Chapter 12 - Nervous Tissue Flashcards
What is the objective of the nervous and endocrine systems?
To keep controlled conditions within limits that maintain life
Which body system is the smallest and most complex?
The nervous system
What are the two main subdivisions of the nervous system?
- Central nervous system
2. Peripheral nervous system
What is the central nervous system composed of?
Brain and spinal cord
What is the brain?
Part of the CNS
Located in the skull
Contains about 100 billion neurons
What is the spinal cord?
Park of the CNS
Connected to the brain (through foramen magnum of the occipital bone)
Contains about 100 million neurons
What is the function of the central nervous system (CNS)?
Processes incoming sensory information
Source of thoughts, emotions and memories
Stimulates muscles to contract
Stimulates glands to secrete
What does the peripheral nervous system consist of?
Cranial nerves, spinal nerves, ganglia, enteric plexuses, sensory receptors
(All nervous tissue outside of the CNS)
What is a nerve?
Bundle of axons located in the PNS
- Cranial nerves connect the brain to the periphery
- Spinal nerves connect the spinal cord to the periphery
How many cranial nerves do we have?
12 pairs
How many spinal nerves do we have?
21 pairs
What is a ganglion? (glanglia)
Cluster of neuronal cell bodies located in the PNS
What is an enteric plexuses? What is the function?
Extensive networks of neurons located in the walls of organs of the gastrointestinal tract
- regulate the digestive system
What is a sensory receptor?
Refers to a specific structure of the nervous system that monitors changes in the external or internal environment
What are examples of sensory receptors?
Touch receptors in the skin
Photoreceptors in the eye
Olfactory receptors in the nose
What are the 3 divisions of the PNS?
- Somatic nervous system
- Autonomic nervous system
- Enteric nervous system
What does the somatic nervous system (SNS) consist of?
- Sensory neurons - convey information from somatic receptors in the head, body wall, and limbs
- special senses: vision, hearing, taste, smell to CNS - Motor neurons - conduct impulses from the CNS to skeletal muscles ONLY - voluntary
What does the autonomic nervous system (ANS) consist of?
- Sensory neurons - convey information from autonomic sensory receptors in the visceral organs (stomach, lungs) to the CNS
- Motor neurons - conduct nerve impulses from the CNS to smooth muscle, cardiac muscle and glands - INVOLUNTARY
What are the two branches of the motor part of the ANS?
- Sympathetic Division
2. Parasympathetic Division
What is the enteric nervous system (ENS)?
The “brain of the gut”
Involuntary
- monitor chemical changes within the GI tract
What do enteric motor neurons govern?
Contraction of GI tract smooth muscle, secretions of the GI tract organs (acid from stomach) and activity of GI tract endocrine cells (secrete hormones)
What are the functions of the nervous system?
- Sensory
- Integrative
- Motor
What is the sensory function?
Sensory receptors detect internal/external stimuli
- ex. increase in blood pressure
Information is carried to brain and spinal cord
What is the integrative function?
Nervous system processes the sensory information
Analysis and then makes decision “integration”
What is the motor function?
Once sensory information is integrated, nervous system may elicit a motor response by activating effectors
- causes muscles to contract or glands to secrete
What is an effector?
Muscle or gland that elicits a response through cranial or spinal nerves
What is an example of an effector of the SNS, ANS, and ENS?
SNS - skeletal muscle
ANS - smooth muscle, cardiac muscle, glands
ENS - smooth muscle, glands and endocrine glands of GI tract
What two kinds of cells types is nervous tissues made up of?
- Neurons
2. Neuroglia
What does a neuron do in the body?
Nerve Cell.
Connects all regions of the body to the brain & spinal cord
Sensing, thinking, remembering, controlling muscles, regulating glands,
Lost ability to divide (mitosis)
What is electrical excitability?
Ability to respond to a stimulus and convert it into an action potential
What is a stimulus?
Any change in the environment that is strong enough to initiate an action potential
What is an action potential (nerve impulse)?
Electrical signal that travels along the surface of the membrane of a neuron
- due to movement of ions (sodium and potassium)
Which nervous system cells posses electrical excitability?
Neurons.
What are the three parts of a neuron?
- Cell body
- Dendrites
- Axon
What is the cell body?
Contains the nucleus surrounded by cytoplasm (lysosomes, mitochondria, golgi complex, free ribosomes, rER)
Also known as Perikaryon or Soma
What are microtubules?
Assist in moving materials between cell body and axon
What is lipofuscin?
Product of neuronal lysosomes that accumulates as the neuron ages
Does not affect function
What are Nissl bodies?
Prominent clusters of rough endoplasmic reticulum and free ribosomes
- Replace cellular components
(material for growth or regenerate damaged axons)
What are neurofibrils?
Provide cell shape and support (intermediate filaments)
What is a dendrite?
The receiving or input portion of a neuron
Short, tapering, highly branched
Contain numerous receptor sites for binding chemical messengers
Contains Nissl bodies, mitochondria, other organelles
What is an axon?
Long, thin, cylindrical projection
Propagates nerve impulses toward another neuron
What is an axon hillock?
Where the axon joins to the cell body
Cone-shaped elevation
What is the initial segment?
Part of the axon closest to the axon hillock
What is the trigger zone?
Nerve impulses arise at the junction of the axon hillock and the initial segment, then travel along the axon to their destination
What does an axon contain?
Mitochondria
Microtubules
Neurofibrils
* no rough ER, so no protein synthesis
What is axoplasm?
Cytoplasm of an axon
What is the axolemma?
Plasma membrane of an axon
What is the direction of flow of information?
Dendrites –> Cell Body –> Axon –> Axon Terminals
What are axon collaterals?
Side braches (typically at a right angle to the axon)
What are axon terminals (telodendria)?
The axon and axon collaterals end by dividing into many fine processes called axon terminals
What is a synapse?
The site of communication between two neurons or between a neuron and an effector cell
What are synaptic end bulbs?
The tips of some axon terminals that swell into bulb-shaped structures
-others exhibit a string of swollen bumps called varicosities
What are synaptic vesicles?
Tiny membrane-enclosed sacs contained in the synaptic end bulbs and varicosities
What is a neurotransmitter? Where is it stored?
A chemical that excites or inhibits another neuron, muscle fibre, or gland cell
Synaptic vesicles
What is slow axonal transport?
Conveys axoplasm in one direction only
-from cell body toward the axon terminal
What is fast axonal transport?
Uses proteins that function as “motors” to move materials along the surfaces of microtubules
- goes in both directions
What is anterograde? Retrograde?
Anterograde - from cell body to axon terminals (forward)
Retrograde - from axon terminals to cell body (backward)
How are neurons classified structurally?
According to the number of processes extending from the cell body
What are the structural classifications of neurons?
- Multipolar neurons
- Bipolar neurons
- Unipolar neurons
What is the structural diversity of neurons?
Diversity in size and shape
Range in diameter from 5 -135 micrometers
Long or short axons
What is a multipolar neuron?
Several dendrites and one axon
- brain, spinal cord, motor neurons
CNS
What is a bipolar neuron?
One main dendrite and one axon
- retina of eye, inner ear, olfactory area of the brain
What is a unipolar neuron?
Dendrites and one axon that are fused together to form a continuous process that emerges from the cell body
- function as sensory receptors (touch, pressure, pain or thermal stimuli)
What is a pseudounipolar neuron?
Unipolar neurons
- begin in the embryo as bipolar neurons
- during development, dendrites and axon fuse together and become a single process
Where is the trigger zone on a unipolar neuron?
Junction of the dendrites and axon
- impluses then propagate toward the synaptic end bulbs
Where are the cell bodies of unipolar neurons located?
In the ganglia of spinal and cranial nerves
in the PNS
What are some examples of unipolar neurons?
Meissner corpuscle - touch receptor
Merkel disc - touch receptor with free nerve endings
Pacinian (lamellated) corpuscle - pressure receptor
Nociceptor - pain receptor
How are neurons classified functionally?
According to the direction in which the nerve impulse (action potential) in conveyed with respect the the CNS
What are the functional classifications of neurons?
- Sensory (afferent) neurons
- Motor (efferent) neurons
- Interneurons (association)
What are sensory neurons?
Either contain sensory receptors at their distal ends (dendrites) or are located just after sensory receptors that are separate cells
- Stimulus –> sensory receptor–> sensory neuron –> (action potential) –> axon –> CNS (through cranial or spinal nerves)
- UNIPOLAR
What are motor neurons?
Convey action potentials away from the CNS to the effectors (muscles and glands) in the PNS through cranial or spinal nerves
-MULTIPOLAR
What are interneurons?
Located in CNS between sensory and motor neurons
Process incoming sensory information from sensory neurons and elicit a motor response
-MULTIPOLAR
How are neuroglia different from neurons?
Smaller than neurons Support, protect and nourish neurons 5-25x more numerous Do not generate action potentials Able to divide 6 types
What are gliomas?
Brain tumors derived from glia (neuroglia)
Highly malignant and grow rapidly
What are the 4 types of neuroglia of the CNS?
- Astrocytes
- Oligodendrocytes
- Microglia
- Ependymal
What are astrocytes?
Neuroglia of the CNS
Star-shaped, largest, most numerous
Have many processes
What are the two types of astrocytes?
- Protoplasmic astrocytes - short branching processes, found in gray matter
- Fibrous astrocytes - long unbranched processes, located in white matter
What are the functions of the astrocytes?
- Contain microfilaments for strength, support neurons
- Processes wrapped around blood capillaries isolate neurons of the CNS from various harmful substances in blood (Blood-brain barrier)
- In the embryo, secrete chemcials that appear to regulate the growth, migration, and interconnection among neurons in the brain
- Maintain proper chemical environment (potassium concentration)
- Role in learning and memory by influencing the formation of neurla synapses
What are oligodendrocytes?
Neuroglia of the CNS
Resemble astrocytes, but smaller and contain fewer processes
What is the function of oligodendrocytes?
Responsible for forming and maintaining the myelin sheath around CNS axons
1 oligodendrocyte can maintain many axons
What is the myelin sheath?
Multi-layered lipid and protein covering around some axons that insulates and increases the speed of nerve impulse conduction
What does myelinated mean?
Axons that have a myelin sheath around them
What are microglia?
Neuroglia of the CNS
Small cells with slender processes that give off numerous spinelike processes
What is the function of microglia?
Function as phagocytes
Remove cellular debris formed during normal development of the nervous system and phagocytize microbes and damaged nervous tissue
What are ependymal cells?
Neuroglia of the CNS
Cuboidal or columnar cells arranged in a single layer that possess microvilli and cilia
Line the ventricles of the brain and central canal of the spinal cord
What is the function of ependymal cells?
Produce, possibly monitor, and assist in the circulation of cerebrospinal fluid
Form blood-cerebrospinal fluid barrier
What are the 2 types of neuroglia of the PNS?
Completely surround axons and cell bodies
- Schwann Cells
- Satellite Cells
What are schwann cells?
Neuroglia of the PNS
Encircle PNS axons, form myelin sheath
What is the function of schwann cells?
Form myelin sheath around axons
1 schwann cell per single axon
BUT 1 schwann cell can enclose as many as 20 or more UNMYELINATED axons
Participate in axon regeneration
What are satellite cells?
Neuroglia of PNS
Flat cells surround the cell bodies of neurons of PNS ganglia
What are the functions of satellite cells?
Provide structural support
Regulate the exchanges of materials between neuronal cell bodies and interstitial fluid
What are the two types of neuroglia that produce myelin sheaths?
- Oligodendrocytes (CNS) - neurolemma not present
2. Schwann Cell (PNS) - form during fetal development
What is the neurolemma (sheath of Schwann)?
Schwann cell wraps around axon, consisting of up to 100 layers of Schwann membrane (myelin sheath) and the cytoplasm and nucleus are in the outermost layer.
The outer nucleated layer that encloses the myelin sheath is the neurolemma
Where is neurolemma ONLY found?
Around axons in the PNS
What is the function of neurolemma?
Aids in regeneration, stimulates regrowth of axon
What are nodes of Ranvier?
Gaps in the myelin sheath, appear at intervals along the axon
Serves to facilitate the rapid conduction of nerve impulses
What are the three parts of a Schwann cell?
- Cytoplasm
- Myelin Sheath
- Neurolemma
What happens to the myeline sheath as you age?
Amount of myelin increases from birth to maturity
Presence greatly increases the speed of nerve impulse conduction
Where is the nucleus of a schwann cell on an unmyelinated axon?
In the center
What is a nucleus?
Cluster of neuronal cell bodies located in the CNS
What is a tract?
Bundle of axons located in the CNS
Interconnect neurons in the spinal cord and the brain
What is white matter?
Composed primarily of myelinated axons
- myelin has a whitish colour
- blood vessels are present
What is grey matter?
Contains neuronal cell bodies, dendrites, UNmyelinated axons, axon terminals, and neuroglia
- Nissl bodies are grayish
- blood vessels are present
Where is white matter in the spinal cord and brain?
Surrounds an inner core of gray matter in spinal cord
Shaped like a butterfly in the brain
Where is gray matter in the spinal cord and brain?
Inner core in spinal cord
Thin shell of gray matter covers the cerebrum and cerebellum
What is the sympathetic division? (PNS)
Motor part of autonomic nervous system (PNS)
Would increase heart rate
Helps support exercise of emergency actions
“fight or flight” responses
What is the parasympathetic division? (PNS)
Motor part of autonomic nervous system (PNS)
Would decrease heart rate
“rest and digest” responses
What are the two types of electrical signals that allow neurons to communicate with each other?
- Graded potentials
2. Action potentials
What are graded potentials used for?
Used for communication over short distances
What are action potentials used for?
Used for communication over long distances
What is the name for an action potential in a nerve cell?
Nerve action potential (nerve impulse)
Describe the function of graded potentials and action potentials as the nervous system allows you to feel a pen in your hand.
- Graded potential develops in a sensory receptor in the skin of the fingers
- Graded potential triggers the axon of the sensory neuron to form a nerve action potential, travels along the axon into the CNS and ultimately causes the release of neurotransmitter at a synapse with an interneuron
- Neurotransmitter stimulates the interneuron to form a graded potential in its dendrites and cell body
- Axon of the interneuron forms a nerve action potential. Nerve action potential travels along axon, which results in neurotransmitter release at the next synapse with another interneuron
- Neurotransmitter release occurs over and over as interneurons in higher parts of the brain are activated. Once the cerebral cortex is trigger - perception occurs and you feel the pen
Describe the function of graded potentials and action potentials if you wanted to write with the pen in your hand.
- Stimulus in the brain causes a graded potential to form in the dendrites and cell body of an upper motor neuron. Graded potential causes a nerve action potential to occur in the axon of the upper motor neuron, followed by neurotransmitter release.
- Neurotransmitter release generates a graded potential in a lower motor neuron (directly supplies skeletal muscle fibres). Graded potential triggers the formation of a nerve action potential and then release of the neurotransmitter at neuromuscular junctions formed with skeletal muscles.
- Neurotransmitter stimulates muscle fibers that control finger movements to form muscle action potentials. Muscle action potentials cause these muscle fibres to contract, which allows you to write with the pen
What is membrane potential?
An electrical potential difference (voltage) across a membrane
What is resting membrane potential?
Electrical potential difference in excitable cells across a plasma membrane
What is the current?
The flow of ions (into and out of cell membranes) constitutes the electrical current
What is an electrochemical gradient?
A chemical and electrical concentration difference
What are the 4 types of ion channels?
- Leak Channels
- Ligand-gated Channels
- Mechanically gated Channels
- Voltage-gated Channels
What are leak channels?
- gated channels randomly open and close
- plasma membranes have many more K+ ion leak channels than Na+ ion leak channels
- membrane is much more permeable to K+ than Na+
- found in all cells (dendrites, axons, cell bodies)
What are ligand-gated channels?
- gated channels open in response to binding of ligand (chemical) stimulus
- neurotransmitters, hormones, ions - all examples of ligands
Location: dendrites of some sensory neurons (pain receptors) and in dendrites and cell bodies of interneurons and motor neurons
What are mechanically-gated channels?
- gated channels open in response to mechanical stimulus (sound, touch, pressure, tissue stretching, vibration)
- force distorts channel from resting position, opening the gate
Location: dendrites of some sensory neurons (touch, pressure and some pain receptors) - in the ear, internal organs and touch/pressure receptors in the skin
What are voltage-gated channels?
- gated channels open in response to voltage stimulus (change in membrane potential)
- participate in the generation and conduction of action potentials
Location: axons of all types of neurons
Why does the resting membrane potential exist?
Small buildup of (-) ions in the cytosol Buildup of (+) ions in the extracellular fluid
True or False:
The greater the difference in charge across the membrane, the larger the membrane potential
True
What is the resting membrane potential in neurons?
in volts
Ranges from -40 to -90 mV
Typically -70mV
What does it mean if a cell is polarized?
A cell that exhibits a membrane potential
What are the three major factors that lead to a resting membrane potential?
- Unequal distribution of ions in the ECF (+) and cytosol (-)
- Inability of most anions to leave the cell (they are too large)
- Electrogenic nature of the Na+/K+ ATPases
What is a graded potential?
Small deviation from the membrane potential that makes the membrane either more polarized (inside more -) or less polarized (inside less -)
What is a hyperpolarized graded potential?
When a response makes the membrane more polarized (inside more negative)
What is a depolarizing graded potential?
When a response makes the membrane less polarized
inside less negative
When does a graded potential occur?
When a stimulus causes mechanically-gated or ligand-gated channels to open or close in an excitable cell’s plasma membrane
Why do graded potential vary in amplitude?
Depends on how many ligand-gated or mechanically gated channels have opened (or closed) and for how long
What kind of graded potential describes a change in membrane potential from -70 to -60 mV?
From -70 to -80 mV?
-70 to -60 mV = depolarizing
From -70 to -80 mV = hyperpolarizing
What is decremental conduction?
The transmission of an electric impulse in which the amplitude of the impulse decreases with distance.
- a graded potential, over short distances
What is summation?
The process by which graded potentials add together
- if two depolarizing graded potentials summate, the net result is a larger depolarizing potential
- if two hyperpolarizing graded potentials summate, the net resultis a larger hyperpolarizing potential
- if one depolarizing and one hyperpolarizing graded potentials summate (and they’re equal), they cancel each other out
What is an action potential?
A sequence of rapidly occurring events that decrease and reverse the membrane potential and then eventually restore it to the resting state
What are the two main phases of an action potential?
- Depolarizing
2. Repolarizing
What happens during the depolarizing stage of an action potential?
Negative membrane potential becomes less negative, reach zero and then becomes positive
What happens during the repolarizing stage of an action potential?
Membrane potential is restored to the resting state of -70mV
What is an after-hyperpolarizing phase? When does it occur?
After repolarization
Membrane potential temporarily becomes more negative than the resting level
What two types of voltage-gated channels open and close during an action potential?
Voltage-gated Na+ channels open, Na+ rushes into the cell (depolarization)
Voltage-gated K+ channels open, K+ rushes out of cell
(repolarization)
What is a threshold?
An action potential occurs in a membrane of the axon of a neuron when depolarization reaches a certain level
What is the subthreshold stimulus?
A stimulus that is a weak depolarization, cannot bring membrane potential to threshold
What is a threshold stimulus?
A stimulus that is just strong enough to depolarize the membrane to threshold
What is a suprathreshold stimulus?
A stimulus that is strong enough to depolarize the membrane above the threshold
What is the “all-or-none” principle?
An action potential either occurs completely or it does not occur
What are the 4 stages of an action potential?
- Depolarizing
- Repolarizing
- After-hyperpolarizing
- Refractory
What happens during the depolarization phase?
Voltage-gated Na+ channels open rapidly.
Inward movement of Na+
Changes membrane potential from -55mV to +30mV
At its peak, inside is 30mV more positive than the outside
Positive feedback
What happens during the repolarization phase?
Na+ channel is in an inactivated state
Voltage-gated K+ channels open
K+ open more slowly (they open as Na+ channels are closing)
Slowing of Na+ inflow and acceleration of K+ outflow causes the membrane potential to change from +30mV to -70mV
What happens during the after-hyperpolarizing phase?
Voltage-gated K+ channels remain open and membrane potential becomes even more negative
(-90mV)
Once K+ channels close, membrane potential returns to resting level
What is the refractory period?
Period of time after an action potential where an excitable cell cannot generate another action potential
(in response to a “normal” threshold stimulus)
What is the absolute refractory period?
A very strong stimulus CANNOT initiate a second action potential
True or False:
Graded potentials exhibit a refractory period
False
Action potentials exhibit a refractory period
Under normal circumstances, what is the maximum frequency of nerve impulses?
10-1000 per second
What is the relative refractory period?
Period of time during which a second action potential CAN be initiated, but only by a larger than normal stimulus
True or False:
Action potential is decremental
False
Action potential keeps it strength and propagates along the membrane
What is propagation?
Action potential keeps it strength, is not decremnetal
Depends on positive feedback
How does positive feedback work on action potentials?
When Na+ ions flow in, they cause voltage-gated channels in adjacent segments to open
Thus, action potential travels along the membrane (like dominos)
Can an action potential travel both ways along the membrane?
No - it can only travel in one direction.
Cannot go back, b/c any region that has just undergone an action potential is temporarily in the refractory period
What are the two kinds of propagation?
- Continuous conduction
2. Saltatory conduction
What is continuous conduction?
Involves step-by-step depolarization and repolarization of each adjacent segment of the plasma membrane
-occurs in unmyelinated axon and in muscle fibres
What is saltatory conduction?
Special mode of propagation
Occurs along myelinated axons
Occurs b/c of uneven distribution of voltage-gated channels
What is the difference between the Nodes of Ranvier and the myelinated axon sheath in terms of voltage-gated channels?
Areas of myelin sheath - have few voltage-gated channels
Nodes of Ranvier -have many voltage-gated channels
*current carried by Na+ and K+ flows across the membrane mainly at the nodes
*each node repolarizes after it depolarizes
What are the two consequences of Nodes of Ranvier?
- Action potential appears to “jump” from node to node, it travels much faster than an unmyelinated axon
- Opening smaller number of channels only at the nodes, it is more efficient
- less ATP is used, minimal flow of Na+ and K+
What are the factors that affect the speed of propagation?
- Amount of myelination - myelinated is faster
- Axon diameter - larger diameter axons propagate faster due to larger surface area
- Temperature - axons propagate action potentials at lower speeds when cooled
How does the frequency of a stimulus affect action potentials?
- Frequency of action potentials - a light touch generates a low frequency of action potentials
- a firm touch elicits action potentials that pass down the axon at a higher frequency - Number of sensory neurons recruited - a firmer touch stimulates a larger number of pressure-sensitive neurons than does a light tough
What are the major differences between graded potentials and action potentials?
Graded: Decremental (not propagated), over short distances, lasts longer, may be hyperpolarizing or depolarizing, no refractory period (summation CAN occur), arise mainly in dendrites and cell body, ligand-gated or mechanically gated
Action: Propagates, long distances, short duration, depolarizing->repolarizing->resting, refractory present (summation CANNOT occur), arise at trigger zones, voltage-gated channels for Na+ & K+
What is a presynaptic neuron?
A nerve cell that carries a nerve impulse toward a synapse
- it is the cell that sends the signal
What is the postsynaptic neuron?
Carries a nerve impulse away from a synapse or an effector cells that responds to the impulse at the synapse
- it is the cell that receives the signal
Why are synapses essential for homeostasis?
Allow information to be filtered and integrated
What is an electrical synapse?
Action potentials (impulses) conduct directly between the plasma membranes of adjacent neurons through gap junctions
What is a gap junction?
Connection between two cells
- contains 100 tubular connexons, that connect the cytosol of two cells together
What are the two main advantages of electrical synapses?
- Faster communication
2. Synchronization
What is a chemical synapse?
Neurons are close to each other, but they do not touch, separated by a synaptic cleft
- presynaptic neuron releases a neurotransmitter that diffuses through the fluid in the synaptic cleft and binds to receptors in the plasma membrane on the postsynaptic neuron
What is the postsynaptic potential?
A type of graded potential
- changes in the membrane potential of the postsynaptic terminal at a chemical synapse
How does the presynaptic neuron communicate with the postsynaptic neuron?
Presynaptic neuron converts an electrical signal into a chemical signal.
Post synaptic neuron receives the chemical signal and generates an electrical signal
Describe the signal steps at a chemcial synapse?
- Nerve impulse arrives at synaptic end bulb of presynaptic axon
- Depolarizing phase open voltage-gated Ca+2 channels, calcium flows in
- Increase in Ca+2 triggers exocytosis of synaptic vesicles, releases neurotransmitter into synaptic cleft
- Neurotransmitter diffuse across synaptic cleft, bind to neurotransmitter receptors on postsynaptic neuron
- Ligand-gated channels open, ions flow across membrane
- Voltage changes across membrane (postsynaptic potential), may be depolarizing or hyperpolarizing
- Depolarizing postsynaptic potential reaches threshold, trigger and action potential in the axon of the postsynaptic neuron
True or False:
Chemical action potentials move in one direction
True
What is an excitatory postsynaptic potential (EPSP)?
Causes depolarization of the postsynaptic membrane
Is excitatory b/c it brings the membrane closer to the threshold
*usually a single EPSP does not initiate a nerve impulse
What is an inhibitory postsynaptic potential (IPSP)?
Causes hyperpolarization of the postsynaptic membrane
Is inhibitory b/c it is more negative and therefore further from the threshold
What are the three ways that a neurotransmitter is removed from the synaptic cleft?
- Diffusion
- Enzymatic degradation
- Uptake by cells
How does diffusion remove neurotransmitters?
Diffuse away from synaptic cleft
If a neurotransmitter is away from the receptor, there is no effect
How does enzymatic degradation remove neurotransmitters?
Certain neurotransmitters are inactivated by degradation
Ex. acetylcholinesterase breaks down acetylcholine
How does the uptake by cells remove neurotransmitters?
Neurotransmitter are actively transported back into the neuron (re-uptake)
What are the two kinds of summation?
- Spatial summation
2. Temporal summation
What is spatial summation?
Summation of postsynaptic potentials in response to stimuli that occur at different LOCATIONS in the membrane of a postsynaptic cell at the same time
What is temporal summation?
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
What can happen if a single postsynaptic neuron receives input from many presynaptic neurons - some of which are excitatory and some inhibitory? (Three results)
- EPSP
- Nerve Impulses
- IPSP
Describe what will happen if a single postsynaptic neuron receives input from many presynaptic neurons and it is EPSP.
EPSP - total excitatory effects are greater than inhibitory effects, but less than threshold level. Following an EPSP, subsequent stimuli are more likely to trigger b/c cell is slightly depolarized
Describe what will happen if a single postsynaptic neuron receives input from many presynaptic neurons and results in a nerve impulse.
Nerve Impulses - total excitatory effects are greater than inhibitory effects, and threshold is reached. Impulses continue to be generated as long as the EPSP is at or above the threshold level
Describe what will happen if a single postsynaptic neuron receives input from many presynaptic neurons and it is IPSP.
IPSP - total inhibitory effects are greater than excitatory effects, membrane hyperpolarizes. Result is inhibition of the postsynaptic neuron, no nerve impulse
What are neurosecretory cells?
Certain neuron in the brain that secrete hormones
How are neurotransmitters classified?
Two classes based on size:
- Small molecule neurotransmitters
- Neuropeptides
What is acetylcholine? (ACh)
Small-molecule neurotransmitter
Excitatory & Inhibitory
Released by PNS and CNS
What is glutamate?
Small-molecule neurotransmitter (amino acid)
Excitatory - opens Na+ channels
Inactivation occurs via re-uptake
In CNS
What is glycine?
Small-molecule neurotransmitter (amino acid)
Inhibitory - opens Cl- channels
What are biogenic amines?
Amino acids that are modified and decarboxylated (carboxyl group removed)
What is norepinephrine? (NE)
Small-molucule neurotransmitter (biogenic amine)
Plays roles in arousal (awakening), dreaming and regulating mood
Excitatory
Also serves as a hormone
What is epinephrine?
Small-molecule neurotransmitter (biogenic amine)
“Adrenaline” - regulates heart rate and blood pressure
Excitatory
Also serves as a hormone
What is dopamine?
Small-molecule neurotransmitter (biogenic amine)
Active during emotional responses, addictive behaviours, & pleasurable experiences
What is catecholamines?
All include an amino group (-NH2) and a catechol ring composed of six carbons and two adjacent (-OH) groups
Inactivation via re-uptake
What are three examples of catecholamines?
- Norepinephrine
- Epinephrine
- Dopamine
What is serotonin?
Small-molecule neurotransmitter (bigenic amine)
Involved in sensory perception, temperature regulation, control of mood, appetite and induction of sleep
Inhibitory
What is nitric oxide? (NO)
Small-molecule transmitter
Excitatory neurotransmitter
Secreted in the brain, spinal cord, adrenal glands and nerves to the penis
Contains a single nitrogen atom
Not synthesized in advance, highly reactive
Lipid-soluable
What are neuropeptides?
Neurotransmitters consisting of 3-40 amino acids linked by peptide bonds
Excitatory or inhibitory
Role in memory & learning, feelings of pleasure, temperature control, sex drive
What are neural circuits?
Functional groups of neurons that process specific types of information
What is a simple series?
A presynaptic neuron stimulates a single postsynaptic neuron
- and so on …
What is a diverging circuit?
A single presynaptic neuron to stimulate many postsynaptic neurons
- muscle fibres or gland cells
- arrangement amplifies the signal
What is a converging circuit?
Several presynaptic neurons synapse with a single postsynaptic neuron
- more effective stimulation or inhibition
- receives input from several sources
What is a reverberating circuit?
Stimulation of the presynaptic cell causes the postsynaptic cell to transmit a series of nerve impulses
- breathing, muscular activities, short-term memory
- feeds back on itself
What is a parallel after-discharge circuit?
A single presynaptic cell stimulates a group of neurons, each of which synapses with a common postsynaptic cell
- can send out impulses in quick succession
What is plasticity?
The nervous systems’ capability to change based on experience
- sprout new dendrites, synthesis of new proteins, changes in synaptic contacts
What is neurogenesis?
Birth of new neurons from undifferentiated stem cells
What two factors result in the lack of neurogenesis?
- Inhibitory influences of neuroglia
2. Absence of growth-stimulating cues present during fetal development
Can repair occur in the PNS?
Yes - if:
- the cell body is still intact
- Schwann cells are functional
- Scar tissue does not form too rapidly
Describe the steps in PNS repair and regeneration.
- 24-48 hours after injury: Nissl bodies break up into fine granular masses
- 3-5 days: part of axon distal to the damaged region become slightly swollen and then breaks up into fragments, myelin sheath also deteriorates
- neurolemma remains
- form regeneration tube - guides growth for new axon
What is multiple sclerosis? (MS)
Disease that causes progressive destruction of myelin sheaths surrounding neurons in the CNS
- autoimmune disease
What is epilepsy?
Characterized by short, recurrent attacks of motor, sensory, or psychological malfunction
- initiated by abnormal, synchronous electrical discharges from millions of neurons in the brain
- reverberating circuits? maybe
