Coordination and the Human Nervous System: Anatomy and Functional Regions

Classification and Structure of Nerves

A nerve is defined as a collection of axons that are enveloped by a protective covering. These nerves arise from the brain and the spinal cord, constituting the Peripheral Nervous System ($PNS$). Nerves are categorized into three distinct types based on the specific axons they contain:

1. Sensory nerves: These contain the axons of sensory neurons only.
2. Motor nerves: These contain the axons of motor neurons only.
3. Mixed nerves: These contain the axons of both sensory and motor neurons.

In various parts of the body, the cell bodies of many neurons often cluster together to form groups enveloped by membranes. These specific groups of cell bodies are known as ganglia ($singular:$ ganglion). As visualized in $Fig. 12.3$, a nerve consists of several axons bundled within its covering.

Divisions of the Nervous System

In humans and other higher animals, the nervous system is organized into two primary components: the Central Nervous System ($CNS$) and the Peripheral Nervous System ($PNS$). The $CNS$ is comprised of the brain and the spinal cord. The $PNS$ is comprised of peripheral nerves that branch out through the body.

Protection for the $CNS$ is provided by skeletal structures: the brain is housed within the cranium (part of the skull), while the spinal cord extends from the posterior side of the brain and is protected within the vertebral column, commonly referred to as the backbone.

Protective Structures and Fluids of the Central Nervous System

The brain is the control center for all life activities in animals. It is enclosed within the cranium and further protected by three layers of membranes collectively known as meninges. These membranes serve the vital function of protecting the brain from harmful substances. Internally, the brain contains ventricles, which are fluid-filled cavities. These ventricles are continuous with the central canal of the spinal cord.

The fluid contained within these ventricles and the central canal is called Cerebrospinal Fluid ($CSF$). The $CSF$ bathes the neurons of both the brain and the spinal cord, acting as a cushion to protect the nervous tissue against physical and mechanical stresses.

Anatomy and Functional Regions of the Forebrain

The human brain is divided into three major regions: the forebrain, midbrain, and hindbrain. The forebrain is the largest and most highly developed area in human beings. It consists of several critical components:

1. Cerebrum: This is the largest part of the brain, divided into two halves called cerebral hemispheres. Communication between these hemispheres is facilitated by a large band of fibers called the corpus callosum. The cerebrum contains billions of neurons packed together and exhibits contralateral control, meaning the left cerebral hemisphere controls the right side of the body, and the right cerebral hemisphere controls the left side. The outer region, the cerebral cortex, is characterized by many folds and convolutions that increase its surface area. It is composed of grey matter (containing cell bodies and non-myelinated axons), while the underlying region is white matter (containing myelinated axons). The cerebrum manages sensory information, processing, memory storage, and voluntary movements. It is the center for intelligence, thinking, reasoning, and decision-making. Furthermore, it contains the speech centre, which is unique to human beings.

2. Thalamus: Wrapped by the cerebrum, the thalamus acts as a processing center for sensory information, particularly from the eyes and ears, as well as general input from the skin and internal organs. It relays this information to the cerebrum and the limbic system.

3. Limbic System: Positioned in an arc between the thalamus and the cerebrum, this system manages emotional and physiological responses including hunger, thirst, fear, anger, tranquility, pleasure, and sexual responses. A portion of the system is also critical for memory formation.

4. Hypothalamus: A vital component of the limbic system located specifically below the thalamus.

Characteristics and Functions of the Midbrain

The midbrain is reduced in size in human beings and is located beneath the cerebral hemispheres. It serves as a relay center, connecting the forebrain with the hindbrain. Its primary functional roles involve controlling reflex movements of the eyes and managing hearing reflexes.