(exactly essay 3) Physiology of excitable cells. Irritability and Excitability. General and specific properties of excitable cells. Membrane potential - ionic basis of the membran epotentials. Measurement fo excitability. Changes in the excitability during excitation. Inhibition. Conduction of excitation

section

excitable tissues

nervous tissue

RMP (resting membrane potential) depends on

characteristics of resting nerve cell membrane

characteristics of muscle

function of muscle

types of muscle

Endocrine tissue

excitable tissues

tissues capable of generating and transmitting electrochemical impulses along the membrane.

1. Nervous - via neurons

2. muscle - nerves conducting mpulses to muscles → conductors

3. Some endocrine tissues- insulin-releasing pancreatic β cells.

nervous tissue

The human nervous system consists of many nerve cells (or neurons) plus supporting (neuroglial) cells.

• Neurons- communicate

• Respond to stimuli

• Extending out from the cell body are processes called dendrites and axons.

• conduct impulses (with dendrites conducting impulses toward the cell body

• axons conducting impulses away from the cell body

• Neurons can respond to stimuli and conduct impulses because a membrane potential is established across the cell membrane.

• Membranes have a RESTING MEMBRANE POTENTIAL. - an unequal distribution of ions (atoms with a positive or negative charge) on the two sides of the nerve cell membrane.

• This POTENTIAL about -70 mV (with the INSIDE of the membrane negative due to more ions outside nerve membrane)

RMP (resting membrane potential) depends on

• Distribution of ions across membrane (extracellular ions and intracellular ions);

• membrane permeability activity of Na-K pump

• SODIUM - POTASSIUM PUMP actively pumps out 3 Na+ out for every 2K+ in → less positive inside

• membrane is more permeable to k+ so they diffuse back out of cell. less name positive

• These gates represent the only way that these ions can diffuse through a nerve cell membrane.

characteristics of resting nerve cell membrane

• all the sodium gates are closed and some of the potassium gates are open.

• This potential will be maintained until the membrane is disturbed or stimulated.

• If there is a strong stimulus greater than the threshold, an AP will occur. → depolarisation = disturbance

• (*The membrane potential of excitable tissues is more negative (-70 to -90 mV) compared to that of non-excitable tissues.)

characteristics of muscle

1. excitability - responds to stimuli (e.g., nervous impulses)

2. contractility - able to shorten in length

3. extensibility - stretches when pulled

4. elasticity - return to original shape & length after contraction or extension

function of muscle

1. Motion - movement of trunk and limits

2. maintenance of posture

3. heat production - shivering → vibration causes friction = heat

types of muscle

• skeletal: voluntary muscle. Moves skeleton and striated muscles = heat

• Smooth: involuntary muscle, muscle of the viscera (e.g., in walls of blood vessels, intestine ) - urinary bladder, spindle-shaped

• Cardiac: involuntary, striated

Endocrine tissue

• The endocrine system includes cells of the glands of the body and the hormones produced by them

• glands are controlled directly by the nervous system as well as by chemical receptors in the blood and hormones produced by other glands.

• The endocrine system regulates its hormones through negative feedback, except in very specific cases like childbirth.

section

define excitation

process when there is an electrical stimulus

action potential

saltatory conduction

inhibition

define excitation

• excitable cells react to irritation or stimulation

• excitation: process of producing an action potential

local respons

-A local response happens when a weak stimulus slightly changes the membrane potential but doesn’t trigger an action potential.

• Na⁺ ions must enter the cell in large enough numbers to open sodium channels and start an action potential.

• This can be caused by mechanical, chemical, or electrical stimulation.

• A weak or subthreshold stimulus (like at point A or B) only causes a small membrane change (e.g., from –95 mV to –85 mV) — no action potential occurs.

• When the stimulus is strong enough to reach the threshold, an action potential is generated.

• A lower threshold means the membrane is more excitable and can respond even to a weaker stimulus if it lasts long enough.

action potential

• At rest, there is no action potential between electrodes A and B — the oscilloscope ray stays on the zero line.

• When a strong stimulus is applied, depolarization begins.

• As depolarization reaches electrode A, its surface becomes negative compared to B — the oscilloscope ray moves upward.

• When the signal reaches electrode B, both electrodes are equally excited — the ray returns to zero.

• When the area near electrode A repolarizes, it becomes positive compared to B — the ray moves downward.

action potential simply

• depolarisation = transient increase in membrane permeability to Na^+ so membrane becomes more positive Na^+ channels open

• Repolarisation = closing of Na^+ channels and opening of K channels, membrane becomes negative again → over shoots

• resting membrane potential = hyperpolarisation so Na/K pump brings back to RMP

saltatory conduction

• AP’s are unable to pass through myelin sheath of nyleinated neurones

• there are gaps between myelin sheath that expose the membrane - nodes of Ranvier

• Impulse jumps across axons - saltatory conduction, increases velocity of nerve transmission

inhibition

• Membrane-stabilizing factors can decrease excitability.

• e.g., a high extracellular fluid calcium ion concentration

• decreases membrane permeability to sodium ions and simultaneously reduces excitability.

• Therefore, calcium ions are said to be a “stabilizer.”

• Tetrodotoxin is a sodium channel blocker-inhibits firing of AP