Human Nervous System: Structure, Divisions, and Actions

The Central Nervous System (CNS)

The Spinal Cord

The spinal cord conducts signals to and from the brain and controls reflex activities.

Meninges: Protective Membranes

There are three membranes containing cerebrospinal fluid, collectively called the meninges, located between the protecting bones and the organs. They are the Dura Mater, the Arachnoid, and the Pia Mater.

CNS Tissues

The CNS is composed of two primary tissue types:

• Grey Matter: Contains neuronal bodies and dendrites. These are control centers that perform specific functions.
• White Matter: Contains neuronal axons covered in myelin.

The Peripheral Nervous System (PNS)

The Peripheral Nervous System connects all the receptors and effectors in the body to the nerve center. It is made up of nerves, which are axons covered by myelin.

Classification of Nerves

Nerves are classified based on the direction of signal transmission:

• Sensory Nerves: Transmit signals from the sensory organs.
• Motor Nerves: Transmit signals to the effector organs.
• Mixed Nerves: Contain both sensory and motor fibers.

PNS Divisions: Somatic and Autonomic

The Somatic Nervous System (SNS)

The SNS connects the sensory receptors to the central nervous system and subsequently to the skeletal muscles.

Functions of the Somatic Nervous System

1. It receives information from external stimuli about the body's position.
2. It transmits signals for voluntary muscle contractions.

Nerve Division Based on CNS Entry

Depending on where they enter the CNS, the nerves are divided into cranial and spinal nerves.

Cranial Nerves

Cranial nerves enter and leave the brain. There are 12 pairs of sensory, motor, and mixed nerves.

Cranial Nerve Innervation

They innervate the head, the upper part of the body, and some internal organs. Along their journey, there are ganglia nerves, which are groups of neuronal bodies.

Spinal Nerves

Spinal nerves leave the spinal cord, forming two roots called the dorsal and ventral roots, which meet to form a nerve cord. There are 31 pairs of mixed nerves. The sensory neurons enter through the dorsal root, and the motor neurons come out of the ventral root.

The Autonomic Nervous System (ANS)

The ANS regulates and controls involuntary actions of the body. The higher control centers are located in the grey matter of the spinal cord, the hypothalamus, and the spinal bulb.

ANS Divisions

The autonomic nervous system is divided into two main branches, each developing opposite actions:

• Sympathetic NS: Associated with the "fight or flight" response.
• Parasympathetic NS: Associated with "rest and digest" functions.

Sympathetic Nervous System

It controls most of the body's organs so that they work effectively, and it prevails in stressful situations.

Parasympathetic Nervous System

Its actions provoke relaxation, causing the organs to work more slowly and less intensely. This happens while resting or relaxing.

Nerve Actions: Reflexes and Voluntary Acts

Reflexes

They are quick and automatic actions in which the brain is not involved. In this process, sensory neurons, motor neurons, and interneurons are used.

Voluntary Acts

They are slower and more elaborate acts in which the cerebral cortex is involved, utilizing a large number of neurons.