Unit 1 - The Nervous System & Neurons

How many neurons are in the brain?

100 billion

How long is a neuron?

10 um

How many watts does the brain use?

20 watts

What’s the most common neurotransmitter?

Glutamate

Resting Voltage

-65mV

CNS

Parts of the nervous system enclosed in bone

PNS

Parts of the nervous system not in bone

Brainstem consists of

Midbrain + pons + medulla

Spine consists of

Cervical + thoracic + lumbar + sacral

Neuron

Fires action potential + axon

Glial Cells

Creates support

Astrocytes

Maintains ionic environment

Microglia

Clears cellular debris

Oligodendrocytes

Myelinates CNS neurons

Schwann Cells

Myelinates PNS neurons

CSF

Aqueous solution surrounding neurons with Na, K, Cl, other ions

What are the main causes of a negative resting potential?

Diffusion and Electrostatic forces

Diffusion of K

K will diffuse out more than in

Electrostatic Forces of K

Enough K flows out creating electrostatic attraction, creating equal flow in and out at a negative voltage

Rostral

Upwards

Dorsal

Behind neuraxis

Caudal

Lower

Ventral

Infront of neuraxis

Superior

Above

Posterior

Behind

Inferior

Below

Anterior

In front of

Ipsilateral

Same side

Contralateral

Opposite side

Decussate

Cross midline

Proximal

Close to reference

Distal

Far to reference

Efferent

Leaving from reference

Afferent

Towards reference

Medial

Near midline

Lateral

Far from midline

Coronal

Side to side

Midline

Sagittal

Neuroscience Rules

1. Symmetry

2. Localization

3. Contralaterality

4. Topography

Localization

Different parts have different functions and spatially separatedq

Contralaterality

Functions for something are crossed

Topography

Neurons are mapped on the brain as the body

Nissl Stain

Shows section of neurons by staining the RNA

How thick is the cerebral cortex in mammals?

2mm

Brodmann’s areas

52 cortical areas identified by the Nissl stain

How is it discovered that everything is through action potentials?

• Local anesthesia

• Electrical brain stimulation

• Strokes

How did Helmhotz measure neuron propagation?

Shocked nerves and seeing the time difference between the muscle contraction

Receptor Potential

Graded tiny potentials that brings neuron to threshold

Depolarization

Voltage becomes positive

Afterhyperpolarization

Past resting potential around -70mV

Threshold Potential

Around -50mV

Na Rushing in because

Postive activation gate on Na gated channels open because it’s no longer attracted to the less negative inside

Na inactivation

Na channel inactivate from the positive gate being repelled from the positive inside

Absolute Refractory Period

Inactivation of Na channels

Falling Phase

K voltage channels open after inactivation from its positive charged gate

Undershoot

K voltage channels close slowly and leaks channels are still open causing hyperpolarization

Lidocaine

Enters Na gated channels and physically blocks them for a certain amount of time

Tetrodoxin

From pufferfish bacteria in ovaries and liver binds to Na channels but remains bounded causing paralysis of lung nerves

Saxitoxin

Butter clam that came in contact with red time made by toxic dinoflagellates which the shells eat causing paralytic poisoning

Voltage Clamp

Measures current voltage and can manipulate the axon’s voltage

Voltage Clamp Set Up

Inserts 2 electrodes into axon, 1 to meausre current and uses a reference as a base, and 2 to add current through

Use of Electrode 2 in Voltage Clamp

Maintains a given voltage, if voltage surpasses threshold, more energy is given to counteract axon’s natural firing

Conventionally, positive charge is plotted as

Negative current

Hodgkin and Huxley

Refined voltage clamp and AP

Nernst Equation

Neuron Conduction Velocity Equation

Goldman-Hodgkin Katz Equation

Rising Phase

Na+ ions enter from positively activated gate, when inside is positive enough, Na channel inactivation closes because it’s repelled from the positive inside

Low Membrane Resistance

Na ions coming in repels K ions that goes out leak channels, making it harder to reach AP (leaky)

High Membrane Capacitance

The negative outside of neuron causes positive ions inside to stick to the membrane (sticky)

High Axial Resistance

Thin axons make it hard for ions to move down

What did invertebrates evolve to increase AP conduction?

Wider axons for selected neurons which increases AP but takes up space

What did vertebrates evolve to increase AP conduction?

Myelin which adds distance between charges, and prevents leaking out to replace it with nodes of ranvier

Louis-Antione Ranvier

Discovered the notes of ranvier

Oligodendrocyte

Myelin in the CNS and wraps around multiple axons

Schwann Cells

Myelin in the PNS and wraps around 1 axon

Theodor Schwann

Discovered Schwann cells and component that cells are made from cells

Multiple Sclerosis

Autoimmune disease causing loss of function in nervous system that vary day to day from demyelination of oligodendrocytes from antibodies

Multiple Sclerosis Risks

Twice as common in woman, and increases chances if an identical twin has it, compared to fraternal

Guillain Barre Syndrome

Multiple sclerosis but in the PNS, which is rarer but condition and recovery can vary drastically

Camillo Golgi believed in

Proponent of reticular theory where all neurons are physically connected

Santiago y Cajal believed in

Proponent of neuron doctrine where neurons have small gaps between them

Synaptic Cleft

Gap between neurons about 20-40nm wide

Otto Loewi

Tested chemical transmission by putting 2 frog vagus nerve in separate containers connected by solution, and simulation of 1 caused same reaction in other nerve indicated some chemical must’ve been released

Bernard Katz

Took muscle cells and stimulated them to see the mV in trails and found at 0.4mV intervals there was a characteristic peak

Poisson Distribution

Discrete probability distribution that shows the probability of a given number of event within an interval

Synaptic Transmission

Positive charge causes Ca gate to unbind and allow Ca ions to come in

Neurotransmitter Release

Vesicles move near the membrane with help of SNARE complex, and Ca binds to synaptotagmin catalyzing membrane fusion

Synaptotagmin

Ca detector protein

Ca Pump

Pumps Ca out using H and ATP

Na/Ca ion Exchanger

Uses Na gradient to pump Ca out

Lily Jan and Yuh Nung Jan

Discovered peptides are neurotransmitters

Glutamate

Common excitatory neurotransmitter

Glumate Receptor Channel

Allosterically binds with glutamate to be equally permeable to K and Na causing overall EPSP

GABA (Gamma-aminobutyric acid)

Most common inhibitory neurotransmitter in cerebral cortex

GABA Receptor Channel

Selectively permeable to Cl when GABA binds causing IPSP

Glycine

Common inhibitory neurotransmitter in CNS

Driving Force Equation

I_x = g_x (V_m - E_x)

If I_x is positive

Outward current, cell loses positive charge

If I_x is negative

Inwards current, cell loses negative charge