Transmission of Nerve Impulses
Honors Anatomy & Physiology
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Electrical Signals in Neurons
• 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
Graded & Action Potentials
• 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
Membrane Potentials
• electrical voltage difference across the
membrane
• flow of ions thru ion channels => flow of
current
Ion Channels
• 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 channels
2. voltage-gated channels
3. ligand-gated channels
4. mechanically gated channels
Leakage 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+)
Voltage-Gated Channels
• open in response to change in membrane
potential
• 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
• work in 1 of 2 ways:
1. directly: ligand molecule itself opens/closes gate
2. indirectly: ligand activates another molecule which in
turn opens/closes gate
Mechanically Gated Channels
• gates open/close in response to mechanical
stimulation in form of:
– vibration (sound waves)
– pressure (touch)
– tissue stretching
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)
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
Generation of Action Potentials
(AP)
• 2 phases: (lasts ~ 1msec)
1. Depolarizing Phase
–
(-) membrane becomes less (-)
2. Repolarizing Phase
–
normal membrane potential restored
AP Voltage-Gated Channels
• 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
Action Potential Characteristics
1. “all-or-none” principle
2. threshold: (~ -55mv) must be reached for
AP to occur
Refractory Period
• period of time after an AP begins during
which an excitable cell cannot generate
another action potential
Propagation of Nerve Impulses
• 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
Local Anesthetics
• 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
Effect of Axon Diameter
• 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
–
found in sensory neurons going from viscera  brain
–
all unmyelinated, found in some sensory for pain from
skin and viscera & in autonomic motor fibers
2. B-fibers: (2-3 μm) travel 15 m/s
3. C-fibers: ( .5-1.5 μm) travel 0.5 – 2 m/s
Signal Transmission @ Synapses
• presynaptic neuron: neuron sending the AP
• postsynaptic neuron: neuron receiving the AP
• synapse: space between the 2
• synapses can be:
1. electrical
2. chemical
Electrical Gap Junctions
• common in visceral smooth muscle,
cardiac muscle, & in developing embryo
• 2 advantages to electrical synapses
1. 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
Chemical Synapses
• 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)
Removal of Neurotransmitter
• occurs in 3 ways:
1. diffusion
2. enzymatic degradation
3. uptake by cells
Neurotransmitters
• 2 classes:
1. small-molecule neurotransmitters
2. neuropeptides
Plasticity in Nervous System
• 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
Regeneration 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
Homeostatic Imbalances in the
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
MS Causes?
• 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 herpes6, Epstein Barr virus, chlamydia
Epilepsy
• 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-epilepticseizu_creation#.UaPsb0B-8TY
Causes of 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 injuries
6. tumors & abcesses of the brain
Medical Terminology
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