Chapter 5

Chapter 5

Neuromuscular Concepts, Proprioception, Kinesthesis

Learning Outcomes: Students will be able to:

Identify the gross anatomy of the ANS, CNS, and PNS, and the spinal cord that serves as the connection between the CNS and PNS.
Label and describe two major ascending sensory neural tracts and one major descending motor neural tract.
Understand and differentiate between dermatome, myotome, cutaneous distribution and motor neuron muscle innervation.
Describe the function of joint and muscle/tendon proprioceptors, the associated responses when activated, and their relationship to injury prevention, posture, performance, and stretching.
Implement the knowledge of selected proprioceptors in performing static and PNF stretching to enhance muscle relaxation during stretching.
Explain and apply the American College of Sports Medicine guidelines for stretching in conjunction with the “Best Stretch” principle.

The Nervous System is a highly complex mechanism responsible for stimulating, controlling, and coordinating all other body systems. It is divided into the Central Nervous System (CNS), the Peripheral Nervous System (PNS), and the Autonomic Nervous System (ANS)

Autonomic Nervous System (ANS) – the ANS is s subcomponent of the PNS and is primarily responsible for controlling visceral structures and consists of the sympathetic nervous system (responses to stress) and the parasympathetic nervous system (conservation of energy through working to restore homeostasis). We will not discuss or study the ANS in this class.

Central Nervous System (CNS) portion of the nervous system consisting of the brain (cerebrum, brainstem and cerebellum) and spinal cord.

Brain: Made up of 3 components

Cerebrum: Responsible for highest mental functions. Is made up of right and left cerebral hemispheres. Each hemisphere has an outer cortex that is divided into four lobes, each with specific functions.

Brainstem: Divided into 3 parts (midbrain, the pons, and the medulla). Controls facial movements, hearing, balance. Also, regulates breathing, heart rhythms, blood pressure, and swallowing.

Cerebellum: Responsible for coordination of voluntary movement, muscle tone and posture.

Figure 1

Peripheral Nervous System (PNS) Portion of the nervous system containing the sensory and motor divisions of all the nerves throughout the body except those found in the CNS division.

There are 3 main types of neurons found in the nervous system. Afferent (sensory) neurons, efferent (motor) neurons, and interneurons. Interneurons are found primarily in the CNS and often connect sensory neurons to motor neurons to create a spinal reflex arc.

Motor neurons create a "motor unit" as each motor neuron innervates several muscle fibers. Some of these motor units are relatively small and some are large. For example, in the eye one motor neuron innervates about 5 muscle fibers. In the hand intrinsic muscles, one motor neuron may innervate 100 or fewer muscle fibers. In contrast, a motor neuron in the thigh may innervate 1,000-2,000 muscle fibers.

Motor units follow the "all or nothing" principle. This means if the threshold for stimulation of a motor neuron is achieved, then ALL of the muscle fibers within that motor unit will depolarize and start developing tension through actin and myosin cross-bridging. If a stimulus is not sufficient to depolarize the motor neuron, then NONE of the muscle fibers in that motor unit will be activated.

Muscle force production is altered via the nervous system through the stimulation of various motor units. Muscle force can be varied by:

Altering the frequency of stimulation of the same motor unit.

Recruiting smaller or bigger motor units in the muscle/muscle group.

Recruiting fewer or more motor units in the muscle/muscle group.

The recruitment pattern of motor units follows the size principle. The smaller neurons are easier to stimulate and are recruited first. These smaller neurons innervate Type I, or slow twitch muscle fibers. These Type I muscle fibers are fatigue resistant and considered endurance fibers. Larger neurons require a greater stimulus to recruit, so they are recruited last. These larger neurons typically innervate Type II muscle fibers, which are fast twitch or power muscle fibers.

As sensory and motor neurons converge together just outside the vertebral column, they form spinal nerves or nerve roots. There are 31 pairs of spinal nerves in the PNS: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1coccygeal. In addition, there are 12 pairs of cranial nerves which primarily are responsible for motor and sensory function of the face (the spinal accessory being an exception_.

As mentioned, these spinal nerves consist of both motor and sensory fibers. The motor fibers innervate specific muscles, while the sensory fibers receive stimuli from certain areas of skin, muscles, tendons, and joints. In the cervical and lumbar regions of the spine, these spinal nerves converge to form a network of neurons called a plexus. Each plexus is responsible for motor and sensory function of a particular area of the body.

3 MAIN NEURAL PLEXUSES

Cervical Plexus (C1-C4)

Responsible for motor and sensory function of the upper shoulders and neck.

Brachial Plexus (C5-T1)

Responsible for motor and sensory function of the upper extremities and most of the scapula.

Lumbosacral Plexus (L1-S1)

Responsible for motor and sensory function of the lower trunk and the lower extremities.

Figure 3

Thoracic Spinal Nerves (not a plexus)

Thoracic Spinal Nerves (T2-T12)

These nerves do not form a plexus but rather maintain a segmental relationship. Each nerve branches into a posterior and anterior ramus.

Anterior Rami – become the intercostal nerves innervating the anterior trunk and intercostal muscles (motor) as well as the skin of the anterior and lateral trunk (sensory).

Posterior Rami – innervate the skin (sensory) and muscles of the back (motor).

Spinal Cord – the common pathway between the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). It is composed of:

1. An inner area of grey matter where neural connections/synapses occur.

2. An outer area of white matter composed of ascending pathways (tracts) that carry sensory information up from lower levels of the spinal cord to higher levels of the spinal cord and/or brain, and descending pathways (tracts) that carry motor information down from the brain to the spinal cord or from higher levels of the spinal cord to lower levels of the spinal cord.

Sensory (afferent) Neurons: transmit impulses from receptors in the skin, joints, muscles, and other peripheral aspects of the body to the spinal cord, and in many cases up to the brain.

Motor (efferent) Neurons: transmit impulses away from the brain and spinal cord to muscle and glandular tissue.

Interneurons: central or connecting neurons that conduct impulses from sensory neurons to motor neurons and are located in the CNS.

Ascending & Descending Tracts of the Spinal Cord

FRONTAL LOBE

Primary Motor Cortex: responsible for voluntary control of skeletal muscle. Origin of upper motor neurons (UMN) that transmit motor impulses down through the internal capsule to the midbrain and pyramidal tracts. After passing through the pons they cross over to the other side of the spinal cord and enter the Lateral Cortical Spinal (LCS) Tract (descending pathway) ending in the anterior horn of the gray matter at desired level in the spinal cord. Here they synapse with a lower motor neuron (LMN) which then exits the spinal cord through the motor root to become a peripheral nerve that eventually innervates skeletal muscles (Figure 8).

Figure 8

PARIETAL LOBE

Primary Sensory Cortex: responsible for receiving and interpreting sensory and proprioceptive input/information. Here is where meaning/understanding is given to sensory information received from the periphery and other parts of the brain. For example – movement, pain, temperature, proprioception, touch, hearing, sight, sensations, taste, etc.

Type C Sensory Neuron: carries sensory information (pain, temperature) from the skin through the sensory root to the posterior horn of gray matter. It then crosses midline and enters the Lateral Spinothalamic (LS) Tract (ascending pathway) traveling up to the thalamus where it is relayed to the Primary Sensory Cortex for interpretation (Figure 9).

Type A Sensory Neuron: carries sensory information (proprioception, vibration) from skin, muscles, tendons, and joints through the sensory root to the posterior horn of gray matter. It then crosses midline and enters the Dorsal Column Medial Leminiscal (DCML) Tract (ascending pathway) traveling up to the Thalamus where it is finally relayed to the Primary Sensory Cortex for interpretation (Figure 9).

Sensory: Dermatomes & Cutaneous Distribution

As previously mentioned, spinal nerves have motor fibers and sensory fibers. The motor fibers innervate certain muscles, while the sensory fibers innervate specific patches of skin.

Dermatome: A patch or band area of skin innervated by the sensory fibers from a single nerve root level.
Cutaneous Distribution: A patch or band of skin innervated by sensory fibers form a specific peripheral nerve.

Right Side of Body: Dermatomes (nerve roots) Left Side of Body: Cutaneous Distribution Patterns (peripheral nerves)

Figure 10

To test for nerve root damage, the corresponding dermatomes supplied by that nerve root may be tested for abnormal sensation. The following are considered to be abnormal sensations: Hypoesthesia = decreased sensation, Hyperesthesia = increased sensation, Anesthesia = reduced sensation of pain via pain medicine/drugs, and Paresthesia = numbness, tingling, burning sensations.

Motor: Myotomes & Motor/Muscle Distribution

UE Myotomes

C1, C2 = Neck flexion

C3 = Neck lateral flexion

C4 = Scapular elevation

C5 = Shoulder abduction

C6 = Elbow flexion, wrist ext.

C7 = Elbow extension, wrist flex.

C8 = Thumb extension

T1 = Finger abduction

LE Myotomes

L1, L2 = hip flexion

L3 = knee extension

L4 = ankle dorsiflexion

L5 = big toe extension

S1 = ankle plantarflexion,

foot eversion

S2 = knee flexion

Motor Distribution

Example:

Musculocutaneous Nerve – Biceps Brachii, Brachialis, Coracobrachialis

The Cutaneous Distribution of the Musculocutaneous nerve is highlighted in red along the lateral forearm.

Figure 11

The myotomes may be tested, in the form of isometric resisted muscle contraction, for weakness of a particular group of muscles (innervated predominantly by motor fibers from a single nerve root level). Results may indicate lesion to a specific spinal cord nerve root level, intervertebral disc herniation pressing a specific spinal nerve root, or a specific level intervertebral foramen stenosis.

Note: Nerves and nerve roots are typically injured through compression or tensile/stretching forces. When a nerve root in the brachial or lumbosacral plexus is damaged, certain patterns of motor and sensory deficits occur in the corresponding limbs. Dermatome and myotome testing is used to evaluate these deficits.

Proprioception (Sensory Neurons)

Proprioception – the process/mechanism by which the body is able to sense movement, joint position and applied force. This helps in the regulation of posture, position, and movement by responding to stimuli originating in the proprioceptors (sensory receptors located in the skin, joints, muscles, and tendons). They provide feedback relative to muscle tension and length, contraction state of muscle, position of the body and limbs, and movements of joints.

There are many types of proprioceptors found throughout the body. We will discuss two major categories.

Joint Proprioceptors

Muscle/Tendon Proprioceptors

Located in and around joint capsules
Provide information about the joint’s static position and its dynamic movement
The two major types of fascial/joint proprioceptors are:

-- Pacinian Corpuscles

-- Ruffini’s Endings

Located within skeletal muscles
Provide proprioceptive awareness about the position and movement of the body and create proprioceptive reflexes to protect muscles and tendons from injury
Two major types of muscle proprioceptors are:

-- Muscle Spindles

-- Golgi Tendon Organs

Joint Proprioceptors are also called mechanoreceptors because they are sensitive to mechanical forces that “deform” the proprioceptors. When the position of a joint changes, the soft tissues around the joint are compressed on one side of the joint and stretched on the other side of the joint. This deformation causes the proprioceptors to be activated, sending a signal to the CNS. Based on which side(s) of the joint has the proprioceptors stimulated and in what pattern this stimulation occurs, the CNS is able to determine which position the joint is in.

Example: When performing knee flexion, the joint proprioceptors located on the anterior side of the joint are stretched (deformed) and the joint proprioceptors located on the posterior side of the joint are compressed (deformed), both sending a signal to the CNS. The CNS interprets the incoming stimuli, knowing that the proprioceptors on the anterior and posterior side of the joint were stimulated and the pattern of how this firing occurred, thus the interpretating that the joint is being flexed. If the joint action had been extension instead, the pattern of anterior and posterior receptors would have interpreted the position as extension. Similarly, impulses from lateral and medial proprioceptors would signal abduction/adduction type movements. Medial and lateral rotational movements would be indicated by their characteristic pattern.

Pacinian corpuscles: provide proprioceptive information only about the movement of the joints.

Ruffini endings: provide proprioceptive information about the movement of the joints and the static end position of the joint movement.

Muscle Proprioceptors are protective in nature and can assist with performance.

Muscle spindles are located within the muscle belly and are sensitive to the amount of stretch and the rate (or speed) of stretch within the muscle. When a muscle is stretched extensively or rapidly, the muscle spindle is also stretched and becomes stimulated, creating an impulse in a sensory neuron that is sent to the spinal cord alerting the CNS that the muscle has been stretched. To prevent the muscle from being over stretched the impulse junctions with a motor neuron in the spinal cord that goes back to the muscle being stretched causing a reflex contraction of the muscle. This reflex is called the stretch reflex or myotatic reflex. Because the reflex increases the strength of muscle contraction immediately after it has been quickly stretched, it can also enhance performance where increased muscle contraction force is desirable. At the same time the stretched muscle is stimulated to contract, an inhibitory signal is sent to the opposite muscle group causing reciprocal inhibition.

Golgi tendon organs (GTO) are located within a tendon of a muscle near the musculotendinous junction. When a muscle contracts and shortens, or is stretched and lengthens, a “pulling force” is placed on the golgi tendon organ. If sufficient tension is placed on the tendon the GTO will be activated. Once activated a sensory neuron that carries the signal to the spinal cord to alert the CNS that the muscle has increased tension in it. Because the increased tension may cause a tearing of the muscle, this impulse junctions with an inhibitory neuron in the spinal cord causing a reflex relaxation of the muscle (autogenic inhibition). This reflex is called the tendon reflex or inverse myotatic reflex. Because this reflex causes muscle relaxation it can help increase the effectiveness of a stretch when trying to improve flexibility.

Stretching Techniques, Proprioceptors, & Reflex Activation

There are basically four stretching techniques (Ballistic, Static, Dynamic, and Proprioceptor Neuromuscular Facilitation). All four types of stretching have been shown to be effective in developing flexibility. However, the use of ballistic stretching to develop flexibility has been controversial because of its activation of the muscle spindle and resulting stretch reflex which may be considered to increase the risk of microscopic muscle membrane tears.

Ballistic Stretching Technique: stretching where there is a rapid bouncing movement in and out of the stretch position, where the end position is not held.

2. Static Stretching Technique: stretching is slow and constant where the end position is held. May be performed actively or passively.

3. Dynamic Stretching Technique: rhythmic moving of the limbs through a ROM generating heat from muscle

contraction and promoting blood flow during the activity.

4. Proprioceptor Neuromuscular Facilitation (PNF) Stretching Technique: stretching performed usually with a

partner and involves both passive movement and active (concentric and isometric) muscle actions in an attempt to engage the muscle

proprioceptors and either autogenic or reciprocal inhibition or both.

Note: All stretching techniques require movement of a body segment to a point of resistance in the ROM. This stretching movement can be done either actively or passively.

Active stretch: when the person stretching supplies the force for the stretch.
Passive stretch: when a partner or stretching machine provides the force for the stretch.

Muscle Spindles & the Myotatic Reflex (Stretch Reflex)

During a ballistic stretching movement, the muscle spindle is activated and a sensory neuron sends an impulse to the spinal cord, where it synapses with a motor neuron which carries a message back to the muscle being stretched causing it to contract. This is called a myotatic reflex (or stretch reflex). Note: Both the myotatic reflex and the recoil effect of stored elastic energy (fascia tissue) contribute to the force of movement.

Muscle spindles are sensitive to the length and rate of change in muscle length. As a load increases, the muscle is stretched to a greater extent, and engagement of muscle spindles results in greater activation of the muscle. This muscle activation process is thought to help protect against muscle tearing, aid in athletic performance, maintaining of functional position, and in maintaining upright positioning.

Figure 14

GTO & the Inverse Myotatic Reflex (Tendon Reflex)

During a static stretch movement (sustained for at least 10 seconds), tension increases within the muscle and a sensory neuron from the Golgi tendon organ (GTO) is activated. Once activated, it sends a signal to the spinal cord where it synapses with an inhibitory interneuron in the spinal cord, which in turn synapses with and inhibits the motor neuron that innervates the same muscle. This is known as Autogenic Inhibition (“self-inhibiting”). The resultant reduction in tension is called the tendon reflex or inverse myotatic reflex.

Note: Because of the slow rate of change of the muscle length the muscle spindles are not activated. Thus, the neural input from GTOs inhibits muscle activation. This inhibitory process is thought to provide a protective mechanism from development of excessive tension and potential muscle/tendon tearing.

Figure 15

Proprioceptive Neuromuscular Facilitation (PNF) Stretching

PNF stretching was originally developed as part of a neuromuscular rehabilitation program designed to relax muscles with increased tone. It has since expanded to the training of athletes, as a method of increasing muscular flexibility. PNF techniques are usually performed with a partner and involve both passive movement and active (concentric and static) muscle contractions. PNF stretching may be superior to other stretching methods because they further facilitate muscular inhibition. Note: Because PNF stretching procedures are designed to affect the contractile components of muscle, not the non-contractile connective tissues, they are more appropriate when muscle spasm limits motion and less appropriate for stretching long-standing, fibrotic contractures.

PNF Hold-Relax (HR) “OR” PNF Contract-Relax (CR)

In this technique the limb/joint is moved to the stretch position and held (static stretch). Then the patient performs an isometric muscle contraction (Hold) “OR” the limb is allowed to move slowly through a ROM via a somewhat resisted concentric muscle contraction (Contract) of the muscle being stretched. The duration of “hold or contract” is approximately 10 seconds, sufficient to activate the GTO and is then followed by the voluntary relaxation (Relax) of the muscle being stretched during the subsequent passive stretch to a further ROM.

Agonist-Contract (AC)

The Agonist-Contract technique usually follows immediately after the Hold-Relax (HR-AC) or Contract-Relax (CR-AC) maneuver, where a concentric muscle contraction of the agonist (opposing muscle to the muscle being stretched) is performed immediately after the Hold-Relax or Contract-Relax phase. This agonist contraction results in reciprocal inhibition (relaxation of the muscle being stretched via activation of the associated inhibitory interneuron). The limb is then moved further into the ROM and a great stretch length is achieved.

A typical PNF stretching session of a particular joint/muscle might look something like this:

1. Warm up the soft tissues/muscles to be stretched by the application of local heat or by active, low intensity exercises and then place the patient in a comfortable, stable position that allows the correct plane of motion in which the movements will be performed.

2. Slowly move the patient’s limb/joint into the stretch position and hold it there (static stretch) for approximately 30 seconds.

3. Then instruct the patient to contract the muscle being stretched and either resist (isometric contraction or Hold) or allow some ROM (concentric contraction or Contract) for approximately 10 seconds followed by instructing the patient to relax and then move the limb/joint through a greater ROM and hold the new position for approximately 30 seconds. You may perform this more than one time.

4. Following a Hold-Relax or Contract-Relax cycle immediately have the patient concentrically contract the agonist muscle (Agonist Contract), which is opposite the muscle being stretched, which will bring the limb/joint through an even further ROM which should then be held at that new greater ROM position for approximately 30 seconds.