Neuro: Somatosensation I

Question 1

What does the somatosensory system convey?

Answer 1

The somatosensory system conveys sensations from the body such as:

• touch
• proprioception (sense of body position)
• heat, cold
• pain, itch

Question 2

What are the two subsystems of the somatosensory system.

Answer 2

• One responsible and mediated by low threshold mechanoreceptors which give us our tactile sensations and sense of proprioception.
• Another subsystem mediated by higher threshold mechanorecepors and thermoreceptors which give us sensations of pain and temperature.

Question 3

What do the peripheral nerves consist of?

Answer 3

• Spinal nerves (31 pairs) which convey sensations from the body to the spinal cord and brain
• Cranial nerves (12 pairs) which conveys equivalent sensations from the head.

Question 4

Describe the structure of a peripheral nerve.

Answer 4

• A nerve is essentially a bundle of axons.
• The nerve itself is covered in epineurium, a connective sheath covering. Inside it are several fascicles, each of which are covered in perineurium. Each fascicle contains multiple axons.

Question 5

How is the spinal cord connected with the spinal nerves?

Answer 5

• Each segment of the spinal cord gives rise to dorsal and ventral routes.
• They emerge from the spinal cord and join to form a mixed spinal nerve.
• Alongside the dorsal roots, there are dorsal route ganglia
• The cell bodies of the sensory neurones in the spinal cord are located in the dorsal root ganglia.
• Their cell bodies give rise to axons travelling into the spinal cord as well as out towards the peripheral receptive field (wherever the specific sensory neurone is innervating the body).
• So it is a complex structure e.g. the cervical spinal cord eventually gives rise to the arm, it is formed from several segments which will join together as the brachial plexus. This is important to know medically because the particular pattern of sensation loss in parts of the arm innervated by different nerves would enable you to deduce the site of damage in the cervical spinal cord.

Question 6

Why are dorsal route ganglion cells important and what are their two distinct systems?

Answer 6

• They are the sensory receptors of the somatosensory system
• Large fibres (large diameter, myelinated, fast conduction): tactile and proprioceptive
• Small fibres (small diameter, thinly-myelinated or unmyelinated, medium or slow conducting): temperature, pain, itch, crude touch

Question 7

Why is the ventral route important?

Answer 7

• Motor information arises from the ventral portion of the spinal cord to the peripheral nervous system.

Question 8

List the receptors of the somatosensory system.

Answer 8

Proprioception

• A-α afferents: large diameter, myelinated, fastest conducting (≤100 m/s)
• These are the sensory afferents coming from muscle spindles (also called group Ia)
• These specific axons sense muscle length which varies with joint position so therefore this gives the brain information about body and limb position (proprioception)

Tactile afferents (discriminative touch)

• A-β afferents: large diameter (second largest in diameter), myelinated, 2nd fastest conducting (30-70 m/s). There are several receptor types within the A-beta classification.
• Superficial (close to skin surface) - Meissner’s corpuscles, Merkel’s discs
• Deep - Ruffinni corpuscles, Pacinian corpuscles

Free nerve endings (low-resolution tactile - temperature, pain)

• Free nerve endings, no specialised capsules e.g. like in Merkels discs etc
• A delta fibres: small diameter (smallest diameter of the myelinated fibres), thinly myelinated, moderate conduction velocity (≤30 m/s)
• C fibres: smallest diameter, unmyelinated, slow conducting (≤1 m/s)

Question 9

Describe two examples of the proprioceptors of the somatosensory system.

Answer 9

Muscle spindles:
- Specialised muscle fibres embedded within the contractile muscle fibres of a voluntary muscle
- The muscle spindle is weakly contractile. The sensory endings (group Ia) are the principle muscle spindle sensory endings.
- If muscle contracts or is lengthened (when it’s
antagonist contracts, the muscle changes in length and consequently the muscle spindle changes in length), sensory endings of the group Ia are highly sensitive to changes in length.
- So nervous system can infer changes in joint angle - as joint position changes, muscles will change in length

Golgi tendon organ:

• These are specialised sensory endings near the joint of a muscle with its tendon
• They are sensitive to tension in a muscle

Question 10

Describe the cutaneous receptors of the somatosensory system.

Answer 10

• Meissner corpuscle - close to the junction between the dermis and epidermis (superficial) (A-beta)
• Merkel’s disks - highly sensitive mechanoreceptor afferents (superficial) (A-beta)
• Pacinian corpuscle (A-beta)
• Ruffini’s corpuscles (A-beta)
  These two are deeper beneath the epidermis and are more sensitive to stretch and vibration but are also still low threshold mechanosensory afferents.
• Free nerve endings class. The peripheral end of the axon ends in little axon terminals within the skin. These are sensitive either to temperature (thermosensitive), mechanosensitive (damaging mechanical stimulation), etc. When the action potentials are initiated in these nerve endings and conveyed back along the A-delta fibres or C fibres towards the spinal cord.

Question 11

What are the two major central pathways of the somatosensory system?

Answer 11

Dorsal column - medial lemniscal system (DCML)

• Mediates discriminative touch (fine tactile perception), vibration, proprioception
• Inputs from A-β (touch) and A-α (proprioceptive) afferent fibres
• So DCML system receives inputs from the larger diameter, faster conducting afferent fibres

Spinothalamic tract (STT, also known as anterolateral system)

• Coarse touch (crude tactile sense), temperature, pain
• Inputs from A-δ (weakly myelinated) and C fibres (unmyelinated)

Question 12

What is the the ventral posterior nuclear

complex?

Answer 12

• The route via which tactile, proprioceptive and nociceptive information reaches the cortex.
• Divided into the medial (VPM) and the lateral parts (VPL)
• VPM receives sensory information from the face
• VPL receives sensory information from the rest
  of the body.
• Both of these target specific regions within the cortex, mainly the primary somatic sensory cortex.

Question 13

Briefly list some Broadmann areas.

Answer 13

• Area 1,2 and 3 comprise the primary somatosensory cortex
• Area 4 is motor cortex
• Area 17 is the primary visual cortex (occipital cortex)