membrane potential

aka

membrane voltage

soma

aka

cell body or cyton or perikaryon

neurons

aka

neurones or nerve cells

E_x

Described mathematically by the Nernst Equation

they will move down their concentration gradient

• constant flow of K+ out of the cell
• constant flow of Na+ into the cell

ionic pumps in mV maintenance[image]

[image]

action potentials

[image]

• Polarized at rest
• Depolarized to carry info
• Hyperpolarized afterward

spatial decrement

[image]

length constant

[image]

action potential

[image]

• Stimulus causes membrane depolarization (Vm increases)

• Works like local response until threshold (Et) is reached

• At Et, rapid continued depolarization

• At Et, irreversible, regenerative (opening Na channels and flow of ions causes still more to open)

• Vm becomes positive = overshoot

• Vm peaks = reversal potential

• Vm drops rapidly, becomes hyperpolarized

• Vm slowly returns to resting state

• Closed at resting Vm
• Depolarization causes opening

• movement of Na causes more channels to open
• positive feedback

• Na channels open easier, Vm near rest
• K channels open slower
• K channels depend only on Vm
• Na channels depend on Vm to open, time to close

• Diminished excitability
• Decreased responsiveness

• No response regardless of stimulus
• Na channels closed

• Large stimulus can excite

• Most Na channels still closed

• Vm below normal must be overcome

• Need “critical” number of Na channels to open to initiate Hodgkin cycle

purpose of refractory period

[image]

axon diameter

• bigger is faster
• increase diameter, decrease resistance

myelination

• glial cells wrap axon
• huge resistance

action potential frequency

[image]

• Frequency depends on rate to reach threshold

• Low intensity stimulus = few AP

• High intensity stimulus = lots of AP

dendrites, cell bodies = passive local changes

axons = action potential

Why are they different?

• must be synthesized and stored in the presynaptic terminal
• must be released by the presynaptic terminal on stimulation and is released in amounts sufficient to exert a defined action on the postsynaptic neuron or effector organ
• must produce a response in the postsynaptic cell that mimics the response produced by the release of neurotransmitter from the presynaptic neuron
• mechanism exists for removing it from its site of action

• sense organ that is located in muscle tissue
• arranged in parallel with the EXTRAFUSAL muscle fibers

1. intrafusal muscle fibers
2. annulospiral nerve ending

• constitute the voluntary striated (skeletal) muscle (i.e. the biceps muscle) that actually performs the work associated with  muscle contraction

• receive a Gamma motor innervation - large, myelinated fivers
• cell bodies of the γ-motor fibers are located in the ventral horn of the spinal cord
• the γ-motor fibers are part of the final common pathway

• within the middle region of the muscle spindle receptor
• nerve endings intertwine and wrap around intrafusal muscle fibers
• this arrangement called the annulospiral ending
• these nerve endings are the peripheral end of large, myelinated, afferent nerve fibers

• the cell body located within the dorsal root ganglion
• upon entering the spinal cord, forms several branches which convey information along pathway

1. to nucleus gracilis & cuneatus via dorsal funiculus
2. to Clarke's nucleus or Lateral Duneate nucleus where 2nd order neurons contribute fibers to spinocerebellar tracts
3. to alpha motor neurons in ventral horn

•   internal resistance to current flow along the axon is low

•   nodes of Ranvier are more widely spaced along its length