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Questions and Answers
What characterizes depolarization in a neuron?
What characterizes depolarization in a neuron?
Which ion's entry primarily leads to the depolarization of neurons?
Which ion's entry primarily leads to the depolarization of neurons?
Which characteristic is a feature of the refractory period following an action potential?
Which characteristic is a feature of the refractory period following an action potential?
What is the primary mechanism through which hyperpolarization occurs in neurons?
What is the primary mechanism through which hyperpolarization occurs in neurons?
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Which factor influences the speed of action potential conduction?
Which factor influences the speed of action potential conduction?
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How does myelination affect action potential propagation?
How does myelination affect action potential propagation?
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What is resting membrane potential primarily determined by?
What is resting membrane potential primarily determined by?
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Which channel type opens in response to mechanical stimulation of a neuron?
Which channel type opens in response to mechanical stimulation of a neuron?
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What outcome occurs when K+ exits the neuron following an action potential?
What outcome occurs when K+ exits the neuron following an action potential?
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Which ions primarily contribute to maintaining resting membrane potential?
Which ions primarily contribute to maintaining resting membrane potential?
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What is the potential difference across a resting cell membrane known as?
What is the potential difference across a resting cell membrane known as?
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Which process results in the mechanical opening of ion channels?
Which process results in the mechanical opening of ion channels?
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Which condition can lead to hyperpolarization in neurons?
Which condition can lead to hyperpolarization in neurons?
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What typically happens to the membrane potential during an action potential?
What typically happens to the membrane potential during an action potential?
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What type of neuron conducts action potentials toward the central nervous system?
What type of neuron conducts action potentials toward the central nervous system?
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Which part of a neuron is primarily responsible for generating action potentials?
Which part of a neuron is primarily responsible for generating action potentials?
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Which glial cell type is responsible for forming the myelin sheath in the CNS?
Which glial cell type is responsible for forming the myelin sheath in the CNS?
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What is the main role of the myelin sheath?
What is the main role of the myelin sheath?
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What distinguishes multipolar neurons from bipolar neurons?
What distinguishes multipolar neurons from bipolar neurons?
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Which part of the neuron receives input from other neurons or sensory receptors?
Which part of the neuron receives input from other neurons or sensory receptors?
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What defines the function of interneurons in the nervous system?
What defines the function of interneurons in the nervous system?
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What is the primary function of astrocytes in the CNS?
What is the primary function of astrocytes in the CNS?
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Which types of glial cells can be found in the PNS?
Which types of glial cells can be found in the PNS?
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What condition is characterized by the gradual loss of myelin sheath in the CNS?
What condition is characterized by the gradual loss of myelin sheath in the CNS?
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What occurs at the synapse?
What occurs at the synapse?
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How are unmyelinated axons typically organized in relation to glial cells?
How are unmyelinated axons typically organized in relation to glial cells?
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Which of the following is not a function of neurons?
Which of the following is not a function of neurons?
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Which component of a neuron is involved in conducting action potentials away from the CNS?
Which component of a neuron is involved in conducting action potentials away from the CNS?
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What is the primary factor that determines whether an action potential is generated in a neuron?
What is the primary factor that determines whether an action potential is generated in a neuron?
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Which event leads to the release of neurotransmitters from the presynaptic terminal?
Which event leads to the release of neurotransmitters from the presynaptic terminal?
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What characterizes an inhibitory postsynaptic potential (IPSP)?
What characterizes an inhibitory postsynaptic potential (IPSP)?
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Which mechanism primarily involves binding to ion channels?
Which mechanism primarily involves binding to ion channels?
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What does spatial summation refer to?
What does spatial summation refer to?
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Which neurotransmitter type is primarily involved in excitatory postsynaptic potentials?
Which neurotransmitter type is primarily involved in excitatory postsynaptic potentials?
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What is the role of presynaptic inhibition?
What is the role of presynaptic inhibition?
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The rapid removal of a neurotransmitter from the synaptic cleft primarily results in what effect?
The rapid removal of a neurotransmitter from the synaptic cleft primarily results in what effect?
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What characterizes temporal summation?
What characterizes temporal summation?
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What is the primary function of neuromodulators?
What is the primary function of neuromodulators?
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Which type of synapse allows ions to flow directly from one cell to another?
Which type of synapse allows ions to flow directly from one cell to another?
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In terms of ion permeability, what does EPSP primarily increase?
In terms of ion permeability, what does EPSP primarily increase?
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What effect do excitatory neurotransmitters have on the postsynaptic cell?
What effect do excitatory neurotransmitters have on the postsynaptic cell?
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Which of the following neurotransmitter types is considered a gasotransmitter?
Which of the following neurotransmitter types is considered a gasotransmitter?
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Study Notes
Membrane Potential
- A measure of the electrical properties of the plasma membrane due to
- Ionic concentration differences across the plasma membrane
- Permeability characteristics of the plasma membrane
- The plasma membrane is polarized and has a potential difference across it
- The resting membrane potential is the potential difference in a resting cell
- The concentration gradient drives the movement of ions through the plasma membrane
- Changes in resting membrane potential can be:
- Depolarization – inside of the cell becomes more positive
- Hyperpolarization – inside of the cell becomes more negative
Depolarization
- The most common cause of depolarization is the influx of Na+ into the cell
- Limited Na+ leak channels exist, so Na+ entry is typically regulated by
- Ligand-gated Na+ channels
- Voltage-gated Na+ channels
- The influx of Ca2+ into the cell can also cause depolarization, which is important for action potential generation in some cardiac muscle cells
- Ca2+ also plays a significant role in action potentials by:
- Regulating voltage-gated Na2+ channels
- Regulating neurotransmitter secretion at the presynaptic terminal
- Hypocalcemia (low blood Ca2+) can cause symptoms like nervousness and uncontrolled skeletal muscle contraction
Hyperpolarization
- The primary cause of hyperpolarization after an action potential is the exit of K+ through voltage-gated K+ channels
- Ligand-gated K+ channels are involved in the mechanism of some inhibitory neurotransmitters
- Hypokalemia (low blood K+) can cause more K+ to exit the cell through leak channels, resulting in symptoms like muscular weakness, abnormal heart function, and sluggish reflexes
- The entry of Cl- into the cell can also cause hyperpolarization, which is used by some inhibitory neurotransmitters
Neuron Communication
- Neuron communication occurs through three phases:
- Graded potentials
- Action potentials
- Synaptic communication
- The synapse is composed of a presynaptic cell and a postsynaptic cell
- There are two types of synapses:
- Electrical synapses
- Chemical synapses
Electrical Synapses
- Occur between cells connected by gap junctions
- Allow ions to flow from one cell to the next
- Not very common in the nervous system
- Found in cardiac muscle and some smooth muscle
Chemical Synapses
- Use chemical messengers (neurotransmitters) to communicate between cells
- Composed of:
- Presynaptic terminal
- Synaptic cleft
- Postsynaptic membrane
- Release of neurotransmitters is triggered by an action potential in the presynaptic terminal, which opens voltage-gated Ca2+ channels, causing the influx of Ca2+ and triggering exocytosis
Neurotransmitter Removal
- Neurotransmitter and receptor equilibrium is important for the duration of neurotransmitter effects
- Rapid removal or destruction of the neurotransmitter results in short-term effects
Receptors in Synapses
- Highly specific receptors on the postsynaptic membrane determine the effect of the neurotransmitter on the cell
- Neurotransmitters can stimulate some cells and inhibit others
Neurotransmitters
- Chemical messengers released from neurons
- Some neurons can secrete more than one type of neurotransmitter
- Characteristics of neurotransmitters:
- Synthesized by the neuron and stored in synaptic vesicles
- Exocytosis into the synaptic cleft is stimulated by an action potential
- Bind to specific receptors on the postsynaptic membrane
- Evoke a response in the postsynaptic cell
Neurotransmitter Classification
- Classified based on:
- Chemical structure
- Effect on the postsynaptic membrane
- Mechanism of action at their target
Neurotransmitter Effect on Postsynaptic Cell
- Excitatory:
- Causes depolarization
- Makes the cell more likely to generate an action potential
- Inhibitory:
- Causes hyperpolarization
- Makes the cell less likely to generate an action potential
Neurotransmitter Mechanisms of Action
- Ionotropic effect: binding to ion channels
- Metabotropic effect: binding to G protein-linked receptors
Postsynaptic Potentials
- Excitatory postsynaptic potential (EPSP):
- Depolarization
- Could generate an action potential
- Typically results from increased permeability of the membrane to Na+
- Inhibitory postsynaptic potential (IPSP):
- Hyperpolarization
- Does not generate action potentials
- Typically results from increased permeability of the membrane to Cl- or K+
Neuromodulators
- Substances released by neurons that influence the likelihood of an action potential being generated in the postsynaptic cell
- Axoaxonic synapses: the axon of one neuron synapses on the presynaptic terminal (axon) of another, allowing for the release of a neuromodulator to influence the action of another neuron
Neuromodulation
- Presynaptic Inhibition: the amount of neurotransmitter released from the presynaptic terminal is reduced
- Presynaptic Facilitation: the amount of neurotransmitter released from the presynaptic terminal is elevated
Summation of Graded Potentials
-
Generation of an action potential is determined by the sum of all graded potentials generated by stimulation of the neuron
-
Spatial summation: multiple action potentials arrive at the same time from separate neurons
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Temporal summation: two or more action potentials arrive very close together from the same neuron### Neuron Structure
-
Cell body (Soma)
- Contains a single, centrally located nucleus with a nucleolus
- Has extensive rough endoplasmic reticulum (Nissl bodies)
- Contains abundant intermediate filaments (neurofilaments) and microtubules forming bundles in the cytoplasm
-
Dendrites
- Processes branching off the cell body
- Short and highly branched
- Tapered from base to tip
- Receive input from other neurons and sensory receptors
- Have dendritic spines, small extensions on the surface where synapses are formed
-
Axons
- Single process extending from the cell body
- Constant diameter with varying length
- Contains cytoplasm called axoplasm and a plasma membrane called axolemma
- Axon hillock is the cone-shaped area where the axon originates from the cell body
- Initial segment is formed by the narrowing of the axon hillock
- Trigger zone (axon hillock and initial segment) is where action potentials are generated
- Presynaptic terminal at the end of the axon houses synaptic vesicles storing neurotransmitters
- Synapse is the point of contact between the axon ending and its effector
Functional Classes of Neurons
- Sensory (afferent) neurons conduct action potentials toward the Central Nervous System (CNS)
- Motor (efferent) neurons conduct action potentials away from the CNS toward muscles or glands
- Interneurons conduct action potentials within the CNS
Structural Classes of Neurons
-
Multipolar neurons
- Have multiple dendrites and a single axon
- Dendrite number varies with branching
- Include motor neurons of the Peripheral Nervous System (PNS) and most neurons in the CNS
-
Bipolar neurons
- Have one dendrite and one axon
- Dendrites often specialize to receive stimuli
- Axons conduct action potentials
- Located in sensory organs such as the retina of the eye and nasal cavity
-
Pseudo-unipolar neurons
- Have a single process that exits the cell body and divides into two branches functioning as a single axon
- Peripheral process extends to the periphery and has dendrites acting as sensory receptors or communicating with sensory receptors
- Central process extends to the CNS
- Most sensory neurons belong to this category
-
Anaxonic neurons
- Lack axons and only have dendrites
- Found in the brain and retina
- Communicate using only graded potentials
Glial Cells of the CNS
-
Astrocytes
- Have cytoplasmic processes extending from the cell body
- Foot processes cover blood vessels, neurons, and pia mater
- Regulate the composition of extracellular brain fluid
- Produce chemicals that promote formation of tight junctions between endothelial cells of capillaries to form the blood-brain barrier
- Play a role in response to tissue damage by limiting inflammation and spread of injury
- Promote development of synapses and help regulate synaptic activity by synthesizing, absorbing, and recycling neurotransmitters
-
Ependymal cells
- Line ventricles of the brain and the central canal of the spinal cord
- Form choroid plexuses, specialized structures in the ventricles, that secrete cerebrospinal fluid
- May have cilia to move cerebrospinal fluid
-
Microglia
- CNS specific immune cells
- Become mobile and phagocytic in response to inflammation
- Phagocytize necrotic tissue, microorganisms, and other foreign substances
-
Oligodendrocytes
- Form the myelin sheath in the CNS
- Cytoplasmic extensions wrap around multiple axons
Glial Cells of the PNS
-
Schwann cells
- Form the myelin sheath in the PNS
- Each Schwann cell wraps around only one axon
- Have a neurilemma, the outermost layer containing the majority of Schwann cell cytoplasm, nucleus, and organelles
-
Satellite cells
- Surround neuron cell bodies in sensory and autonomic ganglia
- Provide support and nutrition
- Protect neurons from heavy metal poisons
Myelinated Axons
- Schwann cells (PNS) or oligodendrocytes (CNS) wrap around axons, forming layers of phospholipids with small amounts of cytoplasm.
- This gives myelinated axons a white appearance.
- Nodes of Ranvier are gaps in the myelin sheath where Schwann cells or oligodendrocytes extend across and connect.
- Myelin sheaths protect and electrically insulate axons.
Unmyelinated Axons
- Axons are not devoid of myelin.
- They rest in invaginations of Schwann cells or oligodendrocytes.
- They are protected by these cells.
Development of Myelin Sheath
- Begins in late fetal development.
- Continues rapidly until the end of the first year after birth.
- Slows and continues after the first year.
Multiple Sclerosis
- Chronic disease of the CNS.
- Characterized by gradual loss of the myelin sheath.
- Slows action potential transmission.
- Impairs control of skeletal and smooth muscle.
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Description
Explore the concepts of membrane potential and the mechanisms of depolarization in cells. Understand how ionic concentration differences and membrane permeability influence electrical properties. This quiz covers essential topics related to resting membrane potential and ion channels.
