NERVOUS CONTROL OF MUSCULAR MOVEMENT STRUCTURE AND FUNCTION OF THE NERVOUS SYSTEM
The information posted on this page is detailed information about the central nervous system and muscular movement structure of the human body. This information will not be covered in this kind of detail in any Red Cross first aid course (more information). This information is supplemental information to the material covered in the basic first aid courses, in particular, head and spinal injuries.
Divisions of The Nervous System
A. Central Nervous System
1. Brain – newer more sophisticated regions are piled on top of older, more primitive regions
(i) Cerebrum constitutes about 80% of total brain weight – cerebral cortex, basal nuclei
(ii) Diencephalon – thalamus, hypothalamus
c) Brainstem – continuous with the spinal cord – medulla, pons, midbrain
2. Spinal cord – long cylinder of nerve tissue which extends down from the brainstem to the second lumbar vertebrae. 45 cm long and 2 cm in diameter. Protected by the vertebral column and associated ligaments and muscles, the spinal meninges and the cerebrospinal fluid.
B. Peripheral Nervous System
Consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves
- Afferent division – conveys information from the sensors in the periphery to the central nervous system (CNS)
- Efferent division
- Somatic nervous system – nerve fibres innervate skeletal-muscle
- Autonomic nervous system – nerve fibres innervate smooth and cardiac muscle and glands
(i) sympathetic division
(ii) parasympathetic division
Neuroglia – comprise about 90% of the cells within the CNS. They occupy about half of the volume of the brain. The four major types of glial cells serve as the connective tissue of the CNS and as such help support the neurons both physically and metabolically. It is estimated that there are approximately 100 billion neurons in the brain and one trillion neuroglia. Along with the endocrine system, the nervous system regulates and coordinates the various functions of the body.
Basic Structure of a Nerve
- Neuron – a nerve cell. A neurons is specialized to transmit electrical signals. It consists of:
- Cell body – soma – contains the nucleus
- Axon – a long fiber that conducts impulses away from the cell body. The term “nerve fiber” is generally used in reference to an axon.
- Dendrite – short projections from the cell body that transmit impulses toward the cell body
The main purpose of the neuron is to pass messages (impulses) from one part of the body to another
- Myelin sheath – a discontinuous sheath around the axon. It is composed mainly of lipid and protein.
- Nodes of Ranvier – spaces between the segments of myelin sheath –> saltatory conduction
- Myelinated nerve fibers have much faster conduction velocities than un-myelinated fibers
- Synapse – the connection of an axon of one nerve to the cell body or dendrites of another nerve.
Neurons can be divided into 3 functional classes:
- Afferent neurons – carry impulses from the sensory receptors into spinal cord or brain
- Efferent neurons – transmit impulses from the CNS out to the effector organs – muscles (motor neurons) and glands
- Interneurons – lie entirely within the CNS. They account for 99% of all nerve cells.
Each spinal nerve is actually a nerve trunk – it contains hundreds of individual afferent and efferent nerve fibers that are bound together by connective tissue sheaths.
The Nerve Impulse
Resting membrane potential – due to the selective permeability characteristics of the nerve cell membrane, a potential difference (voltage) exists between the inside and outside of the nerve fiber. A high concentration of positive sodium ions on the outside of the nerve membrane causes it to be electrically positive, while the inside of the nerve is electrically negative.
Action potential – an appropriate stimulus suddenly causes sodium ions to rush to the inside of the nerve –> reversal of polarity. Once the action potential is started, it spreads along the entire length of the nerve fiber.
Nerve to Nerve Synapses
Nervous information is relayed across the synaptic cleft by means of a chemical transmitter substance. Transmitter substances can be either excitatory or inhibitory in their effects on the post synaptic membrane potential
Spatial vs. temporal summation
Neuromuscular junction – nerve to muscle synapse. The chemical transmitter substance is acetylcholine.
The spinal cord is enlarged in two regions for innervation of the limbs:
- a) The cervical enlargement which extends from the C4 through T1 segments of the spinal cord
- b) The lumbosacral enlargement which extends from the T11 through L1 segments of the spinal cord
Plexus – a network of converging and diverging nerve fibers, or blood vessels. The brain and spinal cord are composed of gray matter and white matter. The nerve cell bodies lie in and constitute the gray matter while the interconnecting tracts of nerve fibers (axons) form the white matter.
Structure of spinal nerves – 31 pairs of spinal nerves are attached to the spinal cord – 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. Each spinal nerve has a dorsal root and a ventral root connected to the spinal cord. The dorsal roots contain afferent (sensory) fibers that carry information from the periphery to the spinal cord and brain. The ventral roots contain efferent (motor) fibers to the skeletal muscle. The cell bodies of the motor axons making up the ventral roots are located in the ventral gray horns of the spinal cord The cell bodies of the sensory axons making up the dorsal roots are outside of the spinal cord in the spinal ganglia.
Ganglion – a collection of nerve cell bodies located outside of the CNS.
Spinal cord injury – transection of the spinal cord results in loss of all sensation and voluntary movement inferior to the point of damage. The patient is quadriplegic if the cord is transected superior to C5. If the transection is above C4, the patient may die of respiratory failure. The patient is paraplegic – paralysis of both lower limbs – if the transection occurs below the cervical segment of the spinal cord.
Deficiency of blood supply (ischemia) to the spinal cord caused by fractures, dislocations, atherosclerosis, etc. affects its function and can lead to muscle weakness and paralysis. When the brain or spinal cord is damaged, in most cases the injured axons do not recover.
MUSCLE SENSE ORGANS
Proprioceptors – conduct sensory information to the CNS from muscles, tendons, ligaments, and joints –> kinesthetic sense
A. Muscle Spindles
Structure – several modified muscle fibers, four to 10 millimeters in length, contained in a capsule, with a sensory nerve spiralling around its center. Spindle fibers (intrafusal fibers) lie parallel to the regular fibers (extrafusal
Function – send information to the CNS regarding the degree of muscle stretch –> activation of the exact number of motor units to overcome a given resistance. With increasing degrees of stretch of the muscle spindle, the frequency of impulse transmission up the afferent neuron to the spinal cord progressively increases.
Three ways that the muscle spindle can activate the alpha motor neurons to cause the muscle to contract:
- Tonic stretch – concerned with the final length of the muscle fibers
- Phasic stretch – spindle responds to the velocity of the change of length
- Gamma system – gamma efferent fibers innervate the contractile ends of the intrafusal fibers. When the alpha motor neurons are activated, the gamma motor neurons are also activated (coactivation).
Gamma system provides the mechanism for maintaining the spindle at peak operation at all muscle lengths.
Muscle spindles are distributed throughout the muscle. Their density varies with the degree of control required by a given muscle.
B. Golgi Tendon Organs
Location – encapsulated in tendon fibers near the junction of the muscle and tendon fibers. They are approximately one millimeter long and 0.1 millimeters in diameter. They are in series with the muscle fibers rather
than in parallel as are muscle spindles. When a muscle contracts, the GTO is stretched.
Functions – the firing rate of the GTO is very sensitive to changes in the tension of the muscle.
- Sensory input from GTO about the tension produced by muscles is useful for a variety of motor acts, such as maintaining a steady grip on an object.
- When stimulated by excessive tension or stretch –> send sensory information to the CNS –> causes the contracted muscle to relax (reflex inhibition) – protect the muscle and its connective tissue harness from damage due to excessive loads
C. Joint Receptors
Supply information to the CNS concerning joint angle, acceleration of the joint, etc.
CONTROL OF MOTOR FUNCTIONS
The cerebral cortex and cerebellum are the main centers employed in learning new motor skills. These areas of the brain initiate voluntary control of movement patterns.
Primary motor cortex – located at the rear of the frontal lobe of the cerebral cortex.
Stimulation of different areas of the primary motor cortex brings about movement in different, specific areas of the body. Contains the motor homunculus. However, no coordinated movement can be elicited. The motor cortex on each side of the brain primarily controls muscles on the opposite side of the body.
Pyramidal tract – corticospinal pathway – long axons which carry impulses from the primary motor cortex where their cell bodies are located directly to lower motorneurons in spinal cord —> spinal nerves. The corticospinal system primarily mediates performance of fine, discrete, voluntary movements of the hands and fingers.
Premotor cortex – one of the three higher areas that command the primary motor cortex. Located on the lateral surface of each cerebral hemisphere in front of the primary motor cortex.
Extra-pyramidal tract – multineuronal pathways – route used to send impulses from the premotor area down to the lower motorneurons of the spinal cord. Instead of synapsing directly with motor neurons, this pathway involves many of the other brain regions, including the cerebellum. These pathways are more concerned with posture and coordination of large muscle groups.
Considerable complex interaction and overlapping of funcion exist between these two systems.
Located behind the brainstem. It functions by means of intricate feedback circuits to monitor and coordinate other areas of the brain involved in motor control. It receives signals concerning motor output from the cortex and sensory information from receptors in muscles, tendons, joints and skin, as well as from visual, auditory and vestibular end organs.
Function – the major comparing, evaluating, and integrating center for postural adjustments, locomotion, maintenance of equilibrium, perceptions of speed of body movement, and general motor coordination.
Damage to the cerebellum results in impaired motor control.