Chapter 12: Nervous Tissue Flashcards
Central Nervous System
Consists of the brain and spinal cord.
Brain contains 85 billion neurons.
Spinal cord contains 100 million neurons.
CNS process many different kinds of incoming sensory information.
Source of thoughts, emotions and memories.
Peripheral Nervous System
Consists of all nervous tissue outside the CNS.
Components: nerves and sensory receptors.
Divided into: sensory and motor divisions.
Nerve
A bundle of 100 to 10000 of axons plus associated CT, blood vessel that lies outside the brain and spinal cord.
Cranial Nerves
PNS
12 pairs.
Emerges from the brain.
Spinal Nerves
PNS
31 pairs.
Emerge from the spinal cord.
Each nerve follows a defined path and serves a specific region of the body.
Sensory Receptors
Refers to a structure of the nervous system
Monitors changes in the external or internal environment.
Examples: touch receptors in skin, photoreceptor in the eye, olfactory receptors in the nose.
Sensory Division
Or afferent
PNS
Converts input into the CNS from sensory receptors in the body.
Provides CNS with sensory information about somatic senses.
Somatic Senses
Tactile, thermal, pain and proprioceptive sensations
Motor Division
Or efferent
PNS
Conveys output from the CNS to effectors (muscles and glands).
Division subdivided into:
1. somatic nervous system
2. autonomic nervous system.
Somatic Nervous System
Conveys output from the CNS to skeletal muscles only.
This part of PNS is voluntary due to motor responses are consciously controlled.
Autonomic Nervous System
Conveys output from the CNS to smooth and cardiac muscles and glands.
Involuntary because its motor responses are not under conscious control.
Contains two main branches:
1. sympathetic nervous system
2. parasympathetic nervous system.
Sympathetic Nervous System
Branch of ANS
Helps support exercise or emergency actions: fight or flight
ie: increases heart rate
Parasympathetic Nervous System
Branch of ANS
Takes care of rest and digest activities.
Ie: slows down heart rate
Enteric Nervous System
Third branch of ANS
Can function independently.
Communicates and is regulated by the other branches of ANS.
Extensive network of over 100 million neurons, confided to the wall of the GI tract.
Helps regulate the activity of the smooth muscle and glands of the GI tract.
3 Basic Functions of Nervous System
- Sensory (input): detect internal or external stimuli. Carried to brain and spinal cord by cranial/spinal nerves
- Integrative (process): processes sensory information by analyzing it and making decisions for appropriate responses, known as integration.
- Motor (output): once sensory information is integrated, motor responses are activated by effectors (muscle and glands) through cranial/spinal nerves.
Integration
Activity performed by integrative (process) function
Making decision for appropriate responses.
Neurons
Nerve cells that process electrical excitability
Electrically Excitability
The ability to respond to a stimulus and convert it into an action potential.
Stimulus
Any change in the environment that is strong enough to initiate an action potential.
Action Potential
Nerve impulse that is an electrical signal that propagates (travels) along the surface of the membrane of a neuron.
Parts of Neuron
Contains 3 parts:
1. Cell body
2. dendrites
3. Axon
Cell Body
Known as perikaryon or soma
Contains a nucleus surrounded by cytoplasm.
Includes cellular organelles.
Nissl Bodies
Prominent cluster of rough endoplasmic reticulum.
Found in the soma of a neuron.
Produces newly synthesized proteins used to replace cellular comments, material for growth of neurons and regenerates damaged axons in the PNS.
Neurofibrils
Found in cytoskeleton
Composed of bundles of intermediate filaments that provide cell shape and support.
Microtubules
Found in cytoskeleton
Assist in moving material between the cell body and axon.
Lipofuscin
Product of neuronal lysosomes
accumulates as neurons age.
A pigment, occurs as clumps of yellowish brown granules in the cytoplasm.
Ganglion
Collection of neuron cell bodies located in the PNS.
Nerve Fiber
General term for any neuronal process that emerges from the cell body of a neuron.
Dendrites
Receiving or input portions of a neuron.
Contain numerous receptor sites for binding chemical messengers from other cells.
If damaged it would affect ability to: trigger AP, release neurotransmitters, transmit messages to other neurons.
Axon
Single neuron
propagates nerve impulses (AP) toward another neuron, a muscle fiber, or a gland cell.
Axon Hillock
Cone-shaped elevation where axons join to the cell body.
Initial Segment
Part of the axon closest to the axon hillock.
Trigger Zone
Area of CNS.
Area where nerve impulses arise at the junction of the axon hillock and initial segment.
If damaged causes failure to propagate an action potential in the axon.
Axoplasm
Cytoplasm of axon
Axolemma
Plasma membrane of axon, surrounds axoplasm.
Axon Collaterals
Side branches that are along the length of the axon. May branch off at a right angle to the axon.
Axon Terminals
Or axon telodendria
Fine processes formed when the axon and its collaterals end by dividing.
Synapse
Site of communication between 2 neurons or between a neuron and an effector cell.
Synaptic End Bulbs
Bulb shaped structures formed when tips of axon terminals swell.
Varicosities
String of swollen bumps found of axon terminals
Synaptic Vesicles
Tiny membrane enclosed sacs, stores neurotransmitters
Found in both synaptic end bulb and varicosities
Neurotransmitter
Chemical stored by synaptic vesicles.
When released excites or inhibits another neuron, muscle fiber or gland cell.
Slow Axonal Transport
Slower transport systems
conveys axoplasm in one direction only: from the cell body toward the axon terminals.
Moves materials about 1-5 mm per day.
Fast Axonal Transport
Transport system that is capable of moving materials a distance of 200-400 mm per day.
Uses proteins that function as motors to move material along the surfaces of microtubules of the neurons cytoskeleton.
Moves material in both directions: always from and towards the cell body.
Anterograde
Fast axonal transport that moves in a foreword direction.
Moves organelles and synaptic vesicles from the cell body into the axon terminals.
Retrograde
Fast axonal transport that occurs in backwards direction .
Moves membrane vesicles and other cellular materials from the axon terminals to the cell body to be degraded or recycled.
Structural Diversity in Neurons
- Neurons display great diversity in size and shape.
- Cell bodies can range from small to large in diameter from 5 um - 135 um.
- Axons can be almost as long as a person is tall.
Structural Classification of Neurons
Classified according to the number of processes extending from the cell body.
1. Multipolar neurons: have several dendrites and one axon. Found in brain, spinal cord and motor neurons.
2. Bipolar Neurons: have one main dendrite and one axon. Found in retina of the eye, inner way and olfactory area.
3. Unipolar Neurons: Have dendrites and one axon that are fused together to form a continues process that emerges from cell body.
Pseudounipolar Neurons
Unipolar neurons being in the embryo as bipolar neurons.
During development the dendrites and axon fuse together and become a single process.
Sensory Receptors
Function of dendrites of most unipolar neurons. Detect sensory stimulus such as touch, pressure, pain or thermal stimuli.
Purkinje Cells
Cell found in the cerebellum
Neurons named by histologist who first described them or for an aspect for their shape and appearance.
Pyramidal Cells
Found in the cerebral cortex of the brain. Have pyramid shaped cell bodies.
Functional Classification of Neurons
Classified according to the direction in which the nerve impulse is conveyed with respect to the CNS.
1. Sensory Neurons or afferent: Forms action potential in its axon and is conveyed into the CNS through cranial or spinal nerves. Unipolar polar structure.
2. Motor Neurons or efferent: convey action potentials away from the CNS to effectors in the PNS through cranial or spinal nerves. Multipolar in structure.
3. Interneurons or association: mainly located within the CNS between sensory and motor neurons. Multipolar in structure.
Neuroglia of CNS
Make up about half the volume of CNS.
Consider as the glue that hold nervous tissue together.
Can be classified on basis of size, cytoplasmic processes and intracellular organization.
Glia
Do not generate or propagate action potentials. Can multiply and divide in mature nervous system.
Gliomas
Brain tumors derived from glia. Tend to be malignant and grow rapidly.
Astrocytes
Star shaped cell that have many processes
The largest and most numerous Neuroglia of CNS.
Processes make contact with blood capillaries, neurons and pia matter.
2 types:
1. Protoplasmic: many short branching processes, found in gray matter.
2. Fibrous: many long unbranching processes, located mainly in white matter.
Oligodendrocytes
Resemble astrocytes but are smaller, and contain fewer processes.
Processes form and maintain the myelin sheath around CNS axons.
Myelin Sheath
Multilayered lipid and protein covering around some axons.
Insulates them and increases the speed of nerve impulse conduction.
Microglial Cells
Or microglia of CNS
Small Neuroglia cells
Slender processes that give off numerous spine like projections.
Function as phagocytes: removes cellular debris
Ependymal Cells
Cuboidal to columnar cells of CNS arranged in a single layer
possess microvilli and cilia.
Line ventricles (spaces filled with cerebrospinal fluid) of the brain and central canal of the spinal cord.
Produce and assist in the circulation of cerebrospinal fluid.
Neuroglia of PNS
Completely surround axons and cell bodies. Two types of glial cells are: schwann cells and satellite cells.
Schwann Cells
Cells that encircle PNS axons.
Form myelin sheath around axons. Each cell myelinated a single axon.
Can also enclose 20 or more unmyelinated axons.
Participate in axon regeneration (easier in PNS than CNS).
If damaged causes slowed action potential transmission in PNS neurons.
Satellite Cells
Flat cells, surround cell bodies of neurons of PNS ganglia.
Provide structural support, regulates exchange of materials between neuronal cell bodies and interstitial fluid.
Myelinated
Axons that are surrounded by a myelin sheath.
Sheath insulates the axon and increase speed of nerve impulses.
Unmyelinated
Axons that are not covered by a myelinated sheath.
Neuroglia that Produce Myelin Sheath
- Oligodendrocytes: in CNS
- Schwann Cells: in PNS. From sheath around axon during fetal development.
Neurolemma
Outer uncleared cytoplasmic layer of a Schwann cell, encloses the myelin sheath.
Only found around axons in the PNS.
Aids in regeneration by forming a tube that guides and stimulates regrowth of the axon.
Nodes of Ranvier
Gaps in the myelin sheath, appear at intervals along the axon.
Each Schwann cell wraps one axon segment between two nodes.
Nucleus
Is a cluster of neuronal cell bodies located in the CNS.
Tract
Is a bundle of axons that is located in the CNS. Tracts interconnect neurons in the spinal cord and the brain.
White Matter
Composed of primarily of myelinated axons. The whitish color of myelin gives white matter its name.
Gray Matter
Contains neuronal cell bodies, dendrites, unmyelinated axons, axon terminals and Neuroglia.
It appears grayish because Nissl bodies impart a gray color and there is little or no myelin in this area.
Electrical Signals in Neurons
Neurons are electrically excitable. They communicate with one another using 2 types of electrical signals
1. Graded potentials: used for short distance communication
2. Action potentials: allow for communication over long distances know as a nerve action potential.
Cerebral Cortex
Outer part of the brain
Upper Motor Neuron
Type of motor neuron that synapses with a lower motor neuron father down in the CNS in order to contact a skeletal muscle.
Lower Motor Neuron
A type of motor neuron that directly supples skeletal muscle fibers.
Membrane Potential
Exhibited by plasma membrane of excitable cells.
A electrical potential difference (voltage) across the membrane.
Current
Flow of charged particles
Leak Channels
Gated Chanel’s that randomly open and close.
Found in all cells
Includes: dendrites, cell bodies and axons of all neurons.
PM is more permeable to K+ then Na+
Ion Channels
When open allow specific ions to move across the PM.
They open and close due to a presence of gates.
The gate is part of the channel protein that can seal the pore or move aside to open the pore.
4 types: leak channels, ligand-gated, mechanically gated and voltage gated.
Ligand-gated Channels
Gated channels that open in response to binding of ligand chemical stimulus.
Ligand examples: hormones, neurotransmitters, particular ions.
Located in:
dendrites of some sensory neurons
Dendrites and cell bodies of interneurons and motor neurons.
Mechanically Gated Channels
Gated channels that open in response to me stimulus such as touch, pressure, vibration or tissue stretching. The force distorts the channel from its resting position, opening the gate.
Located in: dendrites of sensory neurons
Voltage-Gated Channel
Opens in response to change in membrane potential (voltage).
Located in: axons of all types of neurons.
Resting Membrane Potential
Exists due to a small build up of negative ions in the cytosol along the inside of the membrane and an equal build up of positive ions in extracellular fluid along the outside surface of the membrane.
It is an electrical potential (voltage) difference that exists across the PM of an excitable cell under resting conditions.
Resting Potential Range
Ranges from -40 to -90mV.
Typical value is -70 mV
The - indicates the inside of the cell is negative relative to the outside.
Polarized
A cell that exhibits a membrane potential. Most cells in the body are polarized.
Resting Potential Arises from 3 Major Factors
- Unequal distribution of ions in the Cytosol: PM has more K+ leak channels then Na+ leak channels. The number of K+ that leave the cell is greater than Na+ that enter the cell.
- Inability of most ions to leave the cell: Trapped ions cant follow K+ out of the cell because they are attached to non diffusion molecules such as ATP and large proteins.
3.Electrogenic nature of the Na+-K+ ATPases: The electrogenic Na+-K+ ATPase expels 3 Na+ ions for every 2 K+ ions imported.
Graded Potentials
Occurs in the cell body and dendrites. The amplitude depends on the stimulus strength.
Small deviation from the resting membrane potential that makes the membrane either more polarized (inside more -) or less polarized (inside less -).
Are small changed in membrane potential that vary in size and are brought about by external stimuli or neurotransmitter released at synapses.
Hyperpolarizing Graded Potential
When the membrane is more polarized
Depolarizing Graded Potential
When the membrane is less polarized (inside less negative)
Decremental Conduction
The mode of travel by which graded potential die as they spread along the membrane. They die out within only a few mm of their point of origin.
Summation
The process by which graded potential add together.
Net summation: equal amounts of EPSP and IPSP the cell will not depolarize or hyper-polarized so no action potential will generate.
Two types of summation:
1. Spatial: occurs when different presynaptic cells conduct Postsynaptic potentials at different locations at the membrane at different times.
2. Temporal: occurs when Postsynaptic potential occurs at the same location of the membrane at different times.
Action Potential
Or impulse
Is a sequence of rapidly occurring events that decreased and revert the membrane potential and eventually restore to its resting state.
2 main phases:
depolarizing phase
repolarizing phase.
Depolarize Phase of AP
During this phase the negative membrane potential becomes less negative, reaches zero and then becomes positive.
Here voltage gated Na+ channels open rapidly causes depolarizing phase of AP.
Repolarizing Phase of AP
During this phase the membrane potential is restored to the resting state of -70 mV.
After-Hyperpolarizing Phase of AP
This phase occurs following repolarizing phase.
The membrane potential temporally becomes more negative than the resting level.
Threshold for AP
Action potential will occur when the membrane of the axon depolarizes and reaches about -55 mV in many neurons.
Subthreshold Stimulus of AP
A weak depolarization that cannot bring the membrane potential to its threshold.
An action potential will not occur in this response.
Threshold Stimulus of AP
A stimulus that is just strong enough to depolarize the membrane threshold. An action potential will occur with this response.
Suprathreshold Stimulus of AP
A stimulus that is strong enough to depolarize the membrane above threshold. Several action potentials will form with this response.
All or None Principle of AP
An action potential either occurs completely or it does not occur at all.
Example: Principle of action potential where a depolarization reaches threshold an action potential occurs.
If it doesn’t reach depolarization an action potential does not occurs.
Voltage gated Na+ in Depolarizing Phase
Each voltage gated Na+ has 2 separate channels
1. Activation gate
2. Inactivation gate
Refractory Period of AP
The period of time after an AP begins during which an excitable cell cannot generate another AP in response to a normal threshold stimulus.
Absolute Refractory Period
A very strong stimulus occurs but cannot initiate a second AP.
Relative Refractory Period of AP
Period of time during which a second AP can be initiated but only by a larger than normal stimulus.
Propagation of AP
Mode of conduction where the AP keeps its strength as it spreads along the membrane. An AP is not decremental (does not die out).
This type of conduction depends on a positive feedback.
Factors that Speed up Propagation
3 major factors:
1. Amount of myelination: AP propagate more rapidly along myelinated axons
2. Axon Diameter: larger diameter axons propagate AP faster than smaller ones due to large surface area.
3. Temperature: axons propagate AP at lower speeds when cooled.
Two types of Propagation
1.Continuous Conduction: involves step by step depolarization and depolarization of each adjacent segment of the PM. Ions flow through their violate gated channels. Occurs in : unmyelinated axons and muscle fiber.
2. Saltatory Conduction: Specical mode of AP where propagation occurs along myelinated axons. Occurs because of uneven distribution of voltage gated channels
Classification of Nerve Fibers (Axons)
Classified into 3 major groups based on the amount of myelination, diameter and propagation speed.
1. A fibers: largest diameter, myelinated. Brief refractory period and conduction NAP (at 12-130 m/sec) that conduct impulses to skeletal muscles. Associated with touch, pressure, pain sensations.
2. B fibers: medium diameter of 2-3 um, myelinated. Exhibit saltatory conduction (15 m/sec). Longer refractory period. Conduct impulses from viscera to brain and spinal cord.
3. C fibers: smallest diameter 0.5-1.5 m/sec, unmyelinated. Propagation (0.5-2m/sec). Conduct for pain, touch, pressure,heat and cold.
Presynaptic Neuron
Refers to a nerve cell that carries a nerve impulse towards a synapse.
Cell that sends the signal.
Postsynaptic Cell
Is the cell that received a signal.
Postsynaptic Neuron
Nerve cell that carries a nerve impulse away from a synapse.
Effector Cell
Responds to the impulse at the synapse
Synapses Between Neurons
Synapses my be electrical rot chemical band differ both structurally and functionally.
Axodendritic: from axon to dendrites
Axosomatic: from axon to cell body
Axoaxonic: from axon to axon
Electrical Synapses
Contain gap junctions where AP travels making this a faster synapse.
Chemical Synapses
Contain synaptic cleft filled with interstitial fluid. Nerve impulses cannot cross the cleft so an indirect form of communication occurs in a form of neurotransmitter that diffuses across the fluid.
Neurosecretory Cells
Contained in the brain, neurons that secrete hormones.
Neurotransmitter Classes
- Small Molecule Neurotransmitters (SMNT)
Include: acetylcholine, amino acids, biogenetic amines, ATP/other purines, nitric oxide, carbon monoxide - Neuropeptides (NP)
consists of 3-40 amino acids linked by peptide bonds.
Acetylcholine
SMNT
Best studied neurotransmitter, release by many PNS neurons and some CNS neurons.
ACh is an:
Excitatory neurotransmitter at synapses such as neuromuscular junction: opens cation channels
Inhibitory neurotransmitter at other synapses: opens K+ channels
Acetylchinesterase enzyme inactivates ACh
Amino Acids
SMNT
1. Glutamate and aspartate: Neurotransmitters in the CNS that have power excitatory effects.
Most excitatory neurons and half the synapse in the brain communicate via Glutamate
2. Gamma-aminobutyric (GABA) and Glycine: important inhibitory neurotransmitter
Biogenic Amines
SMNT
Aminos acids that are modified and decarboxylated
Norepinephrine: plays a role in arousal (awakening from deep sleep). Is a hormone released in blood by adrenal gland.
Epinephrine: also a hormone released in blood by adrenal gland. Regulates visceral functions.
Dopamine: become active during emotional responses,m addictive behaviors and pleasure able experiences.
Serotonin: concentrated in a part of the brain called rap he nucleus. Involved in sensory perception, temp regulation, control of mood, appetite and sleep.
Catecholamines
SMNT
Contain:
amino group (-NH2)
Catechol ring composed of 6 carbons and 2 adjacent hydroxyl (-OH)
Examples are: norepinephrine, dopamine, epinephrine
Enzyme that breaks this down: monoamine oxidase
ATP and other Purines
SMNT
Ring structure of adenosine portion is a purine ring.
Neurotransmitters: adenosine, triphosphate, diphosphate, monophosphate
These are excitatory neurotransmitters in both CNS and PNS.
Nitric Oxide
SMNT
Simple gas, important excitatory neurotransmitter secreted in the brain, spinal cord, adrenal gland, nerves to the penis and has widespread effects on the body.
Nitric oxide synthase: enzyme that catalyzes formation from NO from amino acids arginine.
Carbon Monoxide
SMNT
Formed as needed excitatory neurotransmitter that diffuses out of cells that produce it into adjacent cells.
Produced in the brain, protects against excess neural activity.
Enkephalins
2 molecules, each a chain of 5 amino acids. Potential analgesic. 200 times stronger than morphine.
Endorphins and Dynorphins
NP
Other opioid peptides. Thought to be bodies natural painkillers.
Also improve memory and learning, feelings of euphoria, control body temp, regulation of hormones.
Substance P
NP
Released by neurons that transmit pain related input from peripheral pain receptors in the CNS, enhancing the perception of pain.
Suppressed by: enkephalin, endorphin
Neural Circuitds
Functional groups of neurons that process specific types of information in the CNS.
Organization of multiple neurons in a complicated pathway.
Simple Series Circuit
A presynaptic neuron stimulates a single Postsynaptic neuron.
Divergence
A single presynaptic neuron may synapse with several Postsynaptic neurons. Permits one presynaptic neuron to influence several Postsynaptic neurons at the same time.
Diverging Circuit
The nerve impulse from a single presynaptic neuron causes the stimulation of increasing numbers of cells along the circuit.
Convergence
An arrangement of several presynaptic neurons synapse with a single Postsynaptic neuron. Permits more effective stimulation or inhibition of the Postsynaptic neuron.
Converging Circuit
The Postsynaptic neurons receives nerve impulses from several different sources but will synapse with a single Postsynaptic neuron.
Influences a small number of neurons.
Reverberating Circuit
Some circuits that are organized so that stimulation of the presynaptic cell causes the Postsynaptic cell to transmit a series of nerves impulses.
Parallel After Discharge Circuit
A type of circuit where a single presynaptic cell that stimulates a group of neurons. Each of which synapses with a common Postsynaptic cell.
Plasticity
NS exhibits the capability to change based on experiences.
Regeneration
The ability to replicate or repair themselves
In PNS: dendrites/myelinated axons can be repaired if cell body remains intact.
If collagen fivers invade the regeneration tube of the PNS neurons, no new axons will be able to be formed.
In CNS: little to no repair of damage to neurons occurs.
Neurogenesis
The birth of new neurons from undifferentiated stem cells. Occurs regularly in some animals.
Damage and Repair in the PNS
Axons and dendrites that are associated with a neurolemma may undergo repair if the cell body is intact.
If the Schwann cells are functions, and if scar tissue formation does not occur too rapidly.
A FIbers
Largest diameter nerves and are myelinated.
Associated with sensory neurons for touch and pressure.
Receptor Potential
Is a type of graded potential that occurs in sensory receptors.
