CNS

CENTRAL NERVOUS SYSTEM

NERVOUS SYSTEM OVERVIEW

  • The nervous system consists of two main parts:
      - Central Nervous System (CNS)
      - Peripheral Nervous System (PNS)
      - The CNS includes the brain and spinal cord.
      - The PNS is further divided into:
        - Autonomic Nervous System
          - Sympathetic
          - Parasythetic
        - Somatic Nervous System

STRUCTURE OF THE BRAIN

  • The brain is divided into three major regions:
      1. Forebrain (Prosencephalon)
         - Telencephalon (Cerebrum)
           - Contains:
             - Cerebral hemispheres
             - Cerebral cortex (surface covered by grey matter)
           - Divided into four lobes:
             - Frontal Lobe (motor functions)
             - Parietal Lobe
             - Temporal Lobe (hearing)
             - Occipital Lobe (vision)
         - Diencephalon
           - Thalamus (receives sensory input except smell)
           - Subthalamus (includes red nucleus, substantia nigra, etc.)
           - Epithalamus (habenular nucleus & pineal gland)
           - Hypothalamus (regulates sleep, hunger, body temperature)
           - Amygdala (controls fear, memory, sexual desire)
           - Hippocampus (learning and memory)
      2. Midbrain (Mesencephalon)
      3. Hindbrain (Rhombencephalon)
         - Cerebellum – maintains posture and balance
         - Pons – regulates breathing rhythm
         - Medulla Oblongata – controls respiration, circulation, digestion, swallowing, coughing

BRAIN WEIGHT AND STRUCTURE

  • The brain constitutes about 3% of body weight and weighs approximately 1.5 kg.

  • The brain and spinal cord are protected by three layers:
      - Pia Mater
      - Arachnoid Mater
      - Dura Mater

  • Subdural space between the dura and arachnoid mater is filled with tissue fluid, and subarachnoid space contains cerebrospinal fluid (CSF).

FUNCTIONS OF THE CEREBRUM

  • Interpret touch, speech, vision, hearing, reasoning, emotions, and the fine control of movements.

THALAMUS FUNCTIONS

  • Receives senses (except smell).

  • Regulates consciousness levels.

  • Manages motor and cognitive functions.

HYPOTHALAMUS FUNCTIONS

  • Regulates:
      - Sleep
      - Hunger
      - Body Temperature

HIPPOCAMPUS FUNCTIONS

  • Learning and memory processes.

AMYGDALA FUNCTIONS

  • Controls fear, memory, and triggers sexual desire.

BRAIN STEM FUNCTIONS

  • Composed of the midbrain, pons, and medulla.

  • Relays signals between brain and spinal cord.

  • Controls basic involuntary functions:
      - Pons – breathing rhythm
      - Medulla – respiration, circulation, digestion, swallowing, coughing
      - Midbrain – vision, hearing, motor control

NEURONS AND THEIR STRUCTURE

  • Neurons are the structural and functional units of the nervous system, with approximately 100 million neurons in the body.

  • Neuron Structure:
      - Cell Body (Soma or Perikaryon)
        - Contains nucleus, Nissl bodies, neurofibrils, mitochondria, Golgi apparatus.
        - The nucleus has nucleolus and lacks centromere, thus cannot divide.
      - Nissl Bodies
        - Basophilic granules present in soma, critical for protein synthesis.
      - Mitochondria
        - Present in soma and axon, responsible for ATP synthesis.
      - Golgi Apparatus
        - Involved in processing and packaging proteins.

NEUROFIBRILS

  • Composed of microfilaments and microtubules, present in soma and nerve processes.

DENDRITES

  • Short processes that transmit impulses towards the cell body.

AXONS

  • Long processes from axon hillock; the first part is highly excitable.

  • Multiple axons form bundles called fasciculi; multiple fasciculi form a nerve trunk, each nerve fiber covered by perineurium; nerve trunk covered by epineurium.

  • Contains cytoplasm (axoplasm) covered by axolemma and neurilemma.

TYPES OF AXONS (NERVE FIBERS)

  1. Myelinated Axons
       - Covered by myelin sheath, absent in Nodes of Ranvier.
       - Impulse conduction via saltatory conduction, jumping from one node to another, facilitating faster impulse transmission.

  2. Non-Myelinated Axons
       - Lack myelin sheath and conduct impulses more slowly.

CLASSIFICATION OF NEURONS

Based on Number of Poles:

  1. Unipolar Neuron – one pole for axon and dendrite.

  2. Bipolar Neuron – two poles; one for axon and one for dendrite.

  3. Multipolar Neuron – many poles; one axon and multiple dendrites.

Based on Function:

  1. Sensory Neuron – transmits sensory information to CNS.

  2. Motor Neuron – transmits motor information to effector organs.

Based on Axon Length:

  1. Golgi Type I Neuron – very long axon.

  2. Golgi Type II Neuron – short axon present in cerebral cortex and spinal cord.

NERVE IMPULSE CONDUCTION

  • Conduction is an active self-propagating process; the active region generates an action potential.

  • Unmyelinated Neurons: Current flows between active and inactive regions leading to depolarization; process called electrotonic conduction.

  • Myelinated Neurons: Current flows from inactive to active node, causing saltatory conduction, which is 50-100 times faster than in unmyelinated fibers.

ASCENDING TRACTS

Sensory Tracts:

  1. Lissauer’s Tract – pain and temperature.

  2. Lateral Spinothalamic Tract – pain and temperature.

  3. Anterior/Ventral Spinothalamic Tract – pressure and deep touch.

  4. Spinoreticular Tract – slow pain and alertness.

  5. Dorsal Column Tract
       - Contains fasciculus gracilis and cuneatus, responsible for fine touch, tactile localization, and discrimination.

Non-Sensory Tracts:

  1. Spinocerebellar Tracts – muscle tone and coordinate movements.

  2. Spinotectal Tract – spinovisual reflex.

  3. Spino-olivary Tract – no significant function.

EXTRAPYRAMIDAL TRACTS

  • Regulate background tone, posture, and equilibrium.

  • Include pathways from subcortical structures like basal ganglia and brain stem.

  • Key tracts:
      - Reticulospinal Tract – regulates muscle tone.
      - Vestibulospinal Tract – facilitates extensor muscles.
      - Rubrospinal Tract – facilitates flexor muscles.
      - Tectospinal Tract – reflex neck movements due to visual stimuli.
      - Medial Longitudinal Bundle – coordinates neck and eye movements.

VISCERAL PAIN

  • Viscera are normally insensitive but can cause diffused, poorly localized pain during inflammation.

Causes of Visceral Pain:

  1. Inflammation (e.g., appendicitis).

  2. Distension (e.g., stones).

  3. Cramping pain (e.g., dysmenorrhea).

  4. Chemical damage (e.g., peptic ulcer).

  5. Spasm of smooth muscles (e.g., intestinal colic).

Pathways of Visceral Pain:

  • Carried by vagus and glossopharyngeal nerves from trachea, pharynx, esophagus.

  • Sympathetic fibers from T1 to L2 segments in thorax.

  • Sacral parasympathetic for pelvic organs.

Features of Visceral Pain:

  • Less localized, more diffuse.

  • Often accompanied by autonomic manifestations (nausea, vomiting).

  • Causes reflex contractions in nearby muscles (guarding), and is often not localized (referred pain).

REFERRED PAIN

  • Pain from a visceral organ felt in a somatic area distant from the source due to shared embryonic origin.

Examples:

  1. Heart pain felt in left shoulder and fingertips.

  2. Diaphragmatic pain referred to right shoulder.

  3. Appendicitis pain referred to umbilicus.

  4. Maxillary sinus pain referred to nearby tooth.

DERMATOMAL RULE

  • Pain is referred to structures developed from the same embryonic tissue as the organ producing it.

THEORIES EXPLAINING PAIN MECHANISM

  • Convergence Theory
      - Suggests neurons from both cutaneous and visceral sources converge on the same nerve pathway in the spinal cord, leading to misinterpretation.

  • Facilitation Theory
      - Suggests persistently activated pathways become more sensitive over time, further complicating pain perception.

BRODMANN’S AREAS IN CEREBRAL CORTEX

Frontal Lobe Areas:

  • Area 4 – Primary Motor Area (initiates voluntary movements).

  • Area 6 – Premotor Area (regulates skilled movements).

  • Area 8 – Frontal Eye Field (controls conjugate eye movement).

  • Area 44 – Broca’s Speech Area (controls speech motor activity, located in left hemisphere for right-handed individuals).

  • Areas 9, 10, 11, 23, 24, 29, 32, 35, 45, 46, 47 – Prefrontal/orbitofrontal cortex (controls thought, memory, and behavior).

Parietal Lobe Areas:

  • Areas 3, 1, 2 – Primary Sensory Area (appreciation of somatic sensations).

  • Areas 5, 7 – Sensory Association Area (analysis of sensations).

Occipital Lobe Areas:

  • Areas 17, 18, 19 – Visual Areas (perception and interpretation of visual stimuli).

Auditory Areas:

  • Areas 41, 42, 22 – Understand auditory stimuli.

HYPOTHALAMUS FUNCTIONS

  • Thermoregulation:
      - Anterior hypothalamus (heat loss center).
      - Posterior hypothalamus (heat gain center).

  • Regulation of Anterior Pituitary:
      - Hormones include:
        - GHRH, GHIH, TRH, CRH, GnRH, PRH, PIH.
      - These regulate various hormones like GH, TSH, ACTH, FSH, LH, Prolactin.

  • Regulation of Posterior Pituitary:
      - ADH and Oxytocin released from hypothalamus and stored/released by posterior pituitary.

  • Water Balance Regulation:
      - Lateral hypothalamus involved in thirst; ADH regulation for water balance.

  • Hunger and Satiety:
      - Lateral hypothalamus (hunger), Median hypothalamus (satiety).

  • Autonomic Nervous System Regulation:
      - Anterior hypothalamus stimulates parasympathetic;
      - Posterior hypothalamus stimulates sympathetic.

  • General and Sexual Behavior Regulation:
      - Involves reward and punishment centers affecting behavior.

  • Biological Clock Role:
      - Regulates daily and monthly cycles (sleep, hormonal rhythms, etc.).

FUNCTIONS OF CEREBELLUM

  1. Coordination of voluntary movements.

  2. Maintains equilibrium.

  3. Predictive functions for limb positioning.

  4. Error detection and correction; connected to both cerebral cortex and muscles.

  5. Regulates rate, range, force, and direction of movement.

  6. Oversees involuntary movements (e.g., knee jerk).

  7. Causes hypertonicity, ataxia, hypotonia, and intention tremor.

THALAMIC FUNCTIONS

  • Relay center for sensory pathways (fine touch, pain, and temperature).

  • Integrates visual, auditory, olfactory, and taste functions.

  • Prepares the groundwork for language development and emotional behavior.

  • Contributes to attention and alertness through the ascending reticular activating system (ARAS).

MEDULLA FUNCTIONS

  • Connects the brain to the spinal cord.

  • Regulates autonomic functions (heart rate, blood pressure, respiration).

  • Cranial nerves IX, X, XI, and XII arise from here, responsible for swallowing, taste, digestion, and neck movements.

PONS FUNCTIONS

  • Relays signals between cerebellum and cerebral cortex.

  • Controls sensation and motor function of the head and neck.

  • Regulates respiration rate and depth.

  • Involved in sleep cycles and body’s fight or flight responses.

VESTIBULAR APPARATUS

Structure:

  • Comprises three semicircular canals, utricle, and saccule.

  • Semicircular canals positioned in three axes:
      - Horizontal, Superior, Inferior.

  • Hair cells activated by movement within the endolymph fluid.

Function:

  • Detects head position concerning gravity and detects angular acceleration (rotational movements).

  • Maintains balance by controlling posture.

  • Works with visual and proprioceptive inputs for spatial awareness.

NEUROGLIA FUNCTIONS

  1. Astrocytes:
       - Support neurons, repair scar formation, form the blood-brain barrier.

  2. Oligodendrocytes:
       - Myelinate nerve fibers.

  3. Microglia:
       - Phagocytic functions to clean up debris.

CEREBROSPINAL FLUID (CSF)

Composition:

  • Volume: 150 ml.

  • pH: 7.35-7.4.

  • Osmolarity: 289 milliosmoles/liter.

  • Pressure: 10 mmHg.

Functions:

  1. Cushions and protects the brain.

  2. Regulates intracranial volume and pressure.

  3. Nourishes the brain; removes metabolites.

  4. Exchanges substances between blood and brain tissue.

CSF Circulation:

  • Pathway:
      - Lateral Ventricle → Foramen of Monro → Third Ventricle → Aqueduct of Sylvius → Fourth Ventricle → Foramen of Luschka/Magendie → Central Canal/Subarachnoid Space.

BLOOD-BRAIN BARRIER (BBB)

  • Function: Protects the brain from harmful substances while allowing nutrient passage.

  • Composed of tight junctions between endothelial cells; absent in areas such as the hypothalamus, allowing for hormonal regulation.

  • The blood-CSF barrier exists at the choroid plexus with tight junctions between choroid cells.

LIMBIC SYSTEM FUNCTIONS

  1. Feeding Behavior:
       - Stimulation of hunger and thirst; satisfaction through limbic activation.

  2. Somatic Responses:
       - Emotional expressions via facial muscles.

  3. Sexual Behavior:
       - Governed by limbic system and hypothalamic integration, influenced by hormones.

  4. Maternal Behavior:
       - Cingulate gyrus importance in breast feeding and offspring protection.

  5. Consciousness and Memory:
       - Recent memory management by hippocampus; emotional regulation by limbic interactions.

SYMPATHETIC AND PARASYMPATHETIC NERVOUS SYSTEMS

Sympathetic Nervous System:

  1. Preganglionic neurons in T1 to L2 segments.

  2. Short preganglionic and long postganglionic neurons.

  3. Effects are widespread and long-lasting (fight or flight response).

Parasympathetic Nervous System:

  1. Originates in cranial nerve nuclei (III, VII, IX, X) and sacral segments (S2, S3, S4).

  2. Long preganglionic and short postganglionic neurons.

  3. Effects localized to specific organs (rest and digest).

Effects Comparison:

  • Parasympathetic:
      - Decrease in heart rate.
      - Bronchoconstriction.
      - Increased gland secretions.
      - Constricts pupils.

  • Sympathetic:
      - Increase in heart rate.
      - Bronchodilation.
      - Decreased gland secretions (except salivary).
      - Dilation of pupils.