ACH's MBBS YEAR 1 SEMESTER 3 WEEK 3 NOTES.docx

7/26/2019 ACH's MBBS YEAR 1 SEMESTER 3 WEEK 3 NOTES.docx

1/20

AFRICAN COLLEGE OF HEALTH (ACH)

MBBS YEAR 1 SEMESTER 3; WEEK 4 NOTES

MONDAY: Neuroanatomy (pg.2)

TUESDAY: Physiology (pg.7)

WEDNESDAY: Ethics (pg.10)

THURSDAY: Neuroanatomy(pg.13)

FRIDAY: Physiology (pg.18)

QUESTIONS? Email: [email protected]


2/20

NEUROANATOMY (MONDAY)

Peripheral nervous system:

The nervous system develops from embryonic segments,

but in the adult state this is obvious only in theconnections of nerve roots with the spinal cord.

Segmental organization:

Spinal nerves have numbers derived from the vertebrae.

The highest spinal nerve penetrates the atlanto-

occipital membrane, above the arch of the atlas, which

is the first cervical vertebra or C1. The second

cervical nerve passes between the atlas (vertebra C1)and the axis (C2).

There are 7 cervical vertebrae. The lowest cervical

nerve is therefore C8. Cervical nerves 1 to 7 go

through foramina above the numbered vertebrae. The

roots of nerve C8 pass below the arch of vertebra C7

and above that of T1. All the thoracic (T1 - T12),

lumbar (L1 - L5) and sacral (S1 - S5) nerves go through

foramina below the equivalently numbered vertebrae. Tocomplete the story, a single coccygeal nerve overlaps

with S5 in supplying the perianal skin.

The most obvious consequence of the segmental

organization of the spinal nerves is seen in the

Dermatomes, which are bands of skin that run

horizontally on the trunk and lengthwise on the limbs.

Each dermatome is centered on the distribution of axons

from a single dorsal root ganglion, but each ganglion

also supplies skin in the dermatomes above and below

its own level.

Consequently, it is necessary to transect three

adjacent dorsal roots or spinal nerves in order to


3/20

completely denervate the skin of one dermatome.

Transection of a single spinal nerve, or destruction of

its ganglion, diminishes but does not abolish sensation

in the affected segment of skin. The cutaneous lesions

of herpes zoster, a common virus that infects certainpain-responsive neurons in individual sensory ganglia,

often neatly map the distributions of dermatomes and

also illustrate the extension of innervation into the

adjacent segments of skin.

The nerve supply to the skin of the limbs is delivered

by cutaneous nerves that are formed in limb plexuses

(brachial and lumbosacral) by complex interchanging and

mixing of fibers from different spinal roots. The areas

supplied by cutaneous nerves bear little resemblance to

the dermatomes. They are sharply demarcated, with

little or no territorial overlapping . The widely

overlapping dermatomes cut across adjacent areas of

skin supplied by cutaneous nerves. A cutaneous nerve

lesion, such as an injury or a mononeuropathy, results

in a well defined area of defective sensation, and

anatomical knowledge can be used to identify theaffected nerve.

Most of the skin of the head is supplied by the three

divisions of cranial nerve V. The areas are sharply

demarcated, and therefore do not correspond to

dermatomes. Cranial nerves VII, IX and X supply small,

overlapping areas of skin of the external ear, and the

dermatome of the second cervical nerve includes parts

of the head, ear, face and neck. (The first cervicalnerve lacks a dorsal root in most people.)

Muscles receive motor and sensory innervation. Most of

the muscles of the limbs are supplied nerves formed in

the limb plexuses from two or more roots. A stretch

reflex (tendon jerk) requires the integrity of both the


4/20

motor and the proprioceptive sensory innervation of the

muscle.

Relation of spinal cord and nerve roots to the

vertebral column:

The vertebral column is longer than the spinal cord,

which ends at the level of the upper border of vertebra

L3 in the newborn and at the upper border of vertebra

L2 in the adult. The lower spinal nerves must therefore

course caudally before passing through their

corresponding intervertebral foramina. Immediately

below the caudal end of the spinal cord, the neural

canal contains the roots of nerves L2-L5, S1-S5 and thecoccygeal nerve.

Cranial nerves:

Although the brain stem from segments (known as

neuromeres), their peripheral distributions and central

connections are most easily understood in terms of the

functions of each nerve. Note that the second cranial

"nerve," despite its traditional name, is not a nerve

but an outgrowth of the brain, as is the retina.

Functions of the cranial nerves:

Cranial Nerve(s) Function(s)

I Olfactory Smell.

II Optic Vision.

III Oculomotor Eye movements

IV Trochlear Downward eye movements.

V Trigeminal Muscles that open Skin of face; and

close the mouth; mouth, teeth,

Tensor tympani muscle nose,


5/20

sinuses, of middle ear. dura mater

of anterior & middle fossa.

VI Abducens Abduction of eye

VII Facial Muscles of face; Lacrimal and nasal

Taste: palate stapedius muscle of

glands (pterygo- anterior 2/3 of

middle ear. palatine ganglion);

tongue. sublingual & sub-

mandibular salivary glands

(submandibular ganglion).

VIII

Vestibulocochlear:

Vestibular Equilibration/Cochlear

Hearing.

IX

Glossopharyngeal

Parotid gland Pharynx, middle

Taste: (Otic ganglion) ear,

posterior third of tongue third of

tongue.

X Vagus Muscles of larynx Slows heart;

Larynx, trachea, Taste: and pharynx

(cardiac ganglia) esophagus, dura

epiglottis. Increases gastric of

posterior acid secretion. fossa.

IX Accessory Trapezius and (Spinal

sternocleidomastoid component)

muscles

XII Hypoglossal Muscles that move the tongue.

1. Afferent fibers in IX and X are of great importance

for regulation of cardiovascular and respiratory

function, but they do not give rise to conscious

sensations, and the physiological functions are not

usually disturbed by unilateral lesions that affect the

nerves or their central connections.


6/20

2. The names of the parasympathetic ganglia are

indicated in parentheses after the functions.

3. The small cranial root of XI carries motor axons

destined mostly for the larynx. These cross over into X

by way of a communicating branch, as the two nerves

pass through the jugular foramen in the base of the

skull. The fibers of the spinal root have their cell

bodies in segments C1-C5 of the spinal cord.


7/20

PHYSIOLOGY (TUESDAY)

SKELETAL MUSCLE PHYSIOLOGY:

Muscle contraction is through depolarization of a nerve

releasing ACh into the neuromuscular junction causingan endpoint potential and triggering an AP in the

muscle and contraction. The strength of the contraction

is due to:

(i) the number of muscle fibers activated;

(ii) frequency of activation;

(iii) initial length of the muscle fiber. This was

studied through the sciatic nerve and the gastrocnemius

muscle (Achilles tendon).

Graded stimulation of the sciatic nerve-recruitment of

motor units

By increasing the stimulus strength (i.e. the electrode

voltage), an increasing number of nerve fibres were

recruited. There was a graded stimulation of the

sciatic nerve due to different thresholds of nervefibres and low voltages only recruited nerve fibres

near the electrode. Any stimulus above maximum tension

is a supra-maximal stimulus.

Effect of varying the frequency of the trains of supra-

maximal stimuli:

When a second stimulus occurs close a first one

without allowing relaxation [increasing frequency],tension is additive. 'Tetanus: twitch' ratio is the

ratio of the peak tension during the tetanus to the

peak tension during a single twitch. 'Tetanus' is a

sustained muscular contraction resulting from a rapid

series of nerve impulses As the frequency of the pulses


8/20

increases, the tension between the twitches does not

return to baseline, so they fuse (tetanus).

The peak tension during the tetanus is greater than the

tension of a single twitch (because in a single twitch

there is not enough time to prestretch the elastic

elements and produce a full force), so as the frequency

increases, the tetanus: twitch ratio also increases.

Tetanus is 4x a single twitch strength. The fusion

frequency is when tension vs. time is a smooth

increase.

Influence of length on isometric contractions:

As the muscle length (and therefore also the passive

tension) increases, the force of the muscle contraction

increases to a point, reaches a peak and then decreases

again. This observation is evidence supporting the

sliding filament binding hypothesis, as it could be due

to sarcomere length -there is an optimal length at

which most of the actin and myosin are overlapping (so

maximum force can be produced by the myosin heads in

the actin binding sites), whereas if the muscle wasstretched too much or not enough, there would be fewer

binding sites available for the myosin heads to attach

[less interference]. This is calculated by measuring

passive tension in the muscle [due to elastic

properties] and subtracting from total tension to find

twitch/active tension.

MEMBRANE POTENTIALS

Concentration differences across membranes of living

cells are energy differences. If a membrane was

impermeable, no electrolytes can move between the two

partitions and no potential difference builds up. Semi-


9/20

permeability allows selective diffusion, transmembrane

potentials can develop.

This experiment creates two chambers to emulate the ICF

[150 mmol/L] and the ESF [variable] separated by a

polysuphonic resin membrane that is selectively

permeable.

Electrodes are: AgCl coated with Ag and a salt bridge

of 3M KCl in agar. It shows how different

concentrations of electrolytes give different

transmembrane potentials.

Both chambers were filled with 150 mM KCl with the ECF

after double washouts filled with 100mM to 1.5mM KCl

measuring the potential.

Using the Nerst equation, we can determine if it's more

selective for K(+) or Cl (-).

*From this we can see as we reduce the concentration of

KCl in the outside bath, either K+ or Cl- has to cross

to balance the concentrations. If K+ crosses into the

outside bath, there would be a negative charge in theICF and a negative membrane potential. Hence Membrane 1

is permeable to cations as it gives a negative charge.

Likewise, if Cl- crossed, it would leave a positive

charge in the ICF. Hence Membrane 2 is permeable to

anions.


10/20

ETHICS (WEDNESDAY)

Matters of life and death:

It has been noted that the right of a competent adult

to consent to and refuse treatment is unlimited,including the refusal of life-sustaining treatment.

Probably the example most familiar to surgeons of this

is Jehovahs Witnesses who refuse blood transfusions at

the risk of their own lives. There can be no more

dramatic example of the potential tension between the

duties of care to protect life and health and to

respect autonomy, with autonomy always constituting the

trump card.

The tension does not stop here, however. For there will

be some circumstances where the protection of the life

and health of patients is judged to be inappropriate,

where they are no longer able to be consulted and where

they have not expressed a view about what their wishes

would be under such circumstances. Here a decision may

be made to with-hold or to withdraw life-sustaining

treatment on behalf of the incompetent patient. The

fact that such decisions can be seen as omissions to

act does not excuse surgeons from morally and legallyhaving to reconcile them with their ordinary duty of

care. Ultimately, this can only be done through arguing

that such omissions to sustain life are in the

patients best interests.

The determination of best interests in these

circumstances will rely on one of three objective

criteria, over and above the subjective perception by

the surgeon that the quality of life of the patient is

poor. There is no obligation to provide or to continuelife-sustaining treatment:

if doing so is futile when clinical consensus

dictates that it will not achieve the goal of extending

life. Thought of in this way, judgments about futility


11/20

should not be linked to evaluations of a patients

quality of life;

if patients are imminently and irreversibly close to

deathin such circumstances it would not be in their

best interest slightly to prolong life (e.g. through

the application of intensive care) when, again, there

is no hope of any sustained success. Not needlessly

interfering with the process of a dignified death can

be just as caring as the provision of curative therapy;

if patients are so permanently and seriously brain

damaged that, lacking awareness of themselves or

others, they will never be able to engage in any form

of self-directed activity. The argument here is backedup by morally and legally reasoning that further

treatment other than effective palliation cannot be in

the best interests of patients as it will provide them

with no benefit. When any of these principles are

employed to justify an omission to provide or to

continue life-sustaining treatment, the circumstances

should be carefully recorded in the patients medical

record, along with a note of another senior clinicians

agreement.

Finally, surgeons will sometimes find themselves in

charge of the palliative care of patients whose pain is

increasingly difficult to control. There will come a

point in the management of such pain when effective

palliation might only be possible at the risk of life

because of the respiratory effects of the palliative

drugs. In such circumstances, surgeons can with legal

justification administer a dose which might be lethal.

The argument employed to justify such action refers to

its double effect that both the relief of pain and

death might follow from such an action. As intentional

killingactive euthanasiais rejected as

professional and legal medical practice throughout most

of the world, a potentially lethal dose is only


12/20

regarded as appropriate when it is motivated by

palliative intent.

Debates rage about whether or not it is realistic in

such circumstances to believe that surgeons can or

should keep all ideas out of their minds about helping

such unfortunate patients to die, especially as we have

seen that clinical decisions are already made that

foreshorten the lives of incompetent patients in

specific circumstances. Deciding whether or not

potentially lethal palliation is justified will require

an evaluationby either the patient, the clinician or

bothof whether or not the life in question is too

valuable on other grounds to risk. Once a negative

conclusion is reached and the risks are incurred, itseems impossible in the face of continued and dramatic

palliative failure then to purport to banish thoughts

of the desirability of death from the scene. What is

clear is that surgeons should document that their

intent is purely palliative through only gradually and

incrementally increasing doses of the drugs that they

administer for this purpose.


13/20

NEUROANATOMY (THURSDAY)

Peripheral nervous system:

The nervous system develops from embryonic segments,

but in the adult state this is obvious only in theconnections of nerve roots with the spinal cord.

Segmental organization:

Spinal nerves have numbers derived from the vertebrae.

The highest spinal nerve penetrates the atlanto-

occipital membrane, above the arch of the atlas, which

is the first cervical vertebra or C1. The second

cervical nerve passes between the atlas (vertebra C1)and the axis (C2).

There are 7 cervical vertebrae. The lowest cervical

nerve is therefore C8. Cervical nerves 1 to 7 go

through foramina above the numbered vertebrae. The

roots of nerve C8 pass below the arch of vertebra C7

and above that of T1. All the thoracic (T1 - T12),

lumbar (L1 - L5) and sacral (S1 - S5) nerves go through

foramina below the equivalently numbered vertebrae. Tocomplete the story, a single coccygeal nerve overlaps

with S5 in supplying the perianal skin.

The most obvious consequence of the segmental

organization of the spinal nerves is seen in the

Dermatomes, which are bands of skin that run

horizontally on the trunk and lengthwise on the limbs.

Each dermatome is centered on the distribution of axons

from a single dorsal root ganglion, but each ganglion

also supplies skin in the dermatomes above and below

its own level.

Consequently, it is necessary to transect three

adjacent dorsal roots or spinal nerves in order to


14/20

completely denervate the skin of one dermatome.

Transection of a single spinal nerve, or destruction of

its ganglion, diminishes but does not abolish sensation

in the affected segment of skin. The cutaneous lesions

of herpes zoster, a common virus that infects certainpain-responsive neurons in individual sensory ganglia,

often neatly map the distributions of dermatomes and

also illustrate the extension of innervation into the

adjacent segments of skin.

The nerve supply to the skin of the limbs is delivered

by cutaneous nerves that are formed in limb plexuses

(brachial and lumbosacral) by complex interchanging and

mixing of fibers from different spinal roots. The areas

supplied by cutaneous nerves bear little resemblance to

the dermatomes. They are sharply demarcated, with

little or no territorial overlapping . The widely

overlapping dermatomes cut across adjacent areas of

skin supplied by cutaneous nerves. A cutaneous nerve

lesion, such as an injury or a mononeuropathy, results

in a well defined area of defective sensation, and

anatomical knowledge can be used to identify theaffected nerve.

Most of the skin of the head is supplied by the three

divisions of cranial nerve V. The areas are sharply

demarcated, and therefore do not correspond to

dermatomes. Cranial nerves VII, IX and X supply small,

overlapping areas of skin of the external ear, and the

dermatome of the second cervical nerve includes parts

of the head, ear, face and neck. (The first cervicalnerve lacks a dorsal root in most people.)

Muscles receive motor and sensory innervation. Most of

the muscles of the limbs are supplied nerves formed in

the limb plexuses from two or more roots. A stretch

reflex (tendon jerk) requires the integrity of both the


15/20

motor and the proprioceptive sensory innervation of the

muscle.

Relation of spinal cord and nerve roots to the

vertebral column:

The vertebral column is longer than the spinal cord,

which ends at the level of the upper border of vertebra

L3 in the newborn and at the upper border of vertebra

L2 in the adult. The lower spinal nerves must therefore

course caudally before passing through their

corresponding intervertebral foramina. Immediately

below the caudal end of the spinal cord, the neural

canal contains the roots of nerves L2-L5, S1-S5 and thecoccygeal nerve.

Cranial nerves:

Although the brain stem from segments (known as

neuromeres), their peripheral distributions and central

connections are most easily understood in terms of the

functions of each nerve. Note that the second cranial

"nerve," despite its traditional name, is not a nerve

but an outgrowth of the brain, as is the retina.

Functions of the cranial nerves:

Cranial Nerve(s) Function(s)

I Olfactory Smell.

II Optic Vision.

III Oculomotor Eye movements

IV Trochlear Downward eye movements.

V Trigeminal Muscles that open Skin of face; and

close the mouth; mouth, teeth,

Tensor tympani muscle nose,


16/20

sinuses, of middle ear. dura mater

of anterior & middle fossa.

VI Abducens Abduction of eye

VII Facial Muscles of face; Lacrimal and nasal

Taste: palate stapedius muscle of

glands (pterygo- anterior 2/3 of

middle ear. palatine ganglion);

tongue. sublingual & sub-

mandibular salivary glands

(submandibular ganglion).

VIII

Vestibulocochlear:

Vestibular Equilibration/Cochlear

Hearing.

IX

Glossopharyngeal

Parotid gland Pharynx, middle

Taste: (Otic ganglion) ear,

posterior third of tongue third of

tongue.

X Vagus Muscles of larynx Slows heart;

Larynx, trachea, Taste: and pharynx

(cardiac ganglia) esophagus, dura

epiglottis. Increases gastric of

posterior acid secretion. fossa.

IX Accessory Trapezius and (Spinal

sternocleidomastoid component)

muscles

XII Hypoglossal Muscles that move the tongue.

1. Afferent fibers in IX and X are of great importance

for regulation of cardiovascular and respiratory

function, but they do not give rise to conscious

sensations, and the physiological functions are not

usually disturbed by unilateral lesions that affect the

nerves or their central connections.


17/20

2. The names of the parasympathetic ganglia are

indicated in parentheses after the functions.

3. The small cranial root of XI carries motor axons

destined mostly for the larynx. These cross over into X

by way of a communicating branch, as the two nerves

pass through the jugular foramen in the base of the

skull. The fibers of the spinal root have their cell

bodies in segments C1-C5 of the spinal cord.


18/20

PHYSIOLOGY(FRIDAY)

SKELETAL MUSCLE PHYSIOLOGY:

Muscle contraction is through depolarization of a nerve

releasing ACh into the neuromuscular junction causingan endpoint potential and triggering an AP in the

muscle and contraction. The strength of the contraction

is due to:

(i) the number of muscle fibers activated;

(ii) frequency of activation;

(iii) initial length of the muscle fiber. This was

studied through the sciatic nerve and the gastrocnemius

muscle (Achilles tendon).

Graded stimulation of the sciatic nerve-recruitment of

motor units

By increasing the stimulus strength (i.e. the electrode

voltage), an increasing number of nerve fibres were

recruited. There was a graded stimulation of the

sciatic nerve due to different thresholds of nervefibres and low voltages only recruited nerve fibres

near the electrode. Any stimulus above maximum tension

is a supra-maximal stimulus.

Effect of varying the frequency of the trains of supra-

maximal stimuli:

When a second stimulus occurs close a first one

without allowing relaxation [increasing frequency],tension is additive. 'Tetanus: twitch' ratio is the

ratio of the peak tension during the tetanus to the

peak tension during a single twitch. 'Tetanus' is a

sustained muscular contraction resulting from a rapid

series of nerve impulses As the frequency of the pulses


19/20

increases, the tension between the twitches does not

return to baseline, so they fuse (tetanus).

The peak tension during the tetanus is greater than the

tension of a single twitch (because in a single twitch

there is not enough time to prestretch the elastic

elements and produce a full force), so as the frequency

increases, the tetanus: twitch ratio also increases.

Tetanus is 4x a single twitch strength. The fusion

frequency is when tension vs. time is a smooth

increase.

Influence of length on isometric contractions:

As the muscle length (and therefore also the passive

tension) increases, the force of the muscle contraction

increases to a point, reaches a peak and then decreases

again. This observation is evidence supporting the

sliding filament binding hypothesis, as it could be due

to sarcomere length -there is an optimal length at

which most of the actin and myosin are overlapping (so

maximum force can be produced by the myosin heads in

the actin binding sites), whereas if the muscle wasstretched too much or not enough, there would be fewer

binding sites available for the myosin heads to attach

[less interference]. This is calculated by measuring

passive tension in the muscle [due to elastic

properties] and subtracting from total tension to find

twitch/active tension.

MEMBRANE POTENTIALS

Concentration differences across membranes of living

cells are energy differences. If a membrane was

impermeable, no electrolytes can move between the two

partitions and no potential difference builds up. Semi-


20/20

permeability allows selective diffusion, transmembrane

potentials can develop.

This experiment creates two chambers to emulate the ICF

[150 mmol/L] and the ESF [variable] separated by a

polysuphonic resin membrane that is selectively

permeable.

Electrodes are: AgCl coated with Ag and a salt bridge

of 3M KCl in agar. It shows how different

concentrations of electrolytes give different

transmembrane potentials.

Both chambers were filled with 150 mM KCl with the ECF

after double washouts filled with 100mM to 1.5mM KCl

measuring the potential.

Using the Nerst equation, we can determine if it's more

selective for K(+) or Cl (-).

*From this we can see as we reduce the concentration of

KCl in the outside bath, either K+ or Cl- has to cross

to balance the concentrations. If K+ crosses into the

outside bath, there would be a negative charge in theICF and a negative membrane potential. Hence Membrane 1

is permeable to cations as it gives a negative charge.

Likewise, if Cl- crossed, it would leave a positive

charge in the ICF. Hence Membrane 2 is permeable to

anions.

[email protected]

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ACH's MBBS YEAR 1 SEMESTER 3 WEEK 3 NOTES.docx