Somatosensory skin receptors
The human skin is equipped with various types of receptors, each with its own narrow specialization in perceiving external stimuli. All information from them is transmitted via afferent pathways.
- Free nerve endings: these are the main structures responsible for the perception of pain and temperature.
- Tactile receptors: this group includes Merkel discs, Meissner corpuscles, and peritrichial nerve endings, which densely wrap around hair follicles.
- Pressure and vibration receptors: Pacinian corpuscles take on this function.
- Thermoreceptors: Krause end bulbs are activated by cold, whereas Ruffini corpuscles specifically respond to warmth.
Receptor apparatus of deep tissues
Muscles, tendons, and fascia also have their own sensory sensors that transmit information about the state of the musculoskeletal system.
Muscle spindles are very thin, encapsulated spindle-shaped structures. They are located between the working (extrafusal) fibers of skeletal muscles. Inside the connective tissue capsule of each spindle are 3 to 10 specific intrafusal muscle fibers. The spindles are rigidly fixed by connective tissue between muscle fascicles, allowing them to move completely synchronously with the muscle. They are wrapped by annulospiral (primary) endings — nerve fibers with a very thick myelin sheath belonging to group IA (having the highest conduction velocity). The main task of the spindles is to respond to stretch.
Golgi tendon organs are localized in the transition zone between the muscle belly and the tendon. They consist of a thin nerve ending (a branch of a thick myelinated fiber) surrounded by collagen fibers within a capsule. They also detect stretch and muscle tension, although their threshold of excitation is significantly higher than that of muscle spindles.
Additionally, Pacinian corpuscles and Golgi-Mazzoni corpuscles are found in deep tissues (the latter specialize exclusively in the perception of pressure).
Peripheral nerve and its sheaths
Afferent impulses from all described receptors travel to the spinal cord via the peripheral processes of the first somatosensory neurons. The cell bodies of these neurons lie in the spinal ganglia. Fibers from a specific area of the body gather into bundles, forming a peripheral nerve.
The composition of a mixed nerve includes:
- Somatic afferent fibers: provide superficial and deep sensitivity.
- Somatic efferent fibers: innervate skeletal musculature.
- Visceral fibers: perform autonomic control of internal organs, smooth muscles of blood vessels, and sweat glands.
The nerve trunk itself has reliable multilayer protection. Endoneurium surrounds each individual nerve fiber. Perineurium covers bundles of fibers and contains nourishing vessels (vasa nervorum). Epineurium is the outer dense sheath of the entire trunk, including adipose tissue and major blood vessels. On a cross section of a nerve, one can see both myelinated segmental fibers (motor and sensory) and unmyelinated fibers (predominantly autonomic).
Roots and nerve plexuses
Before entering the vertebral canal through the intervertebral foramen, the peripheral pathways divide. The anterior root contains efferent fibers leaving the spinal cord, and the posterior root contains afferent fibers entering it.
In the thoracic region, peripheral nerves transition directly into roots. In the cervical and lumbosacral regions, nerve plexuses are formed where a complex redistribution of pathways occurs:
- Sensory fibers from one nerve diverge into different spinal nerves and enter the spinal cord at different levels. Their synapses with the second neuron can be located either near the entry site or significantly higher — up to the brainstem.
- Motor fibers from a single root disperse and enter the composition of several peripheral nerves at once.
The key structural principle is that any peripheral nerve contains fibers from several different radicular segments at once.
Briefly: answers to topical questions
Through which spinal cord tracts is pain and temperature sensitivity transmitted?
Pain and temperature sensitivity is transmitted via the lateral spinothalamic tract and Lissauer's tract.
- Lateral spinothalamic tract (tractus spinothalamicus lateralis) — the ascending pathway of the lateral funiculi through which nerve impulses travel to the CNS.
- Lissauer's tract (tractus posterolateralis) — the marginal zone at the apex of the dorsal horn, serving as the entry and distribution area for thin afferent fibers.
Which neurotransmitters are involved in the transmission of pain impulses in the synapses of the dorsal horns of the spinal cord?
Nociceptive mediators and modulators are involved in the transmission of pain impulses in the synapses of the dorsal horns of the spinal cord.
- Glutamate is a pain mediator, the release of which decreases during presynaptic inhibition.
- Substance P (Substance P) — a tachykinin that exerts a modulatory enhancing effect on the transmission of nociceptive impulses.
- Tachykinins — a group of nociceptive peptides.
Also, at the level of the first relay station (dorsal horns), receptors and effects of GABA, norepinephrine, serotonin, enkephalins, histamine, neurotensin, angiotensin, cholecystokinin, and oxytocin are present.
What is the motor innervation of the intrafusal fibers of the muscle spindle?
The motor innervation of the intrafusal fibers of the muscle spindle is carried out by efferent fibers of $\gamma$-motor neurons of the spinal cord (fusimotor innervation).
- Dynamic $\gamma$-motor neurons — innervate predominantly intrafusal fibers with a nuclear bag.
- Static $\gamma$-motor neurons — innervate mainly intrafusal fibers with a nuclear chain.
These neurons innervate the polar (terminal) parts of the intrafusal fibers. Their activation causes contraction of the terminal parts, which leads to stretching of the central sensory part of the spindle without changing the length of the entire muscle.
To memorize the nerve sheaths from the inner layer to the outer layer, use the "E-P-E" rule: Endoneurium (inside), Perineurium (in the middle of the fascicle), Epineurium (outside the entire nerve).
Class and exam questions
Which skin receptors are responsible for pain perception?
Free nerve endings are responsible for pain and temperature sensitivity.
What is the difference between intrafusal and extrafusal fibers?
Intrafusal fibers are specific receptor elements located inside the capsule of the muscle spindle. Extrafusal fibers are ordinary working fibers of the skeletal muscle that surround the spindle from the outside.
How do the functions of the anterior and posterior spinal roots differ?
The anterior root consists of efferent (motor) fibers exiting the spinal cord. The posterior root is formed by afferent (sensory) fibers carrying information from receptors to the central nervous system.
Why is sensation in the innervation zone not completely lost when a single nerve root is damaged?
A redistribution of fibers occurs in nerve plexuses. As a result, each peripheral nerve receives afferent fibers from several different radicular segments, which ensures the overlapping of innervation zones.
What else to study on this topic
- Biomechanics and synchronous movement of muscle spindles
- Classification of nerve fibers by conduction velocity (group IA)
- Topographic features of nerve transitions into roots in different parts of the spine
- Autonomic unmyelinated fibers within a mixed nerve
- Difference in excitation thresholds of muscle spindles and Golgi tendon organs
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