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Organization of the Nervous System

Although terminology seems to indicate otherwise, there is really only one nervous system in the body. Although each subdivision of the system is also called a “nervous system,” all of these smaller systems belong to the single, highly integrated nervous system. Each subdivision has structural and functional characteristics that distinguish it from the others. The nervous system as a whole is divided into two subdivisions: the central nervous system (CNS) and the peripheral nervous system (PNS).

The Central Nervous System

The brain and spinal cord are the organs of the central nervous system. Because they are so vitally important, the brain and spinal cord, located in the dorsal body cavity, are encased in bone for protection. The brain is in the cranial vault, and the spinal cord is in the vertebral canal of the vertebral column. Although considered to be two separate organs, the brain and spinal cord are continuous at the foramen magnum.

The Peripheral Nervous System

The organs of the peripheral nervous system are the nerves and ganglia. Nerves are bundles of nerve fibers, much like muscles are bundles of muscle fibers. Cranial nerves and spinal nerves extend from the CNS to peripheral organs such as muscles and glands. Ganglia are collections, or small knots, of nerve cell bodies outside the CNS.

The peripheral nervous system is further subdivided into an afferent (sensory) division and an efferent (motor) division. The afferent or sensory division transmits impulses from peripheral organs to the CNS. The efferent or motor division transmits impulses from the CNS out to the peripheral organs to cause an effect or action.

Finally, the efferent or motor division is again subdivided into the somatic nervous system and the autonomic nervous system. The somatic nervous system, also called the somatomotor or somatic efferent nervous system, supplies motor impulses to the skeletal muscles. Because these nerves permit conscious control of the skeletal muscles, it is sometimes called the voluntary nervous system. The autonomic nervous system, also called the visceral efferent nervous system, supplies motor impulses to cardiac muscle, to smooth muscle, and to glandular epithelium. It is further subdivided into sympathetic and parasympathetic divisions. Because the autonomic nervous system regulates involuntary or automatic functions, it is called the involuntary nervous system.

The Central Nervous System

The central nervous system (CNS) is one of the major divisions of the nervous system, responsible for integrating and processing information. It consists of the brain and the spinal cord. The CNS plays a crucial role in coordinating and regulating various bodily functions and behaviors.

1. Brain: The brain is the command center of the nervous system and is responsible for complex cognitive functions, sensory processing, motor control, and regulation of bodily functions. It is protected by the skull and is divided into different regions, each with specific functions. The brain can be broadly categorized into the following regions:

Cerebrum: The largest part of the brain, responsible for higher-level functions such as conscious thought, memory, perception, and voluntary movements.

• Cerebellum: Located at the back of the brain, it is involved in coordinating movements, balance, and posture.
• Brainstem: It connects the brain to the spinal cord and controls essential functions such as breathing, heart rate, and blood pressure.

2. Spinal Cord: The spinal cord is a long, cylindrical bundle of nerve fibers that extends from the brainstem down the spinal column. It serves as a pathway for transmitting signals between the brain and the rest of the body. The spinal cord also carries out certain reflex actions independently, without involving the brain. It is responsible for relaying sensory information from the body to the brain and transmitting motor commands from the brain to the muscles and organs.

The CNS is protected by various structures:

• Skull: The brain is encased within the rigid skull, which provides protection against physical trauma.
• Meninges: The CNS is surrounded and cushioned by three layers of protective membranes called meninges (dura mater, arachnoid mater, and pia mater).
• Cerebrospinal Fluid (CSF): The brain and spinal cord are bathed in cerebrospinal fluid, a clear fluid that circulates around them, providing buoyancy and additional protection.

The CNS functions by receiving and integrating sensory input from the body, processing and interpreting the information, and generating appropriate motor responses. It also plays a vital role in higher-order cognitive functions, emotions, and memory. The CNS works in coordination with the peripheral nervous system (PNS) to regulate and control the body’s functions and responses to the environment.

Brain

The brain is the most complex and vital organ in the human body. It is responsible for controlling and coordinating various bodily functions, processing sensory information, and enabling higher cognitive functions. The brain is protected by the skull and consists of several interconnected regions, each with specialized functions. Here are some key aspects of the brain:

1. Cerebrum: The cerebrum is the largest and most highly developed part of the brain. It is divided into two hemispheres (left and right) and is responsible for higher-level cognitive functions such as conscious thought, perception, memory, language, reasoning, and voluntary movements. The outer layer of the cerebrum is called the cerebral cortex, which is highly convoluted and densely packed with billions of neurons.

   The cerebrum is divided by a deep longitudinal fissure into two cerebral hemispheres. The two hemispheres are two separate entities but are connected by an arching band of white fibers, called the corpus callosum that provides a communication pathway between the two halves.

   Each cerebral hemisphere is divided into five lobes, four of which have the same name as the bone over them: the fontal lobe, the parietal lobe, the occipital lobe, and the temporal lobe. A fifth lobe, the insula or Island of Reil, lies deep within the lateral sulcus.

   

2. Cerebellum:

The cerebellum, the second largest portion of the brain. It is located at the back of the brain, beneath the cerebrum, the cerebellum is responsible for coordinating and regulating movements, balance, posture, and muscle tone. It receives information from various sensory systems and integrates it to control smooth and coordinated movements.

3. Brainstem: The brainstem is the region connecting the brain to the spinal cord. It consists of three main parts: the midbrain, pons, and medulla oblongata. The brainstem is responsible for vital functions such as regulating the cardiovascular system, respiratory system, and basic involuntary functions like swallowing and reflexes. It also serves as a pathway for transmitting signals between the brain and the spinal cord. This region primarily consists of nerve fibers that form conduction tracts between the higher brain centers and spinal cord. The medulla oblongata, or simply medulla, extends inferiorly from the pons. It is continuous with the spinal cord at the foramen magnum. All the ascending (sensory) and descending (motor) nerve fibers connecting the brain and spinal cord pass through the medulla.

4. Limbic System: The limbic system is a collection of brain structures involved in regulating emotions, memory, and basic drives such as hunger, thirst, and sexual behavior. It includes the amygdala, hippocampus, hypothalamus, and parts of the cerebral cortex. The limbic system plays a crucial role in motivation, emotional responses, and forming long-term memories.

5. Thalamus: The thalamus acts as a relay center for sensory information, receiving input from various sensory systems (except olfaction) and directing it to the appropriate areas of the cerebral cortex for processing. It also plays a role in regulating consciousness, attention, and sleep.

6. Hypothalamus: Located below the thalamus, the hypothalamus is a small but vital structure that regulates many essential functions. It controls the autonomic nervous system, body temperature, hunger and thirst, hormone release from the pituitary gland, and plays a role in sleep-wake cycles and emotional responses.

7. Corpus Callosum: The corpus callosum is a large bundle of nerve fibers that connects the two hemispheres of the brain. It enables communication and coordination between the left and right hemispheres, allowing information to be shared and integrated.

The brain is composed of billions of neurons (nerve cells) and glial cells that support and nourish them. Neurons communicate through electrical impulses and chemical signals, forming intricate networks that underlie all brain functions.

It is important to note that the brain is an incredibly complex and active organ, and our understanding of its functions and intricacies is still expanding. Ongoing research continues to unravel the mysteries of the brain and its role in human cognition and behavior.

Ventricles and Cerebrospinal Fluid

A series of interconnected, fluid-filled cavities are found within the brain. These cavities are the ventricles of the brain, and the fluid is cerebrospinal fluid (CSF).

Spinal Cord

The spinal cord extends from the foramen magnum at the base of the skull to the level of the first lumbar vertebra. The cord is continuous with the medulla oblongata at the foramen magnum. Like the brain, the spinal cord is surrounded by bone, meninges, and cerebrospinal fluid.

The spinal cord is divided into 31 segments with each segment giving rise to a pair of spinal nerves. At the distal end of the cord, many spinal nerves extend beyond the conus medullaris to form a collection that resembles a horse’s tail. This is the cauda equina. In cross section, the spinal cord appears oval in shape.

                   Cross-sectional image of the spinal cord

The spinal cord has two main functions:

• Serving as a conduction pathway for impulses going to and from the brain. Sensory impulses travel to the brain on ascending tracts in the cord. Motor impulses travel on descending tracts.
• Serving as a reflex center. The reflex arc is the functional unit of the nervous system. Reflexes are responses to stimuli that do not require conscious thought and consequently, they occur more quickly than reactions that require thought processes. For example, with the withdrawal reflex, the reflex action withdraws the affected part before you are aware of the pain. Many reflexes are mediated in the spinal cord without going to the higher brain centers.

Brain Tumor

Glioma refers to tumors that arise from the support cells of the brain. These cells are called glial cells. These tumors include the astrocytomas, ependymomas and oligodendrogliomas. These tumors are the most common primary brain tumors.

The Peripheral Nervous System

The peripheral nervous system consists of the nerves that branch out from the brain and spinal cord. These nerves form the communication network between the CNS and the body parts. The peripheral nervous system is further subdivided into the somatic nervous system and the autonomic nervous system. The somatic nervous system consists of nerves that go to the skin and muscles and is involved in conscious activities. The autonomic nervous system consists of nerves that connect the CNS to the visceral organs such as the heart, stomach, and intestines. It mediates unconscious activities.

Structure of a Nerve

Cranial Nerves

Twelve pairs of cranial nerves emerge from the inferior surface of the brain. All of these nerves, except the vagus nerve, pass through foramina of the skull to innervate structures in the head, neck, and facial region.

The cranial nerves are designated according to the order in which they appear on the inferior surface of the brain. Most of the nerves have both sensory and motor components. Three of the nerves are associated with the special senses of smell, vision, hearing, and equilibrium and have only sensory fibers. Five other nerves are primarily motor in function but do have some sensory fibers for proprioception. The remaining four nerves consist of significant amounts of both sensory and motor fibers.

Acoustic neuromas are benign fibrous growths that arise from the balance nerve, also called the eighth cranial nerve or vestibulocochlear nerve. These tumors are non-malignant, meaning that they do not spread or metastasize to other parts of the body. The location of these tumors is deep inside the skull, adjacent to vital brain centers in the brain stem. As the tumors enlarge, they involve surrounding structures which have to do with vital functions. In the majority of cases, these tumors grow slowly over a period of years. In other cases, the growth rate is more rapid and patients develop symptoms at a faster pace. Usually, the symptoms are mild and many patients are not diagnosed until some time after their tumor has developed. Many patients also exhibit no tumor growth over a number of years when followed by yearly MRI scans.

Spinal Nerves

Thirty-one pairs of spinal nerves emerge laterally from the spinal cord. Each pair of nerves corresponds to a segment of the cord and they are named accordingly. This means there are 8 cervical nerves, 12 thoracic nerves, 5 lumbar nerves, 5 sacral nerves, and 1 coccygeal nerve.

Each spinal nerve is connected to the spinal cord by a dorsal root and a ventral root. The cell bodies of the sensory neurons are in the dorsal root ganglion, but the motor neuron cell bodies are in the gray matter. The two roots join to form the spinal nerve just before the nerve leaves the vertebral column. Because all spinal nerves have both sensory and motor components, they are all mixed nerves.

Autonomic Nervous System

The autonomic nervous system is a visceral efferent system, which means it sends motor impulses to the visceral organs. It functions automatically and continuously, without conscious effort, to innervate smooth muscle, cardiac muscle, and glands. It is concerned with heart rate, breathing rate, blood pressure, body temperature, and other visceral activities that work together to maintain homeostasis.

The autonomic nervous system has two parts, the sympathetic division and the parasympathetic division. Many visceral organs are supplied with fibers from both divisions. In this case, one stimulates and the other inhibits. This antagonistic functional relationship serves as a balance to help maintain homeostasis