Nervous System Anatomy and Physiology

Introduction
Brain
Spinal Cord
Vertebral Column

 

 

Introduction

The human nervous system is made up of two main components: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS is composed of the brain, the cranial nerves, and the spinal cord. The PNS is made up of the nerves that exit from the spinal cord at various levels of the spinal column as well as their tributaries. The autonomic nervous system (divided into the sympathetic and parasympathetic nervous system) is also considered to be a part of the PNS and it controlls the body's many vegetative (non-voluntary) functions.

 

Brain

The human brain serves many important functions ranging from imagination, memory, speech, and limb movements to secretion hormones and control of various organs within the body. These functions are controlled by many distinct parts that serve specific and important tasks. These components and their functins are listed below.

Brain Cells: The brain is made up of two types of cells: neurons (yellow cells in the image below) and glial cells (pink and purple cells in the image below). Neurons are responsible for all of the functions that are attributed to the brain while the glial cells are non-neuronal cells that provide support for neurons. In an adult brain, the predominant cell type is glial cells, which outnumber neurons by about 50 to 1. Neurons communicate with one another through connections called synapses.

 

Meninges: The bony covering around the brain is called the cranium, which combines with the facial bones to create the skull. The brain and spinal cord are covered by a tissue known as the meninges, which is made up of three layers: dura mater, arachnoid layer, and pia mater. The dura mater is a whitish and nonelastic membrane which, on its outer surface, is attached to the inside of the cranium. This layer completely covers the brain and the spinal cord and has two major folds in the brain, that are called the falx and the tentorium. The falx separates the right and left halves of the brain while the tentorium separates the upper and lower parts of the brain. The arachnoid layer is a thin membrane that covers the entire brain and is positioned between the dura mater and the pia mater, and for the most part does not follow the folds of the brain. The pia mater, which is attached to the surface of the entire brain, follows the folds of the brain and has many blood vessels that reach deep into the brain. The space between the arachnoid layer and the pia mater is called the subarachnoid space and it contains the cerebrospinal fluid.

 

Cerebrospinal Fluid (CSF): CSF is a clear fluid that surrounds the brain and spinal cord, and helps to cushion these structures from injury. This fluid is constantly made by structures deep within the brain called the choroid plexus which is housed inside spaces within the brain called ventricles, after which it circulates through channels around the spinal cord and brain where is it finally reabsorbed. If the delicate balance between production and absorption of CSF is disrupted, then backup of this fluid within the system of ventricles can cause hydrocephalus.

Ventricles: Brain ventricles are a system of four cavities, which are connected by a series of tubes and holes and direct the flow of CSF within the brain. These cavities are the lateral ventricles (right and left), which communicate with the third ventricle in the center of the brain through an opening called the interventricular foramen. This ventricle is connected to the fourth ventricle through a long tube called the Cerebral Aqueduct. CSF then exits the ventricular system through several holes in the wall of the fourth ventricle (median and lateral apertures) after which it flow around the brain and spinal cord.

 

Brainstem: The brainstem is the lower extension of the brain which connects the brain to the spinal cord, and acts mainly as a relay station between the body and the brain. It also controls various other functions, such as wakefulness, sleep patterns, and attention; and is the source for ten of the twelve cranial nerves. It is made up of three structures: the midbrain, pons and medulla oblongata. The midbrain is inovolved in eye motion while the pons coordinates eye and facial movements, facial sensation, hearing, and balance. The medulla oblongata controls vegetative functions such as breathing, blood pressure, and heart rate as well as swallowing.

 

Thalamus: The thalamus is a structure that is located above the brainstem and it serves as a relay station for nearly all messages that travel from the cerebral cortex to the rest of the body/brain and vice versa. As such, problems within the thalamus can cause significant symptoms with regard to a variety of functions, including movement, sensation, and coordination. The thalamus also functions as an important component of the pathways within the brain that control pain sensation, attention, and wakefulness.

Cerebellum: The cerebellum is located at the lower back of the brain beneath the occipital lobes and is separated from them by the tentorium. This part of the brain is responsible for maintaining balance and coordinating movements. Abnormalities in either side of the cerebellum produce symptoms on the same side of the body.

 

Cerebrum: The cerebrum forms the major portion of the brain, and is divided into the right and left cerebral hemispheres. These hemispheres are separated by a groove called the great longitudinal fissure and are joined at the bottom of this fissure by a struture called the corpus callosum which allows communication between the two sides of the brain. The surface of the cerebrum contains billions of neurons and glia that together form the cerebral cortex (brain surface), also known as "gray matter." The surface of the cerebral cortex appears wrinkled with small grooves that are called sulci and bulges between the grooves that are called gyri. Beneath the cerebral cortex are connecting fibers that interconnect the neurons and form a white-colored area called the "white matter."

 

Lobes: Several large grooves (fissures) separate each side of the brain into four distinct regions called lobes: frontal, temporal, parietal, and occipital. Each hemisphere has one of each of these lobes, which generally control function on the opposite side of the body. The different portions of each lobe and the four different lobes communicate and function together through very complex relationships, but each one also has its own unique characteristics. The frontal lobes are responsible for voluntary movement, speech, intellectual and behavioral functions, memory, intelligence, concentration, temper and personality. The parietal lobe processes signals received from other areas of the brain (such as vision, hearing, motor, sensory and memory) and uses it to give meaning to objects. The occipital lobe is responsible for processing visual information. The temporal lobe is involved in visual memory and allows for recognition of objects and peoples' faces, as well as verbal memory which allows for remembering and understanding language.

Hypothalamus: The hypothalamus is a structure that communicates with the pituitary gland in order to manage hormone secretions as well as controlling functions such as eating, drinking, sexual behavior, sleep, body temperature, and emotions.

 

Pituitary Gland: The pituitary gland is a small structure that is attached to the base of the brain in an area called the sella turcica. This gland controls the secretion of several hormones which regulate growth and development, function of various organs (kidneys, breasts, and uterus), and the function of other glands (thyroid gland, gonads, and the adrenal glands).

Basal Ganglia: The basal ganglia are clusters of nerve cells around the thalamus which are heavily connected to the cells of the cerebral cortex. The basal ganglia are associated with a variety of functions, including voluntary movement, procedural learning, eye movements, and cognitive/emotional functions. The various components of the basal ganglia include caudate nucleus, putamen, globus pallidus, substantia nigra, and subthalamic nucleus. Diseases affecting these parts can cause a number of neurological conditions, including Parkinson's disease and Huntington's disease.

 

Cranial Nerves: There are 12 pairs of nerves that originate from the brain itself, as compared to spinal nerves that initiate in the spinal cord. These nerves are responsible for specific activities and are named and numbered as follows:

Cranial nerve I (Olfactory nerve): Smell
Cranial nerve II (Optic nerve): Vision
Cranial nerve III (Oculomotor nerve): Eye movements and opening of the eyelid
Cranial nerve IV (Trochlear nerve): Eye movements
Cranial nerve V (Trigeminal nerve): Facial sensation and jaw movement
Cranial nerve VI (Abducens nerve): Eye movements
Cranial nerve VII (Facial nerve): Eyelid closing, facial expression and taste sensation
Cranial nerve VIII (Vestibulocochlear nerve): Hearing and sense of balance
Cranial nerve IX (Glossopharyngeal nerve): Taste sensation and swallowing
Cranial nerve X (Vagus nerve): Heart rate, swallowing, and taste sensation
Cranial nerve XI (Spinal accessory nerve): Control of neck and shoulder muscles
Cranial nerve XII (Hypoglossal nerve): Tongue movement

 

Pineal Gland: The pineal gland is an outgrowth from the back portion of the third ventricle, and has some role in sexual maturation, although the exact function of the pineal gland in humans is unclear.

 

 

Spinal Cord

The spinal cord is a long, thin, tubular bundle of neurons and support cells that extends from the bottom of the brain down to the space between the first and second lumbar vertebrae, and is housed and protected by the bony vertebral column. The spinal cord functions primarily in the transmission of signals between the brain and the rest of the body, allowing movement and sensation, but it also contains neural circuits that can control numerous reflexes independent of the brain.

General Structure: The length of the spinal cord is much shorter than the length of the bony spinal column, extending about 45 cm (18 inches). It is ovoid in shape and is enlarged in the cervical (neck) and lumbar (lower back) regions. Similar to the brain, the spinal cord is protected by three layers of tissue, called spinal meninges. The dura mater is the outermost layer, and it forms a tough protective coating. Between the dura mater and the surrounding bone of the vertebrae is a space called the epidural space, which is filled with fatty tissue and a network of blood vessels. The arachnoid mater is the middle protective layer. The space between the arachnoid and the underlyng pia mater is called the subarachnoid space which contains cerebrospinal fluid (CSF). The medical procedure known as a lumbar puncture (or spinal tap) involves use of a needle to withdraw cerebrospinal fluid from the subarachnoid space, usually from the lumbar (lower back) region of the spine. The pia mater is the innermost protective layer. It is very delicate and it is tightly associated with the surface of the spinal cord.

In the upper part of the vertebral column, spinal nerves exit directly from the spinal cord, whereas in the lower part of the vertebral column nerves pass further down the column before exiting. The terminal portion of the spinal cord is called the conus medullaris. A collection of nerves, called the cauda equina, continues to travel in the spinal column below the level of the conus medullaris. The cauda equina forms as a result of the fact that the spinal cord stops growing in length at about age four, even though the vertebral column continues to lengthen until adulthood.

Three arteries provide blood supply to the spinal cord by running along its length. These are the two Posterior Spinal Arteries and the one Anterior Spinal Artery. These travel in the subarachnoid space and send branches into the spinal cord that communicate with branches from arteries on the other side.

Function: The spinal cord is divided into 33 different segments. At every segment, a pair of spinal nerves (right and left) exit the spinal cord and carry motor (movement) and sensory information. There are 8 pairs of cervical (neck) nerves named C1 through C8, 12 pairs of thoracic (upper back) nerves termed T1 through T12, 5 pairs of lumbar (lower back) nerves named L1 through L5, 5 pairs of sacral (pelvis) nerves numbered S1 through S5, and 3-4 pairs of coccygeal (tailbone) nerves. These nerves combine to supply strength to various muscles throughout the body as follows:

C1-C6: Neck flexion
C1-T1: Neck extension
C3-C5: Diaphragm
C5-C6: Shoulder movement and elbow flexion
C6-C8: Elbow and wrist extension
C7-T1: Wrist flexion
C8-T1: Hand movement
T1-T6: Trunk muscles above the waiste
T7-L1: Abdominal muscles
L1-L4: Thigh flexion
L2-L4: Thigh adduction (movement toward the body)
L4-S1: Thigh abduction (movement away from the body)
L2-L4: Leg extension at the knee
L5-S2: Leg extension at the hip
L4-S2: Leg flexion at the knee
L4-S1: Foot dorsiflexion (move upward) and toe extension
L5-S2: Foot plantarflexion (move downward) and toe flexion

The spinal nerves also provide sensation to the skin in an organized manner as depicted below.

 

Vertebral Column

General Structure: The vertebral column is made up of 33 vertebrae that fit together to form a flexible, yet extraordinarily tough, column that serves to support the back through a full range of motion. There are seven cervical vertebrae (C1-C7), 12 thoracic vertebrae (T1-T12), five lumbar vertebrae (L1-L5), five fused sacral vertebrae (S1- S5),and four coccygeal vertebrae in this column, each separated by intervertebral disks.

The first two cervical vertebrae have very distinct anatomy as compared to the ramaining vertebrae. The first cervical vertebra, known as the atlas, supports the head; and pivots on the second cervical vertebra, the axis. The seventh cervical vertebra joins the first thoracic vertebra. The thoracic vertebrae provide an attachment site for the ribs, and make up part of the back of the chest (thorax). The thoracic vertebrae join the lumbar vertebrae, which are particularly study and large, as they support the entire upper body weight. At the top of the pelvis, the lumbar vertebrae join the sacral vertebrae. By adulthood these five bones have usually fused to form a triangular bone called the sacrum. At the tip of the sacrum, the final part of the vertebral column projects slightly outward. This is the coccyx, better known as the tailbone. It is made up of three to five coccygeal vertebrae.

A typical vertebra consists of two essential parts: the vertebral body in front and the vertebral arch in the back. The vertebral arch consists of a pair of pedicles, a pair of lamina, a spinous process, and four articular processes (joints) that connect the vertebra to one another, as depicted below.

 

The vertebral bodies, stacked on top of eachother, form a strong pillar for the support of the head and trunk. Between each two vertebral bodies exists a hole, called the intervertebral foramina, which allows for the transmission of the spinal nerves on either side.