Unit 4


General classification & properties

Muscle spindles

The gamma system

Golgi tendon organs

Articular receptors

Cutaneous receptors



We have looked at the neuron and should have an idea how impulses (action potentials) are conducted. However, at this point we have not discussed just how stimuli are converted into impulses. All sensory systems are designed to detect forms of energy and the interface between stimulus and the response of a neuron is normally a structure called a RECEPTOR. The process by which the various sensory receptors convert a particular form of energy into another form is known as transduction.  In other words a receptor converts stimuli into impulses.

Narrated animation about skin receptors

Animation 1


Sensory Modalities

Stimuli can be divided into a range of different types or MODALITIES. Receptors normally respond to only one type of stimuli (or sensory modality), and that type of sensory modality is called the adequate stimulus for a particular type of stimulus. For example, the adequate stimulus for the eyes photoreceptors (rods and cones) is light and not touch (mechanical stimuli).

Below is a list of receptor types verses their adequate stimulus:

Receptor Type

Adequate Stimulus






Mechanical displacement

Temperature change





With an adequate stimulus, receptors produce a receptor potential when stimulated.

The Nature of a Receptor Potential

1. All RP's are graded (ie. RP's differ in magnitude).

2. The magnitude (amplitude) of a RP has a sigmoid  relationship with stimulus strength.

3. The magnitude of the RP is linearly proportional to the firing rate of the associated afferent neuron.

4. All receptors adapt to stimuli. However, at which they adapt varies between slow and fast. (What is the significance for this variation?).

In the graphic illustration of the relationship between the receptor potential and the firing rate along the afferent neuron, as soon as the excitation threshold of the first node of Ranvier is reached the firing rate increases linearly as the magnitude of the RP increases.

For example, a touch receptor in the skin (a mechanoreceptor) – impulse generation

1. The neural part of the Pacinian corpuscle (PC) is  polarized.

2.  The PC is mechanically stimulated.

3. The neural part of the Pacinian corpuscle is depolarized.This forms the receptor potential.

4. The greater the degree of mechanical stimulation, the larger the magnitude of the RP.

5. If the excitation threshold at the first node of Ranvier is reached , then an AP is generated along the entire length of the afferent nerve fibre.  


Receptors also work on the same principles as Adequate Stimulation, spatial and temporal summation as mentioned in unit 2. These are important concepts with respect to in intensity Coding

1) means by which we can determine the intensity with which a particular sensory event occurred

2) property that enables us to distinguish between different sensations (e.g., hard slap vs. light tap, loud vs. soft sound, strong vs. weak muscle

3) the intensity of a stimulus can be conveyed by two mechanisms

Spatial Summation - the stronger the stimulus the larger the number of different sensory receptors fire

Temporal Summation - the stimulated receptors are fired at a higher frequency

Sensory Adaptation –enables us to block out irrelevant sensory information

1) shortly after a sensory receptor registers a stimulus, the firing rate is reduced or, curtailed

2) different sensory receptors adapt at different rates: touch and pressure receptors vs. pain receptors and certain proprioceptors

3) different adaptation rates of receptors determines the nature of the information provided to the CNS

The receptors important for movement are mostly somatosensory receptors.  Please refer to the diagram below.



Ruffini endings

Pacinian corpusles, Meisner corpusles



muscle spindles

Golgi tendon organs

joint receptors

vestibular apparatus


Provide touch and pressure. Distribution is somewhat disproportionate throughout the body - lips and fingers contain many more receptors per sq inch of surface area

Why? - these areas of the body are involved in very fine movements and therefore require greater levels of sensation


Provide uninterrupted knowledge about the general position of our body in space prior to and during movement - also provides us with information regarding where are body segments are relative to each other

Receptors that are stimulated by physical deformation are known as mechanoreceptors

We will limit our discussion to four types of receptors which may be involved in the position and movement of a limb, and involved in proprioception. Each receptor provides different information.


Animation about spindles and GTO

Animation 2


Awesome Site - read carefully - Required




Muscle spindles (important for muscle length and velocity and to a lesser extent, monitoring static length)

Golgi tendon organs (detects muscle force)

Joint afferents (most sensitive to position at extreme joint angles)

Tactile (pressure receptors in muscle and overlying skin, sense flexion or extension of finger)

1st Required QUIZ


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You will have 17 minutes to complete the Required Quiz - use your time wisely!



·        Detect dynamic and static changes in muscle length

·        considered the proprioceptor of greatest importance

·        serves a dual sensory and motor function

·        higher numbers of spindles per square inch of surface area are located in muscles subserving fine movements (tongue, hands)

Function of the Muscle Spindle

Muscle spindles are found in all somatic muscles, within the belly of the muscle and run in parallel with the main muscle fibres.  Whenever skeletal muscle is stretched the muscle spindles are stimulated. They detect changes in the length of muscle fibers as well as the rate of change at which the muscles are lengthening.

  1. The Muscle Spindle consists of 2-10 specialized muscle fibers enclosed in a connective tissue capsule. These muscle fibers are called intrafusal muscle fibers. Voluntary skeletal muscle fibers are called the extrafusal muscle fibers.

  2. Intrafusal Muscle Fibers:

·         Nuclear Bag Fibers:

o        approximately 1-3 fiber per spindle

o        the nuclei are found concentrated in a bag type central part of the fiber (hence the name 'nuclear bag' fibers).

o        the ends of these fibers are striated (contain actin and myosin filaments) and are contractable. The ends of these fibers are also attached to the EXTRAFUSAL muscles.

·         Nuclear Chain Fibers:

o        approximately3-7 fibers per spindle

o        the nuclei are spread in a chain-like fashion in the center of the fiber (hence the name).

o        the ends of these fibers are striated (contain actin and myosin filaments) and are contractable. The ends of these fibers are attached to the ends of the  nuclear bag muscle fibers.

During a muscle stretch the sensory nerve terminals increase their discharge rate as the sensory ending is stretched. This nerve terminal is known as the ANNULOSPIRAL ending, so named because it is composed of a set of rings in a spiral configuration. These terminals (shown in blue on the diagram below) are wrapped around specialised muscle fibers that belongs to the muscle spindle (INTRAFUSAL FIBRES) and are quite separate from the fibers that make up the bulk of the muscle (EXTRAFUSAL FIBRES).

More about the Nuclear Bag fibers and the GAMMA SYSTEM:

A motor supply to the intrafusal muscle (shown above in red). In this region on either side of the central area the intrafusal fibres are able to contract if their motor supply is active. The motor supply comes via efferent fibres that usually fall into the gamma classification of diameters. They are often (and better) referred to as FUSIMOTOR fibres.

Two things can, in principle, cause the annulospiral ending to be stretched and so increase its discharge.

1. A stretch of the muscle as a whole will stretch the spindles within it, and thus the sensory endings.

2. Fusimotor activity will cause contraction of the intrafusal fibres below the fusimotor nerve terminals either side of the central region. This will result in stretch of the central sensory region .

Nuclear chain fibre also have annulospiral sensory endings in the central region (the nuclei are in line). It is a shared branch of the axon that supplied the central area of the nuclear chain fibre. This sensory nerve is of group Ia, the fastest found in the body.

Further out we see that there are other sensory endings, more closely associated with the chain fibres. These fall into the slower group II division of sensory nerves and are referred to as SECONDARY endings in contrast to the centrally located PRIMARY endings.

The two types of intrafusal fibre, (bag and chain) have different mechanical properties, and respond differently to their largely separate fusimotor fibres. They also differ in respect to their sensory endings. Consequently, the information relayed to the CNS by the spindle via group Ia and group II sensory endings is different.

In simple terms, the Ia afferents respond partly to muscle length, but respond more powerfully to changes in length (BLUE). The group II afferents are much better at registering length alone (RED).

Ia afferents can powerfully excite the ALPHA MOTONEURONES of the muscle containing the spindle. This is the basis of the classical STRETCH REFLEX in which extension of the muscle (and thus its spindles) cause a reflex contraction.

The spindle can therefore register muscle length and velocity. Furthermore the sensitivity to both length and velocity can be altered by the CNS via activity in the fusimotor system, the static gamma system controlling length sensitivity and the dynamic gamma system controlling velocity sensitivity.

Golgi tendon organ (GTO)

The GTO detects:

a) the rate of increase of tension on a muscle during active contraction

b) the absolute amount of tension on a muscle during a isometric contraction Prevents damage during excessive force generation

GTOs are relatively simple sensory structures which consist of elaborated nerve ending in a capsule which lies within the tendon at the musculotendinous junctions of somatic muscle. Each skeletal muscle contains a large number - approximately 1 GTO for every 10 muscle fibers.  These ending produce generator potentials and action potential discharges with frequency proportional to the force exerted on the capsule. The tendon organ acts as a "force transducer", and because it is in series with the tendon is only activated in a sustained fashion when the muscle is active. These sensory structures appear to be quite localized, that is they will monitor force within a subdivision of tendon which represents the physical connection of a motor unit to the global tendinous structure of the muscle. Thus the CNS has information about the force output of individual motor units and can make feedback adjustments appropriately in the efferent side.

Figure: muscle spindle structure compared to GTO structure.

GTO’s are innervated with a 1b afferent fiber. This afferent information may also contribute to preventing the muscle from developing too much tension through its inhibitory connections with alpha motor neurons.  See unit 7.

Joint/ Articular receptors

The joint capsules and ligaments of all synovial joints in the skeletal system are well endowed with proprioceptors.  Collectively, joint receptors provide us with information about: a) static position of a joint in space (i.e. joint angles) b) endpoint positions of joints during active movement

Joint Receptors

Receptor Type




Associated Afferents


Type I


ligaments & joint capsule

Slow adapting;
Active at end ranges (passive)

change of direction, amplitude, pressure, velocity to cerebellum



Type II


joint capsule & fat pads

Rapid adapting;
Active at end ranges-dynamic

boost muscle at beginning of movement



Type III



Slow adapting;
Active under tension

protective reflex



Type IV

Free nerve-like

ligaments, joint capsule, fat, periosteum

Slow adapting;

flexion reflex -->prevent further movement

A delta & C

Tactile (cutaneous) receptors

Cutaneous sensations (review skin structures)

Cutaneous receptors:  Glabrous (smooth, hairless skin) and hairy skin both contain a wide variety of receptors for the purpose of detecting mechanical, thermal, or painful stimuli applied to the body surface.  Three types of receptors are common to glabrous and hairy skin:   Pacinian corpuscles, Merkel's discs, and free nerve endings.   The other receptors found in glabrous skin are Meissner's corpuscles.  The most important receptors in hairy skin are the hair follicle endings and Ruffini terminals.

1. Merkel Disk:  Slowly adapting mechanoreceptors structured to respond to maintained deformation of the skin surface.  Typically, an afferent fiber branches to form a cluster of Merkel's discs, situated at the base of a thickened region of epidermis.  Each nerve terminal branch ends in a disc enclosed by a specialised accessory cell called a Merkel cell.  Movement of the epidermis relative to the dermis will exert a shearing force on the Merkel cell.  The Merkel cell plays a role in the sensing of both touch and pressure.

2. Meissner Corpuscle: Terminal branches of a myelinated axon intertwines in a basketlike array of Schwann cells.  Found just underneath the epidermis in glabrous skin (nipples, fingertips, soles of feet), these receptors are quickly adapting mechanoreceptors providing discriminative touch.

3. Ruffini CorpusclesSpray-like dendritic endings in the dermis hairy skin which are involved in the sensation of steady, continuous pressure applied to the skin. 

4.  Pacinian Corpuscles:  Pacinian corpuscles are pressure receptors. They are located in the skin and also in various internal organs. Each is connected to a sensory neuron.

5. Free Nerve EndingsMade up of branching nerve axons, which are entirely or partially surrounded by Schwann cells.  The axon/Schwann cell complex is further surrounded by a basement membrane.  Free nerve endings originate from fine myelinated or unmyelinated fibers that branch extensively in the dermis and may penetrate into the epidermis.  These endings respond to strong mechanical and thermal stimuli, and they are particularly activated by painful stimuli.



(receptive fields)

Adaptation rate


Type I



Type II



Type I


Pacinian Corpuscle

Type II



See Unit 5 to find out where the afferent input from these receptors go…

2nd Required QUIZ


Please take in : www.uh.edu/webct

You will have 31 minutes to complete the Required Quiz - use your time wisely!