5.2 Human Nervous System

Introduction

The human nervous system is a complex network of specialized cells and organs that coordinate all body functions. It enables us to detect changes in our environment, process information, and respond appropriately. The nervous system is divided into two main parts: the central nervous system and the peripheral nervous system. This topic introduces the overall organization of the human nervous system.

Definition

The human nervous system is the organ system that coordinates the body's activities by transmitting signals between different parts of the body. It consists of the central nervous system (CNS) and the peripheral nervous system (PNS).

Main Idea

The human nervous system comprises:

• Central nervous system (CNS): Consists of the brain, which narrows into a long tube called the spinal cord.
• Peripheral nervous system (PNS): Consists of cranial nerves from the brain and spinal nerves from the spinal cord.

The structural and functional unit of the nervous system is the neuron.

Core Concept

Divisions of the Human Nervous System

Detailed Explanation

1. Central Nervous System (CNS)

The central nervous system consists of:

(a) Brain:

• Enclosed within the cranium (skull).
• Covered with three protective membranes called meninges.
• Contains hollow spaces called ventricles.
• Divided into three parts: forebrain, midbrain, and hindbrain.

(b) Spinal Cord:

• A long tube that extends from the base of the brain.
• Protected by the vertebrae of the backbone.
• 31 pairs of nerves originate from the spinal cord.

Simple explanation: The CNS is the control center of the body. The brain processes information, and the spinal cord carries messages to and from the brain.

2. Peripheral Nervous System (PNS)

The peripheral nervous system consists of:

(a) Cranial nerves:

• Nerves that arise from the brain.
• There are 12 pairs of cranial nerves.

(b) Spinal nerves:

• Nerves that arise from the spinal cord.
• There are 31 pairs of spinal nerves.

Simple explanation: The PNS is like a network of wires that connects the CNS to all parts of the body.

Types of nerves:

• Some cranial nerves are sensory, some are motor, and some are mixed.
• All spinal nerves are mixed nerves.

3. Neuron

The structural and functional unit of the nervous system is the neuron.

Simple explanation: Neurons are the cells that carry electrical signals (nerve impulses) throughout the nervous system.

4. Protective Coverings

The brain and spinal cord are covered with three protective membranes called meninges.

What are meninges?

• Three layers of connective tissue.
• Between the inner and middle layer is a fluid called cerebrospinal fluid.
• Cerebrospinal fluid helps cushion the brain from shock.
• Meninges also provide nutrients and oxygen to brain tissue through capillaries.

Simple explanation: The meninges are like protective wrappings around the brain and spinal cord. The cerebrospinal fluid acts as a shock absorber.

Intermediate Level

The human nervous system is divided into:

Key points:

• The brain is enclosed in the cranium (skull).
• The spinal cord is protected by the vertebrae.
• Meninges are three protective membranes.
• Cerebrospinal fluid cushions the brain and spinal cord.
• Neurons are the structural and functional units.
• Cranial nerves arise from the brain.
• Spinal nerves arise from the spinal cord.

Important Points

• - The human nervous system comprises the central nervous system and peripheral nervous system.
• - The central nervous system consists of the brain, which narrows into a long tube called the spinal cord.
• - The peripheral nervous system consists of cranial nerves from the brain and spinal nerves from the spinal cord.
• - The structural and functional unit of the nervous system is the neuron.
• - The brain is enclosed within the cranium (skull).
• - The brain and the spinal cord are covered with three protective membranes called meninges.
• - Between the inner and middle layer, there is cerebrospinal fluid.
• - Cerebrospinal fluid helps to cushion the brain from shock.
• - Meninges also provide nutrients and oxygen to brain tissue through capillaries.
• - The brain contains hollow spaces called ventricles.
• - The brain is divided into three parts: forebrain, midbrain, and hindbrain.
• - There are 12 pairs of cranial nerves.
• - There are 31 pairs of spinal nerves.
• - Some cranial nerves are sensory, some are motor, and some are mixed.
• - All spinal nerves are mixed nerves.
• - The peripheral nervous system can be divided into somatic and autonomic nervous system.
• - The autonomic nervous system is divided into sympathetic and parasympathetic nervous system.

Remember

• - Human nervous system = CNS + PNS
• - CNS = brain + spinal cord
• - PNS = cranial nerves (12 pairs) + spinal nerves (31 pairs)
• - Neuron = structural and functional unit
• - Meninges = three protective membranes
• - Cerebrospinal fluid = cushions brain and spinal cord
• - Brain = enclosed in cranium
• - Spinal cord = protected by vertebrae
• - Cranial nerves = from brain
• - Spinal nerves = from spinal cord
• - Somatic = voluntary
• - Autonomic = involuntary
• - Sympathetic = fight or flight
• - Parasympathetic = rest and digest

Common Mistakes

• 1. Confusing CNS and PNS.
• - CNS = brain and spinal cord.
• - PNS = cranial and spinal nerves.
• 2. Mixing up cranial and spinal nerves.
• - Cranial nerves = from brain (12 pairs).
• - Spinal nerves = from spinal cord (31 pairs).
• 3. Thinking all cranial nerves are mixed.
• - Some cranial nerves are sensory, some are motor, and some are mixed.
• - All spinal nerves are mixed.
• 4. Forgetting the role of meninges.
• - Meninges protect the brain and spinal cord.
• 5. Confusing the somatic and autonomic nervous systems.
• - Somatic = voluntary movements.
• - Autonomic = involuntary functions.
• 6. Mixing up sympathetic and parasympathetic.
• - Sympathetic = fight or flight.
• - Parasympathetic = rest and digest.
• 7. Thinking the brain is not protected.
• - The brain is protected by the cranium, meninges, and cerebrospinal fluid.

Quick Revision

60-Second Revision

• Human nervous system = CNS + PNS
• CNS = brain + spinal cord
• PNS = cranial nerves (12 pairs) + spinal nerves (31 pairs)
• Neuron = structural and functional unit
• Meninges = three protective membranes
• Cerebrospinal fluid = cushions brain and spinal cord
• Brain = enclosed in cranium; divided into forebrain, midbrain, hindbrain
• Spinal cord = protected by vertebrae
• Cranial nerves = from brain
• Spinal nerves = from spinal cord
• **Somatic** = voluntary
• **Autonomic** = involuntary
• Sympathetic = fight or flight
• Parasympathetic = rest and digest
• Warning: Do not confuse cranial nerves (from brain) with spinal nerves (from spinal cord).