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Questions and Answers
What ions contribute to the overall membrane potential in excitable cells?
What is the primary factor in determining the resting membrane potential?
Concentration gradients and relative permeabilities
Graded potentials can be conducted decrementally.
True
What are the two divisions of the nervous system?
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The nervous system is a network of __________ whose main feature is to generate, modulate, and transmit information.
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What is the functional unit of the nervous system?
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Interneurons connect neurons within the CNS.
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Match the component of neural tissue with its description:
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What is the main function of interneurons?
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What is the term used to describe the junction between two neurons?
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Astrocytes are glial cells found in the PNS.
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______ are specialized, macrophage-like cells that perform immune functions in the CNS.
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Match the glial cells with their functions:
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What is a synaptic potential?
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Action potentials and graded potentials are synonymous terms.
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What is the function of graded potentials over short distances?
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The process termed _______ is important in sensation and adds to the graded potential from the first stimulus.
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What is the equation that expresses the effect of voltage V and resistance R on current I?
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What type of materials reduce current flow and are known as insulators?
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Lipids can carry electrical current.
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What is the term used to describe the potential difference across the plasma membranes of neurons at rest?
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Match the ion with its contribution to generating the resting membrane potential:
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What is the result of Cl- entering a cell in terms of membrane potential?
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Increased K+ permeability in the postsynaptic cell produces a depolarization.
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What is the main process of synaptic integration?
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Spatial summation involves inputs from multiple presynaptic neurons firing at the _____ time.
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What prevents the generation of action potentials by blocking voltage-gated Na+ channels?
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Action potentials convey information about the magnitude of the stimulus.
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What is the term for the specific duration in which a neuron is unable to fire another action potential?
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The refractory period limits the number of ____________ an excitable membrane can produce in a given period of time.
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Match the following terms with their definitions:
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What happens to the membrane potential of a postsynaptic neuron at an excitatory synapse?
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What happens to the membrane potential of a postsynaptic neuron at an inhibitory synapse?
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Chemical synapses have a gap between neurons of about 20 nanometers.
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Electrical synapses have a gap between neurons of about 3.5 nanometers.
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The basic structure of a typical chemical synapse includes the synaptic vesicles that contain neurotransmitter molecules in the axon terminals, and a high density of membrane proteins that make up a specialized area called the ________________.
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Match the synapse type with the effect on the postsynaptic cell:
- Excitatory Synapses
- Inhibitory Synapses
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Study Notes
Overview of the Nervous System
- The nervous system is a network of neurons that generate, modulate, and transmit information between different parts of the body.
- It regulates vital body functions, sensation, and body movements, and is responsible for consciousness, cognition, behavior, and memories.
- The nervous system consists of two divisions: the central nervous system (CNS) and the peripheral nervous system (PNS).
Structure and Maintenance of Neurons
- The functional unit of the nervous system is the individual neuron, which generates electrical signals that move from one part of the cell to another.
- Neurons serve as integrators, reflecting the balance of inputs they receive from up to hundreds of thousands of other neurons.
- Neurons have a cell body, dendrites, and axons.
- Dendrites receive incoming information, while axons carry outgoing signals away from the cell body.
- The axon hillock is the location where propagated electrical signals are generated.
- Axons may have branches, called collaterals, and end in axon terminals, which release neurotransmitters.
- Some neurons release chemical messengers from varicosities, which act as points of contact with other cells.
Myelin Sheaths
- Axons of many neurons are covered by myelin sheaths, which consist of 20 to 200 layers of modified plasma membrane.
- Myelin sheaths speed up conduction of electrical signals, conserve energy, and insulate axons.
- In the CNS, oligodendrocytes form myelin on as many as 40 axons, while in the PNS, Schwann cells form individual myelin sheaths.
Axonal Transport
- Axonal transport is the process responsible for moving mitochondria, lipids, synaptic vesicles, proteins, and other organelles to and from the neuron's cell body.
- Axonal transport occurs in two directions: anterograde (from cell body to axon terminals) and retrograde (from axon terminals to cell body).
- Kinesin and dynein are motor proteins that move organelles along microtubules.
Functional and Structural Classes of Neurons
- Neurons can be divided into three functional classes: afferent neurons, efferent neurons, and interneurons.
- Afferent neurons convey information from the tissues and organs to the CNS.
- Efferent neurons convey information away from the CNS to effector cells.
- Interneurons connect neurons within the CNS and integrate information.
- Afferent neurons have only one process, usually considered an axon, which divides into peripheral and central processes.
Glial Cells
- Glial cells surround and support neurons, providing physical and metabolic support.
- Glial cells retain the capacity to divide throughout life.
- There are different types of glial cells: oligodendrocytes, astrocytes, microglia, and ependymal cells.
- In the PNS, Schwann cells are the glial cells.
Neural Growth and Regeneration
- Neuronal development involves proliferation, differentiation, migration, axon guidance, and synapse formation.
- During development, many neurons and synapses degenerate, and the remaining neurons make new connections.
- Neuroplasticity, or brain plasticity, is the ability of the brain to change and adapt in structure and function in response to learning, experience, stimuli, or injury.
- Although neural plasticity varies with age, the ability to produce new neurons is retained in some brain regions throughout life.### Neural Growth and Regeneration
- Regeneration of axons is delayed due to slow axon regrowth rate of about 1 mm per day.
- For example, if afferent neurons from the thumb were damaged in the shoulder area, it might take 2 years for sensation in the thumb to be restored.
- Spinal injuries typically crush rather than cut the tissue, leaving the axons intact.
- In this case, a primary problem is self-destruction (apoptosis) of nearby oligodendrocytes, which lose their myelin sheath, making it difficult for axons to transmit information effectively.
- Severed axons within the CNS may grow small new extensions, but no significant regeneration of the axon occurs across the damaged site, and there are no well-documented reports of significant return of function.
Basic Principles of Electricity
- The predominant solutes in the extracellular fluid (ECF) are sodium and chloride ions.
- The intracellular fluid contains high concentrations of potassium ions and ionized nonpenetrating molecules, particularly phosphate compounds and proteins with negatively charged side chains.
- Electrical phenomena resulting from the distribution of these charged particles occur at the cell's plasma membrane and have a significant function in signal integration and cell-to-cell communication.
- A fundamental physical principle is that charges of the same type repel each other, while oppositely charged substances attract each other and will move toward each other if not separated by some barrier.
Membrane Potentials
- The potential difference across the cell membrane is known as an electrical potential or potential difference (measured in millivolts).
- The movement of electrical charge is called a current.
- The electrical potential between charges tends to make them flow, producing a current.
- The amount of charge that moves (magnitude of the current) depends on the potential difference between the charges and on the nature of the material or structure through which they are moving.
Resting Membrane Potential
- At rest, neurons have a potential difference across their plasma membranes, with the inside of the cell negatively charged with respect to the outside.
- The resting membrane potential (Vm) is the difference in electrical potential across the cell membrane when the cell is not stimulated or in a state of relaxation.
- The magnitude of the resting membrane potential depends on two main factors:
- Differences in specific ion concentrations in the intracellular and extracellular fluids.
- Differences in membrane permeabilities to the different ions, which reflect the number of open channels for the different ions in the plasma membrane.
Contribution of Ion Concentration Differences
- The concentration differences for Na⁺ and K⁺ ions create membrane potentials.
- When the membrane is permeable to only one ion, the concentration gradient for that ion will generate a membrane potential.
- The magnitude of the equilibrium potential for any ion depends on the concentration gradient for that ion across the membrane.
- The Nernst equation describes the equilibrium potential for any ion, which is necessary to balance a given ionic concentration gradient across a membrane so that the net flux of the ion is zero.
Contribution of Ion Permeabilities
- When channels for more than one type of ion are open in the membrane, the permeabilities and concentration gradients for all the ions must be considered.
- The greater the membrane permeability to one type of ion, the greater the contribution that ion will make to the membrane potential.
- The Goldman-Hodgkin-Katz (GHK) equation takes into account individual ion permeabilities and is used to calculate the resting membrane potential of a membrane.
Contribution of Ion Pump
- The Na⁺/K⁺-ATPase pump is essential to maintaining the concentration gradients of ions.
- The pump makes an indirect contribution to the membrane potential because it maintains the concentration gradients that result in ion diffusion and charge separation.
- The pump also has a minor direct role in creating a negative resting potential because with each cycle, it moves three Na⁺ out of the cell for every two K⁺ that it brings in.### Resting Membrane Potential
- The resting membrane potential is maintained at -70mV due to the balance of inward and outward leak of positive ions, despite higher permeability to K⁺.
- The Na⁺/K⁺-ATPase pump sets up concentration gradients for Na⁺ and K⁺, determining the equilibrium potentials for each ion.
- The pump also generates a small electrogenic effect, contributing to the membrane potential.
Development of Resting Membrane Potential
- Step 1: Na⁺/K⁺-ATPase pump sets up concentration gradients and equilibrium potentials.
- Step 2: Greater flux of K⁺ out of the cell than Na⁺ into the cell due to higher permeability to K⁺, resulting in a negative membrane potential.
- Step 3: Dynamic balance is reached, with equal inward and outward fluxes of ions, resulting in a steady resting membrane potential.
Chloride Ions
- Chloride ions (Cl⁻) also contribute to the resting membrane potential.
- In many cells, Cl⁻ channels allow Cl⁻ to shift until its equilibrium potential equals the resting membrane potential.
- In some cells, an active transport system moves Cl⁻ out of the cell, generating a strong concentration gradient and contributing to the negative membrane potential.
Graded Potentials and Action Potentials
- Graded potentials are changes in membrane potential confined to a small region of the plasma membrane.
- They are produced by specific changes in the cell's environment and can vary in amplitude and duration.
- Graded potentials can be depolarizing or hyperpolarizing and can summate to produce a larger response.
Action Potentials
- Action potentials are rapid, large alterations in the membrane potential (up to 100mV).
- They are generated by voltage-gated ion channels and can propagate down the axon.
- Action potentials are all-or-none, meaning their amplitude is fixed and does not vary with the strength of the stimulus.
Action Potential Mechanism
- Step 1: Initial depolarization opens voltage-gated Na⁺ channels, allowing rapid entry of Na⁺ and further depolarization.
- Step 2: Depolarization reaches a critical threshold, opening more voltage-gated Na⁺ channels and causing rapid depolarization.
- Step 3: Inactivation gates close, Na⁺ channels begin to inactivate, and K⁺ channels open, repolarizing the membrane.
- Step 4: K⁺ channels close, and the resting membrane potential is restored.
Voltage-Gated Ion Channels
- Na⁺ channels respond quickly to changes in membrane voltage and have an inactivation gate that limits Na⁺ flux.
- K⁺ channels respond more slowly and help to repolarize the membrane.
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Description
This quiz covers the basics of the nervous system, including neural tissue, neurons, glial cells, and neural growth and regeneration. It is part of the General Physiology course for 2nd-year medical students at the Lebanese University.
