impulse transmission : nerve

saltatory conduction

zRapid transmission of a nerve impulse along an axon, resulting from the action potential jumping from one node of Ranvier to another, skipping the myelin-sheathed regions of membrane.

action potential for

allow communication over both short and long distances

2 phases of generation of action potential

1. Depolarizing Phase 2. Repolarizing Phase

C-fibers

( .5-1.5 µm) travel 0.5 - 2 m/s -all unmyelinated, found in some sensory for pain from skin and viscera & in autonomic motor fibers

Depolarizing Phase

(-) membrane becomes less (-)

B-fibers

(2-3 µm) travel 15 m/s -found in sensory neurons going from viscera à brain

A-fibers

(5-20 µm) travel 12-130 m/s -used by somatic sensory neurons & motor neurons

mechanically gated channels

-gates open/close in response to mechanical stimulation in form of: -vibration (sound waves) -pressure (touch) -tissue stretching

action potential characteristics

-"all-or-none " principle -threshold: (~ -55mv) must be reached for AP to occur

when channels open, what happens?

-: allow specific ions to move across plasma membrane down their electrochemical gradient by diffusion -ions move down their concentration gradient & (+) charged cations move toward (-) charged anions

propagation of a nerve impulse

-AP must travel from trigger zone to axon terminal -this mode of travel called propagation or conduction -uses (+) feedback -when Na+ flows into cell it causes voltage-gated Na+ channels in adjacent segments to open

how voltage-gated channels work

-At resting potential, voltage gated Na+ channels are closed -When membrane is depolarized, conformational changes open the the voltage gated channel.

temperature (factors affecting impulse generation and conduction)

-Cold applied to area also produces anesthetic effect: axons propagate slower -Its prolong time of depolarization -Conduction velocity reduce ~ 2.5 m/s every degree drop -ice partially blocks axon propagation of pain

age (factors affecting impulse generation and conduction)

-Conduction decreases with age -More prominent after 60 yo

nerve location (factors affecting impulse generation and conduction)

-Proximal nerve segment conduction slightly faster than the distal

height (factors affecting impulse generation and conduction)

-Taller individual have slower conduction velocity

factors affecting impulse generation and conduction

-axon diameter -myelination -temperature -age -height -nerve location

leakage ion channels

-channels randomly alternate (open/close) -typically are more K+ leakage channels than Na+ -K+ channels are leakier than Na+ - membranes more permeable to K+ than Na+

membrane potential?

-electrical voltage difference across the membrane -flow of ions thru ion channels => flow of current

CHEMICAL SYNAPSES

-in response to AP axon terminal of presynaptic neuron releases neurotransmitters that diffuse across the synaptic cleft -> bind to receptors in plasma membrane of postsynaptic neuron -> producing a postsynaptic potential -electrical signal (AP) -> chemical signal (neurotransmitters) -> electrical signal (AP)

voltage gated channels

-open in response to change in membrane potential/electrical gradient -important in generation & conduction of action potentials

ligand-gated channels

-open/ close in response to a specific chemical stimulus -ligands include: (neurotransmitters ,hormones, particular ions)

During an action potential

-outside more positive than inside -Na+ influx into the cytoplasm of neuron causing inside depolarization(inside more positive than before) -potassium will diffuse outside thus inside becomes negative again -outside more positive than inside

types of signal transmission at synapse

-presynaptic neuron -postsynaptic neuron -synapse

4 types of ions channel

1) leakage channels 2)voltage-gated channels 3)ligand-gated channels 4)mechanically gated channel

2 ways conduction of nerve impulse can occur

1)continuous conduction(occurs in non-myelinated axon) 2)saltatory conduction(occurs in myelinated axon)

axon diameter classified

1. A-fibers: 2. B-fibers: 3. :

process of action potential

1. Stimulus starts the rapid change in voltage or action potential. 2. Depolarization is caused by a rapid rise in membrane potential opening of sodium channels in the cellular membrane, resulting in a large influx of sodium ions. 3. Membrane Repolarization results from rapid sodium channel inactivation as well as a large efflux of potassium ions resulting from activated potassium channels. 4. Hyperpolarization is a lowered membrane potential caused by the efflux of potassium ions and closing of the potassium channels. 5. Resting state is when membrane potential returns to the resting voltage that occurred before the stimulus occurred.

How does a ligand-gated ion channel work?

1. directly: ligand molecule itself opens/closes gate 2.indirectly: ligand activates another molecule which in turn opens/closes gate

2 types of voltage-gated channels open & then close ( generation of action potential )

1st to open: Na+ then K+ channels open

faster communication

AP passes directly from presynaptic neuron to postsynaptic neuron

action potential def

An action potential is a rapid rise and subsequent fall in voltage or membrane potential across a cellular membrane

then K+ channels open

K+ flows out of cell produces repolarizing phase

axon diameter (factors affecting impulse generation and conduction)

Larger diameter axons propagate impulses faster than smaller diameter ones due to larger surface area

1st to open: Na+

Na+ rushes into cell causes depolarization

Continuos conduction

Occurs in UNmyelinated axons and involves the sequential opening of voltage-gated Na+ channels and voltage-gated K+ channels located in axon plasma membrane along the entire length of the axon.

how can we stimulate neuron

by electrical shock or etc as neurons are electrically excitable

upper motor neuron located

cell body lies in a cns processing center

lower motor neurone

cell body located in a motor nucleus of the brain or spinal cord

electrical gap junctions common in?

common in visceral smooth muscle, cardiac muscle, & in developing embryo

synapses can be

electrical or chemical

synchronization

electrical synapses can coordinate the activity of a group of neurons or muscle fibers

resting membrane potential

exists because there is small build-up of (-) charge just inside plasma membrane (small build-up of (+) charge just outside membrane) -separation of charge source of PE -greater the difference in charge the larger the membrane potential (voltage) -neurons: membrane potentials range from -90 mv to -40 mv -(-) indicates the inside of cell is (-) relative to the outside

Advantages of electrical synapses

faster communication and synchronization

second order neurons

first order neurons synapse on these in the brain or spinal cord

three neurons involve in sensory information

first order, second order, and third order neurone

2 types of electrical signals

graded potential and action potential

postsynaptic neuron

neuron receiving the AP

presynaptic neuron

neuron sending the AP

2. Repolarizing Phase

normal membrane potential restored

graded potential for

only for short distance communication

in resting state gated channel(positivity)

outside more positive than inside

What is the refractory period?

period of time after an AP begins during which an excitable cell cannot generate another action potential

third order neurons

second order neurons synapse on these and found in the thalamus

first order neurone

sensory neurons that deliver sensory information to the cns

synapse

space between the 2

what does both electrical signal depend on

they depend on 2 basic features of plasma membrane in excitable cells: 1. existence of a resting membrane potential 2. presence of specific ion channels

When is action potential generated?

when na+ influx causing depolarization