Ch2 Nervous System

Nervous System

There are billons of neurons.

Neurons regulate almost all physiological variables

Neurons sense and respond to environment

Central Nervous System has the brain and the spinal cord

Peripheral nervous system has afferent divisions (sensing) and efferent division (motor and control)

Sensory receptors > afferent neurons > interneurons > efferent neurons > motor cells

A neuron has dendrites (receive information), cell body, axons, axons terminal, axon hillock, and maybe axon collateral

One way flow of signal

Convergence pathway/Divergence pathway

Ohms Law

$I=GV$

Current is the product of conductance (1 over resistance) and voltage

Nernst Equation for ion equilibrium

$E_X=\frac{RT}{zF}\ln\frac{[X]_{out}}{[X]_{in}}, E_X|_{t=37C}=\frac{58}z\log\frac{[X]_{out}}{[X]_{in}}$

Typical Na conc inside is 15mM, Na conc outside is 145nM, K conc inside is 150mM, and K conc outside is 5mM

$E_k\approx-90mV,\ E_{Na}\approx60mV,\ V_m\approx-70mV$

Goldman-Hodgkin-Katz Equation

$V_{rest}=\frac{G_{Na}E_{Na}+G_KE_K}{G_M}$

Channels:

Na/K ATPase (that pumps 3 Na out and 2K in)

Leak channels (more K leak channel than Na leak channel)

Channels that are always open

Na V-gated channel

open will cause increased membrane potential

Chain-ball model:

There are three states, and will circulate between the three

Closed: When the channel can open and no ions that can flow through

Open: Activated and the ball will quickly inactive the channel

Inactive: No ion cannot pass through and can only go to closed state if repolarized

K V-gated channel

open will cause decreased membrane potential

Process

Depolarization

V goes up

Repolarization

V goes down

Hyperpolarization

V went below RP

Action Potential

all or one

fire if reaches threshold potential and not fire if not reaching threshold potential

unitary (can only fire one at once)

Absolute refractory period: Na channels are still inactive and cannot fire

Relative refractory period: can fire but require more energy

Absolute refractory period keeps move in one direction following AP and forbidding AP from flowing in the other direction.

The elevated membrane potential will make the AP flow (in either direction)

Chemical Synapse are via Neurotransmitters, Electrical Synapses are via gap junction

Steps of passing through chemical synapse (longer than electrical)

AP arrive

V-gated Ca2+ channels opens

Vesicles fuses with the cell membrane and release NT

NT binds with postsynaptic receptors

NT removed from the synaptic cleft

Ionotropic: Ion channels

Metabotropic: via e.g.i GPCR

Excitatory Synapse (EPSP) (compared to IPSP)

cause the postsynaptic neuron more likely to fire.

Echannel is greater than threshold

e.g., AMPA

IPSP NTs

GABA via GABA_A receptors that is Cl- permeable, which Ecl is -80~-60mV

This will either decrease Vm or forces Em cannot go above threshold

There are three types of IPSP

V decrease IPSP

No Change V IPSP

V increase IPSP

When Ecl is greater than 70mV, it still prohibits AP, and thus it is still a IPSP

Knee jerk reflex:

1A muscle spindle stretch receptor, if channel was stretched, it will be pulled apart, and both Na and K can flow through.

This would generate an AP on the 1a afferent neurons if reaching the necessary level.

The receptor is similar to AMPA receptor.

AP propagates to the spinal cords

AP will inhibit flexor muscle in excite extensor muscle

Excitatory Synapse would have Echannel greater than threshold and would cause higher firing rate and firing chance

Inhibitatory Synapse is the direct opposite of Excitatory synapses

GABA Receptor

E_Cl may be greater, lower, or the same as the resting potential, which they anchor membrane potential and reduce the chance and frequency of neuron firings