Surround peripheral nerve fibers and form myelin shealths
Vital to regeneration of damaged peripheral nerve fibers
What are the special characteristics of Neurons
Long-lived (100yrs+)
Amitoitc
High metabolic rate
Plasma membrane functions in : Electrical signaling
Describe the Cell body ( Perikaryon or Soma)
Biosyntheic center of a neuron
Spherical nucleus with nucleolus
Well-developed Golgi apparatus
Rough ER called Nissl Bodies
Network of neruofibrils (neurofilaments)
Axon hillock- cone shaped area from which axon arises
Clusters of cell bodies are called nuclei in the CNS, ganglia in the PNS
What are clusters of cell bodies called in the CNS?
Nuclei
What are clusters of cell bodies called in the PNS?
ganglia
Name the processes
Dendrites and axons
What are Bundles of processes called in the CNS?
Tracts
What are Bundles of processes called in the PNS?
Nerves
Describe a Dendrite
Short, tapering, and diffusely branched
Receptive (input) region of a neuron
Convey electrical signals toward the cell body
Describe the Axon
One axon per cell arising from the axon hillock
Long axons (nerver fibers)
Occasional branches (axon collaterals)
Knoblike axon terminals (synaptic knobs): 1.secretory region of neurons 2. release neurotransmitters to excite or inhibit other cells
Describe Axon functions
Conducting region of a neuron
Generates and transmits nerve impulses (action potentials) away from the cell body
Molecules and organelles are moved along axons by motor molecules (protien carriers) in two directions: 1. Anograde-toward axon terminal 2. Biograde- toward the cell body
What is Anterograde
the movement of molecules and organelles by motor molecules toward axonal terminal
ex: mitochondria, membrane componenets, enzymes
What is Retrograde
the movement of molecules and organelles by motor molecules toward the cell body
ex: organelles to be degraded, signal molecules, viruses, and bacterial toxins
What is a Myelin Shealth?
Segmented protein-lioid sheath around most long or large-diameter axons
What is the function of a Myelin Sheath?
Protects and electrically insulates the axon
Increases speed of nerve impulse transmission
Describe Myelin Shealths in the PNS
Concentric layers of Schwann cells that wrap many times around the axon.
What is the Neurilemma?
It is the peripheral bulge of Schwann cells, the outermost layer. In the PNS
What is the Nodes of Ranvier?
Myelin sheath gaps between adjacent Schwann cells
Sites where axon collaterals can emerge (ocassional)
Are there unmyelinated Axons?
There are some thin and small nerve fibers which are unmyelinated without affection of the function
What are Myelin Sheaths in the CNS formed by?
Formed by processes of oligodendrocytes, not whole cells
True or False, there is a Neurilemma in the Myelin Sheaths of the CNS.
False: there is no neurilemma in the myelin sheath in the CNS as the whole oligodendrocyte is not used.
True or False, Node of Ranvier are present in Myelinated Axons in the CNS.
True
White matter is made up of what?
A dense collection of myelinated fibers
Gray matter is made up of what?
Mostly neuron cell bodies and unmyelinated fibers
What are the three types of neurons (Classified by function) ?
Sensory
Motor
Interneurons
What do Sensory Neurons do?
Transmit impulses from sensory receptors toward the CNS
True or False, Sensory neurons are Efferent.
False, they are afferent
What do Motor Neurons do?
Cary impulses from the CNS to effectors
True or False, Motor Neurons are Efferent.
True
What do Internuerons (association neurons) do?
Shuttle signal through CNS pathways, most are entirely within the CNS
True or False: Neurons are highly irritable
True, neurons are excitable
If a neuron has an adequate stimulus what does it do
It responds to adequate stimulus by generating and action potential ( nerve impulse)
What serves as membrane ion channels?
Protiens serve as membrane ion channels
What are the two main types of ion channels?
Leakage (nongated) channels- always opened
Gated Channels: open and close
What are the three types of gated channels?
Chemically gated (ligand-gated) channels
Voltage-gated channels
Mechanically gated channels
Do Chemically Gated Channels open, close or both, and how?
open with binding of a specific neurotransmitter
Do Voltage-gated channels opem, close, or both, and how?
open and close in response to changes in membrane potential
Do Mechanically gated channels open, close or both, and how?
open and close in response to physical deformation of receptors
What happens when gated channels are open
Ions diffuse quickly across the membrane along their electrochemical gradients
Along chemcial concentration gradients from higher concentration to lower concentration
Along electrical gradients toward opposite electrical charge
Ion flow creates an electrical current and voltage changes across the membrane
What is the potential difference across the membrane of a resting cell?
Approximately -70mV in neurons (cytoplasmic side of membrane is negatively charged relative to outside)
What are the Potential differences genertated by?
Differences in ionic makeup of ICF and ECF
Selective premeability of the plasma membrane
What are the differences in ionic makeup in the cell membrane?
ICG has lower cncentration of Na+ and Cl- than ECF
ICF has higher concentration of K+ and negatively charged proteins (A-) than ECF
Describe the Premeabiltiy of the Cell membrane
Impremeable to A-
Slightyly premeable to Na+ (through leakage channels)
75 times more premeable to K+ ( more leakage channels)
Freely permeable to Cl-
What stabilizes the resting membrane potential?
the Sodium-potassium pump stabilizes the resting membrane potential by maintaining the concentration gradeints for Na+ and K+
Membrane potential changes when
Concentrations of ions across the membrane change
Permeability of membrane to ions changes
What are changes in membrane potentials used for?
the changes are signals which are used to receive, integrate and send information
What are the two types of signals in Membrane potentials?
Action potentials- Long- distance signals of axons
What happens during depolarization?
The membrane potential goes toward ZERO point and then becomes positve
What happens during depolarization?
The membrane potential goes towards the ZERO point and then becomes more and more positive.
Increases the probability of producing a nerve impulse.
What happens during Hyperpolarization?
The membrane potential becomes more negative than RMP
Reduces the probability of producing a nerve impulse
What are Graded potentials?
Short-lived localized changes in membrane potentials
Depolarizations or Hyperpolarizations
Graded potential spreads as local currents change the membrane potential of adjacent regions
Occur when astimulus causes gated ion channels to open
Magnitude varies directly (graded) with stimulus strength
Decrease in magnitude with distance
Short-distance signals
What is Action Potential (AP) ?
Brief reversal of membrane potential with a total amplitude of about 100mV (from -70mV to +30mV)
Principal means of long-distance neural communication
Where does Action Potential occur?
in muscle cells and axons of neurons
True or False, Action Potential decreases in magnitude over distance
False it does not decrease in magnitude over distance
During the Resting state in the Generation of Action Potential what happens?
Only Leakage channels for Na+ and K+ are open
All gated Na+ and K+ channels are closed
Describe the Na+ Channel
Closed at rest
Open fast with depolarization
Describe K+ Channels
Close at rest
Opens slowly with depolerization
What Happens During the Depolarizing Phase?
The depolarized local currents open voltage-gated Na+ Channesl
Na+ influx causes more depolarization
At threshold (-55to -50mV) positive feedback leads to opening of all Na+ channels, and a reversal of membrane polarity to +30mV(spike of action potential)
What happens during Repolarization?
Na+ Channel closes
Membrane premeability to Na+ declines to resting levels
Slow voltage-sensitive K+ gates open
K+ exits the cell and internal negativity is restored
What happens during Hyperpolarization
Some K+ channels remain open, allowing excessive K+ efflux
True or False, Sodium-Potassium pumps always restore the origninal resting membrane potential by maintaing the concentration gradients for Na+ and K+
True
Give a brief summary of Action Potential.
Na+ influx causes a patch of the axonal membrane to depolariz
Local currents occur
Na+ channels toward the point of origin are inactivated and not affected by the local currents
Local currents affect adjacent areas in the forward direction
Depolarization opens voltage-gated channels and triggers an AP
Repolarization wave follows the depolarization wave
Threshold
Membrane is depolarized by 15 to 20mV( around -55 to -55 mV)
Na+ permeability increases
Na influx exceeds K+ efflux
the positive cycle begins
What is the All or none phenomenon?
It is action potentials either completely happening or not based on it reaching or not reaching the threshold
Graded or Action Potentials? Ligand or mechanically gated
Graded
Graded or Action Potentials?
Voltage gated
Action
Graded or Action Potentials? Propagated
Action
Graded or Action Potentials?
Long distances
Action
Graded or Action Potentials?
Not propagated
Graded
Graded or Action Potentials? Short distances
Graded
Graded or Action Potentials? Depends on strenght of stimulus
Graded
Graded or Action Potentials? All or none
Action
What effect does the axon diameter have on conduction?
The larger the diameter fibers the faster the impulse of conduction
What effect does myelination have on the conduction velocity?
Continuous conduction in unmyelinated axons is slower than saltatory conduction in myelinated axons
True of False: Myelin Sheaths insulate and prevent leakage of charge
True
How much faster is conduction in myelinated axons?
Saltatory conduction in myelinated axons is about 30 times faster
Where are voltage gated Na+ channels located?
they are in the nodes
True or false: APs jump slowly from node to node
False, APs appear to jump rapidly from node to node
What is a synapse?
A junction that mediates information transfer from one neuron to A) another neuron or B) an effector cell
What is a presynaptic Neuron
Conducts impulses toward the synapse
What is a postsynaptic Neuron?
transmits impulses away from the synapse
What is an Axodenrtic Synapse?
A sypase b/w the axon of one neuron and the dendrite of another
What is an Axosomatic synape?
A synapse b/w the axon of one neuron and the soma of another
What are Electical Synapases?
Less common than chemical synapses
Neurons ar eelectrically coupled (joined by gap junctions)
Communication is very rapid, and may be unidirectional or bidirectional
Are important in: Embyronic nervous tissue
Whate are Chemical Synapses?
they are specialized for the release and reception of neurotransmitters
Typically composed of two parts : A) Axon terminal of the presynaptic neureon, which contains synaptic vesicles B) Receptor region on the postsynaptic neuron
What is the Synaptic Cleft?
It is a fluid-filled space separating the presynaptic and postsynaptic neurons
Prevents nerve impulses from directly passing from one neuron to the next
Describe the Transmission across the synaptic cleft:
It is a chemical event (as oppesed to an electrical one)
Involves release, diffusion and binding of neurotransmitters
Ensures unidirection communication b/w neurons
Describe Information Transfer
AP arrives at axon terminal of the presynaptic neuron and opens voltage -gated Ca2+ channels
Ca2+ ions promote fusion of synaptic vesicles with axon membrane
Exocytosis of neurotransmitter occurs
Neurotransmitter diffuses and bind to receptors (often chemically gated ion channels) on the postsynaptic neuron
Ion channels are opened, causing and excitatory or inhibitory event
Describe the termination of Neurotransmitter Effects
Within a few milliseconds the neurotransmitter effect is terminated
Degradation by enzymes
Reuptake by astrocytes or axon terminal
Diffusion away form the synaptic cleft
Describe Synaptic Delay
Neurotransmitter must be released, diffuse across the synaps, and bind to receptors
Synaptic delay-time needed to do this (0.3-5.0 ms)
How is the strength of Postsynaptic Potentials determined?
Amount of neurotransmitters released
Time the neurotransmtter is in the area
What are the types of postsynaptic potentials
EPSP- Excitatory Postsynaptic Potentials
IPSP- Inhibitory Postsynaptic Potentials
Describe Excitatory Synapes and EPSPs
Neurotransmitter binds to and opens chemically gated channes that allow simultaneous flow of Na+ and K+ in opposite directions
Na+ influx is greater than the K+ efflux, causing a net DEPOLARIZATION
EPSP helps trigger AP at axon hillock if EPSP is of threshold strength and opens the voltage-gated channels
Describe Inhibitory Synapses and IPSPs and its function.
Neurotransmitter binds to and opens channeslfor K+ or Cl_
Causes a HYPERPOLARIZATION (the inner surface of membrande becomes more negative)
Reduces the postsynaptic neurons ability to produce an action potential
What are Neurotransmitters?
Most neurons make two or more neurotransmitters, which are released at diffenent stimulation frequencies
50 or more neurotransmitters have been identified
How are neurotransmitters classified?
by the chemical structure and by function
Describe Acteylcholine ( Ach)
Excitatory in skeletal muscles and inhibitory in cardiac muscles
Synthesized by enzyme choline acetyltransferase
Degraded by the enzyme acetylcholinesterase (AChE)
Describe Amies, what do they include?
Include: Dopamine, Norepinephrine (NE), Epinephrine, Serotonin, and Histamine
Broadly distrubuted in the brain
Plays key roles in brain function
Describe Amion Acids, and what they include.
GABA- Gamma (y)-aminobutyric acid
Glycine
Aspartate
Glutamate
Describe Peptides (neuropeptides) and what they include.
Substance P: mediatior of pain signals
Endorphins: Act as natural opiates; reduce pain perception
Gut-brain peptides:somatostatin and cholecystokinin
Purines such as ATP do what?
Act in both the CNS and PNS
Produce fast or slow responses
Induce Ca2+ influx in astrocytes
Provoke pain sensation
If a neurotransmitter is Depolarizing what type of effect does it have? Excitatory or Inhibitory?
Excitatory
If a neurotransmitter is Hyperpolarizing what type of effect does it have? Excitatory or Inhibitory?
Inhibitory
What determines whether a neurotransmitter is inhibitory or excitatory?
The receptor type of the postsynaptic neuron
True or False, GABA and glycine are usually excitatory.
False they are usually inhibitory
True or False, Glutamate is usually Excitatory.
True
Where is Acetylcholine excitatory, and where is it inhibitory?
Its Excitatory @ neuromuscular junctions in skeletal muscle
Inhibitory in cardiac muscle
Describe the Direct Action of a neurotransmitter.
Neurotransmitter binds to channel-linked receptor and opens ion channels
Promotes rapid responses
Ex: ACh and amino acids
Describe the Indirect Action of a neurotransmitter.
It needs a sendary messenger
Neurotransmitter binds to a protein-linked receptor and acts through an intracellular second messenger