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UNIT 4 - Receptors

View Unit 4 Learning Objectives

A. Exteroreceptors
B. Proprioceptors
C. Vestibular Receptors

Incoming sensory information from the external surface of the body enters the C.N.S. via receptors dealing with exteroceptive and proprioceptive input.   In addition to this information, input is received from the vestibular, auditory, visual, olfactory and gustatory receptors as well as the interoreceptors serving the viscera.

A. Exteroreceptors

These receptors are situated on the body and respond to various external influences and stimuli. Numerous attempts have been made to allocate specific receptors to the four modalities of cutaneous sensation (touch, pressure, temperature, and pain) but none of these have been very successful. Presented in this chapter is a scheme compiled from many sources (see chart).

B. Proprioceptors

This category of receptors provides information dealing with the force, rate, direction, and range of movement. They are also concerned with reflex muscular activity. They are divided into conscious and unconscious proprioceptors.

1. Conscious Proprioceptors - (see chart)

These receptors are located in the joint capsule, tendons, periosteum, and ligaments.   They are responsible for one's kinesthetic awareness, or our ability to consciously perceive movements in terms of force, rate, direction, and range, including our position sense.   This recognition of movement and joint positions allows us to regulate and monitor our posture and movement and is also used for motor planning and learning.

2. Unconscious Proprioceptors

These proprioceptors are found in the skeletal muscles.   They too, carry information concerning force, rate, direction, and range of movement which is taken directly to the cerebellum.   This information is not received directly by the higher cortical centers of the cerebrum which is why it is considered unconscious.   However, this information does reach higher control centers indirectly through relays from the cerebellum.

a. Muscle Spindle

The muscle spindle consists of a connective tissue capsule that encloses a nuclear bag and numerous nuclear chain fibers. Both the bag and the chain fibers have a central equatorial region which is con-contractile, and polar regions on either end which are contractile containing intrafusal muscle fibers. The muscle spindle is attached to and aligned parallel to the skeletal muscle fibers, known as extrafusal fibers.

1. Nuclear Bag

The nuclear bag fibers are both phasic and tonic receptors responding to the rate of change and the amount of change occurring in a muscle. Information from nuclear bag fibers is picked through the annulospiral endings and conveyed to the cerebellum via heavily myelinated A-alpha or IA nerve fibers.

2. Nuclear Chain

The nuclear chain fibers are tonic receptors and respond to sustained stretch (length change) in the skeletal muscles.   Nuclear chain fibers are equipped with two afferent endings.   The first type is the annulospiral endings, which again relay information via A-alpha or IA nerve fibers, and the second type is the flower-spray endings (secondary spiral endings), which relay information via the A-Beta or II nerve fibers (type II afferent).

NOTE: Unlike any other receptor, the muscle spindle has an efferent component as well. Gamma static efferents nerve fibers are finely myelinated and innervate the contractile portion (polar region) of both the nuclear bag and chain.  Gamma dynamic efferents nerve fibers innervate the polar region of the nuclear bag fiber only.Gamma dynamic efferents provide dynamic sensitivity to the nuclear bag IA fibers to respond to the speed of change. Gamma static efferents fibers provide static sensitivity to both the nuclear bag and chain IA fibers, as well as the type II afferents on the nuclear chain (type II afferents or flower spray endings) to respond to the amount of change (muscle length).

b. Golgi Tendon Organs

Golgi Tendon Organs lie within muscle tendons in series with surrounding muscle fibers immediately beyond tendon attachments to the muscle fibers. The G.T.O.'s send afferent information through large myelinated A-alpha or IB size fibers. There are two functional types of G.T.O. There is a high threshold type, which responds to muscle lengthening and is located in the distal attachments of the tendons. There is also a low threshold type, which responds to muscle contraction, and is found in the proximal attachments of the tendons.

Fig. 1 –

Muscle Spindle Basics, King's College London, Strand, London WC2R 2LS, England, United Kingdom, http://www.kcl.ac.uk/teares/gktvc/vc/lt/mspindle/spin1.htm

Click for Printable PDF Figure 1.

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C. Vestibular Receptors

Vestibular receptors are located in the labyrinths of the inner ear.   They are concerned with balance and equilibrium, that is, the maintenance of posture through learned reflexes that coordinate eye, head, neck, trunk, and extremity movements.   The large myelinated fibers of the eighth cranial nerve supply the neural connections to the vestibular receptors.

1. Phasic Receptors

The semicircular canals respond to movements of the head, especially rotary motions, or changes in acceleration and deceleration of the head in space.

2. Static Receptors

The utricle and saccule respond to the force of gravity acting on the head and linear acceleration and deceleration of the head in space.

Fig. 2 –

Fig. 3 –

Diagram of the interconnected ducts (membraneous labyrinth) of the inner ear, and the vestibulocochlear nerve. (http://www.umanitoba.ca/faculties/medicine/anatomy/EAROBJEC.htm

Click for Printable PDF Figures 2 and 3.

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Touch	Pressure	Pain	Temperature
1. Meissner Corpuscles - fast adapting - fast conduction - fine tactile discrimination - precise location - low threshold	1. Pacinian Corpuscles - fast adapting - fast conduction - precise localization and intensity - low threshold Also allows for: A. Awareness of muscular tension and movement B. Vibratory stimuli (high frequency)	1. Free Nerve Endings A-Delta - fast adapting - fairly fast conduction rate sharp, well localized sensation C- Fibers - slow adapting - slow conduction - provides continuous input - dull but intense - diffuse sensation of pain Both fibers have low threshold	1. Free Nerve Endings A-Delta - fast adapting - fairly fast conduction rate - sharp, well localized sensation of warmth and cold C-Fibers - slow adapting - slow conduction - provides continuous input - sensation is a generalized diffuse feeling of warmth or cold Both fibers have low threshold
2. Merkel's Disc - slow adapting - variable conduction - fine tactile discrimination - continuous input - precise location - higher threshold than Meissner's (need maintained stimulus)	2. Ruffini End Organ (need sustained stimulus) - slow adapting - fast conduction - higher threshold than Pacinian - precise localization and intensity - continuous input Also allows for: A. Awareness of muscular and skin tension, as well as movement B. Protective
3. Free Nerve Endings - slow adapting - variable conduction decreased discrimination and localization but keeps us in continuous contact with our environment at a low level	3. Free Nerve Endings - slow adapting - variable conduction - decreased localization and intensity - keeps us in continuous contact with our environment at a low level - lower threshold than Pacinian
4. Hair End Organs (Petitrichiial endings) - fast adapting - variable conduction - poor discrimination - low threshold - protection and contributes to feel of movement

 

Force	Rate	Direction	Range
1. Pacinian Corpuscle 2. Ruffini End Organ 3. Golgi Massoni 4. Golgi Type Endings Characteristics as listed	1. Golgi Massoni - joint capsule and tendons - fast adapting 2. Ruffini End Organ - joint capsule and tendons - slow adapting - higher threshold than Golgi Massoni - continuous reception of information	1. Pacinian Corpuscle - Periosteum and ligaments - fast adapting 2. Ruffini End Organ - joint capsule and tendons - slow adapting -higher threshold than Pacinian - continuous reception of information	1. Pacinian Corpuscle - Periosteum and ligaments - fast adapting 2. Golgi Type Ending - Periosteum and ligaments - slow adapting - higher threshold than Pacinian - continuous reception of information

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