Chapter 12: Nervous Tissue Flashcards
describe the organization of the nervous system.
I. organization of the nervous system: 3% total body weight; two main
subdivisions: CNS and PNS
II. central nervous system (CNS) – brain and spinal cord; processes incoming sensory information; is also source of thoughts, emotions, memories; most signals that stimulate muscle contractions and gland secretions originate in CNS.
a. brain – located in skull, contains about 85 billion neurons
b. spinal cord – connected to brain through foramen magnum of the
occipital bone, encircled by vertebrae.
III. peripheral nervous system (PNS) – the part of the nervous system that lies outside the CNS, consisting of nerves and ganglia. Divided into SENSORY and MOTOR Somatic nervous system (voluntary) (SNS), MOTOR (INV) autonomic nervous system - SYMPATHETIC and PARASYMPATHETIC (ANS) and enteric nervous system (ENS) - visceral/sensory
describe the three basic functions of the nervous system.
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Sensory Function Sensory receptors detect internal stimuli, such
as an increase in blood pressure, or external stimuli (for example, a raindrop landing on your arm). This sensory information is then carried into the brain and spinal cord through cranial and spinal nerves. - Integrative Function The nervous system processes sensory in-formation 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
Have electrical excitability and ability to respond to stimulus and convert to action potential
Parts of a Neuron
a) Dendrites are the receiving or input portions of a neuron. The plasma membranes of dendrites (and cell bodies) con-tain numerous receptor sites for binding chemical messengers from other cells. Dendrites usually are short, tapering, and highly branched. In many neurons the dendrites form a tree-shaped array of processes extending from the cell body. Their cytoplasm contains Nissl bodies, mitochondria, and other organelles.
Nissl Bodies prominent clusters of rough ER that produce proteins
which are used to replace cellular components, as material growth of
neuron, and to regenerate damaged axons in the PNS.
b) Cell Bodies a nucleus surrounded by cytoplasm that includes typical cellular organelles such as lysosomes, mitochondria, and a Golgi complex.
c) Axons a neuron propagates nerve impulses toward another neuron, a muscle fiber, or a gland cell. An axon is a long, thin, cylindrical projection that oft enjoins to the cell body at a cone-shaped elevation called the axon hillock
Axon Hillock cone-shaped elevation that joins the axon to the cell body
initial segment – part of the axon closest to the axon hillock
- trigger zone* – in most neurons, the junction of the axon hillock and the initial segment, where nerve impulses arise.
- axon collateral* – along the length of an axon, side branches typically at a 90-degree angle to the axon.
- axon terminal* – many fine terminal branches of an axon where synaptic vesicles undergo exocytosis to release neurotransmitter molecules. AKA axon telodendria
Synapse site of communication between two neurons or between a
neuron and an effector cell. Maybe be electrical or chemical.
synaptic end bulb – expanded distal end of an axon terminal that
contains synaptic vesicles. AKA synaptic knob.
3 structural classifications of neurons / 3 functional classifications neurons
STRUCTURAL
*Purkinje cells - exculsivly found in cerbellum
multipolar neuron – usually have several dendrites and one axon. Most brain and spinal cord and all motor neurons are this type.
bipolar neuron – have one main dendrite and one axon. Found in the retina of the eye, inner ear, and the olfactory area of the brain.
unipolar neuron – have dendrites and one axon that are fused together to form a continuous process that emerges from the cell body. More appropriately named pseudounipolar neurons because they begin in the embryo as bipolar neurons but dendrites and axon fuse together during development. Mostunipolar dendrites function as sensory receptors that detect a sensory stimulus
ex. Touch, pressure, pain, temperature. The trigger zone is at the junction of the
dendrites and axon.
FUNCTIONAL
Sensory – afferent neurons. Either contain sensory receptors at their distal ends (dendrites) or are located just after sensory receptors that are separate cells. Most sensory neurons are unipolar in structure. Once a stimulus activates a sensory receptor, the sensory neuron forms an action potential in its axon and conveys it into the CNS through cranial or spinal nerves.
Motor – efferent neurons. Convey action potentials away from the CNS to effectors (muscles and glands) in the PNS through cranial or spinal nerves. Motor neurons are multipolar in structure.
Interneurons – association neurons. 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. Multipolar in structure.
Neuorglia
cells of the nervous system that perform various supportive functions.
About half the volume of the CNS. Generally smaller than neurons. 5-25x more numerus. 6 different types of neuroglia cells, described next. 4 found only in CNS: astrocytes, oligodendrocytes, microglia, ependymal cells. Neuroglia can multiply and divide in the mature nervous system. Fill in spaces formerly occupied by neurons in case of injury or disease. Glia cancers: gliomas, highly malignant, fast growers.
Neuroglia in CNS
Types: 1 )astrocytes, 2) oligodendrocytes, 3) microglial cells, and 4) ependymal cells
- Astrocytes – star shaped cells with many processes. Largest and most numerus of the neuroglia. Two main types of astrocytes: protoplasmic (many short branching processes and found in gray matter) and fibrous astrocytes (many long unbranched processes and located mainly in white matter. Astrocyte processes contact blood capillaries, neurons, and the pia mater (thin membrane around brain and spinal cord)
Functions:
- Supporting neurons – contain microfilaments giving them considerable strength
- Create blood-brain barrier restricting movement of substances between blood and interstitial fluid of the CNS. Done by processes wrapping around blood capillaries and isolating the neurons by secreting chemicals that maintain the unique selective permeability.
- In the embryo, astrocytes 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. Regulate the concentration of ions such as K+, take up excess neurotransmitters, and serve as a conduit for the passage of nutrients and other substances between blood capillaries and neurons.
- May also play a role in learning and memory by influencing the formation of neural synapses.
2. Oligodendrocytes resemble astrocytes but are smaller and have less processes. The processes of oligodendrocytes are responsible for forming and maintaining the myelin sheath around CNS axons.
myelin sheath – multilayered lipid and protein covering around some axons, formed by Schwann and oligodendrocytes, that insulates them and increases the speed of nerve impulse connection.
3. Microglia – AKA microglial cells Small cells with slender processes that give off numerous spine-like projections. Function as phagocytes. Remove cellular debris formed during normal development of the nervous system and phagocytize microbes and damaged nervous tissue.
4. ependymal ells cuboidal to columnar cells, single layer, possess microvilli and cilia. Line the ventricles of the brain and central canal of the spinal cord. Produce, possibly monitor, and assist in circulation of cerebrospinal fluid. Also for the blood-cerebrospinal fluid barrier.
Neuroglia in the PNS
Schwann cells and satellite cells
1. Schwann cells – in PNS. Encircle PNS axons. Form the myelin sheath around axons. Each Schwann cell myelinates a single axon (oligodendrocytes myelinate several axons). A single Schwann cell can also enclose as many as 20 or more unmyelinated axons. Schwann cells participate in axon regeneration which is more easily accomplished in the PNS than in the CNS.
2. Satellite Cells flat cells that surround the cell bodies of neurons of PNS ganglia. Provide structural support and regulate with exchange of materials between neuronal cell bodies and interstitial fluid.
distinguish between gray matter and white matter.
White Matter - myelinated axons
myelinated and unmyelinated – myelinated = axon surrounded by multilayered lipid and protein covering. The sheath electrically insulates the axon of a neuron and increases nerve impulse conduction speed.
nodes of Ranvier – Gaps in myelin sheath. Appear at intervals along the axon. Each Schwann cell wraps one axon segment between two nodes of Ranvier.
Gray Matter - neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglial cells.
Unmyelinated = no covering. 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 in both
- spinal cord - white mater surrounds the inner gray core
describe the cellular properties that permit communication among neurons and effectors.
Communicate using Graded potentials and Action Potentials.
Graded Potentials short distance
Action Potentials long distances
compare the basic types of ion channels and explain how they relate to graded potentials and action potentials.
- electrochemical gradient concentration difference plus an electrical difference. Ions move from higher concentration to lower concentration(chemical part of the gradient) and negatively charged anions move toward a positively charged area (electrical part of the gradient).
- leak channel gates randomly alternate between open and closed positions. Typically, plasma membranes have far more K+ leak channels than Na+ leak channels, and the K+ leak channels are leakier than the Na+ leak channels. Therefore, the membranes permeability to K+ is much higher than Na+. Leak channels are found in nearly all cells, incl. dendrites, cell bodies, and axons of all neurons.
- ligand-gated ion channel opens and closes in response to the binding of a ligand (chemical) stimulus. A wide variety of ligands can open or close ligand-gated channels. Ex. Neurotransmitters, hormones, ions. Ex. Ach opens cation channels that allow Na+ and Ca+ to flow in and K+ to flow out. Located in the dendrites of some sensory neurons, such as pain receptors, and in dendrites and cell bodies of interneurons and motor neurons.
- mechanically-gated ion channel opens or closes in response to mechanical stimulation in the form of vibration (such as sound waves), touch, pressure, or tissue stretching. The force distorts the channel from resting position, opening the gate. Ex: in auditory receptors in ears, receptors that monitor GI tract stretching, and touch and pressure receptors in skin.
- voltage-gated ion channel opens in response to a change in the membrane potential. Participate in the generation and conduction of action potentials in the axons of all types of neurons.
describe the factors that maintain a resting membrane potential.
resting membrane potential exists because of a small buildup of negative ions in the cytosol along the inside of the membrane, and an equal buildup of positive ions in the ECF outside the membrane. The separation of + and – charge is a form of potential energy, measured in volts or millivolts. The greater the difference in charge across the membrane, the larger the membrane potential (voltage). The buildup in charge occurs only very close to the membrane.
Factors that contribute
I. Unequal distribution of ions in the ECF and cytosol – ECF is rich in Na+ and Clions, however in cytosol, the main cation is K+ and the two dominant anions are phosphates attached to molecules and amino acids in proteins. The plasma membrane has more K+ leak channels than Na+ leak channels, the number of K+ that diffuse down their concentration gradient out of the cell and into the ECF is greater than the number of Na+ that diffuse down concentration gradient into
the cytosol. As more and more K+ ions exit, the inside of the membrane becomes increasingly negative, and the outside of the membrane becomes increasingly positive.
II. Inability of most anions to leave the cell – most anions inside the cell are not free to leave. They cannot follow the K+ out of the cell because they are attached to non-diffusible molecules such as ATP and large proteins.
III. Electrogenic nature of the Na+ – K+ ATPases – membrane permeability to Na+ is very low because there are only a few Na+ leak channels. But Na+ do slowly diffuse inward, down concentration gradient. The inward Na+ and outward K+ leaks are offset by Na+-K+ pumps. These pumps help maintain the resting membrane potential by pumping out Na+ as fast as it leaks in, bringing in K+ at the same time. The pumps expel 3 Na+ for every 2 K+ imported so the pumps
are electrogenic, meaning they contribute to the negativity of the resting membrane potential. Their contribution is very small though, only -3mV of -70mV of a typical neuron.
describe how a graded potential is generated.
graded potential –
Short distances, occur in cell body and dendrite
a 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. Graded potentials occur mainly in the dendrites and cell body of a neuron, because typically, mech-gated and ligandgated channels are present in the dendrites of sensory neurons, and ligand-gated
channels are numerous in the dendrites and cell bodies of interneurons and motor neurons. Graded = vary in amplitude, depending on strength of the stimulus. Larger or smaller depending on how many channels have opened or closed and how long each
remains open. Useful for short distance communication only, because they die out within a few mm of origin point.
I. hyperpolarizing graded potential – when the response makes the membrane more polarized (inside more negative)
II. depolarizing graded potential – when the response makes the membrane less polarized (inside less negative)
III. decremental conduction – mode of travel by which graded potentials die out as they spread along the membrane. The opening and closing of channels alters the flow of ions across the membrane, and produces a flow of current that is localized. The current spreads to adjacent regions along the plasma membrane
in either direction from the stimulus source for a short distance and then gradually dies out as charges are lost across the membrane through leak channels.
IV. summation – the process by which graded potentials add together. If two depolarizing graded potentials summate = larger depolarizing graded potential. Two hyperpolarizing graded potentials = larger hyperpolarizing graded potential. One + and one - = cancelled out and overall graded potential disappears.
describe the phases of an action potential.
VOLTAGE GATED NA+ CHANNELS
Action Potential - rapid change in membrane potential involves depolarization followed by repolarization
Resting State
Leakage channels open and all gated channels closed - inactivation gate is open and activation gate is closed
Depolarization
graded potential or some other stimulus voltage gated Na+ channels open rapidly. Na+ rapidly moves into the cell and depolarize membrane (Na K pump can fix this later)
Repolarizing Phase
Na+ gates close and K+ Gates open (outflow). Slowing of Na inflow and accelerating K outflow. Membrane potential changes from +30 to - 70. Repolarization puts Na gates to resting state.
After-Hyperpolarization State
Membrane potential becomes even more negative (-90), K channels close back to -70
Refractory Period
Excitable cell cannot generate another AP. ABSOLUTE RP - even a large stimulus can’t
discuss how action potentials are propagated.
Depends on Positive Feedback.
light touch vs heavy touch frequency of impulses sensory centre
I. Available only in one direction, as the area it just passed is temporarily in the absolute refractory period and cannot generate another action potential.
II. Depends on positive feedback, as discussed above: as Na+ ions flow in, they cause voltage-gated Na+ channels in adjacent sections of the membrane to
continuous conduction – step by step depolarization and repolarization of each adjacent segment of the plasma membrane.
I. Occurs in unmyelinated axons and in muscle fibers
saltatory conduction – the special mode of action potential propagation that occurs along myelinated axons
I. occurs because of the uneven distribution of voltage-gated channels.
II. Few voltage-gated channels are present in regions where a myelin sheath covers the axolemma.
III. At the nodes of Ranvier, the axolemma has many channels.
IV. Current carried by Na+ and K+ flows across the membrane mainly at the nodes
V. Action potential appears to leap from node to node. Travels much faster than in unmyelinated axon.
VI. More energy efficient as a smaller number of channels, only at the nodes, are opened. Therefore less ATP is used by Na+-K+ pumps.
Factors that affect speed propagation:
- *Amount of myelination** – action potentials propagate more rapidly along myelinated axons than along unmyelinated axons.
- *Axon diameter** – larger diameter axons propagate action potentials faster than smaller ones, due to their larger surface area.
- *Temperature** – axons propagate action potentials fast when warm, slower when cooled.
explain the events of signal transmission at electrical and chemical synapses.
ELECTRICAL SYNAPSE
gap junctions
Faster communication
Synchronization
CHEMICAL SYNAPSE
plasma membranes don’t touch - neurotransmitters
transmission of signals at a chemical synapse—7 steps
a. Nerve impulse arrives at a synaptic end bulb (or at a varicosity) of a presynaptic axon.
b. The depolarizaing phase of the nerve impulse opens voltage-gated Ca2+ channels, present in the membrane of synaptic end bulbs. Ca2+ ions are more concentrated in the ECF so Ca2+ flows inward through the opened channels.
c. Ca2+ concentration increase inside the presynaptic neuron serves as a signal that triggers exocytosis of the synaptic vesicles. Neurotransmitter molecules within the vesicles are released into the synaptic cleft as vesicles merge with the plasma membrane. Each vesicle contains 1000’s of neurotransmitter molecules
d. The neurotransmitter molecules diffuse across the synaptic cleft and bind to neurotransmitter receptors (ionotropic or metabotropic) in the postsynaptic neuron’s plasma membrane.
e. Binding of neurotransmitter molecules to their receptors on ligandgated channels opens the channels and allows particular ions to flow across the membrane.
f. Voltage across the membrane changes as ions flow through the opened channel. This change in membrane voltage is the postsynaptic potential. Depending on which ions the channels admit, the postsynaptic potentialmay be a depolarization (excitation) or a hyperpolarization (inhibition). Ex. Inflow of Na+ causes depolarization, but Cl- inflow or K+ outflow causes hyperpolarization because inside of cell becomes more negative.
g. When a depolarization postsynaptic potential reaches threshold, it triggers an action potential in the axon of the postsynaptic neuron.
i. At most chemical synapses, only one-way information transfer
can occur. From presynaptic neuron to postsynaptic neuron or
an effector. Only synaptic end bulbs of presynaptic neurons can
release neurotransmitter, and only postsynaptic neuron’s
membrane has receptor proteins that can recognize and bind
that neurotransmitter.
