Module 5.3- Neuronal communication

Question 1

what are sensory receptors?

Answer 1

cells/ sensory nerve endings that respond to a stimulus in the internal or external environment of an organism and can create action potentials. most are energy transducers that convert one form of energy to another.

Question 2

what is a transducer?

Answer 2

a cell that converts one form of energy into another. each type of transducer is adapted to detect changes in a particular form of energy. other receptors detect the presence of chemicals

Question 3

what is a Pacinian corpuscle?

Answer 3

is a pressure sensor that detects changes in the pressure on the skin.an oval shaped structure that consists of a series of concentric rings of connective tissue wrapped around the end of a nerve cell. when pressure on the skin changes this deforms the rings which push against the nerve endings. when pressure is constant they stop responding

Question 4

What role do ion channels play in maintaining the concentration gradient across the membrane?

Answer 4

Ion channels, which are specialized channel proteins, allow specific ions to diffuse across the membrane. When ion channels are open, ions move according to their concentration gradient. When closed, they maintain the active pump action, creating a concentration gradient across the membrane. Sodium channels allow Na⁺ ions, while potassium channels allow K⁺ ions. These channels may also possess gates to open or close them selectively.

Question 5

How does the sodium-potassium pump maintain the membrane’s potential gradient?

Answer 5

The sodium-potassium pump actively pumps sodium ions (Na⁺) out of the cell and potassium ions (K⁺) into the cell using ATP energy. This action maintains a high concentration of Na⁺ outside and K⁺ inside. The membrane is more permeable to K⁺, so some K⁺ leaks out, while Na⁺ is mostly retained due to low permeability. This results in a negative charge inside the cell, forming the membrane’s potential gradient.

Question 6

How is a nerve impulse initiated?

Answer 6

A nerve impulse is initiated by altering the membrane’s permeability to sodium ions. When gated sodium channels open, Na⁺ ions move into the cell down their concentration gradient, depolarizing the membrane. Depolarization generates a potential change (receptor potential). If the stimulus is strong enough and sufficient channels open, this leads to an action potential.

Question 7

What is the resting membrane potential, and how is it maintained?

Answer 7

The resting membrane potential is the state when the cell membrane is polarized, meaning it is negatively charged inside compared to the outside. This is maintained by the sodium-potassium pump and selective permeability of the membrane. Fewer sodium ions leak into the cell, while potassium ions leak out, creating and sustaining this negative charge.

Question 8

Why is energy required for ion movement, and how does facilitated diffusion differ?

Answer 8

Energy is required to produce a concentration gradient through active transport mechanisms like the sodium-potassium pump, which moves ions against their gradient. Facilitated diffusion, in contrast, does not require energy. It uses channel proteins to allow ions to passively move down their concentration gradient.

Question 9

What are the different types of neurons, and what are their functions?

Answer 9

Motor Neurons: Carry action potentials from the central nervous system (CNS) to effectors such as muscles or glands.
2. Sensory Neurons: Transmit action potentials from sensory receptors to the CNS.
3. Relay Neurons: Connect sensory and motor neurons.
Neurons can also be classified based on the presence of a myelin sheath:

•	Myelinated Neurons: Have a myelin layer that speeds up impulse transmission.
•	Non-Myelinated Neurons: Lack a myelin sheath, resulting in slower impulse transmission.

Question 10

What are the key structural features of a neuron?

Answer 10

1. Length: Neurons are often very long to efficiently transmit action potentials over long distances.
   
   2. Plasma Membrane: Contains gated ion channels for controlled entry or exit of sodium, potassium, or calcium ions.
   3. Sodium-Potassium Pumps: Use ATP to actively transport sodium ions out and potassium ions into the neuron.
   4. Potential Gradient: Neurons maintain a potential difference across their cell surface membrane.
   5. Cell Body: Contains the nucleus, many mitochondria, and ribosomes to support cell functions.

Question 11

How do neurons transmit signals after detecting a stimulus?

Answer 11

Once a stimulus is detected and its energy is converted into a depolarization of the receptor cell membrane:
• The impulse is transmitted to other parts of the body.
• The depolarization spreads as a rapid influx of sodium ions, creating an action potential.
• This process allows the signal to propagate along the neuron.

Question 12

What is the role of dendrites and axons in neurons?

Answer 12

1. Dendrites:
   • Numerous dendrites connect to other neurons and receive electrical signals.
   • They carry impulses toward the cell body.
   
   2. Axon:
      • Carries impulses away from the cell body toward other neurons or effectors.
      • The axon is often surrounded by a myelin sheath for electrical insulation and faster signal transmission.

Question 13

What is the myelin sheath, and what is its function?

Answer 13

• Composition: The myelin sheath is a fatty layer produced by Schwann cells closely associated with neurons.
• Function:
• Insulates the neuron from electrical activity in neighboring neurons.
• Speeds up the transmission of action potentials by enabling saltatory conduction, where impulses jump between gaps in the myelin sheath (Nodes of Ranvier).

Question 14

How do motor, sensory, and relay neurons differ in structure and function?

Answer 14

1. Motor Neurons:
   • Cell body in the CNS.
   • Long axon to carry action potentials to effectors.
   
   2. Sensory Neurons:
      • Long dendron that carries action potentials from sensory receptors to the cell body (outside the CNS).
      • Short axon carries the action potential into the CNS.
   3. Relay Neurons:
      • Short dendrites and axons.
      • Connect sensory and motor neurons to coordinate pathways.

Question 15

What are myelinated neurons, and how do they function?

Answer 15

• Myelinated neurons are insulated by Schwann cells, forming a fatty myelin sheath.
• The sheath wraps tightly around the axon, allowing faster action potential transmission.
• Action potentials “jump” between gaps in the sheath called Nodes of Ranvier (2–3 μm wide, spaced 1–3 mm apart), making transmission rapid through saltatory conduction.

Question 16

How do non-myelinated neurons differ from myelinated neurons?

Answer 16

• Non-myelinated neurons are also associated with Schwann cells but lack tightly wrapped myelin.
• Action potentials travel as a continuous wave rather than jumping between nodes, resulting in slower transmission.
• They are generally shorter and serve functions like breathing, where transmission speed is less critical.

Question 17

Why is myelination beneficial for neurons?

Answer 17

• Increases the speed of action potential transmission:
• Myelinated neurons: 100–120 m/s.
• Non-myelinated neurons: 2–20 m/s.
• Allows rapid response to stimuli by ensuring action potentials reach their destination more quickly.
• Enables efficient long-distance transmission (e.g., neurons up to 1 m long in humans).

Question 18

What is the structure and function of the myelin sheath?

Answer 18

• Made up of Schwann cells wrapped around the axon.
• Provides electrical insulation and prevents ion movement across the membrane, except at Nodes of Ranvier.
• Enables saltatory conduction, where the action potential jumps from node to node, increasing speed and efficiency.

Question 19

What roles do Schwann cells play in myelinated and non-myelinated neurons?

Answer 19

1. Myelinated Neurons:
   • Schwann cells form the tightly wrapped myelin sheath, enabling fast transmission.
   
   2. Non-Myelinated Neurons:
      • Schwann cells enclose multiple axons in a loose arrangement, providing minimal insulation but structural support.

Question 20

What are the functions of non-myelinated neurons?

Answer 20

• Transmit action potentials over short distances, usually at slower speeds.
• Commonly involved in functions where speed is less critical, such as coordinating breathing or digestion.
• Their slower transmission is adequate for these roles, as precise timing is less essential.

Question 21

What is an action potential?

Answer 21

• Definition: A brief reversal of the potential across a neuron’s membrane, changing it from -60 mV (resting potential) to +40 mV.
• Trigger: Occurs when the threshold potential is reached, leading to depolarization and the transmission of a nerve impulse.

Question 22

What is the resting potential, and how is it maintained?

Answer 22

• Resting Potential: The potential difference across a neuron’s membrane at rest, approximately -60 mV.
• Maintenance:
• Sodium-Potassium Pump: Uses ATP to pump 3 sodium ions (Na⁺) out and 2 potassium ions (K⁺) into the neuron.
• Membrane is more permeable to K⁺ than Na⁺, allowing potassium ions to diffuse out.
• Negatively charged proteins and large anions inside the cell also contribute to the negative charge.

Question 23

What roles do sodium and potassium ion channels play in neurons at rest?

Answer 23

• Sodium ion channels are mostly closed at rest, preventing Na⁺ influx.
• Some potassium ion channels remain open, allowing K⁺ to leak out.
• This maintains the concentration gradient and negative resting potential.

Question 24

What are voltage-gated channels, and how do they function?

Answer 24

• Open in response to changes in membrane potential.
• When the threshold potential is reached, sodium voltage-gated channels open, allowing a large influx of Na⁺ into the cell.
• This causes rapid depolarization and the initiation of an action potential.

Question 25

How is an action potential generated?

Answer 25

1. Small depolarizations, called generator potentials, occur when a few sodium channels open.
   
   2. If the depolarization reaches the threshold potential, voltage-gated sodium channels open, causing rapid influx of Na⁺.
   3. This leads to full depolarization and the start of an action potential.

Question 26

What is the “all-or-nothing” principle of action potentials?

Answer 26

• A neuron either generates a full action potential or none at all.
• Action potentials are always of the same magnitude (+40 mV).
• Once initiated, the action potential is self-propagating along the neuron.

Question 27

What role does positive feedback play in generating an action potential?

Answer 27

• Small depolarizations caused by sodium influx trigger the opening of more sodium channels.
• This increases depolarization further, amplifying the response until the threshold potential is reached.

Question 28

How does the synapse influence the initiation of an action potential?

Answer 28

• Synapses release neurotransmitters that open sodium channels on the postsynaptic membrane.
• If enough sodium channels are opened, a generator potential is created.
• When the generator potential reaches the threshold, it initiates an action potential.

Question 29

What happens to potassium ions during an action potential?

Answer 29

• At rest, K⁺ leaks out of the neuron through open channels.
• During repolarization, voltage-gated potassium channels open, allowing a large efflux of K⁺.
• This restores the negative membrane potential after depolarization.

Question 30

Why are sodium-potassium pumps essential for neurons?

Answer 30

• Restore the ion concentration gradient after an action potential.
• Prevents the build-up of sodium inside and potassium outside the neuron.
• Maintains the resting potential, ensuring the neuron is ready for subsequent action potentials.

Question 31

What are the initial stages of an action potential from resting state to threshold?

Answer 31

1. Resting Potential: The membrane starts at -60 mV, with higher Na⁺ outside and higher K⁺ inside.
   
   2. Sodium Ion Channels Open: Some Na⁺ ions diffuse into the cell.
   3. Threshold Potential: The membrane becomes less negative, reaching the threshold of -50 mV.

Question 32

What happens during depolarization in an action potential?

Answer 32

• Positive Feedback: Threshold triggers voltage-gated sodium channels to open.
• Sodium Influx: A large number of Na⁺ ions enter, making the inside of the cell positive.
• Result: Membrane potential rises to approximately +40 mV.

Question 33

How does the membrane repolarize and hyperpolarize after an action potential?

Answer 33

• Repolarization: Sodium channels close, and potassium channels open. K⁺ diffuses out, restoring negativity inside the cell.
• Hyperpolarization: The membrane overshoots slightly, becoming more negative than the resting potential.

Question 34

What is the refractory period, and why is it important?

Answer 34

• Definition: A period during which the neuron cannot generate another action potential.
• Reason: Sodium and potassium ions are in the wrong locations and must be restored by the sodium-potassium pump.
• Importance: Ensures one-way transmission of action potentials and prevents overlapping signals.

Question 35

What are local currents in a neuron, and how are they formed?

Answer 35

Local currents are the movement of sodium ions (Na⁺) along the inside of the neurone membrane towards regions with a lower sodium ion concentration. They are formed when sodium ion channels open, allowing Na⁺ to enter the neuron during depolarization. This creates a slight depolarization along the membrane, causing nearby sodium ion channels to open, amplifying the effect (positive feedback).

Question 36

What happens when an action potential occurs in terms of sodium ion channel activity?

Answer 36

Initial Action Potential: Sodium ion channels open at a specific point in the neurone membrane.
Diffusion of Sodium Ions: Sodium ions diffuse across the membrane into the neuron from a region of higher concentration outside the neurone to the inside. This increases the concentration of sodium ions at the site of the open channels.

Question 37

How do sodium ions move after entering the neurone during an action potential?

Answer 37

3. Sideways Movement of Ions: Sodium ions move sideways along the neurone, creating a local current. This movement spreads the charge difference (depolarization) along the membrane.
4. Propagation of Depolarization: This local depolarization opens adjacent voltage-gated sodium channels, allowing more sodium ions to enter, leading to full depolarization at the next section of the membrane. This propagates the action potential.

Question 38

Why does the action potential move in one direction along the neurone?

Answer 38

The action potential moves in one direction because the concentration of sodium ions behind the action potential remains high. This prevents the depolarization from reversing direction, ensuring the signal travels toward the end of the neurone.

Question 39

What is saltatory conduction and how does it occur?

Answer 39

Saltatory conduction is the process by which action potentials jump between the nodes of Ranvier in myelinated neurons. The myelin sheath acts as an insulating layer, preventing ion diffusion along the axon except at the nodes of Ranvier. Sodium ions diffuse along the axon at these nodes, creating local currents that enable the action potential to “jump” from one node to the next.

Question 40

How does the myelin sheath affect the transmission of action potentials?

Answer 40

The myelin sheath increases the speed of action potential transmission by insulating the axon and allowing action potentials to occur only at the nodes of Ranvier. This reduces the length of the axon that must undergo depolarization, enabling faster propagation compared to unmyelinated neurons.

Question 41

What are the key advantages of saltatory conduction?

Answer 41

Faster Signal Transmission: Myelinated neurons can transmit action potentials at speeds of up to 120 m/s, significantly faster than unmyelinated neurons.
Energy Efficiency: Fewer sodium-potassium pumps are needed because depolarization occurs only at the nodes of Ranvier, reducing energy consumption.

Question 42

What is the all-or-nothing principle in action potentials?

Answer 42

The all-or-nothing principle states that an action potential is only triggered if the stimulus reaches a threshold level of depolarization. All action potentials have the same amplitude of +40 mV, regardless of stimulus strength, provided the threshold is met.

Question 43

How is the intensity of a stimulus encoded by neurons?

Answer 43

The intensity of a stimulus is encoded by the frequency of action potentials. A stronger stimulus opens more sodium ion channels, leading to more frequent action potentials. The frequency of these signals determines the perceived intensity of the stimulus.

Question 44

How do different sensory receptors detect stimuli of varying intensities?

Answer 44

Different receptors are sensitive to specific intensities of stimuli. For example, light-touch receptors are located closer to the surface of the skin, while pressure receptors are deeper. Stimuli that fail to reach deeper receptors will not be detected as pressure.

Question 45

What occurs at the nodes of Ranvier during action potential propagation?

Answer 45

At the nodes of Ranvier, sodium ions diffuse into the axon, causing depolarization. This depolarization creates local currents that spread to the next node, triggering the opening of voltage-gated sodium channels and continuing the action potential.

Question 46

What is a synapse, and what is the role of the synaptic cleft?

Answer 46

A synapse is a junction between two or more neurons, enabling communication between them. The synaptic cleft is a small gap (~20 nm wide) between neurons where the action potential cannot cross directly. Instead, a neurotransmitter is released from the presynaptic neuron, diffuses across the cleft, and triggers a response in the postsynaptic neuron.

Question 47

What is a cholinergic synapse, and how does it function?

Answer 47

A cholinergic synapse uses acetylcholine as its neurotransmitter. When an action potential reaches the presynaptic neuron, acetylcholine is released into the synaptic cleft. It binds to receptors on the postsynaptic membrane, triggering the opening of sodium ion channels and initiating a new action potential.

Question 48

What structures are found in the presynaptic bulb, and what are their functions?

Answer 48

Mitochondria: Provide ATP for active processes like neurotransmitter release.
Smooth Endoplasmic Reticulum: Packages neurotransmitters into vesicles.
Synaptic Vesicles: Contain acetylcholine, ready for release into the synaptic cleft.
Voltage-Gated Calcium Channels: Open when the action potential arrives, allowing calcium ions to enter and trigger neurotransmitter release.

Question 49

What is the structure and function of the post-synaptic membrane in a cholinergic synapse?

Answer 49

The post-synaptic membrane contains specialized sodium ion channels composed of five polypeptides. Two of these polypeptides have receptor sites specific to acetylcholine. When acetylcholine binds, the channels open, allowing sodium ions to enter the neuron and generate an action potential.

Question 50

What is the role of acetylcholine in synaptic transmission?

Answer 50

Answer: Acetylcholine acts as a neurotransmitter that bridges the synaptic cleft. It binds to receptor sites on the post-synaptic membrane, causing sodium ion channels to open and depolarize the neuron, propagating the signal.

Question 51

What is the function of voltage-gated calcium channels in the presynaptic bulb?

Answer 51

These channels open in response to the arrival of an action potential. Calcium ions enter the presynaptic bulb, triggering the fusion of synaptic vesicles with the membrane and the release of acetylcholine into the synaptic cleft.

Question 52

What are the initial steps in the transmission of a signal across a synapse?

Answer 52

1. An action potential arrives at the synaptic bulb.
   
   2. Voltage-gated calcium ion channels open.
   3. Calcium ions diffuse into the synaptic bulb.
   4. Calcium ions cause synaptic vesicles to move to and fuse with the pre-synaptic membrane.

Question 53

What happens during the release and binding of neurotransmitters in the synaptic cleft?

Answer 53

5. Acetylcholine is released by exocytosis.
6. Acetylcholine diffuses across the synaptic cleft.
7. Acetylcholine molecules bind to receptor sites on sodium ion channels in the post-synaptic membrane.
8. Sodium ion channels open, allowing Na⁺ ions to diffuse into the post-synaptic neuron.

Question 54

How is a new action potential generated in the post-synaptic neuron?

Answer 54

Sodium ions diffuse into the post-synaptic neuron, creating a generator potential or excitatory post-synaptic potential (EPSP).
• If sufficient EPSPs occur, the potential across the post-synaptic membrane reaches the threshold.
• A new action potential is created and propagates along the post-synaptic neuron.

Question 55

What is the function of acetylcholinesterase in the synaptic cleft?

Answer 55

Acetylcholinesterase hydrolyzes acetylcholine into ethanoic acid and choline.
• This stops the transmission of signals, preventing continuous action potentials in the post-synaptic neuron.
• Ethanoic acid and choline are recycled back into the synaptic bulb using ATP and re-synthesized into acetylcholine for future use.

Question 56

Why is the recycling of acetylcholine essential in synaptic transmission?

Answer 56

Prevents overstimulation of the post-synaptic neuron.
• Conserves resources by reusing ethanoic acid and choline.
• Ensures efficient synaptic transmission by maintaining neurotransmitter availability for future action potentials.

Question 57

What happens at the synapse when an action potential reaches the end of a neuron?

Answer 57

At the end of the neuron, the pre-synaptic membrane releases neurotransmitter molecules into the synaptic cleft.
• The post-synaptic neuron responds by transmitting the signal further, an example of cell signaling.
• In cholinergic synapses, the signal consists of molecules of acetylcholine.

Question 58

How does the all-or-nothing principle apply to action potentials?

Answer 58

Action potentials either occur fully or not at all, depending on whether the threshold potential is reached.
• They are conducted along the entire length of the neuron without varying in size or intensity.

Question 59

What is an EPSP, and why is it significant?

Answer 59

An Excitatory Post-Synaptic Potential (EPSP) is a small depolarization caused by neurotransmitters in the synaptic cleft.
• A single EPSP is insufficient to trigger an action potential. Multiple EPSPs must combine to reach the threshold potential, a process called summation.

Question 60

What are the two types of summation in synaptic transmission?

Answer 60

1. Temporal Summation: Several action potentials occur in the same pre-synaptic neuron in quick succession.
   
   2. Spatial Summation: Action potentials from different pre-synaptic neurons combine at the same post-synaptic neuron.

Question 61

How do IPSPs affect the generation of action potentials?

Answer 61

IPSPs reduce the likelihood of an action potential by causing hyperpolarization of the post-synaptic membrane.
• This counters the effects of EPSPs, ensuring fine-tuned control of neural signaling.

Question 62

How do EPSPs and IPSPs interact to regulate action potential generation?

Answer 62

• EPSPs and IPSPs compete at the post-synaptic membrane to determine if the threshold potential is reached.
• This interaction ensures precise control over neural signaling and prevents unwanted activation.

Question 63

What is summation in the context of neuronal signaling?

Answer 63

• Summation occurs when the effects of several EPSPs are added together to reach the threshold potential.
• It is essential for transmitting weak signals that would not otherwise trigger an action potential.

Question 64

What is spatial summation, and why is it important?

Answer 64

• Spatial summation occurs when multiple pre-synaptic neurons converge on a single post-synaptic neuron.
• This allows action potentials from different parts of the nervous system to generate a response, such as a warning of danger from multiple stimuli.

Question 65

How can EPSPs be prevented from triggering an action potential?

Answer 65

• The combination of EPSPs can be counteracted by an Inhibitory Post-Synaptic Potential (IPSP), preventing the threshold from being reached.

Question 66

What happens when one pre-synaptic neuron transmits signals to multiple post-synaptic neurons?

Answer 66

• Divergence allows one neuron to send signals to several parts of the nervous system. For example:
• A reflex arc transmits the response to the spinal cord.
• Another signal informs the brain.

Question 67

How do synapses ensure correct signal direction?

Answer 67

• Synapses ensure signals move only forward since vesicles of acetylcholine are in the pre-synaptic bulb, not the post-synaptic neuron.

Question 68

How are low-level signals filtered at synapses?

Answer 68

• Weak signals are unlikely to trigger responses in post-synaptic neurons because they don’t release enough acetylcholine to reach the threshold.

Question 69

How can weak signals be amplified?

Answer 69

Persistent low-level stimuli cause successive action potentials. Over time, the release of acetylcholine allows EPSPs to sum and generate an action potential.

Question 70

What happens during synaptic fatigue?

Answer 70

Repeated stimulation depletes neurotransmitter vesicles, preventing further responses. This helps avoid overstimulation and conserves resources.

Question 71

What is habituation, and why is it beneficial?

Answer 71

• Habituation occurs when synapses adapt to repeated stimuli, reducing the response. It prevents overreaction to unimportant, constant stimuli like background noise.

Question 72

How does repeated stimulation strengthen neural pathways?

Answer 72

Repeated use increases the sensitivity of post-synaptic membranes by adding more receptors, making neurons more likely to respond to specific stimuli.