Nervous System

The nervous system is the body’s communication and control network that receives sensory information, processes it, and sends commands to coordinate actions. It enables organisms to sense their environment, think, move, and respond rapidly to stimuli. In simple animals, the nervous system may be a nerve net (as in Hydra), while in complex animals, it consists of specialized structures like brain, spinal cord, and nerves.

1.0Introduction

The main functions of the nervous system include:

• Receiving stimuli: Detecting changes inside or outside the body.
• Processing information: Integrating and interpreting signals in the brain or ganglia.
• Responding to stimuli: Sending motor signals to muscles or glands.
• Maintaining coordination: Ensuring different organs function harmoniously.

In short, the nervous system enables sensation, thought, and movement — the foundation of behavior and survival.

2.0How does the nervous system work?

The nervous system works by detecting stimuli, processing information, and sending appropriate responses to maintain coordination and homeostasis. This process occurs in three main steps:

1. Sensory Input: Specialized receptors detect changes in the environment (internal or external) and convert them into electrical signals. These signals travel via sensory neurons to the central nervous system (CNS).
2. Integration and Processing: The brain or ganglia receive the incoming signals, analyze the information, and make decisions about the response. This may involve reflex actions, voluntary movements, or involuntary adjustments.
3. Motor Output: Signals are transmitted through motor neurons to effectors like muscles or glands, producing a response — for example, contraction of a muscle, secretion of hormones, or adjustment of organ function.

This entire system relies on neurons and synapses, where electrical impulses travel along axons and neurotransmitters bridge the gap between cells, allowing rapid communication throughout the body.

3.0Nervous System Parts and Functions

The nervous system is structurally and functionally organized into two major parts:

• Central Nervous System (CNS): This is the main processing center. It consists of the brain and the spinal cord. The CNS receives sensory information, processes it, and sends out motor commands. It is protected by a bony skeleton (cranium and vertebral column) and three layers of membranes called the meninges. The cerebrospinal fluid (CSF) also cushions and protects the CNS.
• Peripheral Nervous System (PNS): This is the network of nerves that connects the CNS to the rest of the body. The PNS is responsible for carrying sensory information to the CNS and transmitting motor commands from the CNS to muscles and glands. It is further divided into two functional divisions:

4.0Nervous System Organs

1. Brain

• The main control center.
• Divided into three major parts:
• • Forebrain (Cerebrum, Thalamus, Hypothalamus): Controls thought, memory, and voluntary actions.
  • Midbrain: Connects sensory inputs and controls reflexes.
  • Hindbrain (Cerebellum, Pons, Medulla): Manages balance, coordination, and involuntary functions like heartbeat and breathing.

2. Spinal Cord

• A long, tubular structure extending from the brainstem.
• Conducts impulses between brain and body; also manages reflex actions.

3. Nerves

• Bundles of axons that carry messages.
• Sensory nerves: Transmit impulses to the CNS.
• Motor nerves: Carry commands from CNS to effectors.

5.0Structure and Function of Neuron

The neuron is the fundamental cell of the nervous system. It is specialized to transmit information via electrical impulses. Each neuron has three main parts:

• Cell Body (Soma): Contains the nucleus and other organelles. It is the metabolic center of the neuron.
• Dendrites: Tree-like extensions that receive signals from other neurons and carry them toward the cell body.
• Axon: A long projection that transmits signals away from the cell body to other neurons, muscles, or glands. Axons are often covered by a myelin sheath, an insulating layer that speeds up impulse transmission. The gaps in the myelin sheath are called Nodes of Ranvier.

The junction between two neurons is called a synapse. Communication across a synapse involves the release of chemical messengers called neurotransmitters.

6.0Transmission of Nerve Impulse

The nerve impulse is an electrical signal that travels along the neuron.

1. Resting Potential: The neuron is polarized (inside negative, outside positive).
2. Depolarization: When stimulated, sodium ions enter, reversing the charge.
3. Action Potential: Electrical impulse moves along the axon.
4. Repolarization: Potassium ions move out, restoring original charge.
5. Synaptic Transmission: Neurotransmitters cross the synapse to the next neuron or target cell.

This allows rapid communication, often within milliseconds.

7.0Divisions of the Nervous System

The Peripheral Nervous System (PNS) is further divided into:

Autonomic Nervous System has two parts:

• Sympathetic System: Activates “fight or flight” response.
• Parasympathetic System: Promotes “rest and digest” functions.

8.0Coordination Between Nervous and Endocrine Systems

Both systems work together to maintain homeostasis.

• The nervous system provides rapid, short-term responses via nerve impulses.
• The endocrine system controls slower, long-term responses through hormones.
• The hypothalamus links both systems by controlling the pituitary gland.