15. NERVOUS SYSTEM

NERVOUS SYSTEM: A TEACHER’S COMPREHENSIVE GUIDE

Welcome, future healthcare professionals!

The nervous system is a highly developed network of special cells called neurons. These neurons help in coordinating body activities and passing signals between different body parts. A nerve is simply a group (bundle) of many neurons covered by a protective connective tissue layer. This covering has three layers: Endoneurium (covers each single neuron), Perineurium (surrounds a bundle of nerve fibres), and Epineurium (the outermost fibrous layer covering the entire nerve).

Dpharmguru’s exam insights:

In my years of teaching neuroanatomy, I have observed that students often confuse the three layers of nerve coverings. Remember from inside out: Endoneurium (around each fibre), Perineurium (around a bundle/fascicle), Epineurium (around the whole nerve). This is a classic exam question!

CLASSIFICATION OF NERVOUS SYSTEM

In humans, the nervous system is mainly divided into two parts:

• Central Nervous System (CNS): Includes brain and spinal cord. It receives and processes information, stores memory, and controls all body functions.
• Peripheral Nervous System (PNS): Includes all nerves outside the brain and spinal cord. Sensory nerves carry messages from sense organs to the brain. Motor nerves carry commands from the brain to muscles and glands.

PNS is further divided into:

• Somatic Nervous System: Controls voluntary actions like walking or lifting objects.
• Autonomic Nervous System (ANS): Controls involuntary actions like heartbeat, digestion, etc. It has two divisions:
  • Sympathetic Division: “Fight or Flight” (active during stress).Parasympathetic Division: “Rest and Digest” (active during rest).

CENTRAL NERVOUS SYSTEM (CNS)

The CNS includes the brain and spinal cord — it acts as the control centre of the body. It is covered by three protective layers called meninges and protected by skull bones (for the brain) and vertebral bones (for the spinal cord).

Brain and Its Structure

The brain is the main organ of the nervous system. It is enclosed within the skull (cranium). It is about 1.4 to 1.6 kg in adults and made up of billions of neurons.

The brain has the following main parts: Cerebrum, Cerebellum, Brain stem (midbrain, pons, and medulla), and Diencephalon.

Meninges of Brain

The meninges are three protective membranes covering the brain and spinal cord.

• Pia mater: The innermost delicate layer, rich in blood vessels.
• Arachnoid membrane: The middle, thin layer forming a web-like structure.
• Dura mater: The tough outer layer lying just below the skull.

Functions of Meninges: Protect brain and spinal cord, maintain pressure, and allow exchange of nutrients and wastes.

Dpharmguru’s exam insights:

Remember the meninges from outside to inside: Dura mater (tough mother), Arachnoid mater (spider-like), Pia mater (tender mother). Cerebrospinal fluid flows in the subarachnoid space between the arachnoid and pia mater.

Ventricles of Brain

The brain ventricles are four fluid-filled cavities inside the brain that contain CSF (Cerebrospinal Fluid):

• Two Lateral Ventricles: One in each hemisphere.
• Third Ventricle: In the centre of the brain, between the two thalami.
• Fourth Ventricle: Between the cerebellum and brainstem, connected to the spinal cord.

Cerebrospinal Fluid (CSF)

CSF is a clear, watery fluid that surrounds the brain and spinal cord. It is produced in the choroid plexus and fills the ventricles and subarachnoid space.

Functions of CSF:

• Protection: Cushions the brain against shocks.
• Pressure maintenance: Keeps uniform pressure around the brain.
• Moisture and exchange: Keeps tissues moist and allows exchange of nutrients and waste.
• Chemical stability: Maintains a constant chemical environment.
• Circulation: Helps transport oxygen and nutrients to brain cells.

PARTS OF BRAIN

The brain has four major parts: Cerebrum, Cerebellum, Brain Stem (Midbrain, Pons, Medulla), and Diencephalon.

1. Cerebrum

The cerebrum is the largest part of the brain. It is divided into two hemispheres – right and left – connected by the corpus callosum (a band of white nerve fibres). The outer layer (grey matter) is called cerebral cortex, and the inner layer (white matter) is called cerebral medulla.

Lobes of the Cerebrum:

• Frontal lobe: Controls thinking, movement, and speech.
• Parietal lobe: Receives touch, pain, and temperature sensations.
• Temporal lobe: Responsible for hearing and memory.
• Occipital lobe: Responsible for vision.

Functions of Cerebrum: Controls voluntary muscle actions, processes sensory information, and handles higher brain activities like memory, reasoning, and emotions.

2. Cerebellum

The cerebellum lies below the cerebrum and behind the brainstem. It looks like a smaller brain with two hemispheres and a central part called vermis. It is covered by grey matter (cerebellar cortex) and has a branching white matter pattern called arbor vitae.

Functions of Cerebellum: Maintains body balance and posture, coordinates movement of muscles for smooth actions, and helps in precision of voluntary movements.

3. Midbrain

The midbrain is the upper part of the brainstem that connects the cerebrum with the spinal cord. It contains cerebral peduncles (front part) for motor nerve pathways and tectum (back part) containing superior and inferior colliculi, which control visual and auditory reflexes.

Functions: Controls eye movements and body reflexes related to vision and sound, helps in motor control (through substantia nigra and red nucleus). Degeneration of substantia nigra causes Parkinson’s disease.

4. Medulla Oblongata

The medulla oblongata connects the brain with the spinal cord. It controls many vital involuntary functions such as heart rate and blood pressure (via cardiovascular centre), breathing rhythm (via respiratory centre), reflex actions like coughing, sneezing, and vomiting, and crossing of nerve tracts – right brain controls left body side and vice versa.

5. Pons

The pons lies above the medulla and below the midbrain. It acts as a bridge connecting different parts of the brain.

Functions: Helps control breathing (apneustic and pneumotaxic centres), acts as a relay centre for motor messages. Damage to pons can lead to movement or breathing problems.

6. Diencephalon

The diencephalon connects the cerebrum with the brainstem and surrounds the third ventricle. It includes: Thalamus, Hypothalamus, Epithalamus, Subthalamus, and Basal Ganglia.

• Thalamus: Acts as the relay centre for sensory and motor information between the body and cerebral cortex. Controls alertness, sleep, and wakefulness.
• Hypothalamus: Controls autonomic nervous system, regulates body temperature, controls hunger, thirst, and sleep, regulates hormones through the pituitary gland, maintains circadian rhythm, and manages emotions.
• Epithalamus: Includes the pineal gland (secretes melatonin) and habenular nuclei. Controls hormone secretion and links smell to emotions.
• Subthalamus: Located below the thalamus, connects with basal ganglia and helps regulate body movements.
• Basal Ganglia: Groups of neurons deep in the brain that help control voluntary movements, regulation of muscle tone, and coordination.

Dpharmguru’s exam insights:

The hypothalamus is the master regulator of the body. It controls the pituitary gland (the master endocrine gland), regulates body temperature, hunger, thirst, sleep, and emotions. This is a very important concept in exams!

SPINAL CORD

The spinal cord is the long, cylindrical part of the central nervous system (CNS) that runs inside the vertebral column (backbone). It acts as the main pathway for carrying messages between the brain and the rest of the body. There are 31 pairs of spinal nerves that come out from the spinal cord.

External Structure of Spinal Cord

• Cervical Enlargement: Found in the upper region (C₄–T₁). It gives rise to nerves that supply the upper limbs (arms).
• Lumbar Enlargement: Found in the lower region (T₉–T₁₂). It gives rise to nerves that supply the lower limbs (legs).
• Conus Medullaris: The spinal cord ends in a tapering cone-shaped structure around the level of L₁–L₂.
• Filum Terminale: A thin thread-like structure that anchors the spinal cord to the coccyx.
• Cauda Equina: The bundle of nerve roots that extends from the lower end of the spinal cord, looking like a horse’s tail.

Internal Structure of Spinal Cord

When seen in cross-section, the spinal cord has grey matter in the centre and white matter around it. The grey matter is shaped like the letter “H” or a butterfly and contains nerve cell bodies, dendrites, and non-myelinated axons. The white matter surrounds it and is made of myelinated nerve fibres that carry messages up and down the cord.

Parts of the grey matter:

• Dorsal (posterior) horns: Contain sensory neurons.
• Ventral (anterior) horns: Contain motor neurons.
• Intermediate grey matter: Connects both horns.

Functions of Spinal Cord

• Transmission of impulses: Sensory tracts carry messages to the brain; motor tracts carry commands from the brain to body parts.
• Integration centre: The grey matter acts as the site where sensory and motor information are connected.
• Reflex control: The spinal cord is responsible for reflex actions — quick automatic responses to stimuli.
• Link between body and brain: It connects the CNS with all body parts.

Reflex Activity of Spinal Cord

A reflex action is a quick, automatic, and involuntary response to a stimulus, without involvement of conscious thought.

Examples: Pulling your hand away quickly when you touch something hot, blinking when a strong light flashes in your eyes.

Reflex Arc: The pathway through which nerve impulses travel during a reflex action. It has 5 main parts:

1. Receptor: The sensory organ that detects the stimulus.
2. Sensory neuron: Carries the message from receptor to spinal cord.
3. Integrating centre (Interneuron): Connects the sensory and motor neurons inside the spinal cord.
4. Motor neuron: Carries the response signal from spinal cord to effector organ.
5. Effector: The muscle or gland that performs the action.

Significance of Reflex Action: Protects the body from injury, helps adjust the body to sudden changes in the environment, and reduces brain workload by handling simple actions automatically.

PERIPHERAL NERVOUS SYSTEM (PNS)

The Peripheral Nervous System includes all the nerves that connect the brain and spinal cord (CNS) to the rest of the body — such as muscles, glands, and sense organs. It mainly controls voluntary movements and helps carry sensory and motor information.

Cranial Nerves

There are 12 pairs of cranial nerves that arise directly from the brain. Most supply the head and neck region, except the vagus nerve (10th), which extends to the heart, lungs, and digestive organs.

Dpharmguru’s exam insights:

Remember the mnemonic for cranial nerves: “Oh, Oh, Oh, To Touch And Feel Very Good Velvet, Ah Heaven!” — Olfactory, Optic, Oculomotor, Trochlear, Trigeminal, Abducens, Facial, Vestibulocochlear, Glossopharyngeal, Vagus, Accessory, Hypoglossal. Also, remember the vagus nerve is the only cranial nerve that extends to the abdomen!

Spinal Nerves

There are 31 pairs of spinal nerves, grouped as follows:

AUTONOMIC NERVOUS SYSTEM (ANS)

The autonomic nervous system (ANS) controls all involuntary activities of the body — such as the movement of the heart muscles, smooth muscles, and gland secretions. These actions are automatic and do not require conscious control.

Sympathetic Nervous System (SNS)

The sympathetic nervous system prepares the body to face physical or emotional stress, such as during exercise, fear, pain, or extreme temperature. That’s why it is often called the “fight or flight” system. The SNS originates from the thoracic (T₁) to lumbar (L₂) regions of the spinal cord — hence the name thoracolumbar outflow.

Functions of Sympathetic Nervous System:

Parasympathetic Nervous System (PSNS)

The parasympathetic system is responsible for resting and recovery functions of the body. It helps in energy conservation, digestion, and relaxation — that’s why it is called the “rest and digest” system. It is also known as the craniosacral outflow.

Functions of Parasympathetic Nervous System:

Differences Between SNS and PSNS

Dpharmguru’s exam insights:

The nervous system is the master controller of the body. In exams, focus on the differences between sympathetic and parasympathetic systems, the 12 cranial nerves, and the reflex arc. Remember: Sympathetic = Fight or Flight (increases heart rate, dilates pupils), Parasympathetic = Rest and Digest (decreases heart rate, constricts pupils). These concepts are almost guaranteed to appear!

REFERENCES AND FURTHER READING

• Tortora, G. J., & Derrickson, B. H. (2017). Principles of Anatomy and Physiology (15th ed.). John Wiley & Sons.
• Marieb, E. N., & Hoehn, K. (2019). Human Anatomy & Physiology (11th ed.). Pearson Education.
• Standring, S. (2020). Gray’s Anatomy: The Anatomical Basis of Clinical Practice (42nd ed.). Elsevier.
• Hall, J. E., & Guyton, A. C. (2020). Guyton and Hall Textbook of Medical Physiology (14th ed.). Elsevier.
• National Institutes of Health (NIH). (2022). Nervous System Resources. Retrieved from https://www.nih.gov.

Disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult qualified healthcare professionals for medical concerns.

written by:
Dr. N. Sujith Kumar

Dr. N. Sujith Kumar is a pharmacy educator, Pharm.D graduate, and academic content creator from Jawaharlal Nehru Technological University Kakinada (JNTUK), where he completed his Doctor of Pharmacy (Pharm.D) degree between 2015 and 2021. He has more than 7 years of experience creating pharmacy educational content, writing study materials, and reviewing academic articles for pharmacy students. He has also contributed guest articles to pharmacy education platforms, including PharmD Guru. At D.PharmGuru, his work focuses on simplifying complex Diploma in Pharmacy (D.Pharmacy) subjects into easy-to-understand notes, practical explanations, and exam-oriented educational resources for students across India. His areas of focus include Human Anatomy and Physiology, Pharmaceutics, Pharmacology, Pharmaceutical Chemistry, Hospital and Clinical Pharmacy, and other core D.Pharmacy subjects.