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Lecture 4

CNS sensory and motor (Lecture 4).docx

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McGill University
PHGY 209
Erik Cook

CNS sensory and motor (Lecture 4): • We will focus on the somatosensory system, then the visual and finally auditory systems. Somatic (bodily) sensation is mediated by several types of receptors: • Temperature: Thermoreceptors are free nerve endings [in the skin] containing ion channels that respond to difoerent temperature ranges. o Cold afferents: 0-35 C (activated by menthol) – afferents transmitting information about cold stimuli. o Warm afferents: 30-50 C (activated by capsaicin and ethanol). o o • Extreme temperatures (< 0 C or >50 C) activate pain receptors. o Pain receptors are activated when the sensory stimuli causes tissue damage. • The law of specific nerve energies applies here: we can activate a cold afferent with other than temperature. Indeed, menthol activates cold afferents. Warm afferents can be activated by other chemicals, like ethanol (in a shot of alcohol) and capsaicin (active ingredient in hot chili pepper -> if too hot due to excess of capsaicin, pain receptors can be activated). o Regardless of how we activate the afferents, despite that they were designed to respond to temperature, if activated differently, the perception is still cold/warm. • Pain: Nociceptors are free nerve endings containing ion channels that open in response to intense mechanical deformation, excessive temperature, or chemicals. o Nociceptors become activate when the mechanical deformation of the skin or excessive temperature starts causing (or even before it starts causing) tissue damage. • Pain afferents are highly modulated (enhanced and suppressed). • Visceral pain receptors (inside organs): activated by inflammation. o Overlap between visceral pain receptors and somatosensory pain receptors. This confusion is important in clinical aspects. Nociceptors are sensitized by many chemical mediators: • How pain is encoded and how this information is sent to the CNS? • How nociceptors are sensitized? How they increase their sensitivity to upregulate pain pathways? • Transduction process and how information is being communicated to CNS: 1. You cut your skin with something sharp. 2. Because tissue was damaged, there is an immediate activation of nociceptors, which then sendAPs to spinal cord. nd 3. In the spinal cord, they release substance P (neurotransmitter) onto 2 order neurons. 4. These 2 order neurons send this information up to the brain and we feel the pain. o This is not where most nociceptors are sensitized however. 5. Sensitization of surrounding nociceptors by injured tissue & afferent feedback onto mast cells: on the next day, we touch a surface with our previously injured finger and it really hurts. The wound itself has become more sensitive. Normally, a somatosensory stimulation to your finger wouldn’t cause any pain, but now it does. o This mechanism prevents you from using this part of the body until the tissue heals. o This step is where most nociceptors are sensitized.  As these nociceptors send theirAPs to the spinal cord, they also send APs back to mast cells located nearby. These mast cells will release histamine.  Extracellular chemicals/substances from the damaged tissue surrounding the nociceptors are also released, like serotonin, Bradykinin, Prostaglandin (no need to know the names). This also increases sensitivity of nociceptors. 6. APs are sent back to blood vessels; substance P is released and causes blood vessels to dilate. This is the reason why there is a swelling around the wound. o This also increases the sensitivity of the nociceptors. o Dilation of nearby blood vessels brings in more blood to the area, thus promoting the healing process, and because they swell around the nerve endings, they make the latter more sensitive to mechanical deformation. • The next day, after all these processes have occurred (the 2 circled regions & #6), nociceptors are extremely sensitive and their threshold for transmitting pain is reduced. Normally, it requires tissue damage forAPs to be released, they wouldn’t be activated by simple reaching out and grabbing an object, but now they are. • Hyperalgesia = increased sensitivity to pain. • The process described above is a bottom-up modulation of pain, which increases the pain input to the CNS without our involvement. Dorsal columns: • Understanding the circuitry that sends somatosensory information to the brain is important to fully grasp the top-down regulation of pain. • E.g. You lightly stroke your finger with a feather activating Meissner’s corpuscles. This causes afferents to be activated. These afferents will then send theirAPs along an axon (nerve reaching the spinal cord). o Recall that, on the diagram, we see only a spinal segment with a piece of spinal cord with two corresponding nerves, each spinal nerve innervating a particular strip section the body. Here, it’s a cervical segment. o Cell bodies for the afferents are located outside of the CNS inside the dorsal root ganglion. o White matter contains tracks going up and down the spinal cord. • E.g.(cont’d):Axons come in via the spinal nerve. • Dorsal root contains the axons that enter the spinal cord. The ventral root contains the motor axons that come out to innervate our muscles. These axons come in at the dorsal horn in the gray matter, where cell bodies are found. • Axons quickly go over to the dorsal columns, containing track of axons going up and down the spinal cord on the dorsal side in the back. There, the afferents from the finger at the cervical segment (rather high) would join the other nerves coming from all the way down our body (trunk, legs, toes, all coming up). These axons join up and so this track/group of axons [dorsal columns] carry the same information (ipsilateral touch and proprioception, from the same side of the body). • They go up to the medulla, the beginning of the brain stem and they synapse. • The axons or second order neurons then cross the midline. o Ipsilateral: Same side relative to midline. o Contralateral: Opposite side relative to midline.  As a neurosurgeon, if I cut the dorsal columns, the loss to the patient is ipsilateral loss of touch and proprioception (+ temperature and pain) for the cervical level (arm and hand all the way down one side of the body to the toes.) • The 2 order neurons start going up the brain into the medial lemniscus, which is carrying contralateral information regarding touch and proprioception. It’s a track of axons travelling together whereas column is a specific name for axons travelling up and down the spinal cord. • Then, this information about touch and proprioception synapses in the thalamus. • These neurons then project to the somatosensory cortex. • The cortex thus receives contralateral somatosensory information about touch and proprioception via the dorsal columns. Anterolateral pathway - Temperature and pain: • Instead of tickling your finger with a feather, we now hold it over a candle: ouch! • Nociceptors activate since the o temperature rises above 50 C and send APs through the spinal nerve, cell bodies in the dorsal root ganglion and come into the dorsal root. The synapse in the dorsalndorn of the gray matter with 2 order neurons. Substance P is released, since it’s a nociceptor. If we do not hold our finger so close to the flame so that we do not feel pain, then it’s the thermoreceptors which are activated and we’re not releasing substance P. • The 2 order neurons cross the midline in this spinal segment and then form a track of axons, called the anterolateral column (spinothalamic track), joining axons already coming up from below. • The secondary axons then travel up through the brain stem. Some synapses coming off there (branching into the reticular formation), but most of the information goes to the thalamus. • They synapse in the thalamus and go to the somatosensory cortex, which receives all somatosensory information from the contralateral side of the body. • What happens if we create damage… o In the case of damage to anterolateral column at the cervical level, the patient experiences a contralateral loss of temperature and pain at the level of the lesion and below (on the opposite side of the body).  Not only we have damaged axons entering from this cervical level, but all other axons sending information from the anterolateral column are damaged as well. o If we damage the dorsal root instead, the patient experiences an ipsilateral loss of touch, proprioception, temperature and pain, only at the cervical level, since the tracks com
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