:
The increased concentration of calcium ions in the cytoplasm initiates the contraction of the muscle fiber by binding to troponin and facilitating the interaction between actin and myosin filaments.

Termination of Signal:
Acetylcholine’s action is terminated by the enzyme acetylcholinesterase, which breaks down acetylcholine into acetate and choline.
Choline is actively transported back into the presynaptic neuron and used to synthesize new acetylcholine.

Relaxation of Muscle Fiber:
The removal of calcium ions from the cytoplasm allows the muscle fiber to relax, and the process is ready to be repeated for the next contraction.

Question 24

What are the two fundamental principles of synaptic transmission?
​

Answer 24

1. There is a delay (synaptic delay)​
2. Neurotransmitter release is Ca2+ dependent​

Question 25

Describe the two most common morphological types of chemicals synapses in the CNS are Gray type I and type II​

Answer 25

Type I:​
Usually excitatory (e.g. glutamatergic synapses)​
Round synaptic vesicles​
Asymmetrical membrane differentiations​
Typically contact spines (found on dendrites), less common the shaft of the dendrite.​

Type II:​
Usually inhibitory (e.g. GABAergic synapses)​
Flattened synaptic vesicles​
Symmetrical membrane differentiations​
Contact the cell body of the neuron as well as the shaft of the dendrite​

Question 26

Common Characteristics: Chemical Messengers​

Answer 26

Transmitters in CNS are;
*Always agonists
*Can produce fast or slow events Can excite or inhibit *Determined by receptors Ion channel
- fast-neurotransmission
-2nd messengers - slow -neuromodulation

Question 27

What are the three possible actions that neurotransmitter have?

What are the 4 main groups of neurotransmitter molecules?​

Answer 27

*Excitatory: message continues to be passed along to the next cell.​
*Inhibitory: message is prevented from being passed along to the next cell.​
*Modulatory: Influence the effects of other chemical messengers – they adjust how cells communicate at the synapse and can affect a larger number of neurons at the same time.​

1. Type I neurotransmitters​
   Amino acids (e.g. glutamate= excitatory, glycine=can be inhibitory)​
2. Type II neurotransmitters​
   Monoamines (e.g. dopamine, noradrenaline (American: norepinephrine), serotonin)​
   -Acetylcholine​
   -Purines (e.g. adenosine triphosphate (ATP)
3. Type III neurotransmitters​
   Neuropeptides (e.g. somatostatin, opioids, substance P)​
4. Other neurotransmitters​
   Non-classical (e.g. nitric oxide, carbon monoxide)

Question 28

Common Neurotransmitters​

Answer 28

Glutamate= excitatory, most common neurotransmitter involved in learning and memory, imbalances cause dementia, Parkinson’s and stroke

GABA=inhibitory, calms firing nerves, improves focus, low levels cause anxiety sleep disorders and depression

Acetylcholine= excitatory, involved in thought, learning and memory, activates muscles.

Endorphins=inhibitors, promote a sense of wellbeing, released in response to exercise, natural pain reliever. Low levels are linked to headaches and fibromyalgia

serotonin= mostly inhibitory but can work through excitatory mechanisms, it is linked to mood, contributes to happiness, wellbeing and sleep cycle. medications include SSRI’s SNRI’s for depression and anxiety etc.

Dopamine= excitatory and inhibitory, associated with rewards system and pleasure, linked to addiction e.g substance abuse. It’s important in the control of movement.

Adrenaline=excitatory, fight or flight, increases heart rate and blood flow

Noradrenaline=excitatory, concentration affects attention and response

HISTAMINE =excitatory, Regulates body functions such as wakefulness, feeding behaviour and motivation.​

GLYCINE =inhibitory, Most common inhibitory NT in spinal cord​, Involved in hearing processing, pain transmission and metabolism​

Question 29

Describe the three main specific receptors

Answer 29

*Nicotinic ACH, GABA, and Glycine: Pentamers, with ligand-binding and membrane domains.

*Glutamate: Tetramers with distinct subunits, including an extracellular amino terminus, membrane domain, and M2 segment for channel selectivity.

*ATP (P2X) Receptors: Trimers, with membrane-spanning helices and an extracellular loop for ATP binding.
These structures play crucial roles in neural signaling and communication between cells.

Question 30

Glutamate - Excitatory​

Answer 30

They are composed of two major groups;

*Ionotropic glutamate receptors= All directly gated to ion channels to evoke a response, there is NMDA, AMPA and kainate receptors. The NMDA receptor is the main one. Glutamate binds at the same time that the cell is depolarised so magnesium is released within the pore.
When the cell is not depolarised magnesium serves as a membrane channel blocker, this allows for a resting/blocked state. At a depolarised state magnesium is released, glutamate engages, opening the channel and allowing for sodium and calcium to enter the cell. There is a specific binding site for zinc and PCP too.
For the AMPA and kainate receptors, once glutamate binds, allowing the pore to open to allow for an influx in sodium ion, depending on the subunit composition, calcium ions could enter the cell contributing to excitotoxicity.

*metabotropic glutamate receptors (modulatory)
Glutamate will bind to a receptor coupled to a g-protein depending on the receptor, activating downstream signalling pathways leading to activating function and the opening of ion channels.

Question 31

GABA - Inhibitory​

Answer 31

There are two classes of GABA receptors;
1. GABA-A (ion channels)
2. GABA-B receptors (G-protein coupled)

Inhibitory synaptic action is usually mediated by ionotropic GABA and Glycine receptor channels that are permeable to Chloride (Cl-).​

Binding of GABA:
GABA, synthesized and released by neurons, binds to specific sites on GABA receptors located on the postsynaptic membrane.
Ion Channel Opening:
When GABA binds to its receptor, it induces a conformational change in the receptor protein. This conformational change opens an ion channel associated with the GABA receptor.
Ion Flux:
The opening of the ion channel allows chloride ions (Cl-) to flow into the neuron or, less commonly, potassium ions (K+) to flow out.
The influx of negatively charged chloride ions or efflux of positively charged potassium ions results in a hyperpolarization of the neuron’s membrane potential.
Hyperpolarization:
Hyperpolarization makes it more difficult for the neuron to generate an action potential, reducing the likelihood of the neuron firing and transmitting signals to other neurons.
Inhibitory Effect:
The overall effect of GABA binding to its receptor is inhibitory, preventing the excessive firing of neurons and maintaining a balance between excitatory and inhibitory signals in the brain.

There are two main types of GABA receptors:
GABA-A Receptors:
These are ionotropic receptors that directly control chloride ion channels.
Binding of GABA leads to a rapid opening of the chloride channel, causing a fast inhibitory response.

GABA-B Receptors:
These are metabotropic receptors that act through a second messenger system.
Activation of GABA-B receptors leads to the modulation of ion channels, typically potassium channels, resulting in a slower and more prolonged inhibitory response.

GABA receptors play a crucial role in regulating neuronal excitability and are involved in various physiological processes, including anxiety regulation, muscle tone, and sleep. Dysfunction in GABAergic neurotransmission is implicated in several neurological and psychiatric disorders.

Question 32

How are neurotransmitters removed

Answer 32

1) diffusion=they move out of cleft however this might not be fast enough.
2) enzymes= breaks them down
3) re-uptake pumps (they take neurotransmitter back to axon terminal
4) astrocytes=They also have pumps and synapses to actively get the neurotransmitter out of the synaptic cleft to either be recycled or broken down