Brain and Spinal Cord

Human Physiology

Lesson 42 : Central & Peripheral Nervous System

Brain and Spinal Cord

The brain is part of CNS that regulates important functions of our body. It is the central information processing organ of our body and acts as the ‘command and control system’. Brain controls the voluntary and involuntary movements, balance of the body, functioning of vital involuntary organs (viz. lungs, heart, kidneys, etc), thermoregulation, hunger and thirst, circadian (24-hour) rhythms of our body, activities of several endocrine glands and human behavior. It is also the site for processing all the sensory organs (vision, hearing, speech, memory, intelligence, emotions and thoughts).

The human brain is well protected by the skull. Inside the skull, the brain is covered by three cranial meninges, from outer layer called dura mater, a very thin middle layer called arachnoid and an inner layer which is in contact with the brain tissue called pia mater.

The brain can be divided into three major parts. 1. Forebrain 2. Midbrain 3. Hindbrain

The forebrain consists of cerebrum, thalamus and hypothalamus. Cerebrum forms the major part of the human brain. A deep cleft divides the cerebrum longitudinally into two halves, which are termed as the left and right cerebral hemispheres. The hemispheres are connected by a tract of nerve fibers called corpus callosum. The layer of cells which covers the cerebral hemisphere is called cerebral cortex and is thrown into prominent folds. The cerebral cortex is referred to as the gray matter due to its grayish appearance. The neuron cell bodies are concentrated here giving the colour. The cerebral cortex contains motor areas, sensory areas and large regions that are neither clearly sensory nor motor in function. These regions called as the association areas are responsible for complex functions like intersensory associations, memory and communication. Nerve fibers of the tracts are covered with the myelin sheath, which constitute the inner part of cerebral hemisphere. They give an opaque white appearance to the layer and hence, are called the white matter. The cerebrum wraps around a structure called thalamus, which is a major coordinating centre for sensory and motor signaling. Another very important part of the brain called hypothalamus lies at the base of the thalamus. The hypothalamus contains a number of centers which control body temperature, urge for eating and drinking. It also contains several groups of neurosecretory cells, which secrete hormones called hypothalamic hormones. The inner parts of cerebral hemispheres and a group of associated deep structures like amygdala, hippocampus etc. form a complex structure called the limbic lobe or limbic system. Along with the hypothalamus, it is involved in the regulation of sexual behavior, expression of emotional reactions (e.g., excitement, pleasure, rage and fear), and motivation.

The midbrain is located between the thalamus or hypothalamus of the forebrain and pons of the hindbrain. A canal called the cerebral aqueduct passes through the midbrain. The dorsal portion of the midbrain consists mainly of four round swellings (lobes) called corpora quadrigemina. Midbrain and hindbrain forms the brain stem.

The hindbrain comprises pons, cerebellum and medulla (also called the medulla oblongata). Pons consists of fiber tracts that interconnect different regions of the brain. Cerebellum has very convoluted surface in order to provide the additional space for many more neurons. The medulla of the brain is connected to the spinal cord. The medulla contains centers which control respiration, cardiovascular reflexes and gastric secretions.

Spinal cord is the caudal portion of the CNS and is continued anteriorly as brain stem. It is the main conducting pathways of ascending and descending nerve tracts. It functions as the integrative area for cord reflexes and motor functions. Spinal cord is symmetrically divided into two lateral halves. The dorsal or posterior part is dorsal or posterior medial septum and it receives axons from the receptors in the skin, muscle, tendons, joints and visceral organs. The ventral or anterior part is known as anterior or ventral medial tissue and through which the axons of lower motor neurons. In the middle of the spinal cord there lies a central canal through which cerebrospinal fluid (CSF) flows to the brain. The spinal cord consists of two layers, namely, the gray matter centrally placed surrounded by white matter.

The gray matter is centrally placed and constitutes the projections on each side known as ventral, lateral and dorsal horns. The gray matter is formed by three elements, nerve cells, neuroglia and nerve fibers both axon and dendrites. The ventral horns (motor neuron) innervate the muscle spindle and Ranshaw cells or interneurons transmit inhibitory signals to motor neuron. The lateral horn cells act as relay station for autonomic nerve. They give out preganglionic sympathetic fibers those leaving through ventral root. The dorsal horn cells (sensory neuron), responsible to relay information from organs to the brain. It has dorsal nucleus known as Clark’s column, found in the cervical, thoracic and lumbar region.

The white matter contains mostly myelinated fibers and some unmyelinated fibers that surrounding gray matter. The white matter divided into two lateral halves by dorsal medial septum and ventral median tissues. The lateral halves are divided into three funiculus, namely, dorsal, lateral and ventral funiculus. The ascending and descending tracts are passing through the funiculus.

Spinal nerves

CNS, the brain and spinal cord, is in two-way communication with virtually all body tissues by means of a system of branching peripheral nerves. These are composed of afferent (sensory) fibers, which convey information to the central nervous system from peripheral receptors, and efferent (motor) fibers, which convey instructions from the CNS to peripheral effector organs. The peripheral nerves comprise of 12 pairs of cranial nerves and the considerably larger number of pairs of spinal nerves based on the vertebral formula.

The orderly origin of the spinal nerves reveals the segmentation of the spinal cord. Each nerve is formed by the union of anterior or dorsal and posterior or ventral roots. The anterior or dorsal roots composed of afferent fibers and respond to external stimuli i.e changes within the muscles and other locomotor organs, changes in the internal organs, respectively. The posterior or ventral root composed of efferent fibers passing to effector organ, e.g. muscles and glands.

The anterior or dorsal and posterior or ventral roots join peripherally to form the mixed spinal nerve, which leaves the vertebral canal through the appropriate inter-vertebral foramen. The mixed trunk formed by the union of dorsal and ventral roots divides almost at once into dorsal and ventral rami. The dorsal ramus is distributed to dorsal structures i.e. epaxial muscles of trunk, skin over the back. The ventral ramus is distributed to hypaxial muscles of the trunk, the muscles of the limbs, and the remaining part of the skin.

Both dorsal and ventral rami have connections with their neighbors that form continuous dorsal and ventral plexuses. These plexuses are important for the origin of the nerves to limbs. The brachial and lumbosacral plexuses, give rise to the nerves that are distributed to forelimb and hindlimb structures, respectively. The brachial plexus is formed by contributions from the last three cervical and the first two thoracic nerves. The lumbosacral plexus formed by contributions from the last few lumbar and the first two sacral nerves. The limb plexuses allow for regrouping and reassociation of the constituent nerve fibers, and the nerve trunks that emerge distally are each composed of fibers derived from two or three spinal segments. For example median nerve composed of fibers from spinal nerves C8 and T1, the femoral nerve contains fibers from L4 to L6.

Last modified: Wednesday, 11 April 2012, 5:29 AM