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.

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