Flashcards in Session 6 - Group work Deck (26)
Most peripheral nerves, such as the median nerve are commonly referred to as “mixed spinal nerves”.
Identify 3 general characteristics of spinal nerves that give rise to use of the term “mixed spinal nerves”.
Most mammalian peripheral nerves are composed of mixtures in terms of: a) Directionality in which nerve
impulses are conducted- afferents
b) Functional Modalities represented within the nerve, sensory, motor and
C) Myelination or lack of. Nerves are strictly divisible into two
categories; either myelinated or unmyelinated. Myleinated axons can be further teased into lightly
myelinated, or heavily myelinated
D) The speed with which nerve impulses are conducted. The speed of
nerve impulse conduction is heavily influenced by the level of myelination of the nerve in question. The
speed of conduction in myleinated axons is directly proportional to the fibre cross-sectional diameter, whilst
in unmyelinated axons it is
Name the different nerves stimulated in pain response
A alpha - Motor neurone (spinal reflex arc)
A beta - Mechanoceptor
A gamma - Muscle spindle)
A delta and B - Sharp pain
B - autonomic
C - Dull, throbbing pai
When carrying out nerve conduction investigations of a specific mixed spinal nerve, which sensory
modalities would you expect to be activated at just threshold intensities of stimulation (i.e. minimal
intensities). Justify your answer.
Proprioceptors and in particular, muscle spindle afferents because these are the most heavily myelinated
nerves in the body. Since myelin reduces the capacitance of axons, it follows that the most heavily
myelinated axons would be easiest to bring to threshold by electrical stimulation since the will require
relatively less current to activate them.
6 When carrying out nerve conduction investigations of a specific mixed spinal nerve, which sensory
modalities would you expect to be activated last (i.e. maximal intensities of stimulation). Justify your answer
C-fibres (hence slow pain fibres). These are unmyelinated axons of the body. Their lack in myelin means
that their axons have a relatively high capacitance, thus, making them particularly difficult to bring to
threshold by electrical stimulation. As such, they are likely to be the last axons of the body to be activated
by electrical stimulation
History in this patient suggested that three weeks prior to the onset of pain, the affected hand became
functionally weak. She had difficulty putting her signature to documents, gripping cutlery correctly during
meals and finally her grip had weakened leading to dropping a mug full of tea. What is the explanation for
these difficulties in this patient?
The nerve is under constant compression within the carpal tunnel by the progressively growing ganglion.
This gives rise to a nerve compression syndrome. When nerves are gradually compressed, the most
myelinated axons are most susceptible to the effects of compression, presumably due to disruption of their
blood supply, leading to deprivation of nutritional substrate. In this case, the proprioceptive afferents are
thus, being selectively knocked out of action, leading to the patient being unable to carry out skilled fine
movements like putting her signature to documents. The difficulties may be arising due to lack of afferent
feedback from muscle spindle afferent axons that have succumbed to compression
8 On further questioning by the Neurologist regarding the period between prior to the onset of pain but
subsequent to motor difficulties described in Q6-7, the patient described the problematic hand as not painful
but as if “sometimes it was not mine”. What is the explanation for this lack of awareness of the hand here?
It would appear that the hand had become alien to the patient. This is likely to have arisen from
compression of proprioceptive elements of the nerves leading to a consequent lack of proprioceptive
feedback from the hand. Thus, the brain was no longer aware of proprioceptive feedback from it, hence its
One piece of the patient’s history was that just prior to the onset of severe pain, she recalled that her
hand had become numb to touch in some parts but also remembered the sensation of pins and needles.
What is the explanation for a) Numbness? B) The sensation of pins and needles.
Anaesthesia due to compression of the nerve and its attendant blood supply.
Parasthesia due to compression. The pressure on the axons of the nerve due to compression may have led
them to fire action potentials due to this non-physiological stimulus. Action potentials fired in this way would
not have been encoded correctly and as such, when they occur, the nervous system is unable to make sense
of them, hence the strange feeling of pins and needles.
History in this case indicates that when pain started to be noticeable, it seemed tolerable for a while
but gradually worsened. What is the explanation for this progression in the perception of pain in this patient?
The gradual compression of the nerve led to a gradual and progressive increase in the recruitment of pain
fibres. This would have given rise to the progressive increase in the severity of pain.
What is the explanation for the main cause of this patient’s complaint?
Nerve compression syndrome due to a space-occupying growth within the carpal tunnel
Carpal tunnel syndrome is often associated with repetitive strain injury (RSI) of the wrist. What
contribution would you expect RSI to have made to this patient’s complaint?
None in this case
What does the term “ganglion” refer to in this particular case
A tumour growing axons of a peripheral nerve
What treatment strategies might you suggest would resolve this problem a) acutely and b) long-term?
Acutely- conservatively by trying to manage the pain in the hope that the condition stabilises.
Chronically- if this case does not settle due to progression of the growth, surgical removal of the growth
may be inevitable for thre reasons: a) To relieve pain and suffering b) Reduce the possibility of this acute
pain becoming a case of chronic pain; c) Surgical removal of the growth would avail it for biopsy.
How common are soft tissue injuries as the cause of lower back pain
6 Identify soft tissues of the back that are likely to be implicated in such a case
Muscle Tissue; Ligaments of the sacro-iliac joint; tendons of muscles
7What would be the characteristics of radiation of pain in such a case?
Local tenderness; Stiffness of back muscles: Stiffness of trunk musculature
8 What would be the characteristics of radiation of lower back pain in such a case?
Shooting pain radiating down the leg and buttock. The pain here would trace the physical course taken by
the anatomical layout of the nerve
9 Name two likely sites in which the sciatic nerve or its roots might be trapped.
a) Intervertebral Foramen of the spinal cord (Root values L2-L4); b) Greater Sciatic Foramen; c) As the
nerve passes though the substance muscle of the buttock and in particular, pyrifromis.
0 What further tests (non-imaging) might you carry out in order to ascertain involvement of the sciatic
nerve in such a case?
Straight-Leg raise tes
Taking into account all the information above, what do you suspect the diagnosis to be in such a
presentation of lower back pain?
Muscle/Tendon/ Ligament tears of the muscles of the lower back (not sciatica)
Why is cardiac pain referred the left arm?``
Visceral pain fibres from the heart, travel in the cardiac nerves along with sympathetic afferent
fibres to the superior cervical ganglion of the sympathetic trunk. As the heart develops at the
same segmental level as structures within the dermatome T1, the pain fibres enter the spinal cord
along with somatic afferents from the T1 dermatome of the left side. Within the spinal grey matter
the visceral and somatic afferents converge on secondary fibres in the spinothalamic tracts so
that pain originating in the heart is perceived within the cortex as if it comes from T1 - the left
breast and the medial portion of the left arm
Where on the body surface do we experience pain originating in the diaphragm?
Visceral pain arising from the diaphragm may be felt in the region of the costal margins and in the
Why might this pain be referred to two different regions?
Referred pain in the diaphragm is felt in two places because its peripheral regions are innervated,
along with the skin over the costal margins, by inferior intercostal nerves arising from segments
T5 - T11. The central portion along with skin over the shoulders is innervated by fibres in the
phrenic nerve from segment C5.
Why may an amputated limb still be felt and still give pain?
Following the removal of an arm or leg, patients report sensations from the missing limb -
phantom sensations. These sensations usually involve the distal structures the hand or foot,
where the sensory receptor density was the greatest rather than from the intervening arm or leg,
although the phantom limb usually feels normal in size. For some patients the missing hand or
foot appears to grow directly out of the stump.
Phantom sensations are usually associated with limbs but such sensations can arise from any
part of the body e.g. phantom breasts after mastectomy, or a phantom body after transection of
the spinal cord.
Phantom pain originates within the somatosensory regions of the cerebral cortex and can not
normally be managed by opiate analgesia.
In severe injury such as in a RTA, victims may initially feel a mild pain only. Why?
In critical/stressful situations high order regions of the CNS including the frontal cortex and the
somatosensory cortex can interact with the nociceptive pathway to reduce the sensation of pain.
Fibres from these regions release opiate like neuropeptides including the enkephalins and the
endorphins which act upon cells in the periaqueductal grey matter (PAG) of the midbrain.
Descending projections from the PAG activate serotoninergic fibres and noradrenergic fibres in
the medulla which in turn activate enkephalinergic neurones in the dorsal horn of the spinal cord
and trigeminal nucleus which moderate the nociceptive pathway.
The opioid receptors in the PAG are engaged by the ascending nociceptive fibres forming a pain
modulating feedback loop and by cells in the hypothalamus.
In stressful situations the release of the hormone ACTH from the anterior pituitary is
accompanied by the release of endorphin like chemicals.
Which nerve transmits the sensation of toothache?
Sensory fibres from the upper teeth are carried in the maxillary branch (V2) and from the lower
teeth the mandibular branch (V3) of the trigeminal nerve.