The Bodily Senses

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.

 

Receptors

Receptors are specialized cells for detecting changes in the environment.

Spatial resolution depends on receptor density.

 

Merkel Discs

Ruffini

Meissner

Pacinian corpuscles

temperature receptors

 

mechano cells use TRP channels for generating signals. very important for targeting pain

 

receptor potentials

a generator potential spreads passively along the axon until it reaches an area rich in Na channels and hits threshold. Each cell has a level of 'noise'. a signal must be viewed as something above a given threshold.

 

adaptation

receptors quickly adapt to a given stimuli, with a right-hand shift in the stimulus response curve, which is S-shaped.

All sorts of adaptiation mechanisms are present, including phosphorylation, 2nd messengers, in AP generation, and at synapses.

muscle spindles slowly adapt, while hair receptors rapidly adapt.

 

object perception

objects are perceived by the character and intensity of sensation over a population of cells.

 

 

 

What is Where?

Hairy skin contains Merkel Cells, hair receptors, and nonencapsulated nerve endings.

Smooth (gabrous?!) skin contains pacinian corpuscles, Ruffini endings, Meissner corpuscles, and Merkel cells.

 

 

Nerve Fibres

Peripheral nerves contain various types of sensory nerve fibres bundled together with outgoing musclular signals.

 

Characteristics of the various nerve fibres include:

 

diameter

velocity

myelinated

function

Ia

12-20

70-120

yes

muscle spindle stretch - proprioreception

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

 

 

 

 

 

 

 

 

 

 

 

 

C are very slow, other sensations come in before. can do expt in hot water - feel water before feel hot (one sec or so)

 

 

Most sensory neurons have their cell bodies in dorsal root ganglia, or somatic afferent ganglia for the cranial nerves. This makes for some very long axons!

 

 

Central Nervous System Pathways

Following entry through the dorsal root, sensory neurons synapse in the spinal cord or brain stem. Somatotopy is retained all the way through.

 

The spinothalamic tract carries pain and temperature, itch, and crude touch, while the dorsal columns transmit proprioreception, fine touch, and vibration.

 

Spinothalamic Tract

The spinothalamic tract conveys information about pain, temperature, and crude touch.

Spinal Cord

Neurons enter through the lateral bundle and ascend or descend 1/2 segment through the Tract of Lissauer (dorsolateral tract), which stains faintly because most of the fibres are unmyelinated. 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 nucleus proprius, representing gating systems for pain that can change its perception.

 

Brain stem and Thalamus

Once in the medulla, the substantia gelatinosa becomes continuous with the spinal nucleus of the trigeminal nerve, the head's area of transmitting pain and temperature information.

 

There are both direct and indirect paths to thalamus. The spinothalamic tract conveys information directly to ventral posterolateral nucleus.

The spinoreticular tract synapses in reticular formation of medulla and pons before heading out to thalamus and hypothalamus.

The spinomesencephalic ascends to the superior colliculus in the midbrain.

 

Problems

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.

 

Dorsal column and medial lemniscal system

The dorsal column carries information related to proprioreception, vibration, and fine touch.

Spinal Cord

Pseudounipolar neurons, with cell bodies in the dorsal root ganglion, enter through medial bundle of 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.

 

Brain Stem

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 synapses in the thalamus.

 

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.

 

 

Other ascending pathways

Spinocerebellar tracts are discussed under proprioreception.

 

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