Chapter 12: Nervous Tissue Flashcards
What does the central nervous system consist of?
Brain and spinal cord
What does all nervous tissue outside of the CNS make up?
Peripheral Nervous System (PNS)
What are two main components of the peripheral nervous system?
Nerves and sensory receptors
This is a bundle of hundreds to thousands of axons plus associated associated connective tissue and blood vessels that lies outside the brain and spinal cord
A nerve
There are twelve pairs of ______________________ nerves that emerge from the brain
Cranial nerves
There are 31 pairs of _________________ nerves that emerge from the spinal cord
spinal nerves
This structure of the nervous system monitors changes in the external or internal environment
Sensory receptors
What are the two divisions of the peripheral nervous system?
Motor and sensory
What is another name for the sensory division of the peripheral nervous system?
The Afferent or sensory division
What is the function of the sensory of afferent division of the PNS?
Conveys input into the CNS from sensory receptors in the body
Provides the CNS with sensory information about the somatic senses and special senses
What are the two types of senses that the sensory division of the PNS sends information about the CNS
Somatic senses and special senses
What type of senses are tactile, thermal, pain, and proprioceptive sensations?
Somatic sensations
What is type of senses are small, taste, vision, hearing and equillibrium?
Special sense
What is another name for the motor division of the PNS?
Efferent or motor division
What is the function of the motor or efferent division of the PNS?
Convey output from the CNS to effectors (muscles and glands)
What are the two types of nervous systems that the efferent or motor division of the PNS divides into?
Autonomic nervous system
Somatic nervous system
Which nervous system from the motor division of the PNS convery output from the CNS to skeletal muscles only?
The somatic nervous system (SNS)
What is the action of the somatic nervous system on the skeletal muscles?
Voluntary control
Which nervous system from the motor division of the PNS conveys output fromt eh CNS to smooth muscle, cardiac muscle and glands?
Autonomic nervous system (ANS)
What is the action of the autonomic nervous system?
Involuntary control
How is the autonomic nervous system further broken down?
Sympathetic nervous system
Parasympathetic nervous system
Fight or Flight nervous system?
Sympathetic Nervous System
Rest and Digest nervous system
Parasympathetic nervous system
What is the third branch of the autonomic nervous system?
The Enteric Nervous System (ENS)
Where is the ENS found?
over 100 million neurons confied to the wall of the GI tract that helps regulate the activity of smooth muscle and glands of the GI tract ; can function independently but is regulated by other branches of the ANS
The three basic function groups of the nervous system
- Sensory function
- Integrative Function
- Motor function
This function of the nervous system involves detecting internal or external stimuli and then carrying it to the brain and spinarl cord through cranial and spinal nerves
The sensory function
This function of the nervous system involves processing sensory information by anazlying it and making decisions for appropriate responses (integration)
Integrative function
This function of the nervous system involves eliciting an appropriate motor response by activating effectors (muscles and glands) through cranial and spinal nerves
Motor function
What make up the effectors of the nervous system
muscles that contract and glands that secrete
Two types of cells that nervous tissue is comprised of?
Neurons and neuroglia
These cells support, nourish, and protect neurons
Neuroglia
Three main parts of most neurons
- Cell Body
- Axon
- Dendrite
This part of a neuron contains a nucleus surrounded by cytoplasm that includes typical cellular organelles such as lysosomes, mitochondria, and a golgi complex, as well as free ribosomes and clusters of rough endoplasmic reticulum
The cell body
This is the name for the prominenet clusters fo rough endoplasmic reticulum
Nissl Bodies
What is the site of protein synthesis in the neuron?
The ribosomes
What do Nissl bodies produce (RER)
Proteins to replace cellular components, material for growth of neurons, and regenerate damaged axons
What is the cytoskeleton of the neuron cell body composed of?
Neurofibrils and microtubules
What are neurfibrils and their function?
Intermediate filaments that provide support and shape for the cell
What are microtubules for in the cell
They assist with movement of material from between the cell body and the axon
What is a pigment found in aging neurons that occurs as clumps of yellowish brown granules in the cytoplasm; its a protuct of neuronal lysosomes that accumulates as the neuron ages but does not seem to harm the cell
Lipofuscin
A collection of neuron bodies outside of the CNS
Ganglion
a term for any neuronal process(extension) that emerges from the cell body of a neuron; typically two kinds
Nerve fiber
What are the two common kinds of nerve fibers on a neuron
Multiple dendrites
Single axon
These nerve fibres from a neuron cell body are the receiving or input portion of a neuron
Dendrites
What nerve fiber is usually short, tapering and highly branched?
Dendrite
This part of the neuron cell propogates nerve impulses toward another neuron, a muscle fiber or a gland cell; it is a long, thin, cylindrical projection
Axon
This is the cone shaped elevation where an axon usually joins to the cell body of a neuron
axon hillock
The first part of the axon closest to the attachment at the axon hillock
initial segment
This is a junction between the initial sefment and the axon hillock where nerve impulses usually arise
trigger zone
What organelles does an axon contain?
Mitochondria, microtubules, and neurofibrils
What does it mean that the axon does not have RER present in it
It cannot synthesize protein in the axon
What is the cytoplasm of an axon called?
Axoplasm
What is the membrane of the axon called?
axolemma
Side branches off the axon, usually at 90 degree from axon
Axon collateral
Fine processes that axons and axon collaterals end up branching off into
axon terminials
The site of communication between two neurons or between a neuron and an effector
Synapse
Some tips of the axon terminals swell into bulb shaped structures called
Synaptic end bulbs
A string of swllen bumps on axon terminal
Varicosities
Tiny membrane-enclosed sacs that store neurotransmitters and are found in synaptic end bulbs and varicosities
Synaptic Vesicles
The chemicals stored in synaptic end bulb and varicosities synaptic vesicles; These molecules are released from a sunaptic vesicle that excites or inhibits another neuron., muscle fiber or gland cell
Neurotransmitters
This is the slower system that moves materials about 1-5mm per day from the cell body to the axon only one way. It suplies new axoplasm to developing or regenerating axons and replenishes axoplasm in growing and maturing axons
slow axonal transport
This is the faster transport system of the neuron, it is capable of moving materials 200-400mm per day; uses proteins that function as motors to move materials along the surfaces of microtubules of the cytoskeleton; moves materials in both directions
Fast axonal transport
The forward direction movement of organelles and synaptic vesicles from the cell body to the axon terminals
Antegrade movement
The backward direction that moves membrane vesciles and other cellular material from the axon terminals to the cell body to be degraded or recycled
Retrograde movement
What two ways are neurons classified?
Structurally and functionally
What are the three structural classifications of neurons?
- Multipolar
- Bipolar
- Unipolar
This structural classification of neuron usually have several dendrites and one axon; All motor neurons and most neurons in the brain and spinal cord are this class
Multipolar
This structural classification of neurons has one main dendrite and one axon; found in retine of the eye, inner ear, and olfactory area of the brain
Bipolar neurons
These structural classification of neurons have dendrites and one axon that are fused together to form a continuous single process that emerges from the cell body
Unipolar Neurons
Why are unipolar neurons also known as psuedounipolar neurons?
They begin in the embryo as bipolar neurons and then during development the dendrites and axon fuse together to form a single process
What do the dendrites of most unipolar neurons function as?
Sensory receptors for thins such as touch, pressure, pain, or thermal stimuli
Where is the trigger zone located on a unipolar neuron?
Where the dendrite and axon meet
Where are the cell bodies of most unipolar neurons found?
In the ganglia of spinal and cranial nerves
Where are purkinje cells found?
In the cerebellum
What are Purkinje cells named after?
The hisotogist who discovered them
What are pyramidal cells that are found in the cerebral cortex of the brain called such?
These cells have pyramid shaped cell bodies
What are the three functional classifications of neurons?
- Sensory Neurons (afferent)
- Motor Neurons (efferent)
- Interneurons or association neurons
This functional classification of neurons contain either sensory receptors at their dendrites or are located just after sensory receptors that are seperate cells.
Once stimulus activates action potenial, and it convery messages into the CND through spinal or cranial nerves
These neurons are also typically unipolar
Sensory or Afferent Neurons
This functional classification of neurons convey action potentials away from the CNS to effectors (muscles and glands) in the PNS through cranial or spinal nerves.
These are multipolar in structure.
Motor neurons
This functional classification of neurons are located mainly wihtin the CNS between sensory and motor neuron. They process incoming sensory information and elicit a motor response by activitating the appropriate motor neurons.
Usually multipolar
Interneurons
These cells are much more numerous than neurons, support the function of neurons, make up about half the volume of the CNS, can continue to multiply and divide, do not generate or carry action potentials
Neuroglia
A brain tumor derived from glia, that typically is highly malignant and grows rapidely
Gliomas
4 types of neuroglia found only in the CNS
- Astrocytes
- Oilgodendrocytes
- Microglia
- Ependymal Cells
2 types of neuroglia found in the PNS
- Schwann cells
- Satelitte cells
These CNS neurolgia cells are star shaped, have many processes and are the largest and most numerous of the neurolgia
Astrocytes
What are the two types of astrocytes?
Protoplasmic astrocytes
Fibrous Astrocytes
This type of astrocyte have many short branching processes and are found in grey matter
Protoplasmic Astrocytes
Where are protoplasmic astrocytes found and what is their structure?
In grey matter
Many short branching processes
This type of astrocyte have many long unbranched processes and are located mainly in white matter
Fibrous astrocytes
The processes of astrocytes make contact with what three structures?
Blood capilliaries
Neurons
Pia Mater
This is a thin membrane around the brain and spinal cord
Pia Mater
What component of the astrocyte neuroglia gives it considerable strength that allows it to support neurons?
Microfilaments
What do the processes of astrocytes do to protect the neurons of the CNS from potentially harmful substances in the blood stream?
Wrapped their processes around blood capilliaries and release chemicals that maintain the unique selective permeability characteristics of the endothelial cells of the capilliaries
What function do the astrocytes have in the embryo?
Release chemicals that appear to regulate the growth, migration and interconnection among neurons in the brain
How do astrocytes maintain the appropriate chemical environment for nerve impulse generation?
- regulate concentration of ions
- Take up excess neurotransmitters
- Nutrients wastes and substances pass through them between neurons and blood capillaries
How do astrocytes possibly play a role in learning and memory?
They influence the formation of neural synapses
These CNS neuroglia cells resemble astrocytes but are much smaller and have fewer processes
oligodendrocytes
Oligodendrocytes are responsible for forming and maintaining this
The myelin sheath around CNS axons
This is a multilayered lipid and protein covering around some axons that insulates them and increases the speed of nerve impulse conduction
myelin sheath
Axons that are covered in myelin sheath are said to be what?
Myelinated
This CNS neuroglia cell are small cells with slender process that give off numerous spinelike projections
Microglial cells or microglia
What do microglia function as?
Phagocytes
What do microglia do?
Remove cellular debris formed during normal development and phagocytize microbes and damaged tissues
These neuroglia of the CNS are cuboidal to columnar shaped cells and arranged in a single layer; they have microvilli and cilia. These cells line the ventricles of the brain and central canal of the spinal cord (the spaces filled with CSF)
Ependymal Cells
What is the function of ependymal cells?
Produce, possibly monitor and assist with circulation of CSF; Also form blood-CSF barrier
What are the two types of neuroglia cells found in the peripheral nervous system?
Schwann Cells
Satellite Cells
The neuroglia of the PNS encircle PNS axons and myelin sheath around them; only can cover a single myelinated axon but can enclose as many as 20 unmyelinated axons
Schwann Cells
What do Schwann cells do?
Helps with axon regeneration
These PNS neuroglia are flat and surround the cell bodies of neurons of PNS ganglia
Satellite Cells
What are the 2 main functions of satellite cells?
Structural support
Regulate the exchanges of materials beween neuronal body cells and interstitial fluid
The multilayered lipid and protein covering of some axons; electrically insulates the axon of a neuron and incresaes the speed of nerve impulse conduction
myelin sheath
Axons with myelin sheath
myelinated
Axons without myelin sheath
unmyelinated
The CNS cell that produces myelin sheath
Oligodendrocytes
The PNS cells that produce myelin sheath
Schwann Cells
When do Schwann cells begin to wrap myelin sheath around axons in the PNS?
During fetal development
How do Schwann cells form myelin sheath around an axon?
They wrap themselves several times around the the axon.H
Roughly what length of an axon does one Schwann cell cover?
1mm
What is eventually surrounding the axon?
Multiple layers of glial plasma membrane
What is the outer nucleated cytoplasmic layer of the Schwann cell?
Neurolemma
What is there 100 layers of on the inner portion of the Schwann cell?
Schwann cell membrane
What do the (up to) 100 layers of Schwann Cell membrane create?
The myelin sheath
What contributes to the regeneration of an axon when it is injured, by forming a regeneration tube that guides and stimulates regrowth of the axon?
The neurolemma
What are the gaps in the myelin sheath called that appear at intervals along the axon?
Nodes of Ranvier
How do oligodendrocytes form myelin sheath in the CNS?
The oligodendrocytes extend about 15 process that wrap around CNS axons and form the sheath
Why is there no neurolemma?
Because the oligodendrocyte body and nucleus do not wrap around the axons
What are the two reasons thought to be part of why axons in CNS display little regrowth after injury?
Absense of neurolemma
An inhibitory influence exerted by the oligodendrocytes on axon regrowth
What happens to myelin sheath from birth to maturity?
It continues to grow and increase
What is a cluster of neuronal cell bodies in the PNS called?
Ganglion or ganglia
What is a cluster of neuronal cells bodies in the CNS called?
Nucleus
A cluster of axons in the PNS?
A nerve
A cluster of axons in the CNS?
A tract
What does a tract in the CNS do?
Interconnect the neurons inthe spinal cord and brain
What is white brain matter mostly comprised of that gives it the white color?
Myelinated axons
What does grey matter contain?
Neuronal cell bodies, dendrites, unmyelinated axons, axon terminals and neuroglia
Why is grey matter grey and not white? (2 reasons)
The nissl bodies impart a grey color
There is little or no myelin in that area
Where is white matter found in the spinal cord?
The white matter surrounds the inner core of grey matter
What parts of the brain surface is covered in a thin shell of grey matter?
The cerebellum and cerebrum
What are two ways that the electrically excitable neurons communicate with one another?
- Graded potentials
- Action potentials
What part of the brain processes perception?
Cerebral cortex
What is a lower motor neuron?
The motor neuron that directly supplies skeletal muscle fibres
What 2 basic features of the plasma membrane does the production of a graded potential and action potentials depend on
- Existance of resting membrane potential
- Presence of specific types of ion channels
This is the electrical potential difference (voltage) across the membrane of an excitable cell
resting membrane potential
What constitues the electrical current in living cells?
The flow of ions
A concentration and electrical difference beween the outside of a cell and the inside
electrochemical gradient
What happens when an ion channel opens?
ions move from an area of higher concentration to an area of lower concentration; anions move towards positively charged areas and cations move toward negatively charged areas
What are the 4 types of ion channels that neuron and muscle fibre electrical signals rely on?
- Leak channels
- Ligand-gated channels
- Mechanically gated channels
- Voltage-gated channels
What ion channels operates through randomly alternating of open and closed positions?
Leak Channels
What type of leak channels do plasma membranes typically have and compare them?
More K+ leak channels than Na+ leak channels; K+ leak channels are leakier than Na+
Where are leak channels found?
Nearly all cells, including the dendrites, cell bodies and axons of all types of neurons
This ion channel opens and closes in response to a chemical stimulus. Different channels are stimulated by different chemicals and each one is specific to the ions it allows to cross the membrane.
Ligand-gated channel
Ligand-gated channels can be activated by neurotransmitters, hormones, and particular ions and are typically located:
In the dendrites of some sensory neurons (eg pain receptors) and in the dendrites an dcell bodies of interneurons and motor neurons
These types of ion channels open in response to mechanical stimulation like pressure, vibration or touch and open by the force distorting the channel from its resting position
Mechanically-gated channel
Examples of mechanical gated channel locations
auditory receptors in the ears, receptors that monitor stretching of organs, touch receptors and pressure receptors in the skin
This type of ion channel opens in response to a change in membrane potential
Voltage-gated channels
Where are voltage-gated channels found?
They participate in generation and conduction of action potentials in the axons of all types of neurons
What does a build up of negatively charged ions in cytosol along the inside of the cell memrane and an equal build up of positively charged ions in the extracellular fluid along the outside surface of the cell membrane create?
resting membrane potential
What is the typical value of resting membrane potential in neurons?
Between -40 to -90 but typical is -70mV
A cell that exhibits membrane potential is said to be what?
polarized
What is the range of membrane potential values in body cells that are polarized?
+5mV to -100mV
Three major factors that lead to resting membrane potential:
- Unequal distribution of ions in the ECF and cytosol
- Inability of most anions to leave the cell
- Electrogenic nature of the Na+-K+ ATPases
What is the main cation and the main two anions (attached to molecules) inside the cytosol
Cation is K+
Two dominant anions:
phosphates in ATP
Amino acids in proteins
What is extracellular fluid rich in?
Na+ and Cl- ions
Why can’t most anions leave the cell?
They are attached to non-diffusable molecules like ATP or large proteins
This is a small deviation from the resting membrane potential that makes the membrane either more polarized (inside more negative) or less polarized (less negative inside)
A graded potential
What is a hyperpolarizing graded potential?
when the response makes the membrane more polarized (inside cell more negative)
What is depolarizing graded potential?
When the graded potential causes the membrane to become less polarized
Whena stimulus causes a mechanically-gated or ligand-gated channel to open or close in an excitable cells plasma membrane _________ occurs
A graded potential
What channels are usually found in the dendrites of the sensory neurons?
Ligand-gated and mechanically-gated channels
What channels are numerous on the cell bodies and dendrites of motor and interneurons?
Ligand-gated channels
Where do graded potential mainly occur on a neuron cell
the dendrite and cell body
What does it mean that these potentials are “graded”
they vary in stize depending on the strength of the stimulus and are stronger or weaker depending on how many gated channels are impacted and how long each remians open
When channels open or close they alter the flowof specific ions across the membrane which produces a flow of current that is localized and spreads to adjacent regions along the plasma membrane and then gradually dies out as the charges are lost across the membrane through leak channels
Decremental Conduction
Why are graded potentials only good for short distance communication?
Because they die out within a few millimeters from decremental conduction
The process by which graded potentials add together and can become stronger and last longer
Summation
What is the result of two depolarizing graded potentials summating?
A larger depolarizing graded potential
If two equal, but opposite graded potential summates?
They cancel each other out and the overall graded potential disappears
Graded potentials have different names depending on these two things:
- where they happen
- what type of stimulus causes them
Graded potentials that occur in sensory receptors are called?
receptor potentials
graded potentials that occurs in the dendrites or cell body of a neuron in response to a neurotransmitter is called?
postsynaptic potential
Another name for action potential
impulse
This is a sequence of rapidly occuring events that decrease and reverse the membrane potential and then eventually restore it to a resting state
Action potential (AP) or impulse
The two main phases of action potentials:
- Depolarizing phase
- Repolarizing phase
During this phase of an action potential. the negative membrane potential becomes less negative, reaches zero, and then becomes positive
Depolarizing
During the second phase of the action potential, the membrane potential is restored to the resting state of -70mV
Repolarizing phase
Thisis a pharse that may occur after repolarization during which the membrane potential temporarily becomes more negative than the resting level.
After-hyperpolarizing phase
The two types of voltage-gated channels that open and then close in an action potential:
- voltage-gated Na+ channels
- voltage-gated K+ channels
This is the first channel that opens in an action potential along the axon of a neuron; it allows Na+ to rush into the cell which triggers depolarization
voltage-gated Na+ channel
This is the voltage-gated channel that opens and allows K+ to flow out of the cell, which triggers the repolarization phase of the action potential
Voltage-gated K+ channel
This phase occurs after repolarization when the voltage-gated K+ channel remains open after the repolarization phase ends
After-hyperpolarization phase
When does an action potential occur in the membrane on an axon?
When the depolarization reaches that voltage threshold (about -55mV)
A weak depolarization that cannot bring the membrane potential to threshold and therefore will not trigger an action potential
A subthreshold stimulus
This is a stimulus just strong enough to depolarize the membrane to threshold
threshold stimulus
A stimulus that is strong enough the depolarize the membrane above threshold will cause several action potentials and is called:
a suprathreshold stimulus
What does a greater stimulus cause in terms of action potentials as it will nto increase the amplitude of an action potential
Greater frequency of the action potentials until a maximum frequency is reached
what determines the maximum frequency of action potentials?
absolute refractory period
Where are voltage gated Na+ and K+ channels usually found along the axon?
Axon plasma membrane and terminals
The characteristic of an action potential whereby it will either occur in response to a threshold stimulus or will not because of a subthreshold stimulus is
All-or-none principle
In the depolarizing phase of an action potential, what gradients cause the inflow of sodium ions into the cell?
The chemical (concentration) gradient and the electrical gradient
When sodium flos into the cell during depolarization, what is the change in voltage of the membrane potential?
-55mV activated the action potential
Inflow of sodium brings voltage to +30mV
What are the two separate gates on a voltage-gated Na+ channel?
- Activation gate
- Inactivation gate
What are the states of the gates on the voltage gated Na+ channel when the channel is in resting state?
activated state?
Resting - activation gate is closed; inactivation gate is open
activated state: both are open
What happens to the membrane potential as more channels open and Na is flowing in?
More channels open and the membrane potential depolarizes further
What is happening when the voltage-gated Na+ channel is in an inactivated state?
the inactivation gate closesh
What triggers the opening of voltage-gated K+ channels?
A threshold depolarization level
As Na+ channels are inactivated and K+ channels are activated, Na+ inflow slows and K+ outflow increases and this leads to what change in the membrane potential?
Goes from +30mV to -70mV and repolarizes
What happens to voltage-gated Na+ channels when repolarization happens?
they return to resting state
What is the charge of the membrane potential if the K+ gates remain open briefly during after-hyperpolarizing phase?
-90mV
What two states do potassium voltage gated pumps alternate between?
Open (active) or closed (resting)
The period of time after an action potential that an excitable cell cannot produce another action protneal in response to a normal threshold stimulsu
Refractory period
A period where even a very strong stimulus cannot initiate a second action potential in an excitable cell
absolute refractory period
What does the refractory period coincide with?
activation and inactivation f sodium voltage gated channels
what state must sodium voltage-gated channels be in in order for an action potential to occur?
resting
The larger diameter the axon, the larger surface area, the less time an absolute refractory period lasts and the more action potentials can occur per second
What is the range of maximum frequency of nerve impulses in different axons in normal body conditions
ranges between 10-1000 per second
The period of time during which a second action potential can be initiated, but only be a larger than normal stimulus
relative refractory period
What does the relative refractory period coincide with?
The period when the voltage-gated K+ are open after inactivated Na+ channels have returned to resting state
The action potentials are not continuous, but rather a series of them along the axon through adjacent regions of membrane. This is because of what feedback system
Positive feedback
What prevents action potentials from porpogating backwards towards the cell body?
The temporary absolute refractory period
Why can action potentials communicate longer distances than graded potentials?
Because they do not die out or lose strength
Two types of propogation
- Continuous Conduction
- Saltatory Conduction
This type of conduction involves step by step depolarization and repolarization of each adjacent segment of the plasma membrane
Continuous Conduction
This type of conduction only propogates a short distance and occurs in unmyelinated axons and in muscle fibres
Continuous conduction
Action potentials propogate more rapidly along ___________ axons
myelinated
This is a special mode of action potential propogation that occurs along myelinated axons and occurs because of the uneven distribution of voltage-gated channels
Saltatory Conduction
This area of a myelinated axon has many voltagegated channels along the axolemma
Nodes of Ranvier
This area of myelinated axons does not have many voltage gated channels and therefore this happens to the electric current:
The electric current carried by ions flows through the extracellular fluid surrounding the myelin sheath and through the cytosol from one node of ranvier to the next
Therefore, an action potential in the first nose of ranvier propogates an electric c;urrent through the cytosol which p
Two consequences of saltatory conduction causing the flow of current only at the nodes of ranvier:
- Action potential appears to leap from node to node
- Opening only a few voltage channels at nodes instaed of many at adjacent sections requires much less energy for conduction
3 Major Factors that Affect the Speed of Propogation
- Amount of myelination
- Axon diameter
- Temperature
Action potentials propogate much more rapidly along axons that are ___________________
Myelinated
Action potentials propogate much faster along axons with _________________ diameter due to the ________________ surface areas
larger and larger
Axons propogate action potentials at ___________________ speeds when cooled
Lower
3 Classifications of Nerve Fibres based on the amount of myelination, their diameter and their propogation speeds
- A Fibres
- B Fibres
- C Fibres
These fibres are largest diameter (5-20um) and myelinated
A Fibers
A fibers have a brief absolute refractory period and conduct action potentials at speeds of?
12-130 m/second or 27-290 mi/hr
The axons of sensory neurons associated with touch, pressure, position of joints, and some thermal and pain sensations
Axons of motor neurons that conduct impulses to skeletal muscle
A Fibres
These fibers are axons with 2-3um diameter and are myelinated
B Fibers
B Fibers exhibit saltatory conduction up to 15m/sec; have a somewhat longer absolute refractory period
These fibers conduct sensory nerve impulses from the viscera to the brain and spinal cord
Also all axons of autonomic motor neurons that extend from CNS to the ANS relay stations (Autonomic ganglia)
B Fibers
These nerve axons are the smaller diameter (0.5-1.5um) and are unmyelinated
C Fibers
The speed c fibers propogate is 0.5-2 m/sec (slowest); have longest absolute refractory period
Conduct some sensory impulses for pain, touhc, pressure, heat and cold from skin; pain from viscera
Autonomic motor fibers that extend from autonomic ganglia to stimulate the heart, smooth muscle, and glands
C Fiber
Typical resting membrane potential in neuron versus skeletal and cardiac muscle
-70mV versus closer to -90mV
Duration of a nerve impulse versus muscle action potential versus cardiac action potential
Nerve: 0.5-2msec
Skeletal Muscle: 1-5 msec
Cardiac and smooth muscle fibres: 10-300 msec
How many times faster is the speed of propogation along a myelinated axon of the largest diameter nerve axon versus the propogation speed along the sarcolemma of a skeletal muscle fiber?
18 x
A nerve cell that carriers a nerve impulse towards a synapse; the cell that sends the signal
Presynaptic Neuron
The cell that receives a signal
Postsynaptic cell
A nerve cell that receives a signal at a synapse and carries a nerve impulse away from a synapse
A postsynaptic neuron
A cell that responds to the impulse at the synapse (gland or muscle)
Effector
This synapse is an axon to a dendrite; most synapses between neurons are this
Axodendritic
Some synapses are these two:
axosomatic or axoaxonic
Two types of synapses
chemical or electrical
How do electrical and chemical synapses differ?
Structurally and functionally
WHy are synapses essential to homeostasis?
They allow information to be filtered and integrated
In this type of synapse, action potentials conduct directly between the plasma membrane of adjacent neurons
Electrical Synapses
Structures through which action potentials directly conduct between membranes of adjacent neurons
Gap Junctions
These are like tunnels that connect the cytosol of two cells directly; They are found at gap junctions and there are usually a hundred or so at each one
Tubular connexons
What flows through tubular connexons between cells to spred the action potential?
Ions
Where are gap junctions common?
Visceral smooth muscle, cardiac muscle and the developing embryo; some in the brain
2 main advantages of electrical synapses
- Faster communication
- Synchronization
Why are electrical synapses faster than chemical synapses?
BEcause the action potentials conduct directly through the gap junctions and happen almost instantaneously
What can happen in cardiac or smooth muscle that helps to provide coordinated contraction?
A larger number of neurons or muscle fibres can produce action potentials in unison if they are connected by gap junctionsThe
The name for the gap between post and presynaptic neurons in a chemical synapse
Synaptic cleft
What is the 20-50nm space in the synaptic cleft filled with?
interstitial fluid
What happens in the postsynaptic neuron when it receives the chemical signal from the presynaptic neuron?
A postsynaptic potential
A postsynaptic potential is a type of which potential?>
Graded potential
The reason chemical synapses are slower are relaying signals is called
Synaptic Delay of about 0.5msec
Two types of neurotransmitter receptors on postsynptic neuron plasma membrane
- Ionotropic receptor (a type of ligand-gated channel)
- Metabotropic receptors
This neurotransmitter receptor contains both a neurotransmitter binding site and an ion channel on the same protein.
Ion channel opens with a neurotransmitter (ligand) binds to the receptor site on the protein
Ionotropic Receptor
These type of postsynaptic poentials tend to occur when excitatory neurotransmitters bind to ionotropic receptors that contain cation channels
Excitatory Postsynaptic potentials
What three plentiful cations are able to pass through the cation channels in EPSPs
Na+ K+ and Ca+
Many excitatory neurotransmitters bind to ionotropic receptios that contain cation channels
Many inhibitory neurotransmitters bind to ionotropic receptors that that contain cation channels
This type of Neurotransmitter receptor contains a neurotransmitter binding site but lacks an ion channel as part of its structure
Is coupled to a separate ion channel by a type of membrane protein called a G protein
Metabotropic Receptor
What is the main difference between a metabotropic and an ionotropic neurotransmitter receptor?
THe ionotropic receptor is a proeint that contains both the ion channel and ligand receptor
The metabotropic receptor contains a ligand receptor and has a separate ion channel connected to it by a G protein
3 ways to remove synaptic cleft neurotransmitters
- Diffusion - simply diffuses away from the synaptic cleft and receptors
- Enzymatic degredation - broken down and deactivated by enzymes
- Uptake by cells - actively transported back into the neuron that released them (reuptake) or transported into neighboring neuroglia (uptake)
What are the names of the membrane proteins responsible for reuptake that results in recylcing the neurotramitters back into synaptic vessicles?
neurotransmitter transporters
Two types of summation
- Temporal
- Spatial
This is summation of postsynaptic potentials in response to stimuli that occur at different locations in the membrane of a post synaptic cell at the same time
Spatial summation
This is 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
Results from the buildup of neurotransmitter released by a single presynaptic end bulb two or more times in rapid succession
Temporal Summation
The sum of all the excitatory and inhibitory effects at any given time determines the effect on the postsynaptic neurons
What does this mean
A single postsynaptic neuron receives input from many presynaptic neurons, some inhibitory and some excitatory
If the total excitatory effects are greter than the total inhibitory effects, but less than the threshold level of stimulation, the result is?
An excitatory postsynaptic potential
What can cause the EPSP to generate a nerve impulse?
Subsequent stimuli causing summation because of the paartial depolarization
If the total excitatory effects are greater than the total inhibitory effects and the threshold is reached, one or more ___________________ will be triggered
Nerve impulses or action potentials
If the total inhibitory effects are greater than the excitatory effects, the membrane hyperpolarizes
Inhibition of postsynaptic neuron
Two classes of neurotrasmitters based on size:
- Small-molecule neurotransmitters
- Neuropeptides
Certain neurons within the brain the secrete hormones
Neurosecretory cells
Ach, amino acids, biogenic amines, ATP and other purines, nitric oxide, and carbon monoxide
Small-molecule neurotransmitters
This small-molecule neurotransmitter is found alot in PNS and some in CNS; it is excitatory in some places and inhibitory in others; acetylcholinesterase breaks it down to deactivate it
achetylcholine ACh
These are neurotransmitters in the CNS;
AMino acids
glutamate and asparate are excitatory; many communicate in brain via glutamate; inactivation of glutamate is reuptake by glutamate transporters back into end bulbs and neighbor neuroglia
Excitatory amino acids
Gaba and glycine are important inhibitory amino acids in the CNS; binding to inonotropic receptors opens CL- channels and hyperpolarizes cell membranes
amino acids
These neurotransmitters are made from modifiied and decarboxylated amino acids
Biogenic amines
4 most prevalant biogenic amines
Norepineprhine, epinephrine, dopamine and serotonin
Biogenic amines usually bind to this type of neurotransmitter receptor; and there are many different types of these receptors for each biogenic amine
metabotropic
This biogenic amine plays a role in arousal, dreaming, and regulatin gmood
Norepinephrine
A small number of neurons in the brain use this biogenic amine
Epinephrine
These two biogenic amines also serve as hormones
norepinephrine and epinephrine
These cells release epinephrine and norepinephrine into the blood
the cells of the adrenal medulla inside the adrenal gland
Brain neurons that contain this biogenic amine neurotransmitters are active during emotional responses, addictive behaviors, and pleasurable experiences
Dopamine
These neurons also help regulate skeletal muscle tone and some aspects of movement due to contraction of skeletal muscles
Dopamine releasing neurons
These three biogenic amines / hormones / neurotransmitters are classified chemically as catelcholamines
Norepinephrine, epinephrine and dopamine
Catecholamines contain an amino group (-NH2) and a catechol ring made of six carbons; and two adjacent hydroxyl (-OH) groups
Synthesized from which amino acid??
Tyrosine
Inactivation of catecholamines occur by:
reuptake into synaptic end bulbs and recycled back into synaptic vesicles or destroyed by enzymes
Two enzymes that break down catecholamines
catechol-o-methyltransferase (COMT) and monoamine oxidase (MAO)
An MAO is???
monoamine oxidase - enzyme that breaks down catelcholamines like epinephrine, norepinephrine and dopamine
This biogenic amine neurotransmitter is concentrated in the raphe nucleus of the brain
Serotonin
Serotonin is thought to be involved in seonsry perception, temperature regulation, control of mood, appetite, and the induction of sleep
The biogenic amine is also known as 5-hydroxytryptamine
ATp, ADP, AMp have purine rings and are excitatroy in both CNS and PNS. Most synaptic vesicles that contain ATP also contain another transmitter.
In the PNS, ATP and norepinephrine are usually released together from some ______ neurons
SYmpathetic
In the PNS, ATP is usually released with ACh in the same vesicles in the __________________________ neurons
parasympathetic
What important excitatory neurotransmitter is secreted in the brain, spinal cord, adrenal glands, and nerves to the penis and has widespread effects throughout the body
Nitric Oxide (NO)
What enzyme catalyzes the formation of nictric oxide from the amno acid arginine?
Nitric oxide synthase (NOS)
hat amino acid is nitric oxide catalyzed from by nitric oxide synthase?
Arginine
What is different about how nitric oxide is synthesized in comparison to other neurotransmitters?
It is not prepared in advance but rather made on demand and acts immediately
Why does nitric oxide only exist for less than 10 seconds before it combines iwth oxygen and water to form inactive nitrates and nitrites?
Because it is a highly reacitve free radical
WHat property of nitric oxide allows it to diffuse from cells that produce it into neighboring cells, where it activates an enzyme for production of a second messenger called cyclic GMP
It is lipid soluble
What two neurotransmitters are produced on demand and used immediaty and diffuses out of cells into neighboring cells
nitric oxide and carbon monoxide
Where is carbon monoxide produced and is it excitatory or inhibitory?
In the brain and excitatory
This neurotransmitter might protect against excess neuronal activity, and might be related to dilation of blood vessels, memory, olfaction, vision, thermoregulation, insulin release and antiinflammatory activity
Carbon Monoxide
Where are neuropeptide neurotransmitters of 3-40 amino acids linked by peptide bonds found?
In PNS and CNS
Which neurotransmitter receptors do neuropeptides bind to ?>
Metabotropic receptors
Are neuropeptides excitatory or inhibitaory
Either depending on the receptor it bings to
How are neuropeptides formed?
They are formed int eh neuron cell body, packed into vesicles and transported to axon terminalsWh
What other role do neuropeptides play in the body?
They serve as hormones that regulate physiological responses elsewhere in the body
These are two molecules, eacha chain of five amino acids, that are naturally occuring substances that use the opioid receptors on neurons
enkephalins
Opioid peptides
Endorphins, enkephalins, dynorphins
Opioid peptides such as enkephalins, endorphins and dynorphins are thought to be the bodys what?
Natural pain killers
What neuropeptide is released by neurons that transmit pain related input from peripheral pain receptors intot he CNS, enhancing the perception of pain?
substance P
What two neuropeptides inhibit and suppress substance P, thereby reducing the number of nerve impulses being relayed to the brain for pain sensation?
Endorphins and enkephalin
Functional groups of neurons that process specific types of information
neural circuits
In this type of circuit, a presynaptic neuron stimulates a single postsynaptic neuron; then the second neuron stimulates the next,.etc
SImple series circuit
When a single presynaptic neuron synapses with several postsynaptic neurons; permits one presynaptic neuron to influence several post synaptic neurons at the same time
Divergence
In this circuit, the nerve impulse from a singel presynaptic neuron causes the stimulation of incresaing numbers of cells along the circuit
Diverging circuit
A circuit that involves multiple presynaptic neurons sunapsing with a single postsynaptic neuron, permits more effective stimulation or inhibition of the postsynaptic neuron
Convergence circuit
This type of neural circuit finds the later neurons sending an impulse to the next but also sending the impulse back to the previous neuron for the signal to impulse back through the circuit repeatedly
eg. breathing circuit,
REverberating circuit
This type of neuron circuit has a single presynatpic cell that stimulates a group of neurons, each of which synapses with a common postsynaptic cell
Parallel after-discharging circuit
The ability to change based on experience
plasticity
3 ways change can occur in individual neurons (not all tho)
- sprouting new dendrites
- synthesis of new proteins
- changes in synaptic contacts with other neurons
What two forces can cause neuorn changes to occur?
- chemical and electrical signals
The capability to replicate or repair themselves; limited in mammals neurons
REgeneration
How can repair to damaged dendrites and myelinated axons occur in the PNS?
if cell body is intact
schwann cells that produce myelination remain active
What happens for repair of neurons in the CNS?
Nothing;l there is little or no repair of damage to neurons in the CNS even if the cell body is intact
development of new neurons from undifferentiated stem cells
neurogenesis
hormone like protein that stimulated cells taken from the brains of adult mice to proliferate into both neurons and astrocytes; also known to cause mitosis; and promote wound healing and tissue regeneration
epidermal growth factor
Where do significant numbers of new neurons arise in the human brain?
The hippocampus which is crutial for learning
2 factors influencing almost complete lack of neurogenesis in most areas of the brain and spinal cord
- inhibitory influences from neuroglia (esp oligodendrocytes)
- absense of growth-stimulating cues that were present during fetal development
After axonal damage, what cells proliferate rapidly, forming a type of scar tissue that acts as a physical barrier to regeneration?
Astrocytes
What parts of a neuron in the PNS may undergo repair if the cell body is intact, the schwann cells are functional, and if scar tissue formation does not occur too rapidly
Axons and dendrites that are associated with a neurolemma
What happens in the neuron cell body in the PNS about 24-48 hours after injury to a process of a normal PNS neuron?
Chromatolysis - Nissl bodies break up into granular Masses
What happens to the axon distal to the injured area by the 3-5 day?
Wallerian degeneration - The distal axon becomes swollen and then breaks up into fragments; myelin sheath deteriorates
What remains intact
neurolemma
days 1-2 - chromatolysis
days 3-5 - wallerian degeneration
Following chromatolysis, what is the evidence of healing in the cell body?
macrophages phagocytize the debris
synthesis of RNA and protein accelerate, favoring rebuilding of axon
schwann cells on either side of injured site multiply by mitosis, frow toward each other and may form a regeneration tube
What is the purpose of the regeneration tube formed by the mitosis of schwann cells along the area of injured axon
to guide growth of a new axon from the proximal area across the injured area and into the distal area
what would limit axon growth
is the gap at the injury site is too large or if gap becomes filled iwth collagen fibres
