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What is somatosensation?
Sensory information from the skin and musculoskeletal systems. Pain is the BIG one!!
What factors affect the speed of information processing?
Diameter of axons, degree of axonal myelination, and number of synapses.
What is the difference between sensory information and sensation?
Sensory info is the nerve impulses generated from the original stimuli.
Sensation is awareness of stimuli form the senses.
Sensory receptors are specialized and only respond to a specific type of stimulus, and have an adequate stimulus. Name the three types we talked about:
a. Mechanoreceptors: mechanical input like touch, pressure, stretch.
b. Chemoreceptors: respond to chemicals. Ex) taste. Also ones that respond to damaged cells after injury.
c. Thermoreceptors: Transmit info regarding heat and cold.
When nociceptors are stimulated, what do we feel?
The sensation of pain.
Peripheral sensory neurons have two axons. Name them and what they do:
1. Distal conducts messages from the receptor, to the cell body.
2. Proximal: projects from the cell body into the spinal cord or brainstem
Receptive fields are the area of the skin that is innervated by each neuron. As a general rule, are the receptive fields bigger or smaller distally when compared to the proximal ones?
Distal receptive fields are smaller. This is because there is a greater density of receptors in distal areas like our fingers.
Receptive fields tend to overlap. Why? (2 reasons)
1. Gives us better discrimination. Helps us figure our exactly where the stimuli is on the skin of our body.
2. Neurons dies over time. If receptive fields don't overlap and a neuron dies we will have a "dead spot" where we won't be able to feel anything.
Touch is categorized into two types. Name and describe them:
1. Fine touch: a variety of receptors and subsensations. It is very discriminative, so we will be able to describe what it feels like and locate it easily.
2. Coarse touch: Mediated by free endings throughout the skin. Far less discriminative.
Why does hair make touch easier to feel and describe?
It has a hair follicle nerve ending wrapped around in deeper in the skin. The hair is basically a projection of the nerve ending.
Name the two types of intrafusal fibers and how their nuclei are arranged:
1. Nuclear bag: clumps of nuclei
2. Nuclear chains: nuclei arranged in single file
Where do the primary endings of type 1A neurons attach? The secondary endings of type 2?
1. Type 1A: attach around central region of intrafusal fibers.
2. Type 2: attach on nuclear chain fibers adjacent to primary endings.
What are the two ways the Golgi tendon organs are stimulated?
The GTO responds changed in tension of a tendon. Active contraction and passive stretches will stimulate it.
How are muscle spindles stimulate?
Quick and prolonged stretches of muscle.
How are joint receptors stimulated?
Mechanical deformation of joint capsules and ligaments
For 1A, 1B, 2, A-delta, and C axons: know what type of receptors they use (or where their receptors are located), and what their stimulus is:
1. 1A: receptors= muscle spindles. Stimulus = muscle stretch
2. 1B: receptors= GTO and ligament receptors. Stimulus = tendon tension (GTO) and ligament tension (ligament receptors.)
3. 2: Receptors= Muscle spindles. Stimulation = muscle stretch
4. A-beta: Merkel's discs, hair follicles, etc.. Stimulation= touch and pressure.
5. A-delta: receptors = free nerve endings. Stimulation = cold temperature and coarse touch.
6. C: Receptors = free nerve endings. Stimulation = hot temperature, and itch/tickle.
Describe high fidelity transmission (it is a type of pathway to the brain).
It is a fast transmission that provides a accurate details about the location of this stimulation.
Example: touch pathways, pain pathways, proprioception pathways
What characteristic does a neuron need to have in order to be described as a "pathway" to the brain?
Needs to have long axons that connect distant regions of the nervous system.
What is a tract?
A bundle of axons with the same origin and a common termination.
Name the three types of pathways that bring sensory info to the brain:
1. Conscious relay: These are the important ones because we test them.
2. Divergent: (diffuse systems)- multiple regions of the CNS. Some are conscious and some are unconscious. We do not test these.
3. Unconscious relay: We are never aware of it, so we can't test it.
Go into more depth about conscious relay pathways. Ex. Where do they transmit info to? How many neurons do they usually use?
They transmit info to many locations in the brainstem and cerebrum. Usually use 3 neurons. Conscious relay pathways are transmitted with high fidelity.
We know that divergent pathways transmit info to many locations in the brainstem and cerebrum and do not only use 3 neurons like conscious relay pathways do. What type of sensation is transmitted via divergent pathways in the CNS?
What is the main type of information that unconscious relay pathways transmit? This information is very important even though we are never conscious of it.
Proprioception carried to the cerebellum. This info plays a big role in automatic adjustments of our movements.
There are 4 types of conscious relay pathways. Name them. Are they grouped together in any way?
Touch, proprioception, temperature, and pain. Touch and proprioception are grouped together, and temperature and pain form another group. These groups use different pathways to send info to both the body and head.
We talked about 4 main pathways that go to either the head or the body and relay conscious somatosensation. Name the two pathways to the body (proprioception/touch pathway, and pain/temp pathway) and the two pathways to the head.
a. Pathway to body for proprioception and touch: Dorsal column/medial leminscus (DC/ML)
b. Pathway to body for pain and temperature: Spinal thalamic
c. Pathway to head for proprioception and touch: Trigemino-thalamic
d.Pathway to head for pain and temperature: Spinal-Trigeminal
e. **NOTE: pathways for pain/temp will have word spinal in them. Pathways to head will have word trigeminal in them.
There are 6 rules of for the conscious pathways. Describe the first 2:
1. Rule 1: There are only 3 neurons in the pathway (nor counting interneurons
2. Rule 2: The first neuron is in the PNS: Dorsal root ganglia for the body pathway, and trigeminal ganglia for the head
Describe rules 3 and 4: This is where it gets tricky because these rules are about neuron #2.
1. Rule 3: the second neuron is in the segmental nervous system
- DC/ML: spinal cord
- Spinal Thalamic: brainstem
2. Rule 4: The axon of the second neuron crosses the midline
- DC/ML: Caudal Medulla
- Spinal Thalamic (ALS): Spinal Cord
If an injury occurs at the PNS, what side of the body relative to the injury will we lose pain/temp and proprioception/touch sensations? What about an injury at the spinal cord? Above the spinal cord?
a. PNS: Will lose sensations on same side as injury
b. At spinal cord: Will lose pripro/touch on same side, but pain/temp on opposite side. This is because the spinal thalamic pathway is responsible for pain/temp and the spinal thalamic crosses over at the spinal cord. Right where this injury is occurring. So on your injured side it crosses over and avoids the injury, but on your non-injured side, it crosses over directly into the injured area-thus causing a loss of sensation on the opposite side of the injury.
c. Above spinal cord: Will lose pripro/touch and pain/temp on opposite side. Once again, the injured side axons cross over before the injury (caudal medulla for pripro/touch and spinal cord for pain/temp) and avoids a loss of sensation. But the axons for the non-injured side cross over and run right into the injured area (somewhere above the spinal cord for this example) causing a loss of sensation on the opposite of the injury.
Describe rules number 5 and 6:
1. Rule 5: The third neuron is located in the thalamus-and this makes sense because we know the thalamus is the main relay center for all info going to the cerebrum.
- VPL (ventral postural lateral) for the body pathways (DC/ML and spinal thalamic)
- VPM (ventral postural medial) for the head pathways (Trigeminothalamic and spinal trigeminal)
2. Rule 6: Third neuron terminates in a specific region of the somatosensory cortex (post-central gyrus)
The internal carotid artery supplies the cerebrum with oxygenated blood. But it splits into two segments that supply different areas of the cerebrum. Name the two segments and the areas they supply. What parts of the body are these areas of the cerebrum responsible for controlling sensory info for?
a. Anterior cerebral artery supplies anterior aspect of cerebrum with blood. This aspect of the cerebrum is responsible for controlling sensory functions of the legs.
b. Middle cerebral artery supplies the middle aspect of the cerebrum with blood. It is responsible for controlling sensory function of the arms and face. It is also the most commonly blocked artery by strokes.
Explain the difference between discriminative touch and conscious proprioception:
Touch is ability to discriminate between 2 closely spaced points touching the skin.
Proprioception is the awareness of movements relative to the position of our body parts
Describe what stereognosis is:
Using discriminative touch and proprioception together in order to identify an object. Ex. Knowing what something in your hand is without using your eyes
How many neurons do pathways for touch and proprioception's use? Describe where each neuron conveys information to and from:
3-neuron relay system
1. Primary: Conveys info from the receptors to the medulla
2. Secondary: Conveys info from the medulla to the thalamus
3. Tertiary: Conveys info from the thalamus to the cerebral cortex
What are the dorsal columns? (What makes them up and how are they different?) Remember that we are talking specifically about touch/proprioception right now.
The dorsal column sends info from the first neuron, up the spinal cord to the start of the second neuron. It is split into two fascicles: gracilis and cuneatus.
What is the factor that decides whether info will have gone through the gracilis fasiciles or the cuneatus fascicles before heading up the spinal cord?
It all depends on where the info enters the spinal cord from. Anything that enters the spinal cord below T6 will have come from the gracilis fascicles. Anything that enters the spinal cord above T6 will have come from the Cuneatus fascicles.
We have been talking about primary sensory cortex areas, but we have somatosensory association areas too. How are they different from each other based on what sensory information they tell us:
a. Primary: Discriminates among size, shape, and texture. Basically, what is the object like
b. Association cortex: Uses stereognosis and memory to determine what exactly the object is.
Nociceptive receptors receive and transmit information about what kind of stimulus?
Stimuli that damage or threaten to damage tissues
We know that the first order neurons for spinal thalamic is in the dorsal root ganglion, and for spinal-trigeminal is at the trigeminal ganglion. Where are the second order neurons located for each of these pathways?
a. Spinal thalamic: dorsal horn of spinal cord
b. Spinal-trigeminal: spinal nucleus of trigeminal nerve
Explain what crossed analgesia is:
Occurs when a single lesion causes loss of pain sensation for face and body. The loss of pain sensation for the face occurs on the side ipsilateral to the lesion, but the loss of sensation for the body occurs on the contralateral side to the lesion.
Lateral medullary strokes injure the spinal nucleus of the trigeminal nerve. What sensation loss does this cause (pain/temp or pripro/touch) to where (head or body) and to which side (ipsilateral or contra)?
Pain/temp pathway to the face is lost on the side ipsilateral to the injury
Lateral medullary strokes also cause the loss of pain/temp sensation for the body? Why is this sensation lost even though the injury site was the spinal nucleus of the trigeminal nerve? Which side of the body is sensation lost on?
The anterior lateral tracks that the spinal-thalamic pathway rides up through the spinal cord is very close in proximity to the nucleus of the trigeminal nerve. Since the spinal-thalamic pathway will have already crossed over the mid-line before reaching the injured nucleus of the trigeminal nerve, pain/temp sensation will be lost on the contralateral side of the body.
Describe the first neuron in the medial pain pathways:
A small, unmyelinated C fiber. Receptors are free nerve endings
What parts of the brain do the ascending projection of the medial pain pathway reach? What three tracts in the ALS do they us to accomplish this?
The midbrain, reticular formation and limbic areas are reached via the spinomesencephalic, spinoreticular, and Spinolimbic tracts of the ALS
Information from which of the three tracts do we actually perceive as pain?
The Spinolimbic tract
Remember that the Mesencephalon = midbrain. We have 2 areas of the midbrain that deals with nociception: name them:
Superior colliculus (tectum) and periaqueductal gray
What does the spinomesencephalic tract do?
Carries nociceptive info to the 2 parts of the midbrain (superior colliculus and periaqueductal)
What is the main function of the periaqueductal gray? The superior colliculus?
a. The periaqueductal gray activates pathways that inhibit pain nociception to our brains.
b. The Superior colliculus orientates our eyes and head toward the source of pain input.
The ascending neurons from the spinoreticular tract synapse in the reticular formation. Where is the reticular formation? What does it modulate? Does it reach consciousness?
It is located in the brainstem and modulates arousal, attention and sleep-walking. It does not reach consciousness.
What do axons of the Spinolimbic tract transmit and to where?
Transmits slow pain info to the medial and intralaminar nuclei of the thalamus.
It is important to note that the spinolimbic modulates arousal, withdrawal and responses to pain, similar to the spinoreticular. However there is one big difference. What is it?
The info from the Spinolimbic tract actually reaches consciousness
What is the trigeminoreticulolimbic pathway responsible for transmitting?
Slow pain information transmitted to the face.
There are four cerebellar unconscious pathways: name them. What is their main function?
Their main function is to relay info that is critical for adjusting movements
1. Posterior spinocerebellar pathway
2. Cuneocerebellar pathway
3. Anterior spinocerebellar tract
4. Rostrospinocerebellar tract
Two of these cerebellar pathways are considered high-fidelity pathways. Name the two. What is their main function?
They relay high-fidelity info to the cerebellar cortex. The posterior spinocerebellar pathway, and the cuneocerebellar pathway.
Which pathway transmits info from the legs and lower body? The upper body?
1. Posterior spinocerebellar pathway = lower body
2. Cuneocerebellar pathway = upper body
The other two cerebellar tracks are termed as internal feedback tracts. Name the two pathways, and their main function.
Anterior spinocerebellar tract, and the rostrospinocerebellar tract. They monitor the activity of spinal interneurons and of descending motor signals from the cerebral cortex and brainstem. Basically, these tracks tell the body what to do.
Which pathway transmits info from the lower body (thoracolumber spinal cord)? The upper body (cervical spinal cord)?
1. Anterior spinocerebellar tract = lower body
2. Rostrospinocerebellar tract = upper body
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