About 7 trillion

• Central Nervous System (AKA CNS)
• Peripheral Nervous System (AKA PNS)

• Brain
• Spinal Cord

• All nerves and ganglia outside of and extending into and away from the CNS
• Composed of 12 pairs of Cranial Nerves, 31 pairs of Spinal Nerves

A bundle of nerve fibers (axons) surrounded by a fibrous CT

Swelling in a nerve where neuron cell bodies are concentrated

• Receptors in the nerve endings of Sensory AKA Afferent nerves respond to a stimulus (physical or chemical; internal or external) by converting that stimulus into an electrical signal AKA Nerve Impulse.
• Nerve impulse is conducted (transmitted) via Sensory AKA Afferent nerves into the CNS
• The impulse enters the CNS Integration Center where the impulse is received, interpreted, and responded to
• The response is conducted along Motor AKA Efferent nerves to the appropriate effector (Muscle and/or Gland)

Neurons (AKA nerve cells) and neuroglia (AKA support cells)

• Structural and functional unit of the nervous system
• Length varies from <1.0 mm to >3.0 feet long (longest cell in body), extending from the fingers or toes to/from the spinal cord or from the spinal cord to/from the brain
• All mature neurons are amitotic (unable to undergo mitosis) and cannot regenerate
• Some PNS neurons can repair themselves (NOT REGENERATE - NOTE DIFFERENCE) but CNS neurons cannot
• Neurons have an extremely high metabolic rate, requiring a constant supply of O2 and glucose. Neurons will die after 3-5 minutes of no O2.

• Excitability: responds to a stimulus by generating an electrical signal
• Conductivity: conducts (transmits) the electrical signal
• Secretion: when the electrical signal reaches the terminal end of a neuron (synaptic knob) it secretes a chemical messenger (neurotransmitter) that crosses a gap to stimulate an adjacent cell

• Cell body (AKA soma, neurosoma, or perikaryon)
• Dendrites
• Axon (AKA nerve fiber)

• Soma has a cell membrane, granular cytoplasm, various intracellular intracytoplasmic organelles
• Single central, spherical nucleus with an 'owl's eye' nucleolus. Note: NO CENTRIOLES in mature neuron
• Cytoskeleton consisting of:
• Neurofibrils: bundles of actin proteins - termed neurofilaments - for cell shape and support
• Neurotubules: channels for intracellular transport
• Nissl bodies AKA chromatophilic substances
• Most active and best developed Rough Endoplasmic Reticulum of any of the body's cells
• Other organelles: mitochondria, golgi apparatus, lysosomes
• Lysosomes degrading worn-out organelles produce a brown pigment called lipofuscin which accumulates in old neurons. These lipofuscin granules are 'wear-and-tear' granules

• Typically many dendrites per neuron (one to thousands)
• Wider end of the dendrite extends from the cell body
• Opposite narrow end of the dendrite has receptors
• Primary site for receiving a stimulus and conveying a short-distance signal (graded local potential) TOWARD the cell body
• The more dendrites a neuron has, the more information it can receive

• A single, extension of the cell body ranging from a few mm to >meter in length. Generally has an equal diameter its entire length
• Conducts impulses AWAY FROM the cell body (soma)
• Within the axon itself organelles, ions, nutrients, and neurotransmitters are transported from the cell body to and from the end of the axon
• Axon has a Trigger Zone (of a motor AKA efferent neuron) consisting of:
• Axon Hillock: cone-shaped, slightly elevated area OF THE CELL BODY where the axon originates
• Initial Segment: junction of the axon hillock and axon; nerve impulse traveling AWAY FROM the cell body is generated here
• Axon also has a terminal arborization AKA axon terminal-end branches
• Branched complex at the axon's distal end forming a synaptic knob filled with mitochondria and synaptic vesicles that store and secrete neurotransmitters
• Rarely, there are axon colatterals: branches extending off an axon

• Unipolar: only one extension comes off of the soma (will divide and split later)
• Sensory Neuron is the most common example of Unipolar neuron. Consists of:
• Peripheral fiber/process: this portion of the axon begins as a short 'dendrite-like' sensory receptor that conducts an electrical signal toward the cell body, but bypasses the cell body and continues along the central fiber
• Central fiber/process: this portion of the axon conducts the electrical signal into the CNS
• Bipolar: 2 extensions come off of the soma (1 axon and 1 dendrite)
• Common example: neurons in the eye's retina
• Multipolar: multiple extensions come off of the soma (1 axon, multiple dendrites)
• Most common example: motor neuron

• Sensory AKA Afferent Neurons
• Conduct the impulse from the PNS into the CNS
• Have special receptors at the ends of their dendrites that respond to a stimulus and initiate a local potential that may result in the axon generating and conducting an Action Potential (AKA nerve impulse)
• Interneurons AKA Association neurons
• Conduct the impulse between sensory and motor neurons WITHIN the CNS
• Comrpise 90% of the nerves in the CNS
• Motor AKA Efferent Neurons
• Conduct the responsive impulse from the CNS TO effectors (muscles/glands)

• Both PNS Sensory and PNS Motor nerves each have two divisions:
• Somatic Nervous System (voluntary): Somatic nerves innervate skeletal muscle, skin, bone, and/or joints. NOTE: BOTH Sensory and Motor somatic nerves exist
• Visceral Nervous System (involuntary): AKA Autonomic Nervous System/ANS. ANS nerves innervate glands, cardiac muscle, smooth muscle associated with visceral (internal) organs. NOTE: BOTH Sensory and Motor visceral nerves exist

• Neuroglia outnumber neurons 10:1; protect neurons and help them function (glia = glue)
• PNS has two types of neuroglia:
• Schwann Cell AKA Neurilemmocyte: coils around an axon many times
• Inner layes of Schwann cell's plasma membrane surround and are in direct contact to the external surface of the axon; inner layers contain little or no cytoplasm and are mainly composed of the lipoprotein myelin. The inner layers form the MYELIN SHEATH
• Myelin sheath increases the speed of transmission of nerve impulses; protects and provides electrical insulation of the axon. NOTE: DENDRITES ARE NEVER MYELINATED
• Outer layer of Schwann Cell's plasma membrane is termed the neurilemma and is on the external surface of the myelin sheath; this is where the nucleus and cytoplasm of the Schwann cell are found
• External to the neurilemma is a thin basal lamina (collagen and glycoprotein membrane) and external to this is a thin, fibrous CT sleeve termed the endoneurium 
• Node of Ranvier: small gap along the axon between adjacent Schwann cells
• Internodes: myelin-covered axon segments
• The nerve impulse rapidly travels (jumps) from node of Ranvier to node of Ranvier resulting in increased speed of conduction AKA saltatory conduction. Rate of impulse conduction is mainly dependent upon whether the nerve is myelinated or not and, to a lesser extent, the diameter of the nerve fiber
• If the nerve is myelinated (AKA White Matter): the nerve impulse travels at a velocity of 120m/sec
• If the nerve is unmyelinated (AKA Gray Matter): impulse travels at a velocity of .5-2.0m/sec
• Increased nerve diameter results in faster rate of conduction; decreased diameter results in slower rate of conduction
• Rate of impulse conduction is also affected by other factors such as ions, drugs, etc.
• Staellite Cell: surrounds the Sensory neuron's cell body within PNS ganglia (cluster of cell bodies); provides insulation around the cell body and helps to regulate the chemical environment of neurons

Describe the characteristics of neuroglia AKA support cells AKA glial cells in the Central Nervous System

• Astrocyte: largest, most abundant, star-shaped with many extensions and functions
• Forms a supportive framework in the CNS (covers the entire brain surface and comprises >90% of the tissue in some brain areas)
• foot-like extensions link neurons to capillaries for nutrient exchange and formation of the blood-brain barrier
• secretes nerve growth factor (as does muscle) that promotes neuron growth and synapse formation with their target cells
• helps to control the chemical environment by absorbing neurotransmitters and K+ thus preventing excess levels in the ECF
• assists in immune response via assisting in the formation of scar tissue when neruons are damaged
• Oligodendrocyte: has small, short extensions (octopus-like)
• forms the myelin sheath within the CNS
• no neurilemma
• Microglial cell: smallest neuroglial cell with long, thorny processes
• special form of a macrophage
• Ependymal cell: a single layer of cuboidal-like cells that forms the inner lining of the brain and spinal cord
• Involved in the formation of CSF/cerebro-spinal fluid

• Sympathetic Division
• Parasympathetic Division

• Mobilizes the body during extreme situations such as fear, rage, exercise, excitement, Fight-or-Flight response
• Sympathetic stimulation activates ALL body systems activities EXCEPT digestive system (HR goes up, BP goes up, bronchi and pupils dilate, digestive activities decrease
• Epinephrine AKA Adrenaline is a sympathomimetic drug, i.e. it mimics sympathetic stimulation
• Sympathetic nerves exit the CNS at thoraco-lumbar levels of the spinal cord

• Performs resting and maintenance activities
• Parasympathetic stimulation inhibits all body systems activities except the Digestive system (HR down, BP down, bronchi and pupils constrict, digestive activities increase)
• Parasympathetic nerves exit the CNS at cranio-sacral levels of the spinal cord

• Polarization
• Action Potential (nerve impulse) Generation
• Local Potential Generation

• A resting neuron's cell membrane is electrically charged AKA polarized due to an unequal distribution of ECF and ICF ions
• This Resting Membrane Potential AKA RMP involves the following factors:
• ICF has a higher amount of K+ and proteins. 40x more K+ in ICF than ECF
• ECF has a higher amount of Na+ and Cl-
• In order to reach equilibrium K+ diffuses out of the cell and Na+ diffuses into the cell via diffusion gates. However, equilibrium is prevented from being reached via the active transport action of the Na+/K+ pump/gate
• ATP is used to pump 3 Na+ out of the cell for every 2 K+ that gets pumped in
• This results in the outside of the cell membrane being more positively charged compared to the inside of the cell membrane
• This difference in electrical charges can be measured in mV.
• Thus Resting Membrane Potential is -70mV. NOTE: The minus sign symbolizes that the electrical charge inside the membrane is less than the electrical charge outside of the membrane

• Threshold Stimulus (typically around -55mV, a positive increase of 15mV from RMP) initiates/generates an Action Potential in a neuron
• When the cell membrane is exposed to threshold stimulus it creates a change in membrane permeability
• Na+ voltage-channels rapidly open in a one-way direction, allowing Na+ ions to rush from ECF to ICF while K+ voltage-channels begin to open slowly. This results in depolarization (a reversal in the electrical charge of the cell membrane at the point of stimulation).
• ICF becomes more positive, trending from -55mV to 0mV and finally to +35mV. As the ICFs potential becomes more positive the Na+ voltage channels start to close
• Action potential occurs at the peak of depolarization (+35mV approx) and the nerve is said to 'fire' AKA action potential generated NOTE: This initial action potential stimulates the next adjacent portion of the neuron membrane resulting in its depolarization and the subsequent propagation of waves of action potentials in a domino effect. This is termed the NERVE IMPULSE
• Just before peak depolarization the Na+ voltage channels are all closed and the K+ voltage-channels are now open
• Following action potential generation (again at approx +35mV) the K+ flows from ICF to ECF and restores the original voltage balance AKA Repolarization (+35mV to 0mV to -70mV).
• As K+ voltage-channels are slow to open and slow to close, Hyperpolarization occurs (-70mV to approx -72mV). After the K+ voltage channels completely close, RMP is reestablished as Na+ diffuses into the cell and astrocytes absorb excess extracellular K+
• Summary: Membrane at RMP (-70mV) exposed to threshold stimulus (-55mV) and depolarizes (-55mV to +35mV). At approx +35mV an action potential is generated and the nerve 'fires' which propagates a nerve impulse and, back at the initial site of stimulation, repolarization begins (+35mV to -70mV), going through a period of hyperpolarization (-70mV to -72mV) before RMP is fully restored (-70mV)
• During the interval after the nerve fires until the end of repolarization the nerve is said to be in an Absolute Refractory Period. No 2nd action potential can be generated - even with a superthreshold stimulus. The nerve has not 'rested' AKA returned to its RMP. THIS ACCOUNTS FOR THE ONE-WAY DIRECTION OF A NERVE IMPULSE.
• A second, relative refractory period, occurs during hyperpolarization where a superthreshold stimulus can generte an action potential
• Much like with muscle contractions, neuron firing is an all-or-none response. One it receives a threshold stimulus it will fire at full power or not at all. Increased stimulus intensity does not cause an increase in strength of impulse. Increased number of neurons stimulated results in how strung the impulse is preceived to be.

• Dendrites NEVER generate an action potential, ONLY LOCAL POTENTIALs that occur at the site of stimulation (dendrite's receptors) and travel towards the nerve's trigger zone (Axon Hillock + Initial Segment).
• This is a GRADED potential, i.e. NOT an all-or-none potential. The membrane becomes more negative or more positive but theshold stimulus is NOT REACHED.
• Depolarizing: -70mV to -60mV
• Hyperpolarizing: -70mV to -80mV
• A local potential is a decremental potential, i.e. lasts from one to a few msec and then fades out.

• Neuromuscular synapse
• Neuroglandular synapse
• Nerve synapse AKA neural synapse AKA neuronal synapse
• Axodendritic synapse: between a neuron's axon terminus and another neuron's dendrite
• Axosomatic synapse: between a neuron's axon terminus and another neuron's cell body
• Axoaxonic synapse: between a neuron's axon terminus and another neuron's axon

• Presynaptic neuron: conducts impulses TOWARDS the synapse
• Synaptic Knob: bulbous-end of the presynaptic neuron's axon terminus; has mitochondria and synaptic vesicles containing neurotransmitters
• Synaptic Cleft: small separation/gap between the synaptic knob and the postsynaptic neuron's membrane
• Postsynaptic neuron: conducts impulses away from the synapse; has specific neurotransmitter receptor sites

• The presynaptic neuron's impulse causes Ca++ to diffuse in at the synaptic knob
• Which causes synaptic vesicles to release neurotransmitters into the synaptic cleft
• The neurotransmitters diffuse across the synaptic cleft and bind to the appropriate receptors on the postsynaptic neuron
• These neurotransmitters can have an excitatory or inhibatory effect, depending on the differences of receptors, which cause different changes of membrane permeability by opening or closing various membrane channels

• Acetylcholine (ACh)
• Amino Acids
• Gamma Aminobutyric Acid (GABA)
• Glycine
• Glutamate (Glutamic Acid)
• Aspartate (Aspartic Acid)
• Biogenic Amines
• Catecholamines
• Tyrosine
• L-Dopa
• Dopamine
• Norepinephrine AKA Noradrenaline
• Epinephrine AKA Adrenaline
• Indolamines
• Serotonin
• Histamine
• Neuropeptides
• Substance P
• CCK/Cholecystokinin
• Neuromodulators
• Nitric oxide/NO
• Endorphins
• Enkephalins

• Amino Acids refers to unmodified amino acids
• Gamma Aminobutyric Acid, Glycine, Glutamate (Glutamic Acid), and Aspartate (Aspartic Acid)
• These are found only in the CNS

• Biogenic Amines are modified amino acids
• Biogenic Amines are broadly distributed within the brain, therefore imbalances can lead to mental illnesses
• Large increases of Dopamine -> Schizophrenia
• Large decreases of Dopamine -> skeletal muscle tremors
• Large decreases of Serotonin-receptors -> Possible Alzheimer mood swings and also possible depression
• Catecholamines: derived from a.a. Tyrosine
• Tyrosine -> L-Dopa -> Dopamine -> Norepinephrine AKA Noradrenaline -> Epinephrine AKA Adrenaline
• Indolamines
• Serotonin: derived from a.a. tryptophan
• Histamine: derived from a.a. histidine

• Neuropeptides are short chains of amino acids found in the CNS and, for gut-brain peptides, the digestive tract
• Substance P: mediates pain signals
• CCK/Cholecystokinin: gut-brain peptide
• Neuromodulators: have long-term effects on entire groups of neurons instead of short, rapid effects at a single synapse
• Nitric Oxide/NO: stimulates presynaptic neurons to release more neurotransmitters, especially in the brain's area associated with learning and/or memory
• Endorphins: natural opiates; decrease perception of pain
• Enkephalins: natural euphorics; decrease perception of pain

• NOT an all-or-none potential; GRADED potential: post synaptic membrane becomes more negative or more positive, but threshold stimulus is NOT reached
• Depolarizing: -70mV -> -65mV
• Hyperpolarizing: -70mV -> -75mV
• The local PSP lasts from one to a few mSecs then fades out; it is DECREMENTAL
• Two types of post synaptic potential: Excitatory and Inhibitory
• These effects result from differences of postsynpatic membrane receptors which cause different changes of membrane permeability

• Originates when neurotransmitter causes Na+ to diffuse into the postsynaptic neuron, creating a partial depolarization with lowering of the membrane potential (-70mV to -65mV). NO action potential/nerve impulse is generated
• The EPSP makes the postsynaptic membrane less negative and thus more exiteable i.e. more likely to generate an Action Potential. This is called FACILITATION
• If enough EPSPs hit the receptor segment of the postsynaptic membrane they add up and can eventually reach threshold stimulus and initiate an action potential. This is called SUMMATION.

• Occurs when a neurotransmitter causes Cl- to diffuse into the postsynaptic neuron (or, less commonly, K+ to diffuse out of the postsynaptic neuron) resulting in hyperpolarization (-70mV to -75mV)
• The IPSP makes the postsynaptic membrane more negative, inhibiting the likelihood of generating an Action Potential/Nerve Impulse

• EPSPs produced by aspartate and glutamate
• IPSPs produced by GABA, Glycine, and Dopamine (at motor centers of the brain)
• Acetylcholine (ACh) excites skeletal muscle but inhibits cardiac muscle
• a cholinergic synapse has ACh as its neurotransmitter
• an adrenergic synapse has epinephrine AKA adrenaline or Norepinephrine AKA noradrenaline as its neurotransmitter

• Groups of neurons acting together within the CNS
• Input AKA Presynaptic neuron: one or more
• Interneurons: thousands to millions
• Output AKA Postsynaptic neuron: one or more

• Diverging: one input neuron branches (diverges) to synapse with many interneurons which diverge to synapse with many output neurons (e.g.: single motor neuron can ultimately stimulate thousands of myofibers)
• Converging: many input neurons converge to synapse with a few interneurons, which converge to synapse with one output neuron (e.g.: input from osmoreceptors and barorecptors from blood vessels sensing chemical and pressure changes, plus input from stretch receptors lining the lungs, plus input from other brain areas -> respiratory center in the brain adjusts the rate and depth of breathing)

• Limited to PNS axons
• Only possible if the soma is not damaged and some neurilemma is present
• NOTE: CNS has no Schwann cells, which means no neurilemma capable of fusing, no regeneration tube formation, and no repair.

• Trauma (cut or crush) occurs to a PNS fiber (axon) -> the axon distal to the injury cannot synthesize or receive proteins so it dies (nissl bodies responsible for protein synthesis are mainly in the soma)
• Wallerian degeneration occurs, disintegrating the axon and schwann cells (myelin sheath) at the site of injury and distal to the injury. Macrophages migrate in and clean up debris
• The soma, likely because it can no longer receive nerve growth factor, begins to show abnormalcies such as: swelling, ER break-up -> nissl body dispersion, nucleus shifts off center. This is the point where many damaged neurons die.
• At the site of the trauma, the separated neurilemma ends swell, then fuse to form a regeneration tube connecting the damaged axon ends. Schwann cells produce and secrete nerve growth factors promoting axonal 'sprout' growth plus new myelin sheath
• The regeneration tube guides the growing axon 'sprouts' until synaptic contact is made. Once this occurs, the soma returns to its normal appearance.
• If the gap between the damaged ends of the axon is too large, sprouts may escape and form neuromas

• Nerve: cord-like bundle (or group of bundles) of nerve fibers (AKA axons) held together by layers/sheaths of CT
• Fascicle: single bundle of nerve fibers AKA neurofibers
• Epineurium: holds together/surrounds groups of fascicles
• Perineurium: encloses a single fascicle
• Endoneurium: encloses a single neurofiber's axon (myelinated or not)