Summary USMLE step 1 Physiology of Nerve and Muscle system - Study guide - Study smarter not harder - your guide to pass the exam
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First Aid for the USMLE Step 1 2022, 32E
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SECTION II
NERVE AND MUSCLE
, \()
SECTION II CHAPTER 1
THE NERVE
Nerves and muscles arc studied together because both arc excitable
structures that conduct impulses. but the musc lcsalso conlnu.:t.
Peripheral nerve trunks contain large number so l' indepeudent nerve
fibres that may be either :
(1) Afferent (=sensory) nerve fibres : These transmit nerve impulses
(or signals) from periphcml receptors to the nervous system.
(2) Efferent (= mmor} nerye fibres : These tran.,mi t ncn c impulses
(or signals) from the ncn ous system to the effector organs.
PROPERTIES OF NERVES
(1) Excitability
This is the abi lity or li ving ti ssues to respond to various stimuli. It is
an electric phenomenon, and the electric changes that accompany nerve
excitati on are called the action potential. Such changes are very small and
vel) rapid. so their magnitudes arc measured in mil/it•o/t.\ (m\ '} while their
duration5 arc measured 111 milliseconds ( m~cc ). ·yhe) arc recorded h}
microckctrodcs connected to either a gah•anometer or a cat/lode ray
oscilloscope (C RO ). The latter consists of a cathode emitting a beam of
electrons which is directed to strike a screen (figure 17). This screen i~
covered with a fluorescent phosphorous substance that em its light when
struck by the electrons. The potential changes in the nen c arc picked up
by the microclectrodcs and amplified (by an electronic amplifier). then
they arc transmitted to the CRO where they are recorded ns deflections of
the electronic beam on its screen.
Figure 17: The cathode ra) o~cllto~wpl' (( RO).
, .1 1
Section II - Chapter I Properties o( llt!rl'es
The stimulus
A stimulus is a change in the environment around the ncrYe (or muscle)
which may be either chemical, thermal, meclumical or e/ectric:al. In
laboratories. electrical stimuli are preferred because they can be accu-
ra td y controlled (both in stn.:ngth and duration) nnd , in add ition, they
leave the stimulated structures without damage. The fo llowing 2 types of
electric currents ca n be used lo r stimulation of excitable tissues :
(a) The ga lva nic current : This is a constant (or direct) current
(D.C.) which is obtained from a battery.
(b) The faradic current : This is an alternating current (A.C.) li ke
the induction currents used in laboratories for nerve stimulation.
(2) Conductivity
The physicochemical change produced by vari ous st imuli in the nerve
is called the nen •e impulse. Such impulse is act ive ly conducted along the
nerve fibre and it cun be conducted in both directions ir the nerve was
stimu lated at its middle. I lowcver. in the body, each nerve conducts
impulses in one direction only (motor nerves toward the effector organs
and sensory nerves toward the nervous' system). Conduction in the normal
direction is called orthodromic conduction, but if it occurs in the opposite
direction due to any ca use, it will be ca lled antidromic cmuluction .
(3) All or none law (or rule)
This law states that "A threshold (= minimal) stimulus produces a
maximal response " i. e. a maximal action p otential in nerve and muscle
fibres and a maximal contmc:tiou in muscle fibres. There rore. as long as
other factors that t!f/ect excitahilily remain constant, iu creasiu~ the
intensity of the stimulus abm•e the threshold 1•alue produces no further
increase in the action potential or muscle contraction
The all or none Ia\\ is obeyed in the fo llowing structures:
(a) A single nerl'ejibre.
(b) A single skeletol11111scle.fihre and the motor unit (page 5 1).
(c) The cardiac 11111scle and some smooth muscles (page 78) \Vhich net
as one unit ca ll ed .w uq ,tium (refer to circu lation).
However, nerve trunks and whole skeletal muscles (w hich co ntain
many fibres) do not obey the law. Thi s is because the threshold intensity
for stimulation l'aries in the different types of nen e and muscle fibres
(i.e. it is not equal). Ther(:/ilre, {(the imensiry ofstimulation is increased in
the'ie structures, the response ll'ill also increase till reaching o IIW.rimtmt.
(4) Accommodation (or adaptation)
The nerve tibrc adapts to stimulation by a constant current. so no
response occurs during passage or the cuncnt. and it occurs only on tht:
make (i.e. closing) and break (i.e. opening) ofthe electric circuit.
, Section II - Chapter I Rheobase and cllrOitfL\'ie
(5) lnfatiguabi lity
Nerve fibres arc not fatigued by continuous stimulation (page 70).
OU'-ATION (l iHE)
Fig,u re 18 : The strength duration cun c.
Factors that determine the effectiveness of stimuli
(1) In tensity (= strength) of the stim ulu s : Subthreshold stimuli pro-
duce on ly local respouses that don't initiate acti on potentials (sec later).
(2) Rate of increase in the intensity of stimu li : Subthreshold stimuli
that arc gradua lly increased produce a response onl) "ith a rapid increast:
in the intensity or stimu lr. I r the intensit) is mcreast:d ~lo\\1~. the ncn e
''ill not respond because or the property of accommodation (sec abO\ c).
(3) Duratio n of stimulus {= du r ation of current flO\\ ) : The relation
between the intensity of a stimulating current and the duration (time) of its
llow necessary to set up an impulse is shown in the strength-duration
curve (figure IX). From the curve. the foll ow ing obs~.:rvations arc notic ~.:d:
(a) Within limits, there is a reciprocal relationship between tile cuiT-
eut strength am/ tile dumtion offlow required to produce tm impulse
(b) There is a minimal duration needed for excitation belo\\ which no
excitation occurs'' hate' cr ma) be the strength or the stimulus.
(c) RHEOBASE : Thi" is the minimal .\tren~-:th (or threshold illleu-
si~r ) of a gall'fmic current that c:au set up au impulse. The time rcquin:d
lor excitation whcn using the rheobase is calh.:d tiH.: utili-;.atioutime.
(d) CHRONAXIE : This is the duration or current now required for
excitation when using a strength equal to twice (or double) the rheobase.
Use of Chronaxie : The chronaxie is a good index for tlte degree of
excitability (the short~.:r tht: chronaxie. the greater the e\citnbility and\ icc
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