The Nervous System: Structure, Function, and Divisions
The Nervous System
The nervous system allows the body to react to internal and external environmental changes. It controls and integrates various bodily activities.
Divisions of the Nervous System
Structurally, the nervous system is divided into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).
Functionally, it’s divided into the Somatic Nervous System (SNS) and the Autonomic Nervous System (ANS).
Nerve Tissue
Nerve tissue comprises two main cell types:
Neurons
Neurons are characterized by their excitable plasma membrane. They are specialized for receiving stimuli and conducting nerve impulses.
Morphological characteristics:
- Cell body
- Dendrites (carry impulses toward the cell body)
- Axon (conducts impulses from the cell body to another neuron)
Glial Cells
Glial cells are the support cells of the nervous system. They are actively involved in processing information.
Functions:
- Support neurons
- Retain mitotic capacity (responsible for repair and regeneration)
- Guide and control neuronal migration during development
- Regulate growth and development of axons and dendrites
- Act as insulators (form myelin sheaths)
- Maintain homeostasis and regulate metabolic functions
- Protect neurons (form the blood-brain barrier)
Central Nervous System (CNS)
The CNS comprises the brain and spinal cord.
Protection
The brain and spinal cord are protected by three membranes called meninges:
- Dura mater
- Arachnoid mater
- Pia mater
The meninges are further protected by bone (skull and spine).
Cerebrospinal Fluid
The cavities of the brain and spinal cord are filled with cerebrospinal fluid.
Functions:
- Medium for exchange of substances
- Waste disposal system
- Mechanical damper
Gray and White Matter
Cells in the CNS are arranged to form gray matter (cell bodies) and white matter (dendrites and axons).
Brain
The brain is the most voluminous part of the CNS. It’s divided into two hemispheres separated by the interhemispheric fissure and connected by the corpus callosum.
The surface of the brain (cerebral cortex) consists of gray matter formed into gyri.
White matter lies beneath the cortex.
Deep within the brain are areas of gray matter forming nuclei (e.g., thalamus, caudate nucleus, hypothalamus).
Cerebellum
The cerebellum is located at the bottom and back of the brain, near the brainstem.
Brainstem
The brainstem consists of the midbrain, pons, and medulla. It connects the brain to the spinal cord.
Spinal Cord
The spinal cord is an extension of the brain. It has gray matter inside and white matter outside.
Peripheral Nervous System (PNS)
The PNS consists of cranial and spinal nerves that emerge from the CNS and run throughout the body. It also includes peripheral ganglia (clusters of neuron cell bodies outside the CNS).
Cranial Nerves
There are 12 pairs of cranial nerves. They send sensory information from the neck and head to the CNS and receive motor commands to control the skeletal muscles of the neck and head.
Spinal Nerves
There are 31 pairs of spinal nerves. They send sensory information (touch, pain, temperature, position, and condition of muscles and joints) from the trunk and extremities to the CNS. They also receive motor commands from the CNS to control skeletal muscles.
Somatic Nervous System (SNS)
The SNS includes spinal nerves that send sensory information from the trunk and extremities to the CNS and receive motor commands to control skeletal muscles.
Autonomic Nervous System (ANS)
The ANS regulates involuntary bodily functions, controlling smooth muscle, the heart, and glands.
Branches of the ANS
Sympathetic Branch
Involved in activities that require energy expenditure (fight-or-flight response).
Parasympathetic Branch
Responsible for storing and conserving energy (rest-and-digest response).
Enteric Branch
Regulates gastrointestinal activity and coordinates the peristaltic reflex.
Components of the PNS
Roots
- Cervical
- Thoracic
- Lumbar
- Sacral
Plexus
- Cervical plexus
- Brachial plexus
- Lumbosacral plexus
Nerves
- Cranial nerves
- Upper limb nerves
- Lower limb nerves
Functional Divisions of the Nervous System
Sensory (Afferent) Component
Receives and transmits impulses to the CNS for processing.
Motor (Efferent) Component
Transmits impulses from the CNS to effector organs throughout the body. It’s further divided into the somatic nervous system (SS) and the autonomic nervous system (SA).