Transmission of Nerve Impulses

Honors Anatomy & Physiologyfor copy

• neurons are electrically excitable• 2 types of electrical signals:

1. graded potentials• only for short distance communication

2. action potentials• allow communication over both short &

long distances

Electrical Signals in Neurons

• both depend on 2 basic features of plasma membrane in excitable cells:

1. existence of a resting membrane potential

2. presence of specific ion channels

Graded & Action Potentials

• electrical voltage difference across the membrane

• flow of ions thru ion channels => flow of current

Membrane Potentials

• when open: 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

Ion Channels

• 4 types:1. leakage channels2. voltage-gated channels3. ligand-gated channels4. mechanically gated channels

Ion Channels

• channels randomly alternate (open/close)

• typically are more K+ leakage channels than Na+ & K+ channels are leakier than Na+– (so membranes more permeable to K+

than Na+)

Leakage Ion Channels

• open in response to change in membrane potential

• important in generation & conduction of action potentials

Voltage-Gated Channels

• open/ close in response to a specific chemical stimulus

• ligands include:– neurotransmitters– hormones– particular ions

• work in 1 of 2 ways:1. directly: ligand molecule itself opens/closes gate2. indirectly: ligand activates another molecule

which in turn opens/closes gate

Ligand-Gated Channels

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

Mechanically Gated Channels

• 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)

Resting Membrane Potential

• neurons: membrane potentials range from• -90 mv to -40 mv

• (-) indicates the inside of cell is (-) relative to the outside

• cells with membrane potential are said to be polarized

• most body cells are polarized with membrane potentials vary from +5 mv to -100 mv

Resting Membrane Potential

Generation of Action Potentials (AP)

• 2 phases: (lasts ~ 1msec)1. Depolarizing Phase– (-) membrane becomes less (-)

2. Repolarizing Phase– normal membrane potential restored

• 2 types of voltage-gated channels open & then close:– present mainly in axon plasma membrane &

axon terminals

• 1st to open: Na+– Na+ rushes into cell– causes depolarization

• then K+ channels open– K+ flows out of cell– produces repolarizing phase

AP Voltage-Gated Channels

1. “all-or-none” principle2. threshold: (~ -55mv) must be

reached for AP to occur

Action Potential Characteristics

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

Refractory Period

• AP must travel from trigger zone 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

Propagation of Nerve Impulses

• drugs that block pain & other somatic sensations– cold applied to area also produces anesthetic effect:

axons propagate slower – ice partially blocks axon propagation of pain

• examples:– Lidocaine, Novacaine

• act by blocking the opening of voltage-gated Na+ channels:– sensory nerve impulses cannot travel past obstructed

region so pain signals never make it to the brain

Local Anesthetics

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

• Classified:1. A-fibers: (5-20 μm) travel 12-130 m/s– used by somatic sensory neurons & motor neurons

2. B-fibers: (2-3 μm) travel 15 m/s– found in sensory neurons going from viscera brain

3. 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

Effect of Axon Diameter

• presynaptic neuron: neuron sending the AP

• postsynaptic neuron: neuron receiving the AP

• synapse: space between the 2

• synapses can be:1. electrical2. chemical

Signal Transmission @ Synapses

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

• 2 advantages to electrical synapses1. faster communication– AP passes directly from presynaptic neuron

postsynaptic neuron

2. synchronization– electrical synapses can coordinate the

activity of a group of neurons or muscle fibers

Electrical Gap Junctions

• synaptic cleft: space between filled with interstitial fluid

• 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)

Chemical Synapses

• occurs in 3 ways:1. diffusion2. enzymatic degradation3. uptake by cells

Removal of Neurotransmitter

• 2 classes:1. small-molecule neurotransmitters2. neuropeptides

Neurotransmitters

• nervous system exhibits plasticity: capability to change based on experience

• @ level of individual neurons:– sprouting new dendrites– synthesis of new proteins– changes in synaptic contacts with other

neurons

Plasticity in Nervous System

• limited capacity to replicate or repair itself

• until recently: thought was little or no repair done to damage in CNS: have found neuroglial cells more capable of this than previously appreciated

• PNS: as long as cell body intact & Schwann cell active new axon can be regenerated

Regeneration in Nervous System

• Multiple Sclerosis (MS)• progressive destruction of myelin

sheaths of neurons in the CNS• autoimmune disorder• afflicts ~ 350,000 in USA (estimated)

with:– >2x female/male– white > other races– diagnosis difficult but most diagnosed ages 20

- 50

Homeostatic Imbalances in the Nervous System

• unknown but do see:• genetic susceptibility: having 1st

degree relative with it increases your chances several-fold

• possible association to living farther away from equator (?) so maybe something to do with vitamin D

• possible association with history of herpes-6, Epstein Barr virus, chlamydia

MS Causes?

• characterized by short, recurrent attacks of motor, sensory, or psychological malfunction

• afflict ~1% of world’s population• http://www.dailymotion.com/video/xaigxc_br

ain-animation-of-epileptic-seizu_creation#.UaPsb0B-8TY

Epilepsy

1. brain damage– most frequently from birth trauma

2. metabolic disturbances– hypoglycemia, hypocalcemia, uremia, hypoxia

3. infections– encephalitis or meningitis

4. toxins – alcohol, tranquilizers, hallucinogens

5. head injuries6. tumors & abcesses of the brain

Causes of Epilepsy

1. Guillain-Barre Syndrome (GBS)– acute demyelinating disorder in which macrophages

strip axons of myelin in PNS– may be response of immune system to bacterial

infection

2. Neuroblastoma– most common tumor in infants– immature neurons (neuroblasts) grow in abdomen or

adrenals

3. Neuropathy– any disorder that affects the nervous system but

particularly a disorder of a cranial or spinal nerve example: Bell’s palsy

Medical Terminology