Nervous System I

Anatomy & Physiologyivyanatomy.com

Chapter 10

Basic Structure and Function

Functions of the Nervous SystemSensory Function:• Sensory receptors detect changes in the environment (stimuli)• Information is carried to the CNS on sensory (afferent) neurons

Motor Function:• Nerve impulses are transmitted from the CNS to PNS on motor

(efferent) neurons• effectors (muscles or glands) within the PNS cause a change

(effect)

Integrative Function:• Nervous system maintains homeostasis – detects and responds to

changes in blood pressure, body temp, heart rate, etc.• Higher intellect: problem solving, thoughts, memory, judgment

The Central Nervous System (CNS) • brain and spinal cord.

The Peripheral Nervous System (PNS)• 12 pairs of cranial nerves and • 31 pairs of spinal nerves

• Nerves may be motor (efferent), sensory (afferent), or both (mixed)

The Central Nervous System is Red, while the Peripheral Nervous System is Blue.

Subdivisions of Nervous System

Divisions of the PNS

Sensory (Afferent) Division • Transmits impulses from receptors in the PNS to CNS

Motor (Efferent) Division • Transmits impulses from CNS to effectors in

the PNS

Somatic Nervous System

Somatic Sensory• Senses you’re consciously aware of• Vision, taste, olfaction, smell, hearing• Touch, vibration, pain

Somatic Motor• Controls voluntary (skeletal) activities• Skeletal muscles – voluntary control

Somatic Division is associated with voluntary (skeletal) activities and senses that detect shapes, textures, sounds, and other external and internal forces acting on the body.

Autonomic Nervous System

Autonomic Sensory• Senses that monitor vital conditions within the body• O2/CO2 levels, pH, Blood Pressure, Body Temp

Autonomic Motor – involuntary control• Smooth muscles• Cardiac muscles• glands

Autonomic Nervous System regulates functions of the internal organsHeart rate, digestion, sexual arousal, urination

Divisions of the Autonomic Nervous System

Sympathetic Division• Fight-or-Flight Response• Prepares body for emergency

Parasympathetic Divsion• Rest-and-Digest• Maintains body activities at rest

The Autonomic Nervous System (ANS) is divided into two branches

Cells of the Nervous System

Neurons• Integrate, regulate, and coordinate body functions• Transmit nerve impulses (action potentials)

Neuroglia (glia = “glue”)• Neuroglia provide neurons with nutritional,

structural, and functional support

Neural Tissue contains two Cell Types:

Neurons vary in shape and size

Dendrite

Cell Body (soma)

Nucleus

axon

axon terminal

Myelin sheath

Schwann Cell

3 Parts of a Neuron• Dendrites – receive inputs from other

neurons or other stimuli

• Cell Body (Soma)

• Axon – transmit nerve impulses away from the cell body towards

other neurons, muscles, or glands

Anatomy of a Neuron

Dendrites• Dendrites transmit information towards the cell body• A cell may have many dendrites, few dendrites, or no dendrites• Dendritic spines – additional contact points on dendrites

• A neuron may add more spines, increasing its sensitivity to incoming stimuli, or It may remove spines to decrease its sensitivity to stimuli.

Segment of a dendrite (green) with dendritic spines (yellow)

Anatomy of a Neuron

Anatomy of a NeuronCell Body (Soma)• Contains organelles such as nucleus, mitochondria, Golgi Apparatus,

neurofilaments, and Rough ER• Chromatophilic Substances (Nissl Bodies) – mostly Rough ER, protein

synthesis• Cell body produces proteins for the cell

Axon• Transmits electrical impulses (action potentials)

away from the neuron

• Each neuron has only 1 axon, but it may divide into several branches, called collaterals.

• Axon Hillock (trigger zone) – specialized site connected to the cell body, where electrical impulses (action potentials) are initiated

• Axon terminal – end of the axon.• Contains enlarged synaptic knob (bouton)• Neurotransmitters are stored within secretory

vesicles within the synaptic knob.

Anatomy of a Neuron

Axon• Neurofibrils – microtubules that support long axons

• Neurofibrils aid in axonal transport – transports proteins from the cell body through the axon

• Axoplasm – cytoplasm of the axon

• Axolemma – cell membrane of the axon

Anatomy of a Neuron

Myelination of AxonsMyelin Sheath• Thick fatty coating of insulation surrounding some axons• Myelin sheath greatly enhances the speed of nerve

impulses• Schwann Cells form the myelin sheath in the PNS• Oligodendrocytes form the myelin sheath in the CNS

Schwann cell forming the myelin sheath around an axon within the PNS

Myelination in the PNSSchwann Cells• Schwann Cells myelinate neurons in the PNS

• Schwann Cells wrap around the axon in a jelly-roll fashion, forming a thick layer of lipid insulation, called the Myelin Sheath

• Neurilemma – The cytoplasm and nucleus of the Schwann Cell are pushed outwards, forming an outer layer, called the neurilemma

• Nodes of Ranvier – gaps of exposed axon between adjacent Schwann Cells

Myelin sheathNeurilemma (at the nucleus)

Mitochondrion within axon

axon

◊ Not all axons are myelinated

◊ Myelinated axons in the PNS have a series of Schwann cells lined up along the axon, each having a wrapped coating of myelin insulating the axon

◊ Unmyelinated axons in the PNS are encased by Schwann cell cytoplasm, but there is no wrapped coating of myelin surrounding the axons

Myelination of Axons

Schwann Cell

axon

axon

Schwann Cell

Myelin sheath

Myelination in the CNS

Oligodendrocytes• Oligodendrocytes myelinate

axons in the PNS

• Each oligodendrocyte myelinates multiple axons

• White Matter – mass of myelinated axons within the CNS

• Gray Matter – unmyelinated nervous tissue in the CNS

Oligodendrocyte myelinating several axons.

Gray Matter of the Cerebral Cortex - unmyelinated tissue

White Matter of the Cerebrum - myelinated tissue

Myelination in the CNS

Structural Classification of Neurons

Multipolar Neuron• Many dendrites and 1 axon• Includes most neurons in the

CNS and motor neurons

dendrites

axon

Structural Classification of Neurons

Bipolar Neuron• 1 dendrite and 1 axon• Includes some special sensory neurons,

such as photoreceptors and olfactory neurons.

dendrite

axon

Structural Classification of Neurons

Psuedounipolar Neuron• Contains a single process that acts

as an axon• Peripheral process – conducts

information from the PNS

• Central process – conducts information toward the CNS

• Example includes sensory neurons within the Dorsal Root Ganglia (DRG)

peripheral process

central process

Functional Classification of NeuronsSensory (afferent) neuron• Transmit impulses from the PNS towards the CNS• Most afferent neurons are unipolar. Some are Bipolar.

Motor (efferent) neuron• Transmit impulses from the CNS towards effectors in the CNS

• Somatic Motor Neurons – voluntary control• Autonomic Motor Neurons – involuntary control.

Interneuron (association) • Completely within the CNS• Interneurons link together in the CNS• Interneurons connect sensory neurons to motor neurons

Peripheral Nervous System Central Nervous System

Sensory receptor Sensory neuron interneuron

interneuronMotor neuronEffector (muscle or gland)

NeurogliaGeneral Functions of Neuroglia:• Provide structural and metabolic support for neurons• Guide developing neurons into position • Remove excess ions and neurotransmitters• Strengthen synapses• Neuroglia outnumber neurons 10 to 1

Neuroglia in the CNS vs. PNS• Neuroglia of the CNS include: astrocytes, ependymal cells,

microglia, and oligodendrocytes

• Neuroglia of the PNS include: satellite cells and Schwann Cells

Neuroglia of the CNSAstrocytes:• Star-shaped cell• Attaches neuron to blood vessels• Astrocytes aid in metabolism, strengthen synapses,

and participate in the Blood-Brain-Barrier

Ependymal Cells• Simple cuboidal epithelium with cilia• Lines ventricles of the brain and central canal of spinal

cord• Cover choroid plexuses (capillary networks within CNS)• Regulate the composition of cerebrospinal fluid (CSF)

Neuroglia of the CNSMicroglia:• Normally small cells until activated• Enlarge into macrophages with infection

• Phagocytize foreign material

Microglia (green) surrounding nerve processes (red)

Oligodendrocytes:• Form the myelin sheath within the CNS• Provide structural support

Oligodendrocyte myelinating several axons within the CNS

Neuroglia of the CNS

Schwann Cells:• Form the myelin sheath in the PNS• Greatly increase nerve impulse speed

Satellite Cells:• Surround and support clusters of cell

bodies (ganglia) within the PNS

• Mature neurons do not divide• If cell body is injured, the neuron usually dies

Neuron Regeneration in the PNS:• If a peripheral axon is injured, it may regenerate• Axon separated from cell body and its myelin sheath will degenerate• Schwann cells and neurilemma remain• Remaining Schwann cells provide guiding sheath for growing axon• If growing axon establishes former connection, function will return; if not,

function may be lost

Neuron Regeneration in the CNS:• CNS axons lack neurilemma to act as guiding sheath• Oligodendrocytes do not proliferate after injury• Regeneration is unlikely

Neuroglia and Axonal Regeneration

Disorders of Neuroglia

Multiple Sclerosis:• Autoimmune disease that destroys the

myelin sheath of motor neurons.

• The damaged myelin sheath is replaced with connective tissue, leaving behind scars (scleroses)

• The scars block transmission of underlying neurons, so muscles no longer receive stimuli

• Muscles atrophy and wither over time

White matter lesions (scleroses) of Multiple Sclerosis

Neurons communicate with each other at synapses.

• A synapse is a site at which a neuron transmits a nerve impulse to another neuron

• Presynaptic neuron sends impulse (usually) by releasing neurotransmitters into the synaptic cleft

• Postsynaptic neuron receives impulse• Synaptic cleft separates the 2 neurons

The Synapse

1. A nerve impulse (action potential) travels down the axon to the axon terminal.

2. The action potential opens calcium channels causing calcium to diffuse into the synaptic knob.

3. The calcium influx triggers the exocytosis of neurotransmitters from synaptic vesicles into the synapse.

4. The neurotransmitters diffuse across the synapse and bind to receptors on the post-synaptic cell

5. Neurotransmitter either exerts an excitatory or inhibitory effect, depending on the neurotransmitter and the receptor.

Synaptic Transmission

The cell membrane is usually polarized (charged)

• Inside the membrane is negatively charged relative to outside the membrane

• Polarization is due to unequal distribution of ions across the membrane

• Polarization is maintained by a series of ion pumps and ion channels

• All Cells have a membrane potential.

Cell Membrane Potential

Cell membrane

• Potassium (K+) ions: major intracellular positive ions (cations).

• Sodium (Na+) ions: major extracellular positive ions (cations).• This distribution is largely created by the Sodium/Potassium Pump (Na+/K+ pump) but also by ion channels in the cell membrane.

• Na+/K+ Pump transports Na+ ions out of cell and K+ ions into cell

• Ion channels, formed by membrane proteins, help regulate passage of specific ions into or out of the cell

• Many chemical & electrical factors affect opening & closing of gated channels

Cell Membrane Potential

Ion Channels

Non-Gated (Leak) Ion channels, • Channels are always open, allowing specific

ions to “leak” down their concentration gradient.

• Cells have abundant K+ leak channels, making them permeable to K+.

closed open

Mechanically-Gated Ion channels, • Open in response to physical deformation of

the cell membrane

• Touch, Hearing, Pressure, Vibrations, Etc.

Ion Channels

Na+

Ion Channels

Ligand (molecule)

Ligand-Gated Ion channels, • Open in response to a ligand

(neurotransmitter, hormone, or other molecule)

Na+

closed open

Ion ChannelsVoltage-Gated Ion channels,

• Open and close due to small changes in the membrane potential (millivolts = mV)

• Voltage-gated Na+ channels open when membrane potential reaches -55mV.

• Voltage-gated K+ channels open as the membrane potential approaches +35mV

openclosed

-70mV -55mV

Ion PumpsSodium-Potassium ATPase (Pump)

• Active Transport Mechanism (uses ATP)

• Pumps 3 Na+ out of the cell

• Pumps 2 K+ into the cell

Membrane Potential3 Factors Establish the Membrane Potential1. Na+/K+ ATPase2. Non-gated K+ channels3. Negatively charged proteins and DNA within the cell

Sodium-Potassium ATPase (Pump) • Pumps 3 Na+ out of the cell, but only 2 K+

into the cell.• Net positive charges leaving the cell, making

inside negatively charged.• The Na+/K+ pump only contributes a small

amount (-5mV) to the membrane potential

Cell Membrane Potential3 Factors Help Maintain the Cell Membrane Potential

Non-gated Potassium Channels • Cell has many K+ leak channels, making

it permeable to potassium.

• K+ continually leaks out of the cell, making the inside of the cell more negative.

Na+/K+ PumpK+ leak channel

K+

K+ Na+

ATP

ADP + P

Resting Membrane PotentialResting Membrane Potential (RMP)• RMP = membrane potential of excitable

cells (neurons and muscles) while at rest.

• For a neuron at rest, the RMP is -70mV inside the cell.

Opening/Closing gated-Ion channels cause changes in local membrane potential

Membrane Potential Changes

Hyperpolarization• membrane potential becomes more negative.• e.g. -100 mV

Depolarization• membrane potential becomes less negative.• e.g. -60 mV

Resting Membrane Potential (RMP) of neuron = -70mV

-70mv (RMP)

-70mv (RMP)

Time (ms)

• Local potential changes are graded—the greater the stimulus intensity, the greater the potential change

• If degree of depolarization reaches threshold potential of -55 mV, an action potential results

• If degree of depolarization does not reach threshold potential, an action potential will not occur subhreshold potential

Graded (Local) Potentials

Local Potential Changes

Summation of Graded PotentialsSummation – Graded potentials may add together (summate)

• Spatial summation – stimuli from multiple neurons• Temporal summation – high frequency stimulation from a presynaptic neuron• Combination – stimuli from multiple neurons at a high frequency

• If summation reaches threshold potential (-55mV), it initiates an action potential

Example of Spatial Summation Example of Temporal Summation

Threshold potential

-70 mV (RMP) stimulus Subthreshold

potentials

-55 mV

Action Potential

Dep

olar

izat

ion R

epolarization

Hyperpolarization

3 Phases of an Action Potential

Depolarization• Voltage-Gated Na+ channels open at -55mV (threshold)• Na+ diffuses into the cell

Repolarization• Voltage-Gated K+ channels open as cell

depolarizes towards +30mV • K+ diffuses out of the cell• Na+ channels close

Hyperpolarization• K+ channels remain open, causing an overshoot• Na+/K+ pumps reestablish the RMP.

Action Potential

Na+

K+

At rest, the membrane is polarized (RMP = -70mV). Sodium is mostly outside the cell and potassium is within the cell.

Resting Membrane Potential

Action Potential

Na+

Na+

When a stimulus reaches threshold stimulus (-55mV), voltage-gated Na+ channels open. Sodium rapidly diffuses into the cell, depolarizing the membrane up to +30mV.

Depolarization

K+Na+

Action Potential

K+Na+

Repolarization

As the membrane potential approaches +30mV, voltage-gated K+ channels open and quickly repolarize the membrane. Sodium channels also close at this point.

Action Potential

Na+

K+

Hyperpolarization

K+ channels remain open, causing an overshoot past RMP. Following an action potential, Na+/K+ pumps actively reestablish the Na+ and K+ concentration gradients.

High K+

High Na+

-70mV -70mV -70mV

Action Potential Propagation

Once an action potential is initiated it is propagated along the entire axon at full strength. It does not weaken.

At rest, Na+/K+ pumps maintain a high extracellular Na+ concentration and a high intracellular K+ concentration.

Action Potential Propagation

• Action Potential begins when Axon Hillock depolarizes to threshold potential (-55mV)

• Voltage-Gated Na+ channels open, Na+ diffuses into the cell, depolarizing the region to +30mV

-70mV

+30mVHigh Na+

Action Potential Propagation

High Na+ -55mV -70mV

• Sodium now within the cell diffuses to its adjacent region, depolarizing it to threshold.

• Voltage-Gated Na+ channels in adjacent region open. Na+ diffuses into the cell, causing another action potential in the adjacent region.

Action Potential Propagation

Na+

K+

• Voltage-Gated K+ channels quickly repolarize the axon, following the depolarization.

High Na+ -55mV -70mV

• Again Sodium diffuses to adjacent region, depolarizing it to threshold. Another Action potential follows.

Action Potential Propagation

• The action potential continues sequentially along the entire axon, to the axon terminal.

1. Resting Membrane Potential 1. Na+/K+ pumps, K+ leak channels, and negatively charged proteins maintain RMP = -70mV

2. Graded Potential - Stimulus 2. Neuron receives stimulus initiating graded potentials3. Threshold Potential 3. Graded potentials reach threshold, triggering an

action potential4. Action Potential - Depolarization 4. Voltage-gated, Na+ channels open, sodium diffuses

into cell.5. Action Potential - Repolarization 5. Voltage-gated K+ channels open, potassium diffuses

out of the cell.6. Action Potential - Hyperpolarization 6. Na+/K+ pumps re-establish RMP at region7. Action Potential Propagation 7. Sodium diffusing into the cell generates an electrical

current that stimulates adjacent regions of the membrane. Action potentials occur sequentially along the length of the axon.

Summary of Action Potential

Action Potential propagated along the axon is often called a nerve impulse.

All-or-None Response

All-or-None Response• Action Potentials occur completely or they do not occur at all.

• A stronger stimulation does not produce a stronger impulse.

• Instead, a stronger stimulation produces a higher frequency of nerve impulses (more impulses per second)

weaker stimulus stronger stimulus

Refractory Period: For a brief period following an action potential, a threshold stimulus will not trigger another action potential.

Absolute Refractory Period• no new action potentials can be produced• Occurs while the membrane is changing in sodium permeability• Between the depolarization and repolarization phases

Relative Refractory Period• Action potential can be generated with a high intensity stimulus• Occurs while membrane is reestablishing its resting membrane potential• Lasts from the hyperpolarization phase, until RMP is reestablished

Refractory Period

Impulse Conduction

Speed of a Nerve Impulse Depends on • Diameter of the axon: larger diameter = higher velocity• Myelinated vs Unmyelinated: myelinated neurons are much

faster than unmyelinated neurons.

Unmeylinated Axons must generate action potentials across the entire axon.

The impulse is slow (travels at 1 mile/hour)

Myelinated axons conduct impulses differently than unmyelinated axons.

Impulse Conduction

Unmyelinated AxonsGenerate a series of action potentials along the entire axon.Nerve impulses are slow: travel around 1 mile/hour (0.4 meters/second)

Myelinated Axons• Myelin is an electrical insulator• Action potentials of myelinated axons are only generated

at the nodes of Ranvier.• Nerve impulse through the myelinated portion travels by

electrical conduction• This is called, salutatory conduction• Saltatory conduction increases conduction velocity to

around 285 miles/hour (127 meters/second)

Unmeylinated Axons must generate action potentials across the entire axon. The impulse is slow (travels at 1 mile/hour)

Meylinated axons conduct nerve impulses via salutatory conduction: Electrical conduction through myelin sheath, action potentials only at nodes of Ranvier. Appears as if nerve impulse “jumps” from node-to-node.

electrical conductionNode of Ranvier

meyelin sheath

Impulse Conduction

1. A nerve impulse (action potential) travels down the axon to the axon terminal.

2. The action potential opens calcium channels causing calcium to diffuse into the synaptic knob.

3. The calcium influx triggers the exocytosis of neurotransmitters from synaptic vesicles into the synapse.

4. The neurotransmitters diffuse across the synapse and bind to receptors on the post-synaptic cell

5. Neurotransmitter either exerts an excitatory or inhibitory effect, depending on the neurotransmitter and the receptor.

Synaptic Transmission

Synaptic Transmission

Synaptic TransmissionMost neuron communication occurs when a presynaptic neuron releases neurotransmitters into the synaptic cleft, where the neurotransmitters subsequently bind to receptors on a postsynaptic cell

Local potentials resulting from changes in chemically gated ion channels are called synaptic potentials

Synaptic Potentials

Excitatory postsynaptic potential (EPSP):• Membrane change in which neurotransmitter opens Na+ channels (or

Ca2+) channels• Depolarizes membrane of postsynaptic neuron, as Na+ enters axon• Action potential in postsynaptic neuron becomes more likely

Inhibitory postsynaptic potential (IPSP):• Membrane change in which neurotransmitter opens K+ channels (or Cl-

channels)• Hyperpolarizes membrane of postsynaptic neuron, as K+ leaves axon• Action potential of postsynaptic neuron becomes less likely

EPSPs and IPSPs are added together in a process called summation

Summation occurs at axon hillock (trigger zone)

The integrated sum of EPSPs and IPSPs determines if an action potential occurs

If threshold stimulus is reached an action potential is triggered.

Summation of EPSPs and IPSPs

NeurotransmittersThe nervous system produces at least thirty different types of neurotransmitters.

Examples:1. Acetylcholine – skeletal muscle contractions

2. Monoamines• Norepinephrine

- in CNS it creates a sense of well-being- in PNS it may stimulate or inhibit autonomic nervous system

• Dopamine- in CNS it creates a sense of well-being- Amphetamines increase the levels of norepinephrine and dopamine

3. Amino Acids• GABA – inhibitory neurotransmitter of the CNS

• Many sedatives and anesthesia enhances GABA secretions• Schizophrenia is associated with a deficiency of GABA

4. Gases• Nitric Oxide

• Vasodilation in PNS

Neurotransmitters

Examples:5. Glutamate – primary excitatory neurotransmitter in the CNS

6. Serotonin – primarily inhibitory. Leads to sleepiness.

7. Substance P – pain perception

8. Endorphins & Enkaphalins – reduce pain by inhibiting substance P release

Termination of Neurotransmitter Activity

Enzymatic Degridation • Acetylcholinesterase – decomposes Acetylcholine in the synaptic cleft.• Monoamine Oxidase – decomposes Epinephrine and Norepinephrine.

• Limits the duration of your sympathetic (fight-or-flight) response.

Reuptake• Neuroglia and enzymes transport neurotransmitters within the synaptic

cleft back to the synaptic knob of the presynaptic neuron.• Neurotransmitters are repackaged into new secretory vesicles and used

again.

Drugs that affect Neurotransmitters

Cocaine• Cocaine binds to Dopamine transporters, preventing the reuptake of

Dopamine.• This results in excess dopamine in the synaptic cleft.

Nicotine• Nicotine binds to Nicotinic receptors on dopaminergic neurons, causing

them to release dopamine.

Nerve impulses are processed by the CNS in a way that reflects the organization of neurons in the brain and spinal cord.

Impulse Processing

Neuronal Pool• Organized groups of interneurons within the CNS

• Pools are organized as neuronal circuits that perform a common function, even though they may be in different parts of the CNS.

• May have either excitatory or inhibitory effects on effectors, or other neuronal pools.

Neuronal Pools

Convergence• Several neurons synapse onto one post-synaptic

neuron• Funnels impulses from several areas onto a

single neuron• Information from various sensory receptors may

converge onto a single processing center.

Divergence• Impulses spread from one axon to several post-

synaptic neurons• May amplify a stimulus• May send a one signal to multiple parts of the CNS.

• Neuron in tissue culture By GerryShaw (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/7/70/Neuron_in_tissue_culture.jpg

• Nervous System Divisions Diagram By This SVG image was created by Medium69. Cette image SVG a été créée par Medium69. Please credit this : William Crochot (File:Nervous system diagram.png) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/5/5b/Nervous_system_diagram-en.svg

• Divisions of the Nervous System Diagram By Fuzzform at English Wikipedia [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/0/0b/NSdiagram.png

• Neural Tissue Illustration By Blausen.com staff. "Blausen gallery 2014". Wikiversity Journal of Medicine. DOI:10.15347/wjm/2014.010. ISSN 20018762. (Own work) [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/7/73/Blausen_0672_NeuralTissue.png

• Neuron Illustrated Hand-Tuned Quasar Jarosz at English Wikipedia [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/b/bc/Neuron_Hand-tuned.svg

• Dendritic Spines By Hotulainen P, Hoogenraad CC [CC BY-SA 2.5 (http://creativecommons.org/licenses/by-sa/2.5)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/2/26/Cytoskeletal_organization_of_dendritic_spines_%28ru%29.jpg

• Multipolar Neuron Illustration By BruceBlaus (Own work) [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/1/10/Blausen_0657_MultipolarNeuron.png

• Microtubules Tangled High By ADEAR: "Alzheimer's Disease Education and Referral Center, a service of the National Institute on Aging." [Public domain], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/5/51/TANGLES_HIGH.jpg

• Peripheral Nerve Myelination Illustration By CFCF (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/a/a8/Periferal_nerve_myelination.jpg

Attribution

• TEM of myelinated neuron Roadnottaken at the English language Wikipedia [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/c/c1/Myelinated_neuron.jpg

• Oligodendrocyte Illustration By Artwork by Holly Fischer [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/2/2e/Oligodendrocyte_illustration.png

• Human Brain Sagittal Section By John A Beal, PhD Dep't. of Cellular Biology & Anatomy, Louisiana State University Health Sciences Center Shreveport [CC BY 2.5 (http://creativecommons.org/licenses/by/2.5)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/0/0c/Human_brain_right_dissected_lateral_view_description.JPG

• Multipolar Neuron Illustration By Artwork by Holly Fischer [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/b/bb/Multipolar_Motor_Neuron.png

• Bipolar Neuron Illustration By Artwork by Holly Fischer [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/9/9e/Bipolar_Interneuron.png

• Unipolar (pseudounipolar) Neuron By Artwork by Holly Fischer [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/b/b1/Unipolar_Sensory_Neuron.png

• Central Nervous System By Jordi March i Nogué [1] (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/f/f1/Central_nervous_system_2.svg

• Astrocyte By GerryShaw (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/6/63/Astrocyte5.jpg

• Ependyma By Martin Hasselblatt MD (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC BY-SA 2.5-2.0-1.0 (http://creativecommons.org/licenses/by-sa/2.5-2.0-1.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/b/b7/Ependyma.png

Attribution

• Microglia By GerryShaw (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/0/0b/Microglia_and_neurons.jpg

• By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/c/c3/1210_Glial_Cells_of_the_PNS.jpg

• MRI of MS lesions By Veela Mehta, Wei Pei, Grant Yang, Suyang Li, Eashwar Swamy, Aaron Boster, Petra Schmalbrock, David Pitt [CC BY 2.5 (http://creativecommons.org/licenses/by/2.5) or CC BY 2.5 (http://creativecommons.org/licenses/by/2.5)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/d/d0/Journal.pone.0057573.g005.png

• Synapse Illustration By Edk006 (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/0/08/Neuronal_Synapse.jpg

• Synapse Illustration By Nrets [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/e/e0/Synapse_Illustration2_tweaked.svg

• Resting Membrane Potential By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/d/de/1220_Resting_Membrane_Potential.jpg

• Summation By Sarahadam1 (Own work) [Public domain], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/6/6e/Spacial_and_Temporal_Summation.JPG

• Graded Potentials By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/e/ee/1223_Graded_Potentials-02.jpg

• Action Potential By Original by en:User:Chris 73, updated by en:User:Diberri, converted to SVG by tiZom (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/d/d1/Action_potential_%28no_labels%29.svg

• Action Potentials Stimulus By Original by Curtis Neveu Translated and modified by User:TnoXX [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/7/71/Temporal_summation_uk.png

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