Receptors

Sensory nerve endings – receptors can be classified according the location of the stimulus into:

1) Exteroreceptors – for information from the external world are placed mainly in the skin (hair cells of inner ear and retinal photoreceptors are in the special subgroup of the exteroreceptors - telereceptors).

2) Proprioceptors for information from the musculosceletal system (deep sensation from the muscles, tendons, joint capsules and ligaments).

3)
Interoreceptors for information from the internal organs.


1) Exteroreceptors – skin receptors

 a) free endings – nonencapsulated, they lie freely in the extracellular space or they are closely applied to cells of inervated tisseus. They are covered on most of their surface by the cytoplasm of the Schwann cells. 

 b) encapsulated endings – they are covered by various number of epineurium layers.

1 -  Merkel endings – the axon ends as flattened expansion closely applied to special epidermal Merkel cells.

           Merkelovy buňky

2 -  peritrichal endings – encircling the hair follicles

      3 -  Meissner´s tactile bodies – several dendrites with intertwined branches enbedded among Schwann cells and enclosed in an epineural capsule.   

    Meisnerovo tělísko

4 -  Ruffini corpuscles – one greatly branched nerve fibre surrounded by a capsule.

   Ruffiniho tělísko

5 - Vater – Paccini corpuscles - up to 2 mm large, it is a dendrite encapsulated by Schwann cells and on periphery by many layers of epineurium.

  Paciniho tělísko

6 -  Krause corpuscles  – oval, ramification of one dendrite is placed between Schwann cells, encapsulated by epineurium

 

More complex receptors are placed more deep!

Rozložení receptorů v kůži 

These receptors react to touch, pressure, stretching, vibrations, warmth, cold and pain.

Receptors - according to their adaptation to the stimulus:

1) Slowly adapting receptors – They send impulses during the whole period of stimulation (non encapsulated endings)

2) Fast adapting receptors – They send impulses only on the beginning and cessation of stimulation. (Meissner and Vater Paccini corpuscles)

Generally, the free endings are slowly adapting and more complex encapsulated endings are fast adapting with lot of transitional forms.

Each receptor reacts to several sensation modalities. (touch, pressure, stretching, vibrations, warmth, cold and pain).

Some receptors prefer perception of certain stimulation:

Merkel´s touch cell – touch and pressure

Meisner´s corpuscle – touch, pressure and vibrations under 100 Hz

Vater-Paccini´s corpuscle – stretching, pressure and vibrations (100-400 Hz)

Ruffini´s corpuscle – pressure, stretching

peritrichal ending – touch and pressure on the hair

Krause´s corpuscle – pressure and cold

free endings – pain (nociceptors).

 

Feeligs like itching, tickling, feeling of wet or dry are results of the simultaneous stimulation of many kinds of cutaneous and subcutaneous skin receptors

According to the function, the receptors are classified as: mechanoreceptors, thermoreceptors and chemoreceptors

Signals from the skin receptors around the joints give us the information about the positions of our body parts and about the change in their positions.

2) Proprioceptors

Proprioceptors bring informations from the musculosceletal system (deep sensation from the muscles, tendons, joint capsules and ligaments). Hilton´s law states that every peripherial nerve supplying a muscle sends a branch to the join moved by the muscle and to the skin overlaying the joint. 

Nerve endings in joints

They signal pain (from injury, ischaemia etc.) and performance of movement.

Free nerve endings (pain), Ruffniform (similiar to skin receptors) endigs (streching and pressure), Paciniform corpuscles (beginning and cessation of the movement, acceleration or deceleration of the movement), Golgi tendon organs.

 

Muscle spindle – is a stretch receptor. It is located in interstitial septa among the fibres of the skeletal muscles, near the tendineous insertion of musces. It is composed from 2-20 specialised muscle fibres (intrafusal fibres) in the fibrous capsule. Intrafusal fibres can be bag fibres or chain fibres according to the arrangement of their nuclei.. Motor innervation to intrafusal fibres comes with axons of gamma motoneurons. These fibres serves to the adjustment the muscle spindle sensitivity or by means of the gamma loop mechanism to the movement or change of the muscle tone.

Sensory innervation of the muscle spindle is mediated by fibres of I or II group from the dorsal roots neurons. These dendrites are turned around (annulospiral endings) or diffusely spread (flower spray endings) on the intrafusal fibres. Muscle spindles are stimulated by the streching of the intrafusal muscle fibres in pasive steching of the muscle (e.g. by the antagonist action) or in case of contraction of intrafusal fibres caused by the action of the gamma neurons. Impulses from muscle spindles are convayed by Ia fibres to the spinal cord, where the alfa motoneurons of the corresponding muscle are activated and its extrafusal fibres are contracted. The information of the corresponding muscle is simultaneously convayed to the higher parts of the brain (proprioception). Muscle spindle is continuously stimulated by the weight of the limb (gravity) and by the pull of the antagonists. It is not stimulated in active contraction of the muscle and in such case it decreases the impulse frequency.

Svalove vretenko - barveno stribrem

Muscle spindle – longitudinal section, i – intrafusal fibres, e – extrafusal fibres, S – subcapsular space C – perineural capsule. Stained by silver.

Vřeténko v příčném řezu

Crossection through a muscle spindle, i – intrafusal fibres, e – extrafusal fibres, S – subcapsular space, C – perineural capsule. By stars are signed neural fibres. Stained by toluidin blue.

Svalové vřeténko-schema

The central equatorial part of intrafusal fibres contain nuclei but no myofibrils – noncontractile part. Myofibrils are located in polar segments – contractile segments of intrafusal fibre.

Ia nerve fibre is a thick sensory nerve fibre branching in the muscle spindle. It terminates in the noncontractile central part of the intrafusal fibre and its terminal branches wind around the muscle fibres like spirals and form the annulospiral endings – primary sensory nerve endings.

II nerve fibre is a delicate sensory fibre terminating as flower spray ending – secondary sensory nerve ending predominantly at nuclear chain fibres or at one or both sides of the annulospiral endings.

Both types of intrafusal fibres (nuclear bag fibres and nuclear chain fibres) are supplyed by thin motor Aγ-fibres. Their small motor end plates are located in the contractile polar parts of intrafusal fibres. Stretching in the central equatorial part of the intrafusal fibre is an adequate stimulus for the muscle spindle triggering. Motor innervation of the contractile polar segments of intrafusal fibres causes the stretching of the equatorial segment and the increase of the sensitivity of this proprioceptoru.

Gamma loop mechanism means that gamma motoneurons in ventral horns are activated from higher CNS levels (cortex, cerebellum, brainstem) and it causes the contraction of the intrafusal fibres of certain muscles. Muscle spindles send signals to the spinal cord, where will be activated alfa motoneurons of corresponding muscles and these muscles will contract. This seemingly longer way – by means gamma loop mechanism – being less energy demanding for CNS (for small easily stimulated gamma neurons), is used functionally in many reflexes as well as in voluntary movements. 

Golgi tendon organ – is  formed in the tendons near the musculotendinoeus junction. In a thin fibrous capsule the tendon fibres are interwoven with a branched thicker afferent fibre of group Ib. It has a higher stimulation threshold compared to muscle spindle. Signals from the Golgi tendon organ is convayed to the alfamotoneurons always by means of an inhibitory interneuron. So the alfamotoneuron is „switched off“ by means the interneuron at excessive stretching of the muscle. Golgi tendon organs are much more sensitive to the tension from the active contraction than from the passive muscle stretching. They coooperate with the muscle spindles.

 

 Golgiho šlachové tělísko

Cross section through the Golgi tendon organ placed between the muscle and tendon. M – muscle fibre, G – Golgi tendon organ, T - tendon. Stained by toluidin blue.

Golgiho šlachové tělísko

Electonmicroscopic view of the Golgi tendon organ – cross section. Nerve endings (t) are among the collagen fibres bundles (crosses). Nerve endings are coveredby a Schwann cell (g).

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