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Chapter 13: The Nervous System- Neural Tissue

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Central Nervous System
Brain and spinal cord, which contain control centers for processing and integrating sensory information, planning and coordinating responses to stimuli, and providing short-term control over the activities of other systems. Consists of the central canal, ventricles, and cerebrospinal fluid.
Peripheral Nervous System
Neural tissue outside of the CNS whose function is to link the CNS with the sense organs and other systems. Provides sensory info to the CNS and carries motor commands from the CNS to peripheral tissues and systems. Divided into afferent and efferent.
Afferent Division
Brings sensory info to the CNS. Begins at receptors that monitors environment.
Efferent Division
Carries motor commands to muscles and glands. Begins inside the CNS and ends at a effector (muscle cell, gland cell, or other specialized cell.)
Somatic
Pertaining to the control of skeletal muscle activity or sensory info from skeletal muscles, tendons, and joints.
Visceral
Control of functions such as digestion, circulation, or sensory info from visceral (ventral) organs.
Somatic Nervous System
Controls skeletal muscle contraction (may be voluntary of involuntary)
Autonomic Nervous System
Visceral motor system, regulates smooth muscle, cardiac muscle, and glandular activity.
Neurons
basic functional unit of the nervous system; highly specialized cell, nerve cell. Responsible for the transfer and processing of info in the nervous system.
Structure of Neuron
Cell body attached to axon that ends at 1+ synaptic terminals
Communicates with another cell through synaptic terminal
Region around the nucleus is called the perikaryon
Branching dendrites (dendrite spines in CNS)
Neuroglia
Supporting cells that isolate neurons, provide a framework for the neural tissue, help maintain the intercellular environment and act as phagocytes.
Axon
Long slender cytoplasmic process of a neuron, capable of conducting nerve impulses (action potential)
Neuroglia of CNS (4 types)
1. Astrocytes
2. Oligodendrocytes
3. Microglia
4. Ependymal cells
Astrocytes
Largest and most numerous glial cell.
-Maintain blood-brain barrier
-Provide structural support
-Control interstitial environment (regulate ion, nutrient, and dissolved gas concentration)
-Repair damaged neural tissue
-Guide neuron development
Oligodendrocytes
Looks like an astrocyte but have a smaller cell body and fewer and shorter cytoplasmic processes.
-Myelinate CNS axons which improves speed of nerve impulse
-Structural support
(Myelinate- white matter; Unmyelinated- gray matter)
Microglia
Smallest of glial cells w/ slender cytoplasmic processes with many fine branches.
-Remove waste, cell debris, and pathogens by phagocytosis
Ependymal Cells
Line ventricles of brain and central canal of the spinal cord which are filled with cerebrospinal fluid
-cuboidal/columnar with slender processes that branch to make direct contact with glial cells in neural tissue
-produce, circulate, and monitor CSF
Neuroglia of PNS (2)
Clustered together in masses called ganglia
Axons are bundled and wrapped in connective tissue to form nerves. Insulate all cell bodies and axons in the PNS
Two types: Satellite and Schwann cells
Satellite Cells
-Surround neuron cell bodies in peripheral ganglia
-Regulate the exchange of nutrients and waste between neuron cell body & extracellular fluid.
-Isolate neuron from stimuli other than those provided at synapses
Schwann Cells
Cover every peripheral axon
Axolemma & Neurilemma
Axolemma: Plasmalemma of axon
Neurilemma: superficial cytoplasmic covering of axon provided by Schwann Cell
Gray and White Matter
Gray: Neural tissue dominated by neuron cell bodies
White: Neural tissure dominated by myelinated axons
Myelin
Membranous wrapping, produced by oligodendrocytes in CNS, coats axons and increases the speed of action potential propagation, composed mainly of phospholipids
Neuron Anatomy
Cell body contains a large, round nucleus w/ prominent nucleolus. Cytoplasm consists of perikaryon which contain neurofilaments and neurotubules. Bundles of neurofilaments are called neurofibrils which are cytoskeletal elements that extend into the dendrites and axon.
Dendrites
Stimulated by environmental changes of the activities of other cells.
Cell Body
Contains the nucleus, nucleolus, mitochondria, ribosomes in clusters called Nissl bodies or chromatophilic substance which accounts for gray matter made up of neuron cell bodies, and other organelles and inclusions.
Axon
Nerve fiber, long cytoplasmic process capable of conducting nerve impulse or action potential toward synaptic terminals.
Axon Hillock
In a multipolar neuron, a specialized region that connects the initial segment of the axon to the soma.
Axoplasm
Cytoplasm of axon that contains neurofibrils, neurotubles, numerous small vesicles, lysosomes, mitochondria, and enzymes.
Collaterals
Axon may branch along its length in side branches
Terminal Arborizations and Synaptic Terminals
Main trunk end in a series of terminal extensions which end in a synaptic terminal, where the neuron contacts another neuron or effector.
Synapse and Terminal Bouton
site where the neuron communicates with another cell. A terminal bouton is found where one neuron synapses on another.
Structural Classification of Neurons (4)
1. Axonic: small, can't tell axons from dendrites, only in CNS and special sense organs
2. Bipolar- fine dendrites fuse to form 1. Rare, important in relaying sensory info with smell, hearing, and sight. Unmyelinated axons
3. Pseudounipolar- Cell body lies off to 1 side, continuous axon and dentrites. Sensory neurons of PNS, myelinated axons
4. Multipolar: Several dendrites & single axon w/ 1+ branches. Most common type in CNS
Functional Classification of Neurons (3)
1. Sensory
2. Motor
3. Interneuron
Sensory Neuron
Pseudounipolar w/ bodies located outside the CNS
Form the afferent division of the PNS.
Collect info from external or internal environment & deliver info to the CNS.
Axons (called afferent fibers) extend between a sensory receptor and spinal cord or brain.
Somatic Sensory Neurons
transmit info about the outside world and our position within it
Visceral Sensory Neurons
Transmit info about internal condition and the status of other organs
Receptors (3)
1. Exteroreceptors: provide info about external environment in form of touch, temp, and pressure sensations and more complex special senses of sight, smell, and hearing
2. Proprioceptors: monitor position/movement of skeletal muscles and joints
3. Interoceptors: Monitor digestive, respiratory, cardio, urinary, & reproductive systems & provide sensations of deep pressure and pain as well as taste
Motor Neurons
Multipolar neurons that form the efferent division of nervous system. Stimulates the activity of a peripheral tissue, organ, or organ system. Axons traveling away from the CNS are called efferent fibers.
Visceral Motor Neurons
Innervate peripheral effectors other than skeletal muscles
1. Preganglionic: Axons extending from CNS to a ganglion
2. Postganglionic: Axons connecting ganglion cells w/ peripheral effectors
Interneurons
Situated between sensory and motor neurons, located entirely with the brain and spinal cord. Responsible for the analysis of sensory inputs and the coordination of motor outputs. Excitatory or inhibitory.
Nerve regeneration (in the PNS)
Limited ability to regenerate after injury.
PNS: only a small # of axons can reestablish normal synaptic contacts- permanently damaged
Nerve regeneration (in the CNS)
Complicated because: 1) many more axons are likely to be involved, 2) astrocytes produce scar tissue that can prevent axon growth 3) and astrocytes release chemicals that block regrowth of axons
Wallerian degeneration
Schwann cells participate in the repair of damaged peripheral nerves. Axon distal to injury deteriorates and macrophages eat the debris. Schwann cells in the area divide & form cellular cord that follows where axon was. Schwanns also release growth factors.
Excitability (Nerve Impulse)
Ability of plasmalemma to conduct electrical impulses.
Include- skeletal muscle fibers, cardiac muscle cells, gland cells, axolemma of most neurons.
Action Potential
Electrical impulse that develops after the plasmalemma is stimulated to a certain level, or threshold (-55mV).
Once threshold is reached...
Membrane permeability to sodium and potassium ions changes. Ion movements that result produce a sudden change in transmembrane potential which causes an action potential.
Nerve impulses
axons that action potential travels down in the nervous system
Rate of impulse is dependent on...
1. Presence or absence of myelin sheath. (5-7x faster than unmyelinated)
2. Diameter of axon (the larger that axon, the faster the impulse)
Synaptic Communication
May involve a synaptic terminal and:
1. Dendrite
2. Cell body
3. Axon
Neuroeffector junction
A synapse that permits communication between a neuron and another cell type. (ie. neuromuscular synapse)
Vesicular Synapses
Chemical Synapses. Neurotransmitter is released at presynaptic membrane of a terminal bouton and binds to receptor proteins on postsynaptic membrane which triggers a change in transmembrane potential of the receptive cell.
Acetylcholine
best known neurotransmitter and used by all somatic neuromuscular synapses as well as many vesicular synapses in CNS and PNS
1st and 2nd steps at vesicular synapse
1. Arrival at action potential at terminal bouton triggers release of neurotransmitter from secretory vesicles, through exocytosis at presynaptic membrane
2. Neurotransmitter diffuses across synaptic cleft & binds to receptors on postsynaptic membrane
3rd step at vesicular synapse
Receptor binding results in change in permeability of postsynaptic membrane. Result may be excitatory or inhibitory. Excitatory effects promote the generation of axon potentials & Inhibitory reduces ability to generate action potential.
4th step at vesicular synapse
If excitement is sufficient, receptor binding may lead to the generation of action potential in axon (if postsynaptic cell is a neuron) or sarcolemma (if the postsynaptic cell is skeletal muscle fiber)
5th step at vesicular synapse
Effects of action potential are short lived because the neurotransmitter is either broken down by enzymes or reabsorbed. Additional action potentials must arrive at synaptic terminal and more ACh must be released in synaptic cleft.
Nonvesicular synapse
Electrical synapses. Presynaptic and postsynaptic membrane are tightly bound. Communicating gap junctions permit passage of ions between the cells. Function as it the 2 share a single membrane- therefore they can cross from one neuron to the next quickly.