Bodily Senses
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Introduction
The bodily senses have four major modalities: discriminative touch, proprioreception, nociception, and temperature sense. Due to its importance, proprioreception is discussed in a separate section.
Sensory Receptors
Receptors are specialized cells for detecting changes in the environment. Objects are perceived by the character and intensity of sensation over a population of cells.
The skin
Hairy skin contains Merkel Cells, hair receptors, and nonencapsulated nerve endings.
Smooth skin contains Pacinian corpuscles, Ruffini endings, Meissner corpuscles, and Merkel cells.
Spatial resolution depends on receptor density.
Ruffini
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Meissner
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Free nerve endings
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Pacinian corpuscles
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temperature receptors
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Muscles and joints
There are three types of mechanoreceptors in muscles and joints:
- muscle spindle receptors
- Golgi tendon organs
- joint capsule receptors
A generator potential spreads passively along the axon until it reaches an area rich in Na channels and hits threshold for generating. Each cell has a level of 'noise'. a signal must be viewed as something above a given threshold.
Cells with mechanoreceptors use TRP channels for generating signals. This is very important for targeting pain.
Adaptation
Receptors quickly adapt to a given stimuli, with a right-hand shift in the S-shaped stimulus response curve. Numerous mechanisms are present, including phosphorylation, 2nd messengers, in AP generation, and at synapses. Different receptor types adapt at various rates: muscle spindles adapt slowly while hair receptors do so rapidly.
Nerve Fibres
Peripheral nerves contain various types of sensory nerve fibres bundled together with outgoing muscular signals. Somatotopic (spacially organized) information is retained as nerves enter the spinal cord through dorsal roots.
Characteristics of the various nerve fibres include:
|
diameter |
velocity |
myelinated |
function |
Ia |
12-20 |
70-120 |
yes |
muscle spindle stretch - proprioreception, spinal reflexes |
Ib |
12-20 |
70-120 |
yes |
golgi tendon organs (contraction) |
II (Aβ) |
6-12 |
30-70 |
yes |
Meissner corpuscles, Merkel cells |
III (Aδ) |
1-5 |
5-30 |
yes |
free nerve endings: sharp pain, cold, hair receptors |
IV (C) |
1 |
0.5-2 |
no |
free nerve endings: dull pain, warmth, itch |
Spinal Pathways
Sensory information travels up the spinal cord to the brainstem and finally to the brain. Most sensory neurons have their cell bodies in the dorsal root ganglia of the spinal cord, or the somatic afferent ganglia for the cranial nerves, while axons extend to the site of receptors. Sensory neurons synapse in the spinal cord or brain stem.
The spinothalamic tract carries pain and temperature, itch, and crude touch, while the dorsal columns transmit proprioreception, fine touch, and vibration.
- spinothalamic tract
- spinocerebellar tracts
- dorsal column
- spinocervical tract
Spinothalamic Tract
The spinothalamic tract conveys information about pain, temperature, and crude touch.
Neurons enter through the lateral bundle and ascend or descend 1/2 segment through the Tract of Lissauer (dorsolateral tract), composed of unmyelinated fibres. They then enter the dorsal horn and synapse on 2nd order neurons in the substantia gelatinosa before crossing over. Fibres in the spinothalamic (anterolateral) tract from the lower limb enter first, and as future fibres enter they are pushed progressively lateral.
Important interneurons are present in the substantia gelatinosa and then the nucleus proprius, representing gating systems for pain that can change its perception.
Once in the medulla, the substantia gelatinosa becomes continuous with the spinal nucleus of the trigeminal nerve, the major area of transmitting pain and temperature information about the head.
There are both direct and indirect paths to thalamus.
- The spinothalamic tract and spinal trigeminal tract conveys information directly to ventral posterolateral nucleus
- The spinoreticular tract synapses in reticular formation of the medulla and pons before heading to thalamus and hypothalamus, as well as the cingulate cortex, mediating arousal
- The spinomesencephalic ascends to the superior colliculus in the midbrain
Damage to the spinothalamic tract does not have a large impact on tactile functions, but does cause contralateral analgesia. A cordotomy is the cutting of the spinothalamic tract, though pain returns typically after a few months.
Spinocerebellar Tracts
The posterior spinocerebellar tract (PSCT) carries information from muscle spindles and Golgi tendon organs, synapsing on the thoracic nucleus at the base of the dorsal horn.
It ascend ipsilaterally and travels through the inferior cerebellar peduncle to reach the cerebellar vermis and adjoining areas. It primarily carries proprioreceptive information about the lower limbs. The cuneocerebellar tract travels alongside the PSCT and carries information about the arm.
The anterior spinocerebellar tract (ACST) carries information from proprioreceptors, cutaneous receptors, spinal interneurons, and descending tract fibres. This integration allows it to carry complex information related to attempted movements.
The ASCT crosses in the spinal cord and ascends to the superior pons, entering the cerebellum through the superior peduncle. It then recrosses the midline, ending in the ipsilateral vermis and adjoining areas.
The dorsal column and medial lemniscus also carry proprioreceptive information.
Dorsal column
The dorsal column carries information related to proprioreception, vibration, and fine touch.
Pseudounipolar neurons, with cell bodies in the dorsal root ganglion, enter through medial bundle of the dorsal root and immediately enter the ipsilateral posterior spinal cord.
Axons entering from the lower limb are pushed progressively more medially into the fasciculus gracilis, while axons from the upper limb travel in the fasciculus cuneatus, located more laterally. The fasciculus cuneatus is only present above T6.
The dorsal columns ascend ipsilaterally until they reach the posterior inferior medulla, where they synapse in the nucleus gracilis and cuneatus. They then cross the midline and form the medial lemniscus, a flattened, ribbonlike tract that moves laterally as it rises throughtout the brainstem and into the thalamus.
Sensory information synapses in the ventral posterolateral (VPL) nulceus of the thalamus. Axons then leave through the internal capsule and synapse in the cortex, primarily the primary somatosensory cortex located on the postcentral gyrus.
Problems
Problems with the dorsal column results in a reduced sense of fine touch and and vibration. Perhaps the most affected, however, is a loss of complex discrimination tasks, such as identification of a pattern drawn on the skin or a held object (stereognosis). Proprioreception is also affected, resulting in a characteristic ataxia. Balance is particularly affected if the eyes are closed, providing the basis for Rhomberg's sign.
Spinocervical tract
The spinocervical tract carries information from hair receptors, some tactile receptors, and some nociceptors contralaterally before synapsing in the lateral cervical nucleus. Axons then cross the midline in the medial lemniscus of the medulla before ascending to the ventral posterolateral nucleus of the thalamus.
Central Processing and Perception
There are many steps invloved in pain processing. It causes somatic reflexes, autonomic reflexes, changes in attention, emotion, memory.
Somatic reflexes are automatic responses to address the source of pain. Increased muscle tone occurs.
Flexor and crossed extensor reflexes: cutaneous pain causes limb withdrawal and contralateral limb extension.
Scratch reflex: cutaneous pain on the body causes limb movements to remove source of irritation
Hyperalgesia and Allodynia
Cellular damage and C fibre signaling leads to local vasodilation and substance P release, inducing mast cell degranulation of histamine. This causes local redness and hyperalgesia.
Inflammation-induced kinins, prostaglandins, amines, protons, and ATP can increase pain sensitivity by lowering nociceptor
thresholds. Peripheral inflammation also signals the cell body to increase sensitivity to pain biochemically.
Hyperalgesia and allodynia (pain caused by normally non-painful stimuli, such as touch or warmth) can spread beyond the area covered by the axon reflex, likely through changes in synapses in the spinal cord.
Modulation in the spinal cord
Persistent pain involves peripheral and central sensitization.
Numerous receptors modulate transmitter release, and modulation can be excitatory or inhibitory. Inhibitory neurotransmitters include opiods, cannabinoids, and GABA, while excitatory transmitters include bradykinin and prostaglandins.
Calcium channels in nociceptor terminals are different than those in other synapses, allowing them to be selecively inhibited.
Spinal Gating
Aβ mechanoreceptors synapse on inhibitory neurons, which thereby inhibit the Adelta synapse in the spinal cord. Mechanical stimulation (rubbing) or transcutaneous electrical stimulation excite these large mechanoreceptors and reduce pain.
Descending control
Decsending neurons from the periaqueductal gray and medulla synapse on inhibitory interneurons, which release endorphins (enkephalins) and inhibit the Adelta synapse. There is also some evidence for descending excitatory pain.
DNIC can be controlled by focus.
Resources and References
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