Touch and Pain (The Somatosensory System)

Question 1

What is the somatosensory system?

Answer 1

• Part of the nervous system.
• Responsible for processing sensory information e.g., touch, temp, and proprioception (awareness of movement).

Question 1

What are sensory receptors?

Answer 1

• Specialised structures that respond to stimulation e.g., light, chemical and thermal energy.
• Responsible for converting various forms of energy into electrical signals interpreted by the nervous system.
• Respond to stimulation.

Question 2

What happens in response to stimulation?

Answer 2

• Membrane potential of receptor cells changes
• **Receptor potentials are converted into action potentials in different actions. **
• Receptors can tell us about strength, location, and type/duration of stimulus.

Question 3

How is the sensory system organised?

Answer 3

1 - Association Sensory Cortex
2 - Secondary Sensory Cortex
3 - Primary Sensory Cortex
4 - Thalamus
5 - Receptors
*Complexity of analyses increases, neurons respond to stimuli of greater specificity and complexity.

Question 4

What is the Hypothalamus? Give an example of how it works.

Answer 4

• Receives information and designates it to where it needs to go (sits at the top of the brain stem).
  e.g. For a blue water bottle = photoreceptors in the eyes detect an object in front, but it is the visual cortex that puts information together to interpret and make sense of it is a blue water bottle.

Question 5

What are most sensory receptors in the somatosensory system known as?

Answer 5

= **Mechanoreceptors **

• Receptors that are sensitive to physical distortion e.g., bending or stretching.
• Respond to mechanical stimulation (touch, vibration, and pressure are detected by receptors.

Question 6

Dorsal horn function:

Answer 6

• Receives sensory input from the dorsal root and processes information before transmitting it to higher brain centres.

Question 7

Ventral horn function:

Answer 7

• Sends motor commands before transmitting them to glands, controlling voluntary and involuntary processes.

Question 8

Ventral root function:

Answer 8

• Transmits motor signals from the spinal cord to muscles, exiting from the ventral side and connecting to the ventral horn.

Question 9

Nerve Pathway Near Spinal Cord:

Answer 9

• Nerves split and attach to the dorsal and ventral cords as they approach the spinal cord.

Question 10

What are dermatomes?

Answer 10

• Area of skin that gets its sensation from a specific spinal nerve root.
• Sends signals for things like pressure, pain, and temp.

Question 11

What are Afferent nerves (sensory nerves)?

“A” for arriving…

Answer 11

• Carry signals from sensory receptors (like those in the skin or organs) towards the CNS.
• Transmit sensory information.

Question 12

What are efferent nerves (motor nerves)?

“E” for exiting…

Answer 12

• Carry signals away from CNS.
• Transmit motor commands from CNS to muscle, glands, and organs.
• Direct movements, actions, and physiological responses.

Question 13

What are primary afferent nerves (A - beta fibres)?

Answer 13

• First Nerve: Carries information from receptor to spinal cord.
• Function: Transports stimulation across sensory nerves toward the CNS via the dorsal horn.
• Touch vs. Pain: Information about touch reaches the brain faster than pain due to A beta fibers.

Question 14

What are the different types of Primary Afferent Nerves:

Answer 14

*differ due to the amount of myelination, and diameter:

More myelination = faster the conduction of the electrical impulse.

Wider the diameter = faster the conduction of the electrical impulse.

Question 15

Somatosensory - how does information reach the cortex?

Answer 15

1. Stimulation on body: Detected by mechanoreceptors, which send information across primary afferent nerve (A-beta fibers) to the dorsal horn of the spinal cord.
2. Ascent to medulla: The information ascends to the medulla as it enters the spinal cord.
3. First afferent synapse: Occurs with a second neuron in the medulla.
4. Crossover: The second neuron crosses over and ascends on the opposite side to reach the brain’s thalamus.
5. Third neuron: Receives information and carries it to the primary somatosensory cortex.
6. Cortex analysis: The cortex analyses and interprets the information.

Question 16

What are the 3 neurons involved in somatosensory?

Answer 16

• First order neuron = primary afferent nerve.
• Second-order neuron = first afferent neuron synapses with a second neuron in the Medulla.
• Third-order neuron = from the Thalamus to the Primary Somatosensory Cortex.

Question 17

What are primary afferent nerves?

Answer 17

• Information from the nociceptor that travels to the spinal cord via two types of axons:

Question 18

What are the 2 different primary afferent nerves?

Answer 18

A delta fibres (15-30 m/s) - short duration pricking pains.

**C fibres **(1-2m/s) - long lasting dull pain.

= difference in conduction speed can explain why pain sometimes feels like it has two different phases.

Question 19

What is the primary somatosensory cortex?

Answer 19

• First cortical structure within the somatosensory hierachy.
• Analyses sensory information.
• Lies on the postcentral gyrus, receving inputs from ventral posterior nucleus of thalamus.

Question 20

What did ‘Penfileld’ do?

Answer 20

• First used electical stimulation to map organisation of somatosensory cortex.
• Maps the body across brain area, with regions of the body not being uniformly represrted across brain region.

Question 21

Somatosensory cortex example…

e.g., The hands and lips….

Answer 21

• have large amount of surface area across somatosensory region, despire being smaller features.
• reflects importance for tactile perception, motor control, and communciation.
• Highlights brains ability to adapt to specific needs/demands of body parts.

Question 22

What is the assocation cortex?

Answer 22

• Helps us do complex things like talk, pay attention, control emotions, and move our bodies.
• Integrates different types of sensory information.
• Brain’s command centre for higher-level thinking/behaviour.

Question 23

What can damange to the association cortex result in?

Answer 23

• Neglect syndrome and asomatognosia (patients behave as if their body parts do not belong to them)

Question 24

What is pain (ISAP., 1994)?

Answer 24

• Unpleasant snesory and emotional experience associated with actual/potetial tissue damage.

Question 25

What is congenital insensitivity to pain?

Answer 25

Individuals who cannot experience pain, even if in the most extreme conditions.

Question 26

What can congenital insensitivity to pain lead to?

Answer 26

• No response to injury, and difficulty in differentiating between stimuli.
• Potential low life expectancy (due to the accumilation of injuries and health issues).

Question 27

What are nocieptors/pain receptors?

Answer 27

• Detect noxious/painful stimuli.
• Free nerve endings located in most body tissues.
• Sensory receptors that respond to tissue damange e.g., extremes in temperatures.

Question 28

How does descending pain control work?

Answer 28

• The periaqueductal gray (PAG) in the midbrain receives input from various brain regions, including the cortex and amygdala.
• **Has pain-suppressing effects and can block pain when electrically stimulated. **
• Frontal areas send signals to midbrain, which sends pain-relieving signals to spinal cord’s dorsal horn, blocking pain signals to brian.

Question 29

What happens when PAG is activated?

Answer 29

• Neurons travel down to the raphe nuclei in the medualla, sending pain-suppressing signals.
• **Meduallry neurons then go to the spinal cord’s dorsal horn, reducing painful response. **

Question 30

What is the pathway for touch and pain? What does it do?

Answer 30

Anteriolateral pathway.

• Nociceptors detect information and send it to the primary afferent nerves of pain.

Question 31

How does information reach the cortex after primary afferent nerves of pain?

Answer 31

1. Nociceptors detect painful stimulus.
2. Information ascends the spinal cord via primary afferent nerve.
3. Enters spinal cord through dorsal horn.
4. As they enter, immediately synapse with second neuron at dorsal horn.
5. Second neuron crosses over and travels to Thalamus.
6. From Thalamus, the information is passed onto the Cortex through a third neuron.

Question 32

How does information reach the cortex for pain?

Answer 32

• Information synapses with second neuron at spinal cord, which then crosses over the ascend to the Thalamus.

Question 33

How does information reach the cortex for touch?

Answer 33

• Information synapses and crosses over at the Medulla.
• Touch information ascends ipsilaterally (same side) whereas pain information ascends contralaterally (opposite side) to the stimulus.

Question 34

What is the pain pathway?

Answer 34

• Primary afferents carry pain signals from the body to the spinal cord.
• To the ventral posterior thalamus.
• Finally to the brain’s somatosensory cortex - being known as the anterolateral pathway and is responsble for detecting and locating pain.

Question 35

What happens in the pain pathway?

Answer 35

Here’s the condensed information:

• Pain signals: Travel from the site of injury to the spinal cord via primary afferents.
• Ascent: Signals ascend opposite to the stimulus from the spinal cord.
• Thalamus: Reach the ventral posterior thalamus, a relay station in the brain.
• Processing: Finally, signals are sent to the somatosensory cortex, where the brain processes the pain sensation.

Question 36

What is the difference between nociception vs pain?

Answer 36

Nocieption = injury or tissue damage which can lead to pain.

Pain = an unpleasant sensory and emotional experience assocaited with actual or potential pain.

= can experience pain without tissue damage/have tissue damage and not experience pain.

Question 37

Breecher., (1959)

What is an example study on central modulation (pain)?

Answer 37

• Depsite being severly wounded during action, American service personnel reported little to no pain from the injury.
• Concluded that the meaning behind the wounds inflicted mediated the painful responses.

Question 38

What is peripheral pain modulation?

Answer 38

• Encompasses a variety of mechanims that regulate the transmission of pain signals at the level of the PNS.
• By modulating pain signals before they reach the CNS, peripheral pain modulation can effectively reduce pain perception and provide relief from discomfort.

Question 39

(Melzack and Wall., 1965)

What is the gate control theory?

mechanism that enables peripheral pain modulation..

Answer 39

• Proposed there is a hypothetical gate like mechanism located in the dorsal horn of the spinal cord that influences pain.
• Both touch and pain fibres enter spinal cord from dorsal horn.
• The gate can also be influenced by central factors through desecnding pain pathway e.g., emotional/psychological state.

Question 40

Gate control theory

What happens when the gate opens?

Answer 40

• When the gate opens, there is an experience of pain.
• A Delta and C fibres = if these are active, this opens the gate, enabling pain signals to reach the brain.

Question 41

Gate control theory

What happens when the gate is closed?

Answer 41

• When the gate is closed, there is a suppression of pain.
• A beta fibres (larger touch fibres).
• If these are active, it shuts the hypothetical gate, inhibiting signals of pain to reach the brain.

Question 42

What is the Substantia Gelitanosa?

Answer 42

• Specific region within the dorsal horn of the spinal cotd, processes touch to sensations.
• In the context of the somatosensory touch pathway, the substantia geltinosa serves as an important relay station for incoming sensory signals from the body to the CNS.

Question 43

How does the Substantia Gelitanosa process pain?

Answer 43

1. Painful experience where nocicieptors detect it.
2. Information is sent along the a-delta, and c fibres.
3. Enter the spinal cord, and synapses to a second neuron.
4. This sends signals towards the brain.

Question 44

What is the inhibitory interneuron?

Answer 44

• If it is active, it inhibits other structures.
• Touch is applied to the painful area, as the pain is experienced.
• Mechanoreceptor detect touch and sends signals across.
• a-beta fibres to the brain (synapses at the medulla).

Question 45

What happens if you hurt yourself?

Answer 45

• Action potential is sent across a-delta, and c-fibre = inhibits the inhibitory interneuron.
• If inhibitory strucutre is active, it can no longer supress acitivity of second nueron which oepns the gate so pain reaches the cortex.