Transmission of Nerve Impulses

Honors Anatomy & Physiology

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 potential2. 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

http://www.sumanasinc.com/webcontent/animations/content/electricalsignaling.html

Resting Potential Animation

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

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter14/animation__the_nerve_impulse.html

Action Potentials

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

substances that produce their poisonous effects by acting on nervous system

Japanese pufferfish: produces very lethal neurotoxin TTX (tetrodotoxin) which blocks AP by inserting itself into voltage-gated Na+ channels so they cannot open

Neurotoxins

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

2 types:1. Continuous Conduction2. Saltatory Conduction3. http://www.blackwellpublishing.com/matth

ews/actionp.html

4. http://www.siumed.edu/~dking2/ssb/saltcon.htm

Conduction

http://www.google.com/imgres?q=conduction+propagation+of+a+nerve+impulse+after+it+arises+at+the+trigger+zone&hl=en&biw=1366&bih=532&tbm=isch&tbnid=yrgINNdE7Tn58M:&imgrefurl=http://dc317.4shared.com/doc/R0Ghe_AP/preview.html&docid=Oi2Xex81aF7bPM&imgurl=http://dc317.4shared.com/doc/R0Ghe_AP/preview_html_50fdb4c.jpg&w=800&h=481&ei=j4CjUaSuO4W68ASc5oCgCg&zoom=1&iact=hc&vpx=873&vpy=37&dur=810&hovh=174&hovw=290&tx=145&ty=97&page=2&tbnh=137&tbnw=234&start=15&ndsp=28&ved=1t:429,r:41,s:0,i:214

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

brain3. 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

Axon Diameter & Conduction Speed

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

Signal Transmission at a Chemical Synapse

http://www.mind.ilstu.edu/flash/synapse_1.swf

Transmission of AP across Synapse

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

Damage & Repair of a Neuron

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

MS

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

infection2. Neuroblastoma

◦ most common tumor in infants◦ immature neurons (neuroblasts) grow in abdomen

or adrenals3. Neuropathy

◦ any disorder that affects the nervous system but particularly a disorder of a cranial or spinal nerve example: Bell’s palsy

Medical Terminology