Nervous System Overview

Introduction to the Nervous System

The nervous system is a complex network responsible for coordinating the body's activities by transmitting signals between different parts. It is divided into the central and peripheral nervous systems, each with specialized functions.

• Central Nervous System (CNS): Consists of the brain and spinal cord. Responsible for processing and integrating information.

• Peripheral Nervous System (PNS): Composed of nerves outside the CNS, including cranial and spinal nerves. It connects the CNS to limbs and organs.

Divisions of the Peripheral Nervous System

• Afferent Division: Transmits sensory information from receptors to the CNS. Includes special senses (vision, hearing), somatic receptors (touch, pain), and visceral receptors (internal organs).

• Efferent Division: Carries motor commands from the CNS to effectors (muscles and glands). Subdivided into:

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

  • Autonomic Nervous System (ANS): Regulates involuntary functions of smooth muscle, cardiac muscle, and glands.

Autonomic Nervous System (ANS)

Overview and Structure

The ANS is a division of the PNS that controls involuntary physiological functions. It consists of two main subdivisions: the sympathetic and parasympathetic divisions, which often have opposing effects on target organs.

• Effectors: Smooth muscle, cardiac muscle, and glands.

• Pathway: Involves two neurons in series: a preganglionic neuron (originates in CNS) and a postganglionic neuron (located in a ganglion outside the CNS).

Sympathetic Division

The sympathetic division prepares the body for stressful or emergency situations, often referred to as the "fight or flight" response.

• Origin: Preganglionic neurons arise from the thoracic and lumbar regions (T1–L2) of the spinal cord.

• Ganglia: Three types—sympathetic chain ganglia (paravertebral), collateral ganglia (prevertebral), and adrenal medullae.

• Neurotransmitters: Preganglionic neurons release acetylcholine (ACh); postganglionic neurons typically release norepinephrine (NE).

• Key Responses:

  • Increased heart rate and blood pressure

  • Increased respiratory rate

  • Decreased digestive and urinary activity

  • Mobilization of energy reserves

  • Increased mental alertness and sweat gland activity

Parasympathetic Division

The parasympathetic division promotes rest, recovery, and energy conservation, often called the "rest and relaxation" response.

• Origin: Preganglionic neurons arise from the brainstem (cranial nerves III, VII, IX, X) and sacral spinal cord (S2–S4).

• Ganglia: Located near or within target organs; preganglionic fibers are long, postganglionic fibers are short.

• Neurotransmitters: Both pre- and postganglionic neurons release acetylcholine (ACh).

• Key Responses:

  • Decreased heart rate and blood pressure

  • Increased digestive gland secretion and motility

  • Constriction of respiratory passageways

  • Promotion of urination and defecation

  • Sexual arousal and nutrient absorption

Comparison of Sympathetic and Parasympathetic Divisions

Both divisions innervate many of the same organs but have generally opposite effects. Their coordinated actions maintain homeostasis.

Neurotransmitters and Receptors

Cholinergic and Adrenergic Receptors

• Cholinergic receptors: Bind acetylcholine; found in both divisions at ganglia (nicotinic) and at parasympathetic target organs (muscarinic).

• Adrenergic receptors: Bind norepinephrine or epinephrine; found at sympathetic target organs.

Summary Table: Neurotransmitters and Receptors

Functional Anatomy of the Brain (Brief Overview)

• Frontal Lobe: Primary motor cortex; voluntary control of skeletal muscles.

• Parietal Lobe: Primary sensory cortex; perception of touch, pressure, pain, taste, and temperature.

• Temporal Lobe: Auditory and olfactory cortex; perception of hearing and smell.

• Occipital Lobe: Visual cortex; perception of visual stimuli.

Key Terms and Definitions

• Neuron: The basic functional unit of the nervous system, consisting of a cell body (soma), dendrites, and an axon.

• Ganglion: A collection of neuron cell bodies outside the CNS.

• Synapse: The junction between two neurons or a neuron and an effector cell, where neurotransmitters mediate communication.

• Effector: A muscle or gland that responds to neural stimulation.

Example: Autonomic Pathway Structure

• Sympathetic Pathway: CNS → short preganglionic neuron → sympathetic ganglion → long postganglionic neuron → effector (e.g., heart, smooth muscle)

• Parasympathetic Pathway: CNS → long preganglionic neuron → ganglion near/within target organ → short postganglionic neuron → effector

Summary

• The autonomic nervous system is essential for involuntary regulation of body functions.

• Sympathetic and parasympathetic divisions have distinct anatomical and functional characteristics but work together to maintain homeostasis.

• Understanding neurotransmitters and receptor types is crucial for comprehending how these systems influence target organs.