Nervous System Overview
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Nervous System Overview

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Questions and Answers

What are the two major systems of the nervous system?

Central nervous system (CNS) and peripheral nervous system.

What are the major components of the central nervous system?

The major components are the brain and spinal cord.

What is the role of dendrites in a neuron?

Dendrites receive incoming signals and process them to send information to the soma.

Where is the initial segment of a neuron located and what is its significance?

<p>The initial segment is located at the starting part of the axon, and it is where action potentials are generated.</p> Signup and view all the answers

What function does the myelin sheath serve?

<p>The myelin sheath insulates the axon to enhance the speed of electrical impulses.</p> Signup and view all the answers

What are nerve endings and what occurs there during signal transmission?

<p>Nerve endings, or presynaptic terminals, release neurotransmitters in response to action potentials.</p> Signup and view all the answers

Name the four major lobes of the human brain.

<p>The four major lobes are parietal, frontal, temporal, and occipital.</p> Signup and view all the answers

How many neurons are estimated to be in the human brain?

<p>There are estimated to be over 100 billion neurons in the human brain.</p> Signup and view all the answers

What is the role of Schwann cells in the peripheral nervous system?

<p>Schwann cells wrap around the axon to form the myelin sheath.</p> Signup and view all the answers

What happens at the nodes of Ranvier during action potential conduction?

<p>Action potentials 'jump' between the nodes of Ranvier where the axon is unmyelinated.</p> Signup and view all the answers

Which ion concentrations are higher inside and outside a neuron at rest?

<p>K+ concentration is higher inside the cell and Na+ concentration is higher outside.</p> Signup and view all the answers

What is the typical resting membrane potential (RMP) of neurons?

<p>The typical resting membrane potential of neurons is around -70 mV.</p> Signup and view all the answers

How does the Na+/K+ ATPase pump contribute to membrane potential maintenance?

<p>The pump actively moves Na+ out and K+ in, maintaining the correct ion gradients.</p> Signup and view all the answers

What initiates an action potential in a neuron?

<p>An action potential is initiated when a threshold level of stimulus is reached.</p> Signup and view all the answers

What are the two types of potentials generated by neurons?

<p>Neurons generate local (non-propagated) potentials and propagated action potentials.</p> Signup and view all the answers

Why is the resting membrane potential primarily determined by K+ concentrations?

<p>The resting membrane potential is influenced by the higher permeability of the membrane to K+ due to more open K+ channels.</p> Signup and view all the answers

What does the All-or-None law state about action potentials?

<p>It states that an action potential either occurs fully or not at all.</p> Signup and view all the answers

What triggers the opening of ligand-gated channels?

<p>Ligand-gated channels open when a neurotransmitter or chemical binds to them.</p> Signup and view all the answers

What happens during the depolarizing stimulus phase of an action potential?

<p>Some voltage-gated Na+ channels open, allowing Na+ to enter the cell, raising the membrane potential.</p> Signup and view all the answers

During which phase do Na+ channels quickly inactivate?

<p>Na+ channels quickly inactivate during the rapid depolarization phase.</p> Signup and view all the answers

What causes repolarization during an action potential?

<p>Repolarization occurs when voltage-gated K+ channels open, allowing K+ to exit the cell.</p> Signup and view all the answers

What defines the absolute refractory period?

<p>The absolute refractory period is the time after an action potential when no stimulus can trigger another response.</p> Signup and view all the answers

How does the strength-duration relationship work?

<p>Weak stimuli require longer durations to trigger a response, while strong stimuli can trigger a response in a shorter duration.</p> Signup and view all the answers

What is the membrane potential during after-hyperpolarization?

<p>During after-hyperpolarization, the membrane potential becomes more negative than the resting level.</p> Signup and view all the answers

What is the All or None Principle in relation to action potentials?

<p>The All or None Principle states that an action potential will only occur if the stimulus reaches the threshold level; if it does not, no action potential is generated.</p> Signup and view all the answers

Explain how conduction occurs during an action potential.

<p>During an action potential, positive charges flow into the area, decreasing polarity and triggering a local response that propagates the signal.</p> Signup and view all the answers

Describe how myelinated axons conduct action potentials differently from unmyelinated axons.

<p>Myelinated axons conduct action potentials via saltatory conduction, where depolarization jumps between nodes of Ranvier, making signal transmission faster.</p> Signup and view all the answers

How does decreased sodium (Na+) outside the cell affect action potentials?

<p>Decreased Na+ lowers the strength of the action potential but minimally impacts the resting membrane potential.</p> Signup and view all the answers

What effect does increased potassium (K+) outside the cell have on neuronal excitability?

<p>Increased K+ lowers the threshold for action potentials, making the neuron more excitable.</p> Signup and view all the answers

What role do glial cells play in the nervous system?

<p>Glial cells support neurons by providing nutrients, insulating them, and removing pathogens and dead cells.</p> Signup and view all the answers

What is the function of the nodes of Ranvier in myelinated axons?

<p>The nodes of Ranvier allow for saltatory conduction, enabling faster action potential propagation.</p> Signup and view all the answers

How does decreased calcium (Ca2+) outside the cell influence membrane excitability?

<p>Decreased Ca2+ increases membrane excitability, making it easier for the neuron to fire an action potential.</p> Signup and view all the answers

What is the primary function of the Blood Brain Barrier (BBB)?

<p>To separate the central nervous system from peripheral blood circulation.</p> Signup and view all the answers

What role do neurotrophins play in the nervous system?

<p>They support the growth, survival, and function of neurons.</p> Signup and view all the answers

Describe what happens to axons after a peripheral nerve injury.

<p>The axon degenerates beyond the injury, but the distal stump survives and undergoes axonal sprouting.</p> Signup and view all the answers

What is a key factor that affects axonal growth during peripheral nerve regeneration?

<p>Neurotrophins produced by the distal stump promote axonal growth.</p> Signup and view all the answers

How does the regenerative capability of CNS axons compare to that of peripheral axons?

<p>CNS axons have a much lower regenerative capability compared to peripheral axons.</p> Signup and view all the answers

What factors contribute to an unfavorable environment for CNS axonal regeneration?

<p>Astrocytic proliferation, microglial activation, scar formation, inflammation, and immune cell invasion.</p> Signup and view all the answers

What happens when a regenerating axon reaches its target in peripheral nerve regeneration?

<p>A new functional connection is formed at the target site.</p> Signup and view all the answers

What is the main focus of treatment for brain and spinal cord injuries?

<p>Rehabilitation rather than reversing nerve damage.</p> Signup and view all the answers

Study Notes

Nervous System Overview

  • Divided into two major systems: Central Nervous System (CNS) and Peripheral Nervous System (PNS).
  • CNS:
    • Brain (encephalon)
      • Cerebrum:
        • Cerebral cortex (grey matter)
        • White matter
        • Basal ganglia
        • Lateral ventricles
      • Diencephalon:
        • Epithalamus
        • Thalamus
        • Hypothalamus
        • Subthalamus
        • Contains the 3rd ventricle
      • Brainstem:
        • Midbrain (cerebral aqueduct)
        • Pons
        • Medulla oblongata
      • Cerebellum:
        • Contains the 4th ventricle
    • Spinal cord
  • PNS:
    • Somatic:
      • Sensory (general and special)
      • Motor
    • Autonomic Nervous System:
      • Sympathetic and parasympathetic
  • Neuron:
    • Functional unit of the nervous system (>100 billion in the human brain)
    • Variations in size, type, and specificity.
    • Composed of nucleus, cytoplasm, neuronal membrane, and cytoskeleton.
    • Excitability: ability to generate action potentials.
    • Other excitable cells: skeletal, smooth, and cardiac muscle cells, pancreatic secretory cells.
    • Functional Zones of Neurons:
      • Dendrites: receive signals, process information, and send it to the cell body (soma).
      • Initial Segment: where action potentials are generated at the start of the axon.
      • Axon: transmits impulses from the axon hillock to nerve endings.
      • Nerve Endings (Presynaptic Terminals): end in synaptic knobs (terminal boutons) where action potentials trigger the release of neurotransmitters stored in vesicles (synthesized in the cell body).

Myelin Sheath

  • Covers many axons.
  • Protein-lipid complex that insulates the axon.
  • In the PNS, Schwann cells wrap around the axon to form the myelin sheath.
  • Gaps in myelin are called nodes of Ranvier.
  • Action potentials "jump" between nodes for faster conduction (saltatory conduction).

Excitation and Conduction

  • Key feature of neurons: excitable membrane.
  • Neurons generate two types of potentials:
    • Local (non-propagated) potentials like synaptic responses.
    • Propagated action potentials: essential for communication within the nervous system.
  • Action potentials: primary method of signal transmission, traveling without losing strength (constant amplitude and velocity).
  • Conduction: self-propagating process where the signal moves continuously along the nerve.

Membrane Potential Maintenance

  • Factors that maintain membrane potential:
    • Unequal ion distribution across the membrane (concentration gradient).
    • Membrane permeability to specific ions through channels or pores in the lipid bilayer.
  • Neuron Ions:
    • Higher K+ concentration inside the cell.
    • Higher Na+ concentration outside the cell.
  • Na-K ATPase pump actively maintains this concentration difference:
    • Moves Na+ outside the cell.
    • Moves K+ inside the cell.
  • Passive ion movement:
    • K+ moves out when K+ channels are open.
    • Na+ moves in when Na+ channels are open.
  • At rest, the membrane is more permeable to K+:
    • More open K+ channels than Na+ channels.
    • Main determinant of the resting membrane potential (close to the equilibrium potential for K+).

Resting Membrane Potential (RMP)

  • Electrical difference across the plasma membrane of a living cell when it is not stimulated.
  • Measured as the potential inside the cell compared to the outside.
  • Typically around -70mV in neurons.
  • Na+/K+ ATPase pump maintains ion gradients for RMP.

Action Potential (AP)

  • Electrochemical signal that allows the nerve to transmit impulses over a distance.
  • Occurs when:
    • Change in voltage across the cell membrane.
    • Ionic gradients and membrane permeability create the conditions for the action potential.
    • A threshold level of stimulus is reached.
  • Transmitted without losing strength, maintaining constant amplitude and velocity.
  • Follows the All-or-None law: happens fully or not at all.

Neuronal Membranes

  • Two types of ion channels:
    • Ligand-gated channels: open when a neurotransmitter or chemical binds to them.
    • Voltage-gated channels: open in response to a change in electrical charge across the membrane.

Ion Conductance

  • Depends on:
    • Permeability: how easily the ion can pass through the membrane.
    • Electrical resistance of the membrane.

Steps of Action Potential

  1. Resting State: neuron is at rest, no ion movement across the membrane.
  2. Depolarizing Stimulus:
    • Stimulus causes some voltage-gated Na+ channels to open, allowing Na+ to enter the cell.
    • Membrane potential reaches threshold.
  3. Rapid Depolarization:
    • More Na+ channels open, leading to further depolarization in a positive feedback loop.
    • Rapid rise in membrane potential (upstroke).
  4. Na+ Channels Inactivate:
    • Membrane potential moves toward +60mV (Na+ equilibrium potential) but doesn't reach it because Na+ channels quickly inactivate.
  5. Repolarization:
    • At the peak (overshoot), the membrane potential reverses.
    • Voltage-gated K+ channels open, allowing K+ to exit, causing repolarization.
    • K+ channels open more slowly and stay open longer than Na+ channels.
  6. After-Hyperpolarization:
    • Slow closure of K+ channels causes a brief period of hyperpolarization (membrane potential becomes more negative than resting).
  7. Return to Resting State:
    • Membrane potential returns to its resting level after the K+ channels close.

Threshold Intensity

  • Minimum strength of a stimulus needed to trigger an action potential.

Strength-Duration Relationship

  • Weak stimuli need a longer duration to trigger a response.
  • Strong stimuli can trigger a response with a shorter duration.
  • This relationship forms the strength-duration curve.

Refractory Periods

  • Absolute Refractory Period:
    • From the start of the action potential until about one-third of repolarization.
    • No stimulus, no matter how strong, can trigger a response.
  • Relative Refractory Period:
    • After the absolute refractory period, during the final part of repolarization.
    • A stronger than normal stimulus can trigger a response.

All or None Principle

  • Action potential only occurs if the stimulus reaches the threshold level.
  • If the stimulus is too weak (subthreshold), no action potential will be generated.
  • Once the threshold is reached, the action potential will occur and its size will remain constant, even if the stimulus gets stronger.
  • Slowly increasing stimuli may not trigger an action potential because the nerve can adapt to the gradual change.

Conduction of the Action Potential

  • During an action potential, positive charges from surrounding areas flow into the region where the action potential is happening.
  • This flow reduces the polarity of the membrane in front of the action potential, triggering a local response.
  • When the firing threshold is reached, a new action potential is initiated, propagating the signal further.

Conduction in Myelinated Axons

  • Conduction relies on circular current flow.
  • Myelin acts as an effective insulator, so current flows only at the gaps between myelin (nodes of Ranvier).
  • Depolarization "jumps" from one node to the next, a process known as saltatory conduction.
  • Saltatory conduction is much faster, allowing myelinated axons to conduct signals up to 50 times faster than unmyelinated fibers.

Clinical Notes

  • Decreased Na+ outside the cell:
    • Lowers the strength of the action potential but has minimal impact on the resting membrane potential.
  • Increased K+ outside the cell (Hyperkalemia):
    • Lowers the threshold for action potential, making the neuron more excitable.
  • Decreased K+ outside the cell (Hypokalemia):
    • Hyperpolarizes the membrane (makes it more negative), reducing the likelihood of an action potential.
  • Calcium (Ca2+) effects:
    • Decreased Ca2+ (Hypocalcemia): Increases membrane excitability.
    • Increased Ca2+ (Hypercalcemia): Decreases membrane excitability.

Glial Cells or Neuroglia

  • Non-neuronal cells in the CNS and PNS that do not produce electrical impulses.
  • CNS Glial Cells:
    • Oligodendrocytes
    • Astrocytes
    • Ependymal cells
    • Microglia
  • PNS Glial Cells:
    • Schwann cells
    • Satellite cells
  • Functions:
    • Surround neurons and hold them in place.
    • Supply nutrients and oxygen to neurons.
    • Insulate one neuron from another.
    • Destroy pathogens and remove dead neurons.
  • Role in neurotransmission and synaptic connections.

Blood Brain Barrier (BBB)

  • Multicellular vascular structure that separates the central nervous system (CNS) from the peripheral blood circulation.

Neurotrophins

  • Family of proteins that play a key role in the growth, survival, development, and function of neurons in both the CNS and PNS.
  • Important functions in the immune and reproductive systems.

Axonal Injury and Regeneration

  • Peripheral nerve Damage:
    • Can often be repaired.
    • Axon degenerates beyond the injury, but the connective tissues of the distal part (distal stump) usually survive.
    • Axonal sprouting begins from the proximal stump, growing toward the distal stump.
    • Schwann cells guide this growth, releasing growth-promoting factors that attract the axon to the distal stump.
    • Inhibitory molecules in the surrounding tissue (perineurium) ensure that the regenerating axons grow along the correct path.
    • Neurotrophins produced by the distal stump further promot axonal growth.
    • When the regenerating axon reaches its target (e.g., a neuromuscular junction), a new functional connection is formed.
    • Enables significant, though not complete, recovery.
    • Fine motor control may be permanently affected if some motor neurons connect to the wrong motor fibers.
    • More successful than regeneration within the CNS.
  • CNS Axonal Injury and Regeneration:
    • The proximal stump of a damaged axon can form short sprouts, but the distal stump rarely recovers, and damaged axons are unlikely to form new synapses.
    • CNS neurons lack the growth-promoting chemicals necessary for regeneration.
    • CNS myelin actively inhibits axonal growth.
    • Factors that hinder regeneration:
      • Astrocytic proliferation.
      • Activation of microglia.
      • Scar formation.
      • Inflammation and immune cell invasion.
    • Treatment for brain and spinal cord injuries focuses on rehabilitation rather than reversing nerve damage.

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Description

Explore the intricate structure of the Nervous System, focusing on the Central Nervous System (CNS) and Peripheral Nervous System (PNS). This quiz covers key components including the brain, spinal cord, and neurons, as well as their functions and divisions. Test your knowledge on the foundational elements that support human cognition and response.

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