NERVOUS SYSTEM
The nervous system is a highly advanced network of specialised cells called neurons that help in coordinating body functions and transmitting signals between different body parts. A single nerve is a collection of various neurons.
Layers of Connective Tissue in Nerves
- Endoneurium: Surrounds each individual neuron
- Perineurium: Surrounds the bundle of fibres
- Epineurium: Surrounds and encloses numerous nerve fibre bundles
CLASSIFICATION OF NERVOUS SYSTEM
The nervous system is broadly classified into the Central Nervous System (CNS) and Peripheral Nervous System (PNS).
CENTRAL NERVOUS SYSTEM (CNS)
The CNS includes the brain and spinal cord and is the processing centre of the body. It is surrounded by three layers of membranes known as meninges.
Meninges of Brain
- Pia Mater: Innermost layer; delicate connective tissue richly supplied with blood vessels
- Arachnoid Membrane: Middle layer; thin, vascular, spider web-like structure
- Dura Mater: Outermost layer; thick, fibrous, along inner side of cranium
Ventricles of Brain
- Lateral Ventricles (2): C-shaped chambers in each cerebral hemisphere
- Third Ventricle: Narrow cavity along midline, superior to hypothalamus
- Fourth Ventricle: Located in hindbrain between brain stem and cerebellum
Cerebrospinal Fluid (CSF)
- Mechanical Protection: Absorbs shock; provides buoyancy for brain
- Chemical Protection: Optimal chemical environment for neuronal signalling
- Circulation: Aids in exchange of nutrients and waste products
PARTS OF BRAIN
Cerebrum
Cerebrum forms the major portion of the brain. It is divided into two cerebral hemispheres separated by the longitudinal fissure and connected by the corpus callosum. The outer rim (cerebral cortex) is grey matter; underlying white matter is cerebral medulla.
Lobes of Cerebrum
- Frontal Lobe: Controls voluntary activities, emotion, concentration, speech (Broca’s area)
- Parietal Lobe: Perception of touch, pain, temperature, and proprioception
- Temporal Lobe: Hearing, auditory perception, memory
- Occipital Lobe: Vision, visual perception
Functions of Cerebrum
- Controls all voluntary activities
- Receives and processes sensory stimuli
- Associated with higher brain functions (memory, intelligence, reasoning, learning)
- Controls mental activities, sensory perception, and voluntary muscle contraction
Cerebellum
Cerebellum is butterfly-shaped, located inferior to cerebrum and posterior to pons and medulla. It is divided into two cerebellar hemispheres by the vermis. The cerebellar cortex has grey matter arranged in folia; white matter is called arbor vitae.
Functions of Cerebellum
- Maintains body posture and equilibrium
- Controls and coordinates movements of multiple muscle groups
Midbrain
The midbrain connects the cerebrum to the hindbrain. It contains the tectum (superior and inferior colliculi) and cerebral peduncles. It controls eye movement, visual/auditory functions, and body movements. Degeneration of dopamine-producing neurons in substantia nigra causes Parkinson’s disease.
Medulla Oblongata
- Pyramidal tracts cross from one side to the other (right brain controls left body)
- Cardiovascular centre controls heart rate and contraction force
- Respiratory centre controls rate and depth of respiration
- Vasomotor centre controls diameter of blood vessels
- Reflex centres for vomiting, coughing, and sneezing
Pons
Pons is located above medulla and below midbrain. It contains accessory breathing centres (apneustic and pneumotaxic centres) and serves as a motor relay centre.
Diencephalon
Thalamus
- Primary relay station for sensory impulses to cerebral cortex
- Regulates sleep and wakefulness
- Controls motor systems
Hypothalamus
- Autonomic nervous system regulation (heart rate, blood pressure, digestion)
- Temperature regulation (shivering, sweating)
- Hormone regulation (pituitary gland control)
- Thirst and hunger regulation
- Sleep regulation (circadian rhythm)
- Emotional responses, memory, learning
- Sexual behavior and reproduction
Epithalamus
- Pineal gland secretes melatonin (regulates reproductive cycle, sleep/wakefulness)
- Habenular nuclei relate smell to emotional responses
SPINAL CORD
The spinal cord is the elongated part of the CNS extending from the lower end. It has 31 pairs of spinal nerves (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal).
Functions of Spinal Cord
- Contains sensory and motor tracts
- Conducts impulses to/from brain
- Site for integration of EPSPs and IPSPs
- Connects CNS to sensory receptors, muscles, and glands
- Mediates all reflex activities
Reflex Arc Components
- Receptor organ (detects stimulus)
- Sensory neuron (transmits to CNS)
- Integrating centre (inter-neuron in grey matter)
- Motor neuron (transmits from CNS)
- Effector organ (muscle or gland responds)
PERIPHERAL NERVOUS SYSTEM (PNS)
Cranial Nerves (12 Pairs)
- I Olfactory (Sensory) – Smell
- II Optic (Sensory) – Vision
- III Oculomotor (Motor) – Eye movement
- IV Trochlear (Motor) – Eye movement
- V Trigeminal (Mixed) – Facial sensation, mastication
- VI Abducens (Motor) – Eye movement
- VII Facial (Mixed) – Taste, facial expression, salivation
- VIII Vestibulocochlear (Sensory) – Hearing, equilibrium
- IX Glossopharyngeal (Mixed) – Taste, salivation, swallowing
- X Vagus (Mixed) – Visceral functions (heart, lungs, digestion)
- XI Accessory (Motor) – Neck and shoulder movement
- XII Hypoglossal (Motor) – Tongue movement
AUTONOMIC NERVOUS SYSTEM (ANS)
Sympathetic Nervous System (Fight or Flight)
- Thoracolumbar outflow (T₁ to L₂)
- Short preganglionic, long postganglionic fibres
- Releases norepinephrine (adrenergic fibres)
- Effects: tachycardia, bronchodilation, vasoconstriction, mydriasis, decreased peristalsis, ejaculation
Parasympathetic Nervous System (Rest and Digest)
- Craniosacral outflow (CN III, VII, IX, X and S₂-S₄)
- Long preganglionic, short postganglionic fibres
- Releases acetylcholine (cholinergic fibres)
- Effects: bradycardia, bronchoconstriction, miosis, increased peristalsis, erection, salivation
