© 2006 zillmusom



            A.  Movements can be performed without thinking about them - Many parts of the nervous system (in brain stem and spinal cord) function without requiring conscious awareness or control.  Some parts of the spinal cord and brainstem can generate patterns of movement independent of higher centers, i.e. walking. Clinical correlate: Individuals with extensive damage to cortex can still produce complex motor actions in a vegetative state.


            B.  Automatic Reactions - Rapid adjustments to movements (ex. after tripping when walking) can also be made without thinking about them.  Neural circuits in spinal cord and brain stem can produce Automatic Reactions using information from muscle receptors about position and muscle tension.   Examples of automatic reactions:


                        1.  Adjustments when standing and walking - Rapid contractions of muscles of leg and trunk occur to maintain balance (ex. when standing on one foot or when standing in a moving bus).  Muscle contractions can be initiated by reflex connections of muscle sensory neurons (muscle spindles).  These reactions are rapid as muscle spindles have the fastest conducting axons in the peripheral nerves.


                        2. Regulating muscle tensions - The force developed by contractions of muscles are automatically controlled so that they do not cause damage to tendons or bones (example: lifting a very heavy object).  Sense organs that monitor forces in muscle tendons (Golgi tendon organs) can contribute to these reactions; also conduct rapidly.


                        3. Stepping on a Nail - More complex reactions can be triggered by painful stimuli; reactions can involve muscle contractions in both legs (see Reflexes Lecture). 


            C.  Kinesthesia - Muscle sensory receptors also contribute to the sense of body position and movement (called kinesthesia) and provide information for fine control of movements (example: ballet dancing). Sense organs that signal a person's or animal's own body position or movements are called proprioceptors (proprio = self, proprioception = sense of yourself). Signals from muscle sensory receptors are transmitted in pathways that reach the cerebellum and cortex (dorsal columns).


            D. Two major types of receptors associated with muscles: Overview:


                        1. Muscle spindles - sensory endings around specialized skeletal muscle cells; complex structure and function; sensory neurons are sensitive to stretch and signal changes in muscle length.  Some neurons also indicate velocity of movement (velocity = change in length/time).  Sensitivities of sense organs are adjusted by gamma motor neurons that innervate the specialized muscle cells.


                        2. Golgi tendon organs - located in muscle tendons; anatomy simple; sensitive to tension in tendon; signal force produced by muscle. 




            A. Muscle contraction - muscles contract by activation of alpha (a) motor neurons (Lower motor neurons); when muscle cells contract they shorten in length and generate forces that are transmitted to the skeleton by tendons (or connective tissue attachments). 


Note: Determining body position from muscle lengths - Using simple geometry, you can calculate limb position (joint angles) if you know the lengths of muscles. If know how fast lengths of muscles are changing (velocity = change in length/time), you can calculate rate of limb movement.  The nervous system apparently does these calculations (at supraspinal levels).  


            B. Isotonic contraction (iso =same, tonic = force or load) - Contraction against moderate (constant) load, ex. lifting an object.  Muscle contracts and produces large changes in length; forces on tendon are constant.   


            C. Isometric contraction (iso = same, metric = length) - Contraction against large load, - ex. trying to lift an immovable object.  Muscle length remains constant (almost) and large forces are generated on tendon.


            D. Muscle tonus = amount of tension in muscle at rest.  Normal muscle has a certain amount of tension at rest and resistance to being stretched (due, in part, to activity in alpha (a) motor neurons at rest). 


Clinical Note: Tonus can be tested by slowly stretching a muscle.  When normal muscles are completely relaxed, the muscle cells can easily be stretched, but the tendons are stiff; when alpha motor neurons are activated, the muscle cells become stiff. Changes in muscle tonus can be a clinical sign: ex.  increased tonus occurs in Upper motor neuron disorders (lesions to descending pathways). 


III. MUSCLE SPINDLES - sense organs that contain specialized muscle cells and sensory and motor nerve endings; found inside muscle; orientation of spindles is parallel to regular muscle cells (termed IN PARALLEL). Number of spindles in a muscle varies from around 20 to several hundred.  Muscle spindles are found most densely in muscles that are used for fine control (ex. interosseus muscles of hand).    The size of muscle spindles varies from 0.5 to 10mm (about 20% of length of regular muscle cells).


Terminology: What is a spindle? A shape is that is long but wider in the middle (this is the shape of a tool, called a spindle, used  in spinning yarn by hand); the Latin word for a spindle is fusus and things shaped like a spindle are called fusiform.   Muscle cells inside the muscle spindle are called intrafusal cells; all the remaining regular skeletal muscle cells in the muscle are called extrafusal cells. (Note about terminology in texts: a muscle fiber is a muscle cell). 


            A. Structure of Muscle Spindle and Contractile Properties of Spindle Muscle Cells


                        1. Specializations of Intrafusal muscle cells - Spindle muscle cells are skeletal muscle cells that are different from regular muscle cells; nuclei are in the middle (equatorial region) of the cell; middle region does not contain myofibrils and does not contract; ends of cells (polar regions) contain contractile filaments and are able to contract


                        2. Types of muscle cells in spindle - differ in morphology/contractile properties; number of muscle cells is variable


                                    a. Dynamic bag muscle cells (dynamic nuclear bag cells) - typically 1 per muscle spindle; nuclei arranged in cluster in middle; contract rapidly when activated.


                                    b. Static bag muscle cells (static nuclear bag cells) - typically 1 per spindle; nuclei arranged in cluster in middle; contract slower.


                                    c. Chain muscle cells (nuclear chain cells) - from 5 - 10 per spindle; nuclei arranged in row in middle like a chain; smallest muscle cells; contract slowly.


                        3. Motor Innervation of muscle spindle - Spindle muscle cells receive own innervation by motor neurons: Gamma (g) motor neurons innervate only spindle muscle cells; Beta motor neurons innervate both spindle and regular (extrafusal) muscle cells but much less is known about them.


                                    a. Gamma motor neurons - axons of gamma (g) motor neurons are smaller (1-8 microns diameter) than alpha motor neurons; however, up to 30% of all axons in ventral roots are from gamma motor neurons; only innervate spindle muscle cells so do not contribute to force of contraction; gamma motor neurons innervate ends (polar regions) of spindle muscle cells; Two types of gamma motor neurons -


                                                i. Gamma dynamic motor neurons - innervate Dynamic bag muscle cells.

                                                ii. Gamma static motor neurons - innervate Static bag muscle cells and Chain muscle cells.


                        4. Strange property of all intrafusal muscle cells - When regular muscle cells (extrafusal cells) contract, they shorten; when spindle muscle cells (intrafusal cells) contract, they shorten overall but the middle of the cell is stretched (middle does not have myofilaments). 


            B. Sensory innervation of muscle spindle - Two types -   Differ in distribution and responses. 


                        1. Primary (Ia) ending (called annulospiral because of its shape); one per spindle; innervates all spindle muscle cells; very large axon (12-20 micron diameter), very fast conduction of action potentials.


                        2. Secondary (II) ending (called flower spray ending) - variable number (typically 1-5); 6-12 microns diameter; fast but not as fast as Ia; innervate Static bag muscle cells and Chain muscle cells; do not innervate Dynamic bag muscle cells.


                        3.  Responses of muscle spindles - both Primary (Ia) and Secondary (II) endings respond best to passive stretch of muscle.


                                    a. Why? Spindle sensory neurons are mechanoreceptors.  Elongation of nerve endings activates stretch-sensitive ion channels; channels open when the membrane is mechanically stretched and produce a receptor current, which depolarizes the membrane potential and can trigger action potentials. 


Clinical Note: Muscle spindle sensory neurons can be very effectively activated by a tap on the tendon of a muscle when it is relaxed; produces a very fast, very small lengthening of the muscle; can activate all the muscle spindles in a muscle simultaneously; tapping on a tendon does NOT activate Golgi tendon organs.


                        4. Properties of Ia and II endings differ:


Note:  Ramp stretch is used to characterize responses of sensory neurons. Muscle is stretched at a constant rate (dynamic phase) and then held at a new length (static phase). 


                                    a. Primary (Ia) sensory neurons


                                                i. discharge intensely when stretch is occurring (dynamic phase); sensory neurons encode velocity of stretch during this time (the faster the stretch the more they fire); this is called the dynamic response


                                                ii. in the static phase, Ia neurons encode magnitude of change in length (how much the spindle has stretched); called the static response


                                    b. Secondary (II) sensory neurons only encode magnitude of the change in length (how much the spindle has stretched) and are much less sensitive to velocity in dynamic phase.


Problem: Active muscle contraction (produced by activating only alpha motor neurons) - Because intrafusal muscle cells are in parallel with extrafusal muscle cells, if only alpha motor neurons are activated,  intrafusal muscle cells go slack and both I and II sensory neurons stop discharging or decrease firing.  This is not very useful.

Solution: Alpha-gamma co-activation


                        5. Alpha-Gamma Coactivation - Gamma motor neurons (motor neurons that innervate only muscle spindles) are activated at the same time as alpha motor neurons; when gamma motor neurons are activated, they take up slack in intrafusal cells and spindle becomes taut; this is used to adjust spindle sensitivity to muscle length; sensory neurons then discharge when the muscle is stretched from any position.


Note: Alpha-Gamma Coactivation forms a mechanism for telling the difference between desired position or movement (set by gamma activity) and actual position or movement; if you move your arm or leg with regular muscle cells and spindle muscle cells contracting at the same rate, you can tell if you encounter an obstacle or perturbation.


                        6. Selective activation of different types of gamma motor neurons - Gamma dynamic motor neurons can selectively enhance sensitivities of Ia sensory neurons; Gamma dynamic motor neurons innervate dynamic bag muscle cells that receive only Ia sensory innervation; Firing gamma dynamic motor neurons takes up slack the in dynamic bag cells and brings Ia fibers closer to firing threshold; This increases Ia discharge to movement velocity and magnitude of change in length; Gamma dynamic motor neurons are activated in behaviors in which rapid compensation may be necessary (walking on a thin rail).  Gamma static motor neuron only increases discharges to magnitude of change in length NOT velocity (due to properties of spindle muscle cells).


IV. GOLGI TENDON ORGANS - sensory endings sensitive to force of contraction; size = 0.5 mm long by 0.1 mm wide.


            A. Structure and Force Detection - Golgi tendon organs are innervated by large sensory neurons (Type 1b) that end in muscle tendons or connective tissue attachments (near where muscle cells attach, called myotendinous junction).  Branches intertwine with collagen fibers. Large forces applied to the tendon cause it to become taut. The collagen fibers squeeze together (like a rubber band becoming thinner) causing endings to depolarize and neuron to discharge. This occurs when muscle contracts against a large load or when strong forces are applied to a contracted muscle.  Firing is maintained for as long as force is developed. Golgi tendon organs are IN SERIES with muscle fibers.


            B. Responses of tendon organs - when muscle contracts against a large load (isometric contraction), tendon organs fire intensely; when contract against a moderate load (isotonic contraction), tendon organ firing reflects amount of force needed to move load.  Passive stretch or tendon tap does NOT excite tendon organs. 






Number of sensory neurons per sense organ



Effect gamma dynamic motor neuron




motor neuron






All intrafusal muscle cells

Movement Velocity and Length

Increase  sensitivity


Increase sensitivity



Secondary II

1- 5

Chain Cells and Static Bag cells not Dynamic Bag cells

Length NOT velocity

No effect

Increase sensitivity

Golgi Tendon Organ



Muscle tendon at junction with muscle cells

Muscle Force



Note: Gamma Dynamic Motor Neurons innervate Dynamic Bag cells that only have Ia sensory neurons, not II sensory neurons






Muscle Spindle

Golgi Tendon Organ

Passive stretch relaxed muscle (rapidly)

Increase discharge

Little or no increase

Passive stretch contracted muscle (attempted)

No discharge

Increase discharge

Isotonic contraction (alpha motor neuron activation only)

Discharge decreases or stops

Discharge reflects load


Isotonic contraction (with alpha-gamma co-activate)

Maintain discharge

Discharge reflects load

Isometric contraction

No discharge

Increase discharge

Gamma motor neuron activate only

Increase sensitivity

(or produce discharge)

No increase

Note: 1) Isotonic contraction = lift an object; muscle length shortens, force is constant; 2) Isometric contraction = attempt to lift immovable object; muscle length constant, force is  large