Study Notes: The Nervous System – Anatomy & Physiology

The Nervous System

Overview of the Nervous System

The nervous system is a complex network responsible for coordinating all activities in the body. It is divided into the central nervous system (CNS) and the peripheral nervous system (PNS).

• Central Nervous System (CNS): Consists of the brain and spinal cord. It is the main control center for processing information.

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

Central Nervous System

Blood-Brain Barrier

The blood-brain barrier is a selective barrier that protects the brain from harmful substances while allowing essential nutrients to pass through.

• Structure: Formed by the least permeable capillaries in the body.

• Function: Permits water, glucose, and amino acids to pass through, but blocks many potentially harmful substances and metabolic wastes.

• Importance: Maintains a stable environment for the brain and acts as a barrier against some substances.

The Spinal Cord

The spinal cord is a long, thin, tubular structure that extends from the foramen magnum of the skull to the first or second lumbar vertebra. It serves as a major pathway for information traveling between the brain and the rest of the body.

• Cauda Equina: A collection of spinal nerves at the inferior end of the vertebral canal.

• Function: Provides a two-way conduction pathway to and from the brain.

• Spinal Nerves: 31 pairs arise from the spinal cord, each serving a specific region of the body.

Spinal Cord Anatomy

The spinal cord is protected by meninges and surrounded by vertebrae. It contains both white and gray matter, which are involved in transmitting and processing neural signals.

• Cervical and Lumbar Enlargements: Regions where nerves serving the upper and lower limbs arise.

• Meninges: Three protective membranes (dura mater, arachnoid mater, pia mater) surround the spinal cord.

• Cauda Equina: Bundle of spinal nerves and nerve roots at the lower end of the spinal cord.

Peripheral Nervous System (PNS)

Structure and Function

The PNS consists of nerves and ganglia outside the CNS. It connects the CNS to the limbs and organs, facilitating communication throughout the body.

• Nerves: Bundles of axons (neurons) found outside the CNS, surrounded by connective tissue sheaths (endoneurium, perineurium, epineurium).

• Ganglia: Clusters of neuron cell bodies located outside the CNS.

Types of Nerves

Nerves in the PNS can be classified based on the direction of signal transmission and the type of fibers they contain.

• Sensory (Afferent) Nerves: Carry impulses toward the CNS.

• Motor (Efferent) Nerves: Carry impulses away from the CNS.

• Mixed Nerves: Contain both sensory and motor fibers.

Cranial Nerves

Cranial nerves primarily serve the head and neck, with the exception of the vagus nerve, which extends to the thoracic and abdominal cavities.

• Total: 12 pairs of cranial nerves.

• Function: Most are mixed nerves, but three are sensory only (olfactory, optic, vestibulocochlear).

Spinal Nerves and Plexuses

Spinal nerves arise from the spinal cord and are organized into plexuses, which are networks of nerves serving motor and sensory functions for specific regions.

• Plexuses: Cervical, brachial, lumbar, and sacral plexuses.

• Major Nerves: Axillary, radial, ulnar, median (brachial plexus); femoral, obturator (lumbar plexus); sciatic, tibial, fibular (sacral plexus).

Autonomic Nervous System (ANS)

Overview

The autonomic nervous system is a division of the PNS that controls involuntary body functions, including the activity of cardiac and smooth muscles and glands.

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

• Autonomic Nervous System: Regulates involuntary functions.

Structure of the ANS

The ANS consists of two-neuron chains: a preganglionic neuron (originates in the CNS) and a postganglionic neuron (extends to the organ).

• Preganglionic Neuron: Cell body in the brain or spinal cord.

• Postganglionic Neuron: Axon extends to the effector organ.

Divisions of the ANS

The ANS is divided into the sympathetic and parasympathetic divisions, which have opposing effects on body functions.

• Sympathetic Division: Prepares the body for "fight or flight" responses during stress or emergency.

• Parasympathetic Division: Promotes "rest and digest" activities, conserving energy and maintaining daily body functions.

Parasympathetic Division

The parasympathetic division is also known as the craniosacral division. Its neurons originate in the brainstem (cranial nerves III, VII, IX, X) and sacral regions of the spinal cord.

• Function: Stimulates salivation, digestion, and other maintenance activities.

• Neurotransmitter: Acetylcholine is released at both pre- and postganglionic synapses.

Sympathetic Division

The sympathetic division is also known as the thoracolumbar division. Its neurons originate from thoracic and lumbar regions (T1-L2) of the spinal cord.

• Function: Increases heart rate, dilates airways, inhibits digestion, and prepares the body for action.

• Neurotransmitters: Preganglionic neurons release acetylcholine; postganglionic neurons release norepinephrine or epinephrine.

Comparison of Sympathetic and Parasympathetic Divisions

The following table summarizes the main differences between the two divisions:

Feature	Sympathetic Division	Parasympathetic Division
Origin	Thoracolumbar (T1-L2)	Craniosacral (brainstem & sacral spinal cord)
Function	"Fight or flight" (emergency, stress)	"Rest and digest" (maintenance, energy conservation)
Neurotransmitter	Acetylcholine (preganglionic), Norepinephrine/Epinephrine (postganglionic)	Acetylcholine (both pre- and postganglionic)
Main Effects	Increases heart rate, dilates pupils, inhibits digestion	Decreases heart rate, stimulates digestion, constricts pupils

Key Terms and Definitions

• Neuron: A nerve cell; the basic unit of the nervous system.

• Synapse: Junction between two neurons where information is transmitted.

• Ganglion: Cluster of neuron cell bodies outside the CNS.

• Myelin Sheath: Insulating layer around axons that increases the speed of nerve impulse transmission.

• Reflex Arc: The neural pathway involved in a reflex action.

Example: Reflex Arc

A classic example of nervous system function is the reflex arc, such as the knee-jerk reflex. It involves sensory neurons, interneurons in the spinal cord, and motor neurons, allowing for rapid, involuntary responses to stimuli.

Additional info: Some content was inferred and expanded for completeness and clarity, including definitions and the comparison table.