NERVOUS SYSTEM (INTRODUCTION)

The Nervous System

The nervous system is the major controlling, regulatory, and communicating system in the body. It is the center of all mental activity including thought, learning, and memory. Together with the endocrine system, the nervous system is responsible for regulating and maintaining homeostasis. Through its receptors, the nervous system keeps us in touch with our environment, both external and internal.

Like other systems in the body, the nervous system is composed of organs, principally the brain, spinal cord, nerves, and ganglia. These, in turn, consist of various tissues, including nerve, blood, and connective tissue. Together these carry out the complex activities of the nervous system.

The various activities of the nervous system can be grouped together as three general, overlapping functions:

1. Sensory

2. Integrative

3. Motor

Millions of sensory receptors detect changes, called stimuli, which occur inside and outside the body. They monitor such things as temperature, light, and sound from the external environment. Inside the body, the internal environment, receptors detect variations in pressure, pH, carbon dioxide concentration, and the levels of various electrolytes. All of this gathered information is called sensory input.

Sensory input is converted into electrical signals called nerve impulses that are transmitted to the brain. There the signals are brought together to create sensations, to produce thoughts, or to add to memory; Decisions are made each moment based on the sensory input. This is integration.

Based on the sensory input and integration, the nervous system responds by sending signals to muscles, causing them to contract, or to glands, causing them to produce secretions. Muscles and glands are called effectors because they cause an effect in response to directions from the nervous system. This is the motor output or motor function.

Structural Classification of the Nervous System

The nervous system is divided into:

Central Nervous System (CNS) – brain and spinal cord

Peripheral Nervous System (PNS) – cranial nerves, spinal nerves, and ganglia

Nerve Tissue

The nervous tissue is made up of two types of cells which are:

1. Neurons

The neuron is the structural and functional unit of the nervous system. It transmits nerve impulses.

Structure of a neuron includes:

Cell body (soma)

Dendrites (receive impulses)

Axon (transmits impulses)

Myelin sheath (insulates axon)

Nodes of Ranvier (speed up conduction)

2. Neuroglia (Glial Cells)

These cells are non-conductive and provide support, nourishment, and protection for neurons

Classification of Neurons

Functionally, neurons are classified as:

1.Afferent (sensory) – carry impulses to the CNS

2. Efferent (motor) – carry impulses from the CNS

3. Interneurons – connect neurons within the CNS

Organization of the Nervous System

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.

Brain

The brain is the control center of the nervous system, located in the cranial cavity. It is responsible for coordinating body activities, interpreting sensory information, and enabling thought, memory, and emotions.

Major Parts of the Brain

Cerebrum (thinking, memory, voluntary actions)

Cerebellum (balance and coordination)

Brainstem:

Midbrain

Pons

Medulla oblongata (controls vital functions)

Spinal Cord

The spinal cord is a long cylindrical structure extending from the brainstem through the vertebral canal.

Functions:

Conduction pathway – carries impulses to and from the brain

Reflex center – controls rapid, automatic responses (reflex actions)

It connects the brain to the rest of the body through spinal nerves and plays a key role in movement and sensation.

Spinal Cord and Reflex Arc

The spinal cord acts as the main center for reflex actions. A reflex arc is the pathway that nerve impulses follow during a reflex.

A reflex is a rapid, automatic response to a stimulus.

In many reflexes, the spinal cord receives sensory information and sends a response immediately, without waiting for the brain. This makes the response very fast and automatic.

Reflex Arc Components

Receptor

Sensory neuron

Interneuron

Motor neuron

Effector

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.

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.

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

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.

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.

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.

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.

Sympathetic

Fight or flight

Increases heart rate

Dilates pupils

Inhibits digestion

Parasympathetic

Rest and digest

Decreases heart rate

Constricts pupils

Stimulates digestion