The Nervous System

Divisions of the nervous system

Neuroanatomic references

The nervous system mediates a tremendous range of functions:

• the unconscious control of visceral functions
• sensory perceptions
• voluntary movement
• behavior,
• еmotions
• abstract thinking.

There is great regional specialization within the nervous system.

Classification of nervous system:

The CNS consists of spinal cord and the brain .

The PNS consists of neurons and glia, which are located outside the meninges. Structures in the PNS include spinal nerves, cranial nerves, nerve ganglia and sensory receptors.

The brain has three major parts:

1. The brainstem is a conduit for the flow of information between the cerebrum and the spinal cord. The brainstem includes the medulla oblongata, pons and midbrain. It is a site of regulation of vital body functions, including breathing and consciousness. The loss of brainstem function is fatal.
2. The cerebrum is the seat of thoughts, perceptions, and voluntary actions.
3. The cerebellum is a key part of the motor system that is required for the maintenance of equilibrium and the coordination of muscle actions.

1. The spinal cord

The spinal cord is attached to the brainstem and passes downward within the bony vertebral column. If the spinal cord is transected, there is total loss of sensation and paralysis of muscles below the affected spinal level.

Segments:

1.        Cervical: CI – CVIII 
2.        Thoracic: ThI – ThXII
3.        Lumbar: LI – LV
4.        Sacrum: SI – SV
5.        Coccyx: CoI

The functions of the spinal cord:

1.         the major conduit of sensory and motor information between the brain and the periphery
2.         In the spinal cord some reflexes localized (stretch reflexes, protective and autonomic reflexes).

The spinal cord is connected to the periphery via spinal nerves, which are part of the peripheral nervous system.

Each spinal nerve attaches to the spinal cord by two branches, a dorsal root and a ventral root.

Dorsal roods provide afferent (sensory) information (to the spinal cord), Ventral roods provide efferent (motor) information (from the spinal cord).

The localization of spinal cord into the spinal canal

A horizontal view through the spinal cord reveals a characteristic pattern of gray matter and white matter. Gray matter consists of nerve cell bodies (reflex function); white matter consists of nerve axons covered in myelin (conduction function).

The neurons of spinal cord:

1.         The dorsal horn receives afferent input from the dorsal nerve roots (sensitive function).
2.         The reticular formation is located between the lateral and posterior horns; it performs activating function.
3.         The lateral horns are the centers of autonomic (vegetative, involuntary) nervous system.
4.         The ventral horn sends efferent axons to the ventral nerve roots. The ventral horn contains motoneurons that innervate muscles.

The white matter consists of several defined nerve tracts arranged in columns that carry information either to or from the brain:

•        The dorsal columns carry only sensory information via ascending tracts.
•        The ventral and lateral columns convey both sensory and motor information via ascending and descending tracts, respectively.

The spinal cord performs reflex and conduction functions.

There are the centers of all motor involuntary reflexes (except head musculature) and the centers of vegetative sympathetic and partly of parasympathetic nervous system: vasomotor and perspiration centers that regulate the functions of heart and bronchi, urination and defecation (act), and genital organs action.

2. Medulla

There are nuclei of the VIII-XII cranial nerves; so it is responsible for appropriate functions (Hearing, Gag sense and swallowing, Parasympathetic functions and coughing, Shrugging shoulders, Tongue protrusion).

The medulla oblongata is linked with chemoreceptors and baroreceptors of vessels, interorecepters of internal organs and vestibuloreceptors. Due to these links the medulla oblongata controls the functions of respiratory and cardiovascular systems.

There is a caudal part of reticular formation in the medulla oblongata; it performs the activating functions.

The medullary pyramids are the site of decussation (crossing from one side to the other) of this motor pathway, which explains why movement of one side of the body is controlled by the opposite cerebral hemisphere. These tracts carry axons of motor neurons that command movement; the corticospinal tract travels from the motor cortex to the spinal cord.

The functions of medulla oblongata:

3. Hindbrain structures

The rostral half of the hindbrain (metencephalon) becomes the pons and the cerebellum.

( A ) The pons

The anterior (ventral) surface is called medullary pyramids.

These tracts carry axons of motor neurons that command movement; the corticospinal tract travels from the motor cortex to the spinal cord. 

Locus coeruleus - consists of noradrenergic neurons. It is responsible to physiological reaction of tension and anxiety; it also participates in mechanisms of sleep. It is “alarm center” that becomes most active when new environmental stimuli appear. 

The functions of pons:

•        conduction (prevail);

•        provides pose maintenance and keeps equilibrium of body;

•        provides neck muscles tonus;

•        regulates mastication and swallowing;

•        plays an important role in activation of the cortex (including the state of anxiety);

•        limits sensory inflows of nerve impulses to cerebral hemispheres during sleep.

( B ) The cerebellum

The cerebellum (“little brain”) receives an enormous number of axons from both the forebrain and spinal cord. Descending inputs from the forebrain relay information about the intentions for movements, and ascending inputs from the spinal cord relay information about the position of the body in space (known as proprioception).

The cerebellum processes this information to determine the  correct sequence of muscle contractions needed to coordinate accurate movement.

4.Midbrain structures

( A ) The tectum

The dorsal part of the midbrain is called the tectum, which has two conspicuous pairs of swellings:

1. The superior colliculus receives sensory input directly from the eye, and is involved in the control of eye movements.

2. The inferior colliculus is a relay station for sensory input from the ear before it passes to the thalamus and onward to the cortex.

These centers provide startle response in the body – momentary mobilization of all body at eager activity at occurrence of danger.

•    The ventral part of the midbrain is called the tegmentum (or cerebral peduncles), and contains two areas involved in motor control, the red nucleus and the substantia nigra

( B ) Red nuclei

provide coordination and automatism of motions, muscle tonus

( C ) Substantia nigra

one of the brain sites that produce dopamine. Dopamine is a transmitter of neurons in the cerebral “support system”. It is naturally produced in large quantities during positive (by human subjective conception) experience – delicious food intake, pleasant ‘touch’ sensations, and so on. In addition the substantia nigra regulates muscular tone (especially during sleep), provides homeostasis; it is a part of antinociceptive system and system that forms sleep.

 5. The reticular formation and diffuse modulatory systems

The reticular formation is an anatomic concept that describes networks of neurons that extend throughout the core of the brainstem. Neurons in the reticular formation determine the level of CNS activity.

The activity of neurons in the reticular formation has an important influence on the degree of wakefulness because lesions may result in loss of consciousness and even coma.

6. The thalamus

The main function of the thalamus is to relay motor and sensory signals to the cerebral cortex. The thalamus divides sensory signals by significant (they come to the cortex) and insignificant (they inhibit).

7. The hypothalamus

The hypothalamus is a command center for the ANS, and also for much of the endocrine system, through its control of the pituitary gland. It is a key site for the control of homeostatic functions and motivated behaviors, including eating, circadian rhythms, and the sex drive.

The homeostatic functions of the hypothalamus:

•        Secretion of hormonal factors controlling the pituitary gland

•        Activation of sympathetic nervous system

•        Water and electrolyte balance

•        Body temperature regulation

•        Eating behavior

•        Drinking behavior and thirst

•        Sexual behavior

•        Circadiam rhythms

8. The limbic system

The limbic system is a functional concept, which includes a series of structures that are intimately associated with the experience and control of emotions and with learning and memory. 

The hippocampus is important in the conversion of short-term memory to long-term memory.

The amygdaloid nucleus (amygdala) is involved in strong emotions, including fear and aggression, and linking emotions with memories.

Dysfunction of the limbic system and diffuse modulatory systems underlie psychiatric diseases such as major depression, bipolar disorder (or manic–depressive disorder), and schizophrenia.

9. Forebrain structures

Many areas of the cortex can be assigned specific functions (e.g., vision or somatic sensation). Large areas of cortex cannot be assigned specific functions and are called association cortex; these areas are involved with interpretation and meaning of sensory information, formation of intentions, and “higher functions” of the mind (thinking, intellect).

10. The autonomic nervoussystem