The Nervous System

Overview

The nervous system is a complex network responsible for monitoring changes in the body, processing information, and dictating responses. It is divided into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS), each with specialized structures and functions.

Simplified Functions of the Nervous System

Three Overlapping Functions

• Sensory Input:

  • Sensory receptors detect changes (stimuli) inside and outside the body.

  • The information gathered is called sensory input.

  • Example: Touching a hot object activates sensory receptors in the skin.

• Integration:

  • The nervous system processes and interprets sensory input.

  • This function is known as integration.

  • Example: The brain interprets the sensation of heat and determines an appropriate response.

• Motor Output:

  • The nervous system activates effector organs (muscles or glands) to respond.

  • The response is called motor output.

  • Example: Withdrawing your hand from a hot object.

Nervous Tissue

Components of Nervous Tissue

Nervous tissue is specialized for communication and consists primarily of neurons and supporting cells (neuroglia).

• Neurons: The functional units of the nervous system, responsible for transmitting electrical and chemical signals.

• Neuroglia: Supporting cells that provide structural and metabolic support to neurons.

Neuron Structure

Major Parts of a Neuron

• Cell Body (Soma): Contains the nucleus and most organelles; biosynthetic and receptive center.

• Dendrites: Branching extensions that receive signals and transmit them toward the cell body.

• Axon: A single, long process that conducts impulses away from the cell body to other neurons or effector organs.

• Axon Hillock: The region where the axon originates from the cell body; site of action potential initiation.

• Myelin Sheath: Insulating layer formed by Schwann cells (in PNS) or oligodendrocytes (in CNS) that increases impulse speed.

• Node of Ranvier: Gaps in the myelin sheath where action potentials are regenerated.

• Axon Terminals: The secretory region where neurotransmitters are released to communicate with other cells.

Neuron Processes

Axons and Dendrites

• Axon:

  • Each neuron has a single axon.

  • Functions as the impulse generator and conductor, transmitting signals away from the cell body.

• Dendrites:

  • Extensive branches from the cell body.

  • Transmit chemical or electrical signals toward the cell body.

Structure of a Typical Neuron

Functional Regions

• Receptive Region: Dendrites and cell body receive incoming signals.

• Conducting Region: Axon transmits the action potential.

• Secretory Region: Axon terminals release neurotransmitters to communicate with other cells.

Diagram Description

The typical neuron consists of a cell body with dendrites extending outward, a single axon originating from the axon hillock, covered by myelin sheath (with nodes of Ranvier), and ending in terminal branches.

Key Terms and Definitions

• Stimulus: Any change in the environment that can be detected by sensory receptors.

• Sensory Input: Information gathered by sensory receptors about internal and external changes.

• Integration: The process by which the nervous system processes and interprets sensory input.

• Motor Output: The activation of effector organs in response to integration.

• Neuron: A nerve cell specialized for communication.

• Dendrite: A neuron process that receives signals.

• Axon: A neuron process that sends signals.

• Myelin Sheath: Insulating layer that speeds up nerve impulse transmission.

Example: Sensory-Motor Pathway

• Step 1: Sensory receptors in the skin detect a stimulus (e.g., heat).

• Step 2: Sensory input is transmitted to the CNS.

• Step 3: The CNS integrates the information and decides on a response.

• Step 4: Motor output is sent via motor neurons to muscles, causing withdrawal from the stimulus.

Additional info:

• Neurons are highly specialized for rapid communication and can be classified by structure and function (e.g., sensory, motor, interneurons).

• Neuroglia (not shown in detail here) include astrocytes, oligodendrocytes, microglia, and ependymal cells in the CNS, and Schwann cells and satellite cells in the PNS.