The nervous system presentation dawn part 2

transcript

THE NERVOUS

SYSTEM

DAWN V TOMY M.Pharm

Asst.Professor,

Dept. Of Pharmacology

St.JOSEPH’S COLLEGE OF PHARMACY,CHERTHALA.

Part -2

Cell Membrane Potential

•A cell membrane is usually electrically charged, or

polarized, so that the inside of the membrane is negatively

charged with respect to the outside of the membrane (which is

positively charged).

• This is as a result of unequal distribution of ions on the

inside and the outside of the membrane.

Distribution of Ions

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

• Sodium (Na+) ions are the major extracellular positive ions (cations).

• This distribution is largely created by the Sodium/Potassium Pump

(Na+/K+ pump).

• This pump actively transports 3 sodium ions out of the cell and 2

potassium ions into the cell.

Resting Potential

• Resting Membrane Potential

(RMP):

• 70 mV difference from

inside to outside of cell

• It is a polarized

membrane

• Inside of cell is negative

relative to the outside of

the cell

• RMP = -70 mV

• Due to distribution of

ions inside vs. outside

• Na+/K+ pump restores

AxonCell body

Low Na+

Axon terminalLow K+

High K+

High Na+

+––

–+–

–+–+

–70 mV

+––

–+–

–+–+

–70 mV

Low Na+

Low K+ High K+

High Na+

Impermeant

anions

Local Potential Changes

• Caused by various stimuli:• Temperature changes

• Light

• Pressure

• Environmental changes affect the membrane potential by

opening a gated ion channel

• Channels are 1)Leakage channels

2) chemically gated, 3) voltage gated, or 4) mechanically gated.

Gate-like mechanism Protein

(b) Channel open(a) Channel closed

membrane

Fatty acid

Phosphate

Local Potential Changes

• If membrane potential becomes more negative, it is hyperpolarized.

• If membrane potential becomes less negative, it is depolarized.

• Graded (or proportional) to intensity of stimulation reaching

threshold potential.

• Reaching threshold potential results in a nerve impulse, starting an

action potential.

–62 mV

Neurotransmitter

–55 mV

Na+ Na+

Trigger zone (axon hillock)

Chemically-gated

Na+ channel

Presynaptic

neuron

Voltage-gated

Na+ channel

Action Potentials

•At rest, the membrane is

polarized (RMP = -70)

• Sodium channels open

and membrane

depolarizes (toward 0)

• Potassium leaves

cytoplasm and

membrane repolarizes

• Threshold stimulus

reached (-55)

• Brief period of

hyperpolarization (-90)

Region of depolarization(b)

Region of repolarization(c)

–0K+

Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+

K+ K+ K+ K+ K+ K+ K+

Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+

K+ K+ K+ K+ K+ K+ K+ K+

Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+

K+ K+ K+ K+ K+

Na+ Na+ Na+

Threshold

stimulus

K+ K+ K+ K+ K+

Na+ Na+ Na+

K+ K+ K+

Na+ channels open

K+ channels closed

K+ channels open

Na+ channels closed

Action Potentials

Direction of nerve impulse

– – – – – – – – –

– – – – –– – – –

– – – – – – – – –

+ + + + + + + +

+ + + + + + + + +

++ + + + + + + +

++ + + ++ + + +

Region of

action potential

Action Potentials

Milliseconds

2 3 4 5 6 7 8

Action potential

Hyperpolarization

Resting

potential

Resting potential

reestablished

Refractory Period

• Absolute Refractory Period• Time when threshold stimulus does not start another

action potential

• Relative Refractory Period• Time when stronger threshold stimulus can start another

action potential

All-or-None Response

• If a neuron axon responds at all, it responds completely –

with an action potential (nerve impulse).

•A nerve impulse is conducted whenever a stimulus of

threshold intensity or above is applied to an axon

•All impulses carried on an axon are the same strength

Impulse Conduction

The Synapse

• Nerve impulses pass

from neuron to neuron at

synapses, moving from a

pre-synaptic neuron to a

post-synaptic neuron.

Dendrites

Impulse

Synaptic

Axon of

presynaptic

neuron

Cell body of

postsynaptic

neuron

Axon hillock of

Postsynaptic neuron

Axon of

presynaptic

neuron

Synaptic Transmission

• This is where released neurotransmitters cross the

synaptic cleft and react with specific molecules called

receptors in the postsynaptic neuron membrane.

• Effects of neurotransmitters vary.

• Some neurotransmitters may open ion channels and

others may close ion channels.

Synaptic Transmission

• Neurotransmitters are

released when impulses

reach synaptic end bulbs

Mitochondrion

Synaptic knob

Synaptic cleft

Neurotransmitter

Presynaptic neuron

Direction of

nerve impulse

Synaptic

vesicles

Cell body or dendrite

of postsynaptic neuron

Synaptic

vesicle

Vesicle releasing

neurotransmitter

membrane

Polarized

membrane

Depolarized

membrane

Ca+2Ca+2

Synaptic Potentials

• EPSP

• Excitatory postsynaptic potential

• Graded

• Depolarizes membrane of postsynaptic neuron

•Action potential of postsynaptic neuron becomes more likely

• IPSP

• Inhibitory postsynaptic potential

• Graded

• Hyperpolarizes membrane of postsynaptic neuron

•Action potential of postsynaptic neuron becomes less likely

Summation of EPSPs and IPSPs

• EPSPs and IPSPs are added

together in a process called

summation

• More EPSPs lead to greater

probability of an action

potential

Nucleus

Neuron

cell body

Presynaptic

Neurotransmitters

Neuropeptides

• Neurons in the brain or spinal cord synthesize neuropeptides.

• These neuropeptides act as neurotransmitters.

• Examples include:

• Enkephalins

• Beta endorphin

• Substance P

Neurotransmitters

The nervous system presentation dawn part 2

Health & Medicine