Fundamentals of the Nervous System and Nervous Tissue

Introduction: Why This Matters

Understanding neurotransmitter function is essential for comprehending how drugs and diseases affect the nervous system. The nervous system is the master controlling and communication system of the body, utilizing both electrical and chemical signals to coordinate rapid and specific responses.

• Electrical and chemical signals: Neurons communicate via these signals for fast, targeted actions.

• Immediate responses: Most nervous system responses occur almost instantly.

Nervous System’s Function

The nervous system is responsible for gathering information, processing it, and producing responses. This is achieved through three main steps:

• Sensory Input: Sensory receptors detect stimuli from inside or outside the body, sending information to the central nervous system (CNS).

• Integration: The CNS processes and interprets sensory input, deciding on an appropriate response.

• Motor Output: The CNS sends signals to effectors (muscles or glands) to produce a response.

• Example: Touching a hot surface activates sensory receptors, the CNS processes the information, and motor output causes you to withdraw your hand.

Organization of the Nervous System

The nervous system is organized into two main divisions, each with specialized functions:

• Central Nervous System (CNS): Consists of the brain and spinal cord; responsible for integration and control.

• Peripheral Nervous System (PNS): Includes cranial and spinal nerves; connects the CNS to the rest of the body.

The PNS is further divided into:

• Sensory (Afferent) Division: Transmits sensory information from receptors to the CNS.

• Motor (Efferent) Division: Transmits motor commands from the CNS to effectors.

The Motor Division includes:

• Somatic Nervous System: Controls voluntary movements of skeletal muscles.

• Autonomic Nervous System (ANS): Regulates involuntary functions (e.g., heart rate, digestion) and is subdivided into:

  • Sympathetic Division: Mobilizes body systems during activity ("fight or flight").

  • Parasympathetic Division: Conserves energy and promotes "rest and digest" functions.

Neuroglia (Glial Cells)

Nervous tissue contains two principal cell types: neuroglia and neurons.

• Neuroglia (Glial Cells): Small, supportive cells that surround and protect neurons.

• Neurons (Nerve Cells): Excitable cells that transmit electrical signals.

Four main types of neuroglia support CNS neurons:

• Astrocytes: Most abundant; support and brace neurons, regulate exchanges between neurons and capillaries, and control the chemical environment.

• Microglial Cells: Defensive cells; monitor neuron health and can transform into phagocytes to remove debris.

• Ependymal Cells: Line the central cavities of the brain and spinal cord; may be ciliated to help circulate cerebrospinal fluid (CSF).

• Oligodendrocytes: Form myelin sheaths around CNS nerve fibers, increasing the speed of impulse transmission.

Additional info: Neuroglia in the Peripheral Nervous System (PNS) include satellite cells (support neuron cell bodies) and Schwann cells (form myelin sheaths around peripheral nerve fibers).