Neurophysiology

Lectures 4, 5, 6, 7

What is the fundamental unit of the nervous system

Neurons

3 structural classes of neurons

1. multipolar

2. bipolar

3. unipolar / pseudounipolar

Multipolar neuron

multiple processes from cell body, multiple dendrites and one axon
“normal”

Bipolar neuron

two main processes

Unipolar or pseudounipolar neuron

one main process from cell body

3 functional classes of neurons

1. sensory (afferent - “away")

2. interneuron

3. motor (efferent - “enter”)

Sensory neurons

• Transmit information about sensory stimuli from receptors to CNS

• Specific structure and function depends on sensory system

Interneurons

• Connects neurons to one another- sensory to motor

• Often short, unmyelinated axons and complex dendrites

Motor neurons

• Transmits info from CNS to effector organ (muscle or gland)

• Initiates muscle contractions or gland secretions

• Cell bodies located in CNS with long myelinated axons to reach distant targets

Neuronse use ______ to transmit _____

electricity, signals

Additional; Neurons are specialized to use changes in membrane potential to communicate signals across long distances

Excitable cells maintain a ______

membrane potential

Membrane Potential

voltage difference across cell memrbane

• measured in V or mV

• inside of cell negative relative to outside of cell

How do membrane potentials arise

from unequal distribution of ions across the membrane

How does the cytoplasm function in membrane potentials?

Cytoplasm has a net neutral charge, separated charges accumulate along membrane surfaces

How does the plasma membrane function as a capacitor?

a component that stores electrical energy by accumulating electric charges on two closely spaced surfaces that are insulated from each other

Ions move across membranes according to __________

electrochemical gradients

Equilibrium potential of an ion

• The membrane potential at which an ion reaches equilibrium (i.e. the net flow of an ion is zero)

• It also determines the membrane potential at which the direction of ion flow reverses (i.e. the electrical gradient exceeds the concentration gradient)

How can equilibrium potential be calculated?

Use Nernst equation

How can ion movement be predicted

Compare Eion (equilibrium potential for that ion) and Vm (membrane potential)

How is membrane potential calculated?

Goldman equation

What is typical resting membrane potential

-70 mV, this is actively maintained thru dynamic equilibrium

What factors contribute to maintaining -70 mV RMP (3 main ones)

• Distribution of ions near plasma membrane

  • more (+) outside; more (-) inside (especially large, impermeable anions)

• Na/K Pump

  • Direct effect = Between -2 and -5 mV

  • Indirect effect = Maintains ionic gradients by active transport

• Leaky channels – Predominant determinant of RMP

What are “leaky channels”

constitutively active channels that allow facilitated diffusion

Does the RMP ever reach Ek?

no it approaches it, but NEVER reaches it

What does changes in membrane permeability cause

electoral signals

When ion channels open and close, what happens to membrane potential?

rapid changes in membrane potential

3 main ways to describe changes in potential relative to resting potential

1. depolarization

2. hyperpolarization

3. repolarization

Depolarization

membrane potential becomes LESS negative

• (+) ions enter, and (-) ions exit

Hyperpolarization

membrane potential becomes MORE negative

• (+) ions exit, (-) ions enter

Repolarization

membrane potential returns to RMP

What id driving force, and what does it determine

difference between the membrane potential (Vm) and the ion equilibrium potential (Eion), determines ion flow.

• larger the difference, stronger the driving force, the higher the rate of ion movement

Signal path (IMAGE)

What can signal reception lead to

Grade potentials

• Occur in dendrites and cell bodies

• Vary in magnitude and duration - Proportional to strength of stimulus

• Transient changes that occur locally

• Short distance signals

• Can be excitatory or inhibitory

Graded potentials decay over short distances, why?

Electrotonic: passive current due to electrical interactions within the cell

Threshold potential

membrane potential required to initiate an action potential

• depolarization required to activate NaV channels

• usually around -55 mV

• @ axon hillock

EPSP - excitatory post synaptic potential

increased likelihood of reach threshold

• ligand gated Na+ channels

IPSP - inhibitory post synaptic potential

decreases likelihood of reaching threshold

• ligand gated Cl- channels

Action Potential

• All-or-none mentality

• Long distance signals - Maintain amplitude

• Same magnitude and duration for a given cell

• Occur in axons

• Caused by opening and closing of voltage-gated channels

FOR EXAM KNOW HOW TO DRAW

Gating of voltage-gated Na+ channels is determined by what two mechanisms?

• Activation gate

  • voltage sensor

  • opens @ threshold

• inactivation gate

  • closes from cytoplasmic side

Voltage-gated Na+ channels display intrinsic inactivation (shut off after usage)

What does depolarization of membrane do?

activates voltage gated K+ channels

2 types AP refractory periods

Absolute and relative

Absolute refractory period

time during which another AP CANNOT be triggered

• No matter the stimulus strength

• Occurs from onset of AP until the end of Na+ channel inactivation

Relative refractory period

time during which a stronger stimulus is necessary to trigger an AP

• More depolarizing current is required to reach threshold

• Due to hyperpolarization