Chapter 12 Flashcards
Central nervous system (CNS)
Consists of the brain and spinal cord. The CNS processes many different kinds of incoming sensory information. It is also the source of thoughts, emotions, and memories. Most signals that stimulate muscles to contract and glands to secrete originate in the CNS.
Peripheral nervous system (PNS)
Consists of all nervous tissue outside the CNS. Components of the PNS include nerves and sensory receptors.
What two systems does the PNS get divided into?
- Somatic nervous system (SNS)
- Autonomic nervous system (ANS)
Somatic nervous system (SNS)
Conveys output from the CNS to skeletal muscles only. Is voluntary.
Autonomic nervous system (ANS)
Conveys output from the CNS to smooth muscle, cardiac muscle, and glands. Is involuntary.
What are the three branches of the ANS?
- Sympathetic nervous system
- Parasympathetic nervous system
- Enteric nervous system (ENS)
Sympathetic nervous system
Helps support exercise or emergency actions; “fight-or-flight” responses.
Parasympathetic nervous system
Takes care of “rest-and-digest” activities.
Enteric nervous system (ENS)
An extensive network of over 100 million neurons confined to the wall of the gastrointestinal (GI) tract. Helps regulate the activity of the smooth muscles and glands of the GI tract. Can function independently, but communicates with and is regulated by the other branches of the ANS.
Nerve
A bundle of hundreds of thousands of axons plus associated connective tissue and blood vessels that lie outside the brain and spinal cord.
Cranial nerves
Nerves the emerge from the brain; 12 pairs.
Spinal nerves
Nerves that emerge from the spinal cord; 31 pairs.
Sensory receptors
A structure of the nervous system that monitors changes in the external or internal environment (Eg. Touch receptors in the skin, photoreceptors in the eye, and olfactory (smell) receptors in the nose).
What are the three basic functions of the nervous system?
- Sensory function
- Integrative function
- Motor function
Sensory function
Detect internal stimuli or external stimuli. This sensory information is then carried into the brain and spinal cord through cranial and spinal nerves.
Integrative function
Processes sensory information by analyzing it and making decisions for appropriate responses – an activity known as integration.
Motor function
Once sensory information is integrated, the nervous system may elicit an appropriate motor response by activating effectors (muscles and glands) through cranial and spinal nerves. Stimulation of the effectors causes muscles to contract and glands to secrete.
Neurons
Form complex processing networks within the brain. Connect all regions of the body to the brain and spinal cord. Responsible for functions such as sensing, thinking, remembering, controlling muscle activity, and regulating glandular secretions. Can’t undergo mitotic divisions. Possess electrical excitability. Vary in size. Generate/propagate action potentials.
Electrical excitability
The ability to respond to a stimulus and convert it to an action potential.
Stimulus
Any change in the environment that is strong enough to initiate an action potential.
Action potential
An electrical signal that propagates (travels) along the surface of the membrane of a neuron. It begins and travels due to the movement of ions (such as sodium and potassium) between interstitial fluid and the inside of a neuron through specific ion channels in its plasma membrane.
What are the 3 main parts of a neuron?
- Cell body
- Dendrites
- Axon
Cell body
Receives stimuli and produces EPSPs and IPSPs through activation of ligand-gated ion channels.
Nissi body
Free ribosomes and prominent clusters of rough endoplasmic reticulum in neuronal cell bodies. The ribosomes are sites of protein synthesis. Newly synthesized proteins produced by Nissl bodies are used to replace cellular components, as material for growth of neurons, and to regenerate damaged axons in the PNS.
Neurofibrils
Composed of bundles of intermediate filaments that provide the cell shape and support.
Microtubules
Assist in moving materials between the cell body and axon.
Lipofuscin
A pigment that occurs as clumps of yellowish-brown granules in the cytoplasm. Lipofuscin is a product of neuronal lysosomes that accumulates as the neuron ages, but does not seem to harm the neuron.
Ganglion
Collection of neuron cell bodies outside the CNS.
Nerve fiber
A general term for any neuronal process (extension) that emerges from the cell body of a neuron. Includes dendrites and axons.
Dendrites
Receive stimuli through activation of ligand-gated or mechanically-gated ion channels; in sensory neurons, produce graded potentials; in motor neurons and interneurons, produce EPSPs and IPSPs.
Axon
Propagates nerve impulses from initial segment (or from dendrites of sensory neurons) to axon terminals in self-regenerating manner; impulse amplitude does not change as it propagates along axon.
Trigger zone
Composed of the axon hillock and initial segment. Integrates EPSPs and IPSPs and, if sum is depolarization that reaches threshold, initiates action potential.
What is the difference between axoplasm and axolemma?
Axoplasm: cytoplasm of an axon.
Axolemma: plasma membrane of an axon.
Axon collaterals
Side branches along the length of the axon. May branch off, typically at a right angle to the axon.
What is the role of the axon terminals, synaptic end bulbs, and variocosties?
Inflow of Ca2+ caused by depolarization phase of nerve impulse triggers exocytosis of neurotransmitter from synaptic vesicles.
Synapse
The site of communication between two neurons or between a neuron and an effector.
Synaptic vesicles
Membrane-enclosed sacs that store neurotransmitters.
Neurotransmitter
A molecule released from synaptic vesicles that excite or inhibit other neurons, muscle fibers, or gland cells.
Slow axonal transport
Slower system which moves materials about 1-5mm per day between the cell body and axon terminals. Moves materials in one direction only - cell body to axon terminals.
Fast axonal transport
Faster system which moves materials about 200-400mm per day between the cell body and the axon terminals. Uses proteins that function as “motors” to move materials along the surfaces of microtubules of the neuron’s cytoskeleton. Moves materials in both directions - away from and toward the cell body.
Fast axonal transport that occurs in an ______ direction moves organelles and synaptic vesicles from the cell body to the axon terminals. Fast axonal transport that occurs in a ______ direction moves membrane vesicles and other cellular materials from the axon terminals to the cell body to be degraded or recycled.
Anterograde (forward); retrograde (backward)
What are the three structural classifications of neurons?
- Multipolar neurons
- Bipolar neurons
- Unipolar neurons
Multipolar neurons
Usually have several dendrites and one axon. Most neurons in the brain and spinal cord are of this type, as well as all motor neurons.
Bipolar neurons
Have one main dendrite and one axon. They are found in the retina of the eye, the inner ear, and the olfactory area of the brain.
Unipolar neurons
Have dendrites and one axon that are fused together to form a continuous process that emerges from the cell body. The dendrites of most unipolar neurons function as sensory receptors that detect a sensory stimulus such as touch, pressure, pain, or thermal stimuli.
Purkinje cells
Neurons found in the cerebellum.
Pyramidal cells
Neurons found in the cerebral cortex of the brain, which have pyramid-shaped cell bodies.
What are the three functional classifications of neurons?
- Sensory neurons (afferent neurons)
- Motor neurons (efferent neurons)
- Interneurons (association neurons)
Sensory neurons
AKA afferent neurons; forms an action potential in its axon when an appropriate stimulus activates a sensory receptor. The action potential is conveyed into the CNS through cranial or spinal nerves. Most are unipolar neurons.
Motor neurons
AKA efferent neurons; convey action potentials away from the CNS to effectors (muscles and glands) in the PNS through cranial or spinal nerves. Are multipolar neurons.
Interneurons
AKA association neurons; are mainly located within the CNS between sensory and motor neurons. Integrate (process) incoming sensory information from sensory neurons and then elicit a motor response by activating the appropriate motor neurons. Most are multipolar neurons.
Neuroglia
Smaller than, but greatly outnumber, neurons. Support, nourish, and protect neurons, and maintain the interstitial fluid that bathes them. Can undergo mitotic divisions. Do not generate/propagate action potentials.
Gliomas
Brain tumors derived from glia. Tend to be highly malignant and grow rapidly.
Astrocytes
Star shaped cells that have many processes and are the largest and most numerous of the neuroglia.
What are the five main functions of astrocytes?
- Are strong and support neurons.
- Help to isolate neurons of the CNS from harmful substances in the blood.
- Secrete chemicals that appear to regulate growth, migration, and interconnection among neurons in the brain.
- Help maintain the appropriate chemical environment for the generation of nerve impulses.
- May play a role in learing and memory by influencing the formation of neural synapses.
What are the two types of astrocytes? Where are they found?
- Protoplasmic astrocytes (short branching processes found in gray matter)
- Fibrous astrocytes (long unbranched processes found in white matter)
Ogliodendroctyes
Resemble astrocytes but are smaller and contain fewer processes. Processes of oligodendrocytes are responsible for forming and maintaining the myelin sheath around CNS axons.
Myelin sheath
A multilayered lipid and protein covering around some axons that insulates them and increases the speed of nerve impulse conduction. Such axons are said to be myelinated. Are produced by Schwann cells in the PNS and oligodendrocytes in the CNS.
Microglial cells/microglia
Small cells with slender processes that give off numerous spinelike projections. They function as phagocytes. Like tissue macrophages, they remove cellular debris formed during normal development of the nervous system and phagocytize microbes and damaged nervous tissue.
Ependymal cells
Are cuboidal to columnar cells arranged in a single layer that possess microvilli and cilia. These cells line the ventricles of the brain and central canal of the spinal cord. Functionally, ependymal cells produce, possibly monitor, and assist in the circulation of cerebrospinal fluid. They also form the blood–cerebrospinal fluid barrier.
Schwann cells
Cells that encircle the PNS axons. Like oligodendrocytes, they form the myelin sheath around axons. A single oligodendrocyte myelinates several axons, but each Schwann cell myelinates a single axon. A single Schwann cell can also enclose as many as 20 or more unmyelinated axons (axons that lack a myelin sheath). Schwann cells participate in axon regeneration, which is more easily accomplished in the PNS than in the CNS.
Satellite cells
Flat cells that surround the cell bodies of neurons of PNS ganglia. Provide structural support and regulate the exchanges of materials between neuronal cell bodies and interstitial fluid.
Neurolemma
The outer nucleated cytoplasmic layer of the Schwann cells which encloses the myelin sheath. Is found only around axons in the PNS. When an axon in injured, the neurolemma aids regeneration by forming a regeneration tube that guides and stimulates regrowth of the axon.
Nodes of Ranvier
Gaps in the myelin sheath. Appear at intervals along the axon.
Nucleus
Cluster of neuronal cell bodies located in the CNS.
What is the difference between gray matter and white matter?
White matter: composed primarily of myelinated axons. The whitish color of myelin gives white matter its name. Blood vessels are present.
Gray matter: contains neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia. It appears grayish, rather than white, because the Nissl bodies impart a gray color and there is little or no myelin in these areas. Blood vessels are present.
Describe the steps in this image:
- As you touch the pen, a graded potential develops in a sensory receptor in the skin of the fingers.
- The graded potential triggers the axon of the sensory neuron to form a nerve action potential, which travels along the axon into the CNS and ultimately causes the release of neurotransmitter at a synapse with an interneuron.
- The neurotransmitter stimulates the interneuron to form a graded potential in its dendrites and cell body.
- In response to the graded potential, the axon of the interneuron forms a nerve action potential. The nerve action potential travels along the axon, which results in neurotransmitter release at the next synapse with another interneuron.
- This process of neurotransmitter release at a synapse followed by the formation of a graded potential and then a nerve action potential occurs over and over as interneurons in higher parts of the brain (such as the thalamus and cerebral cortex) are activated. Once interneurons in the cerebral cortex are activated, perception occurs and you are able to feel the smooth surface of the pen touch your fingers.
- A stimulus in the brain causes a graded potential to form in the dendrites and cell body of an upper motor neuron, a type of motor neuron that synapses with a lower motor neuron farther down in the CNS in order to contract a skeletal muscle. The graded potential subsequently causes a nerve action potential to occur in the axon of the upper motor neuron, followed by neurotransmitter release.
- The neurotransmitter generates a graded potential in a lower motor neuron, a type of motor neuron that directly supplies skeletal muscle fibers. The graded potential triggers the formation of a nerve action potential and then release of the neurotransmitter at neuromuscular junctions formed with skeletal muscle fibers that control movements of the fingers.
- The neurotransmitter stimulates the muscle fibers that control finger movements to form muscle action potentials. The muscle action potentials cause these muscle fibers to contract, which allows you to write with the pen.
Resting membrane potential
An electrical potential difference (voltage) that exists across the plasma membrane of an excitable cell under resting conditions.
What are the four types of ion channels?
- Leak channels
- Ligand-gated channels
- Mechanically gated channels
- Voltage-gated channels
Leak channels
Gated channels that randomly open and close. Found in nearly all cells, including dendrites, cell bodies, and axons of all types of neurons.
Ligand-gated channels
Gated channels that open in response to binding of a ligand (chemical) stimulus. Found in dendrites of some sensory neurons such as pain receptors and dendrites and cell bodies of interneurons and motor receptors.
Mechanically-gated channels
Gated channels that open in response to mechanical stimulus (such as touch, pressure, vibration, or tissue stretching). Found in dendrites of some sensory neurons such as touch receptors, pressure receptors, and some pain receptors.
Voltage-gated channels
Gated channels that open in response to voltage stimulus (change in membrane potential). Found in axons of all types of neurons.
What three major factors does the resting membrane potential arise from?
- Unequal distribution of ions in the ECF and cytosol.
- Inability of most anions to leave the cell.
- Electrogenic nature of the Na+-K+ ATPases.
Graded potential
Small deviation from the resting membrane potential that makes the membrane either more polarized (inside more negative) or less polarized (inside less negative). Occurs when a stimulus causes mechanically-gated or ligand-gated channels to open or close in an excitable cell’s plasma membrane. Occur mainly in the dendrites and cell body of a neuron.
What is the difference between a hyperpolarizing graded potential and a depolarizing graded potential?
Hyperpolarizing graded potential: when the response makes the membrane more polarized (inside more negative).
Depolarized graded potential: when the responses makes the membrane less polarized (inside less negative).
Summation
The process by which graded potentials add together.
Action potential
Sequence of rapidly occurring events that decrease and reverse the membrane potential and then eventually restore it to the resting state.
What are the two main phases of action potentials?
- Depolarizing phase
- Repolarizing phase
Depolarizing phase
The negative membrane potential becomes less negative, reaches zero, and then becomes positive. Caused by Na+ rushing into the cell after voltage-gated Na+ channels get opened.
Repolarizing phase
The membrane potential is restored to the resting state of -70mV. Caused by K+ flowing out of the cell after the voltage-gated K+ channels get opened.
After-hyperpolarization phase
The phase that is said to possibly follow the repolarization phase. During this phase, the membrane potential temporarily becomes more negative than the resting level. Caused by the voltage-gated K+ channels remaining open after the depolarization phase ends.
What is the difference between subthreshold stimulus, threshold stimulus, and suprathreshold stimulus?
Subthreshold stimulus: a weak depolarization that cannot bring the membrane potential to threshold. An action potential will not occur.
Threshold stimulus: a stimulus that is strong enough to depolarize the membrane above threshold. An action potential will occur.
Suprathreshold stimulus: a stimulus that is strong enough to depolarize the membrane above threshold. Several action potentials will form.
What are the two types of propagation?
- Continuous conduction
- Saltatory conduction
Continuous conduction
Occurs along an unmyelinated axon. Ionic currents flow across each adjacent segment of the membrane.
Saltatory conduction
Occurs along a myelinated axon. The action potential (nerve impulse) at the first node generates ionic currents in the cytosol and interstitial fluid that open voltage-gated Na+ channels at the second node, and so on at each subsequent node.
Compare the origin, types of channels, conduction, amplitude, duration, polarity, and refractory period between graded potentials and action potentials
What is the difference between axodendritic, axosomatic, and axoaxonic?
Axodendritic: synapses from axon to dendrite; most common.
Axosomatic: synapses from axon to cell body.
Axoaxonic: synapses from axon to axon.
Describe the steps in this image:
- A nerve impulse arrives at a synaptic end bulb of a presynaptic axon.
- The depolarizing phase of the nerve impulse opens voltage-gated Ca2+ channels, which are present in the membrane of synaptic end bulbs. Because calcium ions are more concentrated in the extracellular fluid, Ca2+ flows inward through the opened channels.
- An increase in the concentration of Ca2+ inside the presynaptic neuron serves as a signal that triggers exocytosis of the synaptic vesicles. As vesicle membranes merge with the plasma membrane, neurotransmitter molecules within the vesicles are released into the synaptic cleft. Each synaptic vesicle contains several thousand molecules of neurotransmitter.
- The neurotransmitter molecules diffuse across the synaptic cleft and bind to neurotransmitter receptors in the postsynaptic neuron’s plasma membrane.
- Binding of neurotransmitter molecules to their receptors on ligand-gated channels opens the channels and allows particular ions to flow across the membrane.
- As ions flow through the opened channels, the voltage across the membrane changes. This change in membrane voltage is a postsynaptic potential. Depending on which ions the channels admit, the postsynaptic potential may be a depolarization (excitation) or a hyperpolarization (inhibition) postsynaptic potential.
- When a depolarizing postsynaptic potential reaches threshold, it triggers an action potential in the axon of the postsynaptic neuron.
What are the two types of neurotransmitter receptors?
- Ionotropic receptors
- Metabotropic receptors
Ionotropic receptors
A type of neurotransmitter receptor that contains a neurotransmitter binding site and an ion channel.
Metabotropic receptors
A type of neurotransmitter receptor that contains a neurotransmitter binding site and is coupled to a separate ion channel by a G protein.
What is the difference between spatial summation and temporal summation?
Spatial summation: summation of postsynaptic potentials in response to stimuli that occur at different locations in the membrane of a postsynaptic cell at the same time.
Temporal summation: summation of postsynaptic potentials in response to stimuli that occur at the same location in the membrane of the postsynaptic cell but at different times.
Divergent circuit
In this type of circuit, the nerve impulse from a single presynaptic neuron causes the stimulation of increasing numbers of cells along the circuit.
Converging circuit
In this type of circuit, the postsynaptic neuron receives nerve impulses from several different sources.
Reverberating circuit
In this type of circuit, the incoming impulse stimulates the first neuron, which stimulates the second, which stimulates the third, and so on. Branches from later neurons synapse with earlier ones. This arrangement sends impulses back through the circuit again and again. Involved in breathing, coordinated muscular activities, waking up, and short-term memory.
Parallel after-discharge circuit
In this type of circuit, a single presynaptic cell stimulates a group of neurons, each of which synapses with a common postsynaptic cell. A differing number of synapses between the first and last neurons imposes varying synaptic delays, so that the last neuron exhibits multiple EPSPs or IPSPs. May be involved in precise activities such as mathematical calculations.
Can peripheral nerves that are damaged be repaired?
Axons and dendrites that are associated with a neurolemma may undergo repair if the cell body is intact, if the Schwann cells are functional, and if scar tissue formation does not occur too rapidly.
