Neuromuscular System

 

Nerves-

sensory nerves-send sensory information to the spine

motor nerves-send signals to the muscles

 

MOTOR UNIT- is the motor nerve and all of the fibers it innervates.

Different muscles have different innervation ratios-

1:10 for occular muscles- smaller motor units

1:10,000 for quadriceps muscles - larger motor units

This is what results in fine versus gross motor movement.

 

Can increase force of muscular contraction by increasing

1) spatial summation - bring in more motor units

2) temporal summation- make the motor units firing increase their increase the rate of firing.

 

Electromyography measures this firing rate.

Firing frequency and synchronization:

synchronization means that many motor units may fire at the same time which increases the chance of a powerful forceful movement.

temporal summation

spatial summation

 

Sensory Receptors:

proprioceptors- sensory receptors- sense changes in the environment.

 

Cutaneous receptors- deep pressure and touch

 

Joint and skin receptors- joint- ruffinni endings

skin- touch- (meissner)

pressure( pacinian)

pain(free nerve endings)

 

Musculotendinous receptors-

Golgi tendon organs- sensitive to stretch of the tendon due to muscular contraction and causes the muscle to relax.

Muscle spindle- responsive to active or passive stretch. as well as the rate (phasic) and length (tonic) of stretch.

Labyrinthine and neck proprioceptors-

labyrinthine system- orientation of the head

neck proprioceptors- orientation of the neck relative to the head.

 

 

REFLEXES-

preprogrammed motor patterns or nerve muscle patterns

- They do not require voluntary activation

 

Examples-

Stretch reflex- stretching the muscle results in more muscular force. This is due to activation of the muscle spindles which make the muscle fibers contract more strongly.

Reciprocal inhibition- inhibition of tension development in the antagonist muscles resulting from activation of muscle spindles.

Tendon reflex- when the tendon ( GTOs are stretched as well) is stretched the agonist relaxes and the antagonist contracts. This may inhibit follow through in a beginner trying to generate force from a throw.

Babinski reflex- if you tickle the bottom of the foot it will flex the toe upward. This dissapears in young children after the age of 6 months. Injury to the corticospinal tract (path from motor cortex to motor neurons) results in this flexion response after this age. (Taber, 1997)

Flexion reflex- pain stimulus results in withdrawal of limb

Grasp reflex- object in the hand- tend to grasp it- Larry's baby

Extensor thrust reflex- Pressure on the surface of the hands or feet tend to extent the limbs

Righting reflex- labyrinthine reflexes- inner ear

tend to lift up head as you start to fall over or if you are upside down.

Flexion of head ( head down) results in flexion of upper limbs, extension of lower ones.

Extension of head (head up) results in extension of upper limbs, flexion of lower ones.

Spinal reflexes- Walking is an example- pre programmed responses from the limbs to the spine and back again without travelling to the brain.

Crossed extensor reflex- as one limb flexes the contralateral limb extends.

Tonic neck reflex- look toward the extended arm, opposite arm flexes.

Reflexes can be overcome voluntarily

Reflexes may return in times of fear or motor cortex damage.

Electromyography (EMG) measures the electrical signal generated by a muscle. This can tell you about the patterns of muscle activity.

Unskilled movement results in more co-contraction during a movement which can be measured by EMG. This results in jerkier movements than skilled movement. You can see this with overlapping muscular activity.

Skilled movement is characterized by more sequential movement patterns.

EMG activity prior to movement- when landing muscles activate in preparation for landing.

Sometimes can get hurt more by unexpected hits because you are not braced to receive them.

 

There are two main types of EMG

 

Surface- electrodes are on the surface silver coated with plastic

1) measures from superficial muscles

2) measures from a large area of muscle

3) sometimes more than one muscle

 

Indwelling- electrode inserted into muscle

1) measures deep muscle

2) measures from a smaller area

3) can measure from a specific muscle or motor unit.

In a clinical setting:

1) Indwelling needle electrodes- or

2) fine wire electrodes-

 

 

Patterened electrical stimulation-The reverse of EMG - is used

to activate muscles on the verge of activation or to activate muscles that do not have voluntary control ( as in the case of parapalegics or individuals who have had a stroke)

 

Some things that will affect measurement accuracy of EMG:

1) electrode placement

2) fatigue

3) excitement level

4) muscle fatigue (ion levels)

5) temperature (emg decreases 2.4 m/s for every deg centigrade drop)

6)age (very young (less than 4 yrs) < adult> 60 or older)

EMG quantities vary from day to day

 

EMG cannot be used to determine muscle force except to determine force of an isometric contraction after it is normalized to the Maximum voluntary contraction of a particular muscle, on a particular day using an isometric contraction.

 

EMG can be used to determine:

1) when muscles are active

2) how active they are

3) changes in muscle activity levels by thinking about them (biofeedback)

4) nerve or muscle damage.

 

 

EMG can be used to measure

1) motor conduction (conduction from nerve to muscle)

2) sensory conduction (conduction between two points on the nerve)

3) radiculopathies(spinal cord nerve compression)

4) myopathies (muscle disease)

5) anterior horn cell disease (spinal cord disorders)

6) peripheral neuropathies (peripheral nerve disorders)

7) entrapment syndromes (nerve compression anywhere)

click below to see more about specific clinical EMG techniques:

http://www.teleemg.com/Chapters/mindex.htm

 

 Some EMG characteristics which are important in rehabilitation

1) looking for small or innapropriate EMG response.

Ex. continuous low level EMG

from electrical stimulation look for:

1) increase of decrease in amplitude

2) time delay

3) shape

 

Patterned Electrical Stimulation-

1) to produce movement which cannot be produced voluntarily

2) to keep nerve pathways active

3) perhaps lead to voluntary movement

 

Biofeedback

Multiple devices

1) using multiple devices increases the chance of getting reliable information

2) can verify the results of the other devices

3) limited by the number of channels

4) sometimes they can interact electronically.

analogue body signal > digital signal with computer readouts

 

Biofeedback Modalities

1)EMG- muscle activity measured as an electrical signal

2) peripheral temperature- a correlate of peripheral vasoconstriction- ex: Reynaud’s syndrome

3) heart rate

4) respiration

5) ECG- electrocardiogram-

6) EEG- electroencephalograph- brain wave activity

 

HR and Respiration are often positively correlated. When one goes up so does the other. Frequency of HR is a multiple of breathing- some people have stronger entrainment than others. Stronger link between HR and breath rate. There is some reason to believe that people with less entrained HR and breathing are less likely to get space sick.

 

BIOFEEDBACK-

1) monitor

2) measure

3) report- this can be in different modalities

sight

sound

 

 

Might want a person to imagine themselves doing something and then show them their metabolic or physiological responses while they are thinking about certain things.

They train biathletes to shoot in between heart beats.

 

Autonomic Nervous System

1) Sympathetic nervous system

causes nervous system stimulation when activated.

Fight or flight response ^ HR and ^ breathing rate

2) Parasympathetic nervous system

causes relaxation when activated decreased HR and breathing rate.

These nervous systems are primarily regulated by hormones in your body.

increased Norepinephrine increases sympathetic nervous system response.

 

Dive reflex- put your face in very cold water and HR , BP, breathing decrease. causes decreased oxygenation of tissues especially to the periphery of the body.

 

The Neuromuscular Junction

Calcium must be present during depolarization to produce transmitter released.

Acetylcholine- ACh- is the transmitter for the neuromuscular junction.

Time course for synaptic transmission

Figure 13-5

Other neurotransmitters such as (Norepinephrine, Serotonin, etc.)

act at the nerve junctions to affect many functions such as:

sleep wake cycles

depression

fluid shifts

growth

lactation

Table 14-2 and 14-3

 

Diseases of the Neuromuscular junction-

 

Myasthenia Gravis-(severe muscle weakness)- autoimmune response where antibodies are produced against ACh receptors.

Characterized by:

1) weakness of eyelids or eye muscles and limb muscles

2) weakness varies from day to day ( remission and exacerbation)

3) no EMG signs of denervation, or other motor unit problems

4) symptoms reversed by drugs that inhibit ACh-esterase (Edrophonium)

 

Lambert Eaton Syndrome-

  • amount of neurotransmitter is lost due to loss of Calcium channels.

     

  •  

    Diseases of the Motor Unit-

    These can happen anywhere along the motor unit-

    such as: neuron

    peripheral nerve

    muscle cell.

     

     

    Diseases of Motor neuron- neurons degenerate progressively-

    non-painful muscular weakness.

    Does not affect the sensory or autonomic nervous system.

    Starts with wasting of small muscles.

    Typically older men are the patients in this disease.

    Example: Lou Gherig’s disease or lateral sclerosis.

     

     

    Peripheral neuropathies- numbness, pins and needles, or tingling. Demylenating-myelin sheath degeneration. This leads to slowed nerve conduction velocities.(multiple sclerosis)

    Results in - 1)Fasciculation-one or more motor units twitch. Produces visible twitching. 2)Fibrillation-only one motor unit twitches- detectable only by EMG.

    This neuropathy can be either:

    1)Acute- Guillian Barre syndrome-(swine-flu shots)often after respiratory infection.

    2)Chronic- diabetes, b12 deficiency, lead poisoning, alcoholism, carcinomas of lung.

     

    Some factors can be fixed if the cause is changed.

     

    Myopathies- diseases of skeletal muscle- muscular dystrophies

    Four major types based on clinical and genetic patterns.

    1) Duchenne- starts in legs, male only, x-linked gene, fast loss.

    2) facioscapulohumeral- shoulder girdle and face, both sexes, adolescent, much milder, slower loss.

    3)Myotonic- myotonia- delayed relaxation of muscle after vigorous contraction caused by repetitive firing of muscle action potential.

    4) limb-girdle dystrophy- more than one type combined. Various onset ages and sexes, slow loss.

     

    In general, some losses are due to loss of physiology properties ( ability to use glycogen) rather than or in addition to, loss of muscle fibers.

     

    Reactions of neurons to injury-

    Severing or crushing a nerve can cause loss of function in the motor unit.

    1)Synaptic transmission is lost rapidly because the nerve terminal can't synthesize protein, needs transport by the cell body.

    2)Degeneration of distal axon occurs slowly- acutely damaged nerves distal to the injury may remain active up to 72 hours after injury.

    3)Glial cells and macrophages remove the debris.

    Nerve Growth Factor- is the best neurotrophic factor-maintains synaptic connections

    Alzheimer's disease may reduce NGF

    peripheral nerves can regenerate.

    glial scars can prevent regeneration of the axon.

     

    Electrical Stimulation:

    regenerating retinal axons of hamsters respond to

    1)light.

    2)But respond more highly to direct electrical stimulation to the nerve

    Patterned electrical stimulation is often used to try to get reactions from the brain and/or muscle after:

    1)Stroke- to form new neural connections to healthy brain tissue.

    2) Nerve crush or severing- to maintain muscle tone

     

     

    Parkinson's Disease

    Degeneration of the Basal Ganglia

    Reduction in dopamine, serotonin, norepinephrine

    Symptoms:

    tremor at rest,

    rigidity,

    akinesia ( difficulty initiating movement),

    bradykinesia (slowness in excuting movement),

    Treatment- l-dopamine

    http://www.nlm.nih.gov/medlineplus/parkinsonsdisease.html

     

    Alzheimer's

    70% of cases of dementia related to alzheimer's

    Atrophy of the cortex

    60-90% loss of ACh

    neurofibrillary tangles of the hippocampus- recent memory

    senile plaques

    neuronal cell loss

    decreased brain weight

    http://www.alzheimers.org/