Neuroscience: Axonal Transport and Neuron Types

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Questions and Answers

What characterizes an electrical synapse?

It is the most common type of synapse in the adult nervous system.

It allows signals to pass in only one direction.

It uses neurotransmitters to transmit signals.

It utilizes gap junctions to conduct electricity. (correct)

Which neurotransmitter is NOT commonly associated with chemical synapses?

Acetylcholine

Insulin (correct)

Epinephrine

Glutamate

How do chemical synapses differ from electrical synapses in signal transmission?

Chemical synapses rely on direct electrical conduction.

Chemical synapses can transmit signals in both directions.

Chemical synapses transmit signals through neurotransmitter release. (correct)

Chemical synapses are more common in the embryonic nervous system.

What is the resting membrane potential in a typical nerve cell?

$-70mv$ Signup and view all the answers

What primarily causes the more negative charge inside a resting nerve cell?

Higher efflux of potassium ions compared to sodium ions. Signup and view all the answers

What is the threshold voltage that must be reached for the action potential to occur?

-55 Signup and view all the answers

What occurs during depolarization of the membrane potential?

35 Signup and view all the answers

How does an excitatory postsynaptic potential (EPSP) affect the membrane potential?

-70 Signup and view all the answers

What is the main ion that contributes to the repolarization of the membrane potential?

4 Signup and view all the answers

What happens during hyperpolarization?

-70 Signup and view all the answers

What is the primary outcome of signal convergence on a neuron?

It allows a single neuron to receive inputs from multiple sources. Signup and view all the answers

What mechanism prolongs the excitation of a neuron after the initial stimulus has ceased?

Synaptic transmission failure. Signup and view all the answers

What characterizes a reverberatory (oscillatory) circuit?

It relies on positive feedback to sustain activity in a circuit. Signup and view all the answers

In synaptic after-discharge circuits, what allows excitatory signals to reach the output neuron at staggered times?

Divergence followed by convergence. Signup and view all the answers

What is an example of prolonged excitation as a result of after discharge?

Holding a hand in place after a reflex action. Signup and view all the answers

What type of transport does kinesin facilitate in neurons?

Anterograde transport Signup and view all the answers

Which type of neuron carries information from target organs to the CNS?

Sensory neuron Signup and view all the answers

What is the primary function of dynein in axonal transport?

Transport of nerve growth factors Signup and view all the answers

What defines the peripheral nervous system?

Structures that connect the CNS to target organs Signup and view all the answers

How are nerves classified according to their structure?

Myelinated and non-myelinatd Signup and view all the answers

What impact can a synapse have on nerve impulses?

Change a single impulse into repetitive patterns Signup and view all the answers

Which of the following neurons is located within the CNS?

Interneuron Signup and view all the answers

Which type of nerve fibers are typically unmyelinated?

Type C fibers Signup and view all the answers

What is the role of transport proteins in axonal transport?

To facilitate movements of molecules along the axon Signup and view all the answers

What occurs at the axon hillock when the graded potential reaches the threshold level?

Opening of Na+ voltage channels Signup and view all the answers

Which classification describes nerves based on the transmitters they secrete?

Cholinergic and adrenergic Signup and view all the answers

What is the result of inhibitory postsynaptic potential (IPSP)?

Increased intracellular negativity Signup and view all the answers

Which best describes divergence in neuronal pools?

One neuron sends signals to excite multiple neurons Signup and view all the answers

What can lead to hyperpolarization in a neuron?

Cl- influx and K+ efflux Signup and view all the answers

Which process allows a signal to be transmitted to different areas from the same neuronal pool?

Divergence into multiple tracts Signup and view all the answers

What happens when the graded potential is not strong enough to reach the axon hillock?

The Na+ voltage channels remain closed Signup and view all the answers

Which of the following statements about action potentials is true?

They are propagated in a non-graded manner Signup and view all the answers

In impulse processing, what characterizes impulse convergence?

Multiple inputs to a single output Signup and view all the answers

Study Notes

Axonal Transport

Proteins are transported within neurons via vesicles along the cytoskeleton, which consists of three types of fibers: microtubules, neurofilaments, and microfilaments
Anterograde transport carries material from the soma (cell body) to the axon terminal and dendrites; this is done by the protein kinesin
- Fast axonal transport is for vesicles and organelles
- Slow axonal transport is for regenerative materials used to repair the axon
Retrograde transport carries materials from the axon towards the soma; this is done by the protein dynein
- Functions: carries trophic substances (like nerve growth factors) and waste products

Functional Classification of Neurons

Sensory (afferent) neurons: carry information from target organs to the central nervous system (CNS)
Motor (efferent) neurons: carry information from the CNS to the target organs
Interneurons: process information within the CNS (brain and spinal cord)

Nerves

Nerves connect the neurons of the CNS to target organs
Form the peripheral nervous system
Composed of bundles called fasciculi, which are made up of neuronal axons

Classification of Nerves

Structure: Myelinated vs. Non-myelinated (myelinated nerves conduct faster due to saltatory conduction)
Distribution:
- Somatic: To skeletal muscles
- Autonomic: To smooth muscles and glands
Origin:
- Cranial nerves: Originate from the brain
- Spinal nerves: Originate from the spinal cord
Neurotransmitter:
- Cholinergic: Release acetylcholine
- Adrenergic: Release norepinephrine
Function:
- Sensory: Receive information
- Motor: Send motor commands
- Mixed: Contain both sensory and motor fibers
Fiber Diameter:
- Thickest fibers: Myelinated and conduct with the highest velocity
- Type C fibers: Non-myelinated and conduct with the lowest velocity

Central Nervous System Synapse and Synaptic Transmission

Information is transmitted in the CNS via nerve action potentials (nerve impulses)
Impulses pass from one neuron to another through a functional contact called a synapse
A neuron can have 1,000-10,000 synaptic interactions with other cells
Synapses can:
- Block impulse transmission
- Convert a single impulse into multiple impulses
- Integrate impulses from multiple neurons into a complex pattern

Types of Synapses

Chemical synapses: The most common type, involves neurotransmitter release and receptor activation
Electrical synapses: Fast transmission due to direct electrical connections (gap junctions) between cells
- Allow signals to pass in both directions
- Rare in the adult CNS, more common during embryonic development

Chemical Synapse

Components: Presynaptic axon terminal, synaptic cleft, postsynaptic receptors
Process: Neurotransmitters are released from the presynaptic neuron, diffuse across the synaptic cleft, and bind to receptors on the postsynaptic neuron
One-way transmission: From presynaptic to postsynaptic neuron
Major neurotransmitters: Acetylcholine, norepinephrine, epinephrine, histamine, GABA, glycine, serotonin, glutamate

Resting Membrane Potential and Action Potential

Resting membrane potential: Difference in electrical charge across the cell membrane; inside is more negative than outside
Nerve cell resting potential: -70 mV
Resting potential is caused by: Unequal ion concentration and permeability across the membrane, with K+ efflux exceeding Na+ influx
Action potential: A temporary reversal of the membrane potential, where the inside becomes positive
Action potential initiation:
- Must be a stimulus to open Na+ channels
- Influx of Na+ depolarizes the membrane until the threshold potential (-55 mV) is reached, triggering the opening of voltage-gated Na+ channels
- Na+ influx drives membrane potential to +35 mV (depolarization)
- Na+ channels close, K+ channels open, and K+ efflux repolarizes the membrane
- Hyperpolarization: Membrane potential may become more negative than the resting potential

Generation of Nerve Action Potential

Neurotransmitter release from presynaptic neuron
Neurotransmitters bind to receptors on postsynaptic neuron, which can be excitatory or inhibitory
Excitatory Postsynaptic Potential (EPSP):
- Neurotransmitters bind to excitatory receptors (ligand-gated Na+ channels)
- Increase Na+ permeability
- Influx of Na+ depolarizes the membrane
- Graded potential: Amplitude varies with stimulus strength
- Local depolarization: Conducts passively along the soma membrane - If the EPSP is strong enough to reach the axon hillock and the threshold (-55 mv) is reached, it triggers an action potential - Action potentials are "all-or-none": Either occur or don't occur - Soma does not generate action potential because it has limited voltage-gated Na+ channels

Inhibitory Postsynaptic Potential (IPSP)

Neurotransmitters bind to inhibitory receptors (ligand-gated Cl- channels and/or K+ channels)
Increase Cl- permeability or K+ permeability
Chloride influx and potassium efflux: hyperpolarize the membrane, making it more negative
Inhibitory effect: Hyperpolarization counteracts excitatory graded potentials, preventing the neuron from reaching threshold and firing an action potential

Impulse Processing: Neuronal Circuits

The CNS is composed of neuronal pools, containing mostly interneurons
Pools can be small or large, receive input from sensory neurons, process information, and generate output signals
Key processes:
- Impulse Divergence: One neuron sends signals to multiple neurons – amplifies the signal - Amplifying divergence: Spread of a signal to increasing numbers of neurons in successive orders
  - Divergence into multiple tracts: Signal transmitted in different directions to different areas
- Impulse Convergence: Multiple inputs excite a single neuron – allows for summation of signals
- Impulse Prolongation and Repetitive Discharge: Signals continue after the stimulus has stopped, prolonging the excitation
  - Reverberatory (Oscillatory) circuit: Positive feedback within the circuit re-excites the outputs, resulting in rhythmic activity
  - Synaptic after-discharge circuit: Divergence followed by convergence, with signals arriving at different times to sustain excitation
- Excitatory and inhibitory output signals: An input signal can result in excitatory outputs to some neurons and inhibitory outputs to others

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Description

Explore the fascinating processes of axonal transport in neurons, including anterograde and retrograde transport mechanisms. Learn about the different types of neurons and their functional classifications, such as sensory, motor, and interneurons. Test your understanding of these crucial concepts in neuroscience.