*somatic sensations : PAIN *Anteriolateral spinothalamic tract (STT) : a sensory pathway that is positioned anteriorly and laterally in the spinal cord. *This pathway carries a variety of sensory modalities: 1-pain 2-Temperature ( always coupled with pain but has a minimal significance ) 3-Itching 4-Tickiling 5-sex sensation *All those sensory modalities are transmitted via the LATERAL part of the Spinothalamic tract. While the VENTRAL part transmits crude touch (non-discriminative touch). Touch examination is not important clinically as it has no diagnostic value. Because loss of touch sensation can't be exclusively determine wither the lesion is in the dorsal column or in the spinothalamic tract. *Spinothalamic tract : * From the pain receptors (NOCICEPTORS ), sensory fibers(1 st order neurons )enter the spinal cord posterior horn bypassing the soma in the dorsal root ganglia. 1
*They synapse forming the 2 nd order neurons which cross immediately to the opposite side of the cord through the anterior commissure. NOTE : the fibers cross the spinal cord at the level of entry or (one to two ) segments above. *Then they ascend as the anterior or lateral spinothalamic tract. *They pass through the brainstem without synapsing. *As they reach the THALAMUS (ventropostrolateral part), the 2 nd order neurons synapse forming the 3 rd order neurons that join fibers from the dorsal column. *Then through the Thalamocortical radiation to AREA 3,1,2 (Somatosensory area) All spinal cord lesions cause CONTRALATERAL loss of pain sensation. ( Analgesia ) WHY?? Because fibers cross in the spinal cord 2
*What is pain?? - pain is a protective mechanism. Imagine a person sleeping and suffering from appendicitis, without the pain sensation which wakes him up to seek medical care, the appendix would rupture silently causing peritonitis and septicemia. -pain has a psychological aspect as it is associated with unpleasant sensory and emotional experiences. (patients with chronic pain are usually depressed and irritated ). -pain always indicates tissue damage, either ACTUAL or POTENTIAL. (burning your finger is an actual tissue damage that induces pain, while being bitten by an insect is a potential tissue damage which without treatment will cause celluar death and tissue damage). -It's very important to make sure that the patient is not exaggerating, afraid or anxious. -pain is subjective, thus it can't be measured like other symptoms as elevated temperature or blood pressure. *pain perception vs. reaction to pain : Imagine if we apply the same painful stimuli to a group of people, would they sense the pain intensity equally? if they didn t, does this imply differences in the pain transmission pathway ( receptors, neurons, spinal cord, synapses, cortex ) or it s just simply ''different people REACT to pain differently ''?? *The are no differences in the pain pathway between people (pain perception). BUT, people really differ in their( reaction) to pain. -for example : in the delivery room, although the labor is very intensely painful, some women experience a 3
calm childbirth while others delivery is hugely disastrous. So as a physician you must pay attention to the real injury not to the patient's reaction to pain. *Reaction to pain and cultural values : Here, in the middle east, we raise girls to react intensely to pain, while we inhibit this reaction in male children so they learn not to express their pain. *Congenital insensitivity to pain (congenital analgesia ): -A rare condition in which a person can't feel ( and has never felt ) physical pain. Those patients have a short life span and need continuous protection. -Children with this condition often suffer oral cavity damage (having bitten off the tip of their tongue, lips ), they may suffer from fractures and internal bleeding without knowing. IT'S A VERY BAD CONDITION!! So one can see that pain is really protective. *Transmission of pain : Pain is transmitted via A(delta) and C fibers. A(delta)fibers C fibers Fast sharp(cutting) pain of rapid onset and offset. localized Travels 20-30m/sec Usually superficial but it could be perceived subcutaneously. Mainly elicited by a mechanical or a thermal pain stimulus. Dull slow pain. Can't be localized Travels.5-2m/sec Mainly transmit visceral pain(pain from deep structures) ex: colic pain Mainly elicited by a chemical pain stimuli or a persistent mechanical or thermal stimuli. 4
* A(delta) fibers are fast in comparison to C fibers but generally when we compare them to A(alpha) fibers which transmit at 120m/sec they are slow. *A sudden painful stimuli often gives a double pain sensation; a sharp fast pain that is transmitted by A(delta) fibers, followed by a slow pain transmitted by C fibers. *A perfect example of slow pain is when you stub your toe on the coffee table. You feel the jolt of impact (proprioception and Pacinian corpuscles), and you have approximately a heartbeat to think, "This is really going to hurt." That heartbeat is the C-fiber travel time from your toe to your brain. When the signal hits, the pain is severe and lasts for quite a while. *slow transmission of pain is due to SYNAPSES because they delay conduction. A(delta) FIBERS Entre the dorsal horn (lamina marginalis), excite 2 nd order neurons, that bypass lamina 5 and cross immediately to the opposite side of the cord via the anterior commissure, and continue to the thalamus. C FIBERS Entre the dorsal horn (lamina gelatinosa), make multiple synapses with interneurons, then pass to lamina 5 and excite 2 nd order neurons that give rise to axons that join the fibers from the fast pain pathway, they cross to the opposite side of the cord via the anterior commissure, and continue to the thalamus. 5
*INTERNEURONS : neurons that start and end in the same place ( either excitatory or inhibitory). *Tract of lissauer (intersegmental connection) : contains centrally projecting axons from dorsal root ganglion cells carrying discriminative pain and temperature information.these axons enter the spinal column and ascend or descend one or two spinal segments in this tract before penetrating the grey matter of the dorsal horn, where they synapse on second-order neurons. The axons of these secondorder neurons cross the midline and ascend in the anterolateral quadrant of the contralateral half of the spinal cord, where they join the spinothalamic tract. *Fast fibers: 99% of them only synapse once. *Slow fibers: 99% of them make multiple synapses. *Sensory input from the face : -Trigeminal nerve ( cranial nerve no.5 ). -It has a huge nucleus ( the largest cranial nucleus ) in the Pons. -The nucleus is subdivided into : 1-The main sensory nucleus. 2-The mesencephalic nucleus. 3-The spinal nucleus of trigeminal. 1) Discriminative touch : this pathway is similar to the dorsal column medial lemniscus, fibers bypass the (soma) in the trigeminal ganglia and enter into the main sensory nucleus ( principle nucleus ) where they 6
synapse, then cross and ascend joining the somatosensory fibers from the dorsal column on its way to the thalamus. 2)Proprioception: the proprioceptive axons in the trigeminal nerve are the stretch and Golgi tendon receptors from the muscles of mastication*carrying unconscious proprioception*. ( All the muscles of facial expression are innervated by the facial nerve ). *Fibers bypass the trigeminal ganglia, entering the mesencephalic nucleus, where the soma is located (This is a strange characteristic among PRIMARY somatosensory neurons, as the soma is located inside the brain NOT outside it as usual ). *So the Mesencephalic nucleus is essentially a dorsal root ganglia that has been pushed into the CNS, so no synapses occur within it. *The fibers then synapse in the nearby motor nucleus where they can initiate a stretch reflex for the muscles of mastication. THOSE ARE AFFERENT FIBERS OF THE REFLEX. *Tapping on the tendon of the Messeter muscle jaw closure.. JAW REFLEX. 3)Pain : sensory fibers enter the trigeminal ganglia bypass the soma trigeminal nucleus fibers descend to reach the spinal cord ( C1-C2) synapse in the spinal nucleus of trigeminal ( caudalis part ) * it reaches the spinal cord, hence the name spinal nucleus * ascend then cross to the other side. 7
*Cross section in the spinal cord : 1)The dorsal column medial lemniscus : note the spatial arrangement of the fibers (sacral, lumbar, thoracic, cervical ) from medial to lateral respectively. 2)The spinothalamic tract: note the spatial arrangement of the fibers ( cervical, thoracic, lumbar, sacral ) from medial to lateral. Clinical correlation : a tumor compressing the spinal cord from the outside affects the sacral region early in its course. While a spinal cord tumor initially affects the cervical region. *Visceral ( slow pain ) can't be localized : Example : a patient suffering from an epigastric pain can't tell wither this pain is cardiac ( MI) or gastric ( peptic ulcer ).etc 8
*This inability to distinguish the real source of pain is due to : When C fibers reaches the brainstem they give information to the reticular formation ( to provoke a state of alertness and consciousness ), they also give information to the limbic system ( emotional centers ). ** PAIN IS PERCIEVED SUBCORTICALLY** So, due to these extensive connections and synapses, by the time the signals reach the cortex one can't distinguish the real site of injury. *In the past and before all the nowadays diagnostic techniques, surgeons used to perform open and see operations in order to found the diseased organ. *CLINICAL CASE : A patient with right sided body analgesia and left sided face analgesia, where is the lesion?? *Left lower medulla, as the ST tract has already crossed from the left to the right side. BUT the facial pain fibers haven't yet crossed to the opposite side. So you can make a provisional diagnosis of a left lower medullary tumor. 9
*Lesion above the level of crossing ( pons / upper medulla / thalamus) contralateral body and face loss of all sensaory modalities. *Due to the thalamocortical radiation a parietal lobe injury ( somatosensory cortex) leads to contralateral partial body ( mainly upper limb) and face loss of sensory modalities. Done by : Bayan Al-araj 10