Two basic types of nerve cells

Neurons-transmit electrical signals

Neuroglia (glial cells)-support the neurons

4 types of neuroglia-CNS

Astrocytes

Microglial cells

Ependymal cells

Oligodendrocytes

2 types of neuroglia-PNS

Satellite cells

Schwann cells

Neuroglia compared to Neurons

Neuroglia are smaller, darker then neurons

they outnumber neurons 10 to 1

making up at least 50% of brain and spinal cord mass

Astrocytes-CNS

"star-shaped"

most abundant and versatile

Jobs: support and brace neurons, guide young neurons, synapse formation, adjust capillary permeability, adjust "chemical environment" by absorbing ions, information processing

Microglial cells-CNS

oval cells with long, thorny processes

Jobs: monitor health of neurons, transform into a macrophage (**because cells of immune system are denied access to CNS)

Ependymal cells-CNS

can be squamous, cuboid, or columnar, most are ciliated

Jobs: line central cavities of brain and spinal cord, cilia help to circulate CSF

Oligodendrocytes-CNS

Job: wrap around neurons of CNS, insulating them and forming myelin sheath

**cannot regenerate like schwann cells

*does not wrap around Nodes of Ranvier

Satellite cells-PNS

*surrounds neuron cell bodies of PNS

Job: is thought to help guide young neurons like the astrocytes

Schwann cells-PNS

AKA neurolemmocytes

Jobs: wrap around nerve fibers in PNS forming myelin sheath; similar to oligodendrocytes of CNS; regenerate damaged peripheral fibers

Neurons (nerve cells) characteristics

conduct nerve impulses, in CNS and PNS, last a lifetime, have high metabolic rate (O2)

**Amitotic-once they reach maturity, they lose ability to divide; except the olfactory epithelium and some hippocampal regions that have stem cells

Nerve cell anatomy parts

Neuron cell body

Nissl body (rough ER)

Microtubules and Neurofibrils

Inclusions

Dendrites and Axons

Neuron Cell body

AKA (ALSO KNOWN AS) perikaryon or soma

do not have centrioles, which is why they are amitotic

Most are in CNS, in receptive region

Nissl Bodies

AKA Rough ER

AKA chromatophilic substance (because it stains darkly with basic dyes)

Microtubules and Neurofibrils

help maintain shape and integrity of cell

Inclusions

little packages of metabolic byproducts that accumulate in the cell

Some are pigments: melanin and lipofuscin

Melanin inside inclusions

red iron-containing pigment

Lipofuscin inside inclusions

golden-brown pigment that accumulates with age

AKA "aging pigment"

Nuclei

clusters of nerve cell bodies in CNS

Ganglia

clusters of nerve cell bodies in PNS

Processes of nerve cell (neuron) anatomy

CNS contains both neuron cell bodies and their processes; tracts

PNS contains mostly neuron processes; nerves

Bundles of neuron processes in the CNS

tracts

Bundles of neuron processes in the PNS

nerves

2 Types of Nerve cell processes

Dendrites

Axons

Dendrites

short, branching

main receptive or input regions with graded potential

**Motor neurons have 100's--dendritic spines are points of synapses with other neurons

Graded potential

NOT action potentials, but are a type of short-distance signal

Axons

**Conducting region of neuron that sends action potentials

1 per neuron, can have many branches

starts with axon hillock, a funnel-shaped region by cell body

long ones (3-4 ft in leg) are called nerve fibers

branches are called axon collaterals

ends in thousands of terminal branches (telodendria) with knob-like end called an axon terminal

What happens when axons get cut

axon contains the same organelles found in the dendrites and cell body with 2 exceptions (no nissl bodies AKA rough ER, no golgi apparatus)

**so axons will quickly decay if cut

Axonal Transport

single bidirectional transport system that brings stuff up and down axons

*motor protein that uses ATP

*goes along microtubules at 15 inches per day

directions: retrograde and anterograde

Retrograde

transport back to the cell body

**polio, herpes simplex and tetanus toxin use this to read cell body

Anterograde

toward axon terminals

Myelin Sheath and Neurolemma

Jobs: whitish fat covers long nerve fibers, protects and electrically insulates nerve fiber, increases speed of nerve impulse transmission

*dendrites always unmyelated, axons either way

Myelin sheath and neurolemma composition-PNS

Schwann cells make it up, Nodes of Ranvier (gaps in between these cells), can wrap around 15 nerve cell axons

Gray Matter-CNS

contains mostly nerve cell bodies and unmyelinated fibers

Myelin sheath and neurolemma composition-CNS

Oligodendrocytes make it up, can wrap around 60 nerve cell axons with widely spaced nodes of ranvier

White Matter-CNS

dense collections of myelinated fibers

Classification of Neurons-Structural

Multipolar neurons

Bipolar neurons

Unipolar neurons

Multipolar neurons

have 3 or more processes

*most common type! (about 99%)

Bipolar neurons

2 processes-cell body in middle and dendrite

*rare (retina of eye and olfactory mucosa)

Unipolar neurons

1 short process emerging from cell body and divides into peripheral and central processes

*found in ganglia or PNS used as sensory neurons

Classification of Neurons-Functional

Sensory (afferent) neurons

Motor (efferent) neurons

Interneurons or association neurons

Sensory (Afferent) neurons

transmit impulse into CNS

unipolar

located in PNS...ganglia

Motor (Efferent) neurons

carry impulse away from CNS

multipolar

cell bodies in CNS...nuclei

Interneurons or Association neurons

between motor and sensory neurons

multipolar and in CNS

Neurophysiology Components

Voltage

Current

Resistence

Voltage

"potential energy" from separation of charges

measured in volts or with tiny stuff like a nerve, millivolts

Current

flow of electrical charge from one point to another

used to do work

Resistance

hindrance to charge flow

Ohm's Law

Current (I) = Voltage (V) / Resistance (R)

Electrical currents reflect flow of ions across cell membranes

Plasma membranes maintaining membrane potential use ion channels

**Open channels allow ions to move along their electrochemical gradients

Types of Charged Channels

Chemically (Ligand) gated ion channels

Voltage gated ion channels

Mechanically gated ion channels

Leak channels

Chemical (Ligand) gated ion channel

**found on nerve dendrites and cell bodies

transmembrane--open or close in response to a chemical neurotransmitters like ligands in neurons, help react quickly to messages

**uses lock and key fit, site far from channel

ion permeability changes, let potassium, sodium, calcium pass through to evoke intracellular electrical signal

Voltage gated ion channels

**Found on nerve axons and axon hillock

**rely on difference in membrane potential

Change potential to let Action potentials occur

Starts at a resting potential with Sodium-potassium ATPase

*reverses resting membrane potential, potassium leaves, which removes positive charges

closes channel by ball-and-chain method

RESULT: more negative cell and an action potential

Resting Membrane Potential

*graded potential

+ and - charges inside and outside the nerve cell, greater negative charge inside

*leak channels allow ions to diffuse down concentration gradients--many more potassium leak channels than sodium leak channels--so more permeable to potassium ions

sodium and potassium gradients, more sodium ions outside, more potassium ions inside

**measured using a voltmeter, on average the charge is -70 millivolts

Depolarization and Hyperpolarization

*graded potentials for short distances

*action potentials for long distances

anything that produces a change in ion permeability can change membrane potential

Depolarization

loss of difference in charge in a nerve cell where sodium ions enter cell

Hyperpolarization

increase in potential difference

Spread and Decay of graded potentials

Action Potential

**send signals over long distances

AP transmission and generation in skeletal muscle cells and neurons are the SAME.

**To have this occur, enough voltage gated channels need to open, graded potentials at the axon hillock transition into action potentials

**NO dissipation over distance, unlike graded potentials

**ALL of these of from -70mV to +30mV

Depolarization of an Action Potential

restores resting electrical conditions, but does NOT restore resting ionic conditions by Sodium-potassium pump

Amounts of sodium needed for Action potential is a 0.012% change in intracellular Na+ concentrations

Saltatory Conduction

in myelinated nerves and motor neurons

action potential is propagated along axon's entire length from each node of ranvier and is self-propagating

30x faster than continuous conduction seen in unmyelinated axons

Propagation of an Action Potential

reversal of charges across a membrane, positive charge from axon hillock to the next segment of axon to trigger action potential in the next axon, then axon hillock returns to resting membrane potential

---action potential travels from one axon to the next --with action potential also moving from one segment of an axon to the next

Graded Potential

positive charge

dissipates over distance, Action potentials go from -70mV to a range of values up to +30mV

Refractory Period of an action potential propagation

**Cannot trigger an action potential going backwards on the gradient

--the ball-and-chain method where the ball closes off the channel so no charge can go through it again...until they reset for another action potential

Relationship between stimulus strength and action potential frequency

**any voltage that is NOT strong enough to open enough sodium channels will NOT generate an action potential

*increasing axon width will also increase action potential speed

*once threshold is reached for an action potential, the stronger the stimulus, the more frequently the action potentials are generated

Absolute Refractory Period in Action Potentials

**when sodium channels open until sodium channels reset

*this patch of membrane can NOT respond to another stimulus

Relative Refractory Period in Action Potentials

**most of the sodium channels have closed

*still repolarizing, but a STRONG stimulus can cause an action potential

Action Potential in Bare Plasma Membrane

Action Potential in Unmyelinated Axons

https://www.youtube.com/watch?v=pbg5E9GCNVE

Action Potential in Myelinated Axons

MS-Multiple Sclerosis

autoimmune disease-due to myelin sheath in CNS being gradually destroyed and hardened (sclerosis)

axons are not affected though

onset in young adults!

1st sign is visual (temporary blindness)

problems controlling muscles (weak, clumsy)-from peripheral motor nerve demyelination

Synapse (tiny gaps) Types

Electrical

Chemical

Electrical Synapse

**less common

neurons joined like this are electrically coupled

action potential transmission is very rapid

unidirectional OR bidirectional

Chemical Synapse

transmit action potentials to postsynaptic neurons by specific chemicals called neurotransmitters

*inside synaptic vesicles at axon terminals

neurotransmitters shoot across a tiny gap called a synaptic cleft (30 to 50nm wide)

*only unidirectional

Excitatory Post Synaptic Potentials

Inhibitory Post Synaptic Potentials

Action potential vs. Graded potential!!

VERY important for test!

Types of Neurotransmitters

Acetylcholine

Norepinephrine

Dopamine

Serotonin

Acetylcholine

nicotinic and muscarinic subtypes

*when prolonged, you can get titanic muscle spasms because acetylcholinesterase is blocked (sarin nerve gas and insecticides can do this)--also inhibited by the botulism toxin

Alzheimers and Acetylcholine connection

less overall acetylcholine with people who have this disease

Norepinephrine

release enhanced by amphetamines

removal from synapse blocked by cocaine and antidepressants

**low levels in depression

Dopamine

release and removal similar to Norepinephrine

**low levels in Parkinson's disease

Serotonin

acts like a brake on a bicycle

blocked by LSD and Prozac (treats depression)

blocking an inhibitor makes it activate!

Differences between action potentials and graded potentials