Lecture Notes on The Central Nervous System

Chapter 11: The Central Nervous System

Section 11.1: General Characteristics of the Central Nervous System

• Central Nervous System (CNS) comprises the brain and spinal cord.

• Functions of the Brain:

  • Largest and most complex part of the nervous system.

  • Controls sensation, perception, movement, and thinking.

  • Consists of:

  • Cerebrum

  • Diencephalon

  • Brainstem

  • Cerebellum

  • Brainstem: Connects the brain to the spinal cord.

• Peripheral Nervous System (PNS):

  • Connects the CNS to the body via peripheral nerves.

• Meninges:

  • Membranes that protect the brain and spinal cord, located between the bone and the soft tissue of the nervous system.

Quick Review: Nerve Cells

• Axons: Carry nerve impulses in one direction, away from the cell body.

• Types of Neurons:

  • Impulses to the brain or spinal cord are carried on unipolar sensory neurons.

  • Impulses from the brain/spinal cord to the body are carried on multipolar motor neurons.

• Myelin: White, fatty coating that speeds up nerve impulses. Only axons are myelinated, but not all axons are.

White and Gray Matter

• White Matter: Consists of myelinated axons that carry impulses rapidly from one location to another.

• Gray Matter: Contains neuron cell bodies and/or unmyelinated axons, functions to make decisions and generate impulses.

  • Structure in Brain: Outer layer is gray matter, inner core is mostly white matter.

  • Structure in Spinal Cord: Gray matter is located in the center (shaped like a butterfly) and white matter is on the outside.

Section 11.2: Meninges

• Three Layers of Meninges:

  • Dura Mater:

  • Outer layer, tough and dense connective tissue.

  • Contains dural sinuses (carry venous blood).

  • Epidural space (spinal cord only) contains adipose tissue and blood vessels.

  • Subdural hematoma: Accumulation of blood between dura and arachnoid mater, can compress and damage brain tissue.

  • Arachnoid Mater:

  • Middle layer, web-like.

  • Subarachnoid space: Contains cerebrospinal fluid (CSF).

  • Pia Mater:

  • Inner layer, attached to the surface of brain and spinal cord, contains blood vessels and nerves, nourishes CNS.

• Meningitis: Infection/inflammation of arachnoid and pia mater, which can be viral or bacterial, causing severe brain damage or death.

Section 11.3: Ventricles and Cerebrospinal Fluid (CSF)

• Cerebrospinal Fluid (CSF): Clear, watery fluid produced in four ventricles.

  • Ventricles: Interconnected cavities within the cerebral hemispheres and brainstem, continuous with the central canal of the spinal cord.

  • Four ventricles:

    • Two Lateral Ventricles (first and second) located in the cerebral hemispheres.

    • Third Ventricle surrounds the diencephalon.

    • Fourth Ventricle is located within the brainstem, anterior to the cerebellum.

  • Connections:

    • Interventricular foramina connect the third and lateral ventricles.

    • Cerebral aqueduct: Connects third and fourth ventricles.

• CSF Production: Secreted by choroid plexuses (capillaries of pia mater) covered by ependymal cells, creates a blood/CSF barrier.

• Functions of CSF:

  • Nutritive and protective for CNS neurons.

  • Helps maintain stable ionic concentrations in the CNS.

  • Circulates in ventricles, central canal of spinal cord, and subarachnoid space.

  • Absorption by arachnoid granulations within dural sinuses; volume ~140 mL.

Clinical Application 11.1: Traumatic Brain Injury (TBI)

• TBI Types:

  • Concussion: Mild TBI from one-time injury, typically no lasting symptoms.

  • Chronic Traumatic Encephalopathy (CTE): Resulting from repetitive mild TBIs, symptoms appear years later affecting memory and behavior.

  • Blast-related brain injury: Severe TBI from explosions in combat situations leading to cognitive decline.

Clinical Application 11.2: Cerebrospinal Fluid Pressure

• Continuous secretion and reabsorption maintain constant pressure in ventricles.

• Interference (by infection, tumor, blood clot) may lead to increased intracranial pressure (ICP).

• Pressure monitoring through a spinal tap/lumbar puncture.

• Hydrocephalus in infants treated with shunt insertion.

Section 11.4: Brain

• Brain Functions:

  • Sensory function, perception, motor commands to skeletal muscles, higher mental functions (memory, reasoning), coordination of muscular movement, regulation of visceral activities, personality.

• Brain Development:

  • Neural tube gives rise to the CNS; brain forms from three vesicles: forebrain (prosencephalon), midbrain (mesencephalon), hindbrain (rhombencephalon).

  • Each vesicle divides further into:

  • Forebrain: telencephalon (cerebrum, basal nuclei) and diencephalon (thalamus, hypothalamus).

  • Midbrain: remains as midbrain.

  • Hindbrain: metencephalon (cerebellum, pons) and myelencephalon (medulla oblongata).

Major Portions of the Adult Brain

• Cerebrum

• Diencephalon

• Cerebellum

• Brainstem

Structural Development of the Brain: Table 11.2

Structure of the Cerebrum

• Largest part of the brain.

• Cerebral Hemispheres: Two halves separated by falx cerebri.

• Corpus Callosum: Connects the two hemispheres.

• Gyri: Ridges or convolutions on the surface.

• Sulci: Shallow grooves; e.g., central sulcus.

• Fissures: Deep grooves that separate brain areas.

• Five Primary Lobes:

  • Frontal lobe

  • Parietal lobe

  • Temporal lobe

  • Occipital lobe

  • Insula

• Cerebral Cortex: Thin outer layer of gray matter, containing 75% of neuron cell bodies in the nervous system.

• White Matter: Lies beneath the cerebral cortex, connecting neuron cell bodies to other parts of the nervous system.

Functions of the Cerebral Cortex

• Higher mental functions like interpreting sensory impulses, initiating voluntary movements, memory storage and retrieval, reasoning, and personality establishment.

• Divided into sensory, association, and motor areas, with overlaps present.

Sensory Areas of the Cortex

• Cutaneous sensory area: In parietal lobe, interprets skin sensations.

• Sensory speech area (Wernicke’s area): Temporal/parietal lobe, understanding language.

• Visual area: Occipital lobe, interprets vision.

• Auditory area: Temporal lobe, interprets sound.

• Taste and Smell Sensory Areas: Found in association with the cortex deep within temporal lobes.

Decussation in the CNS

• Nerve fibers typically cross over in the brainstem and spinal cord, with the right brain interpreting sensations from the left side of the body and vice versa.

Association Areas of the Cortex

• Definition: Areas not primarily motor or sensory.

  • Connect to other cortical areas and structures in the brain.

  • Help interpret sensory experiences, memory, reasoning, judgment, and emotional responses.

• Frontal Lobe Association Areas: Concentration, planning, problem-solving, emotional behavior, consequences judgment.

• Parietal Lobe Association Areas: Speech understanding and word choice.

• Temporal Lobe Association Areas: Interpret sensory experiences, store auditory and visual memories.

• Occipital Lobe Association Areas: Analyze visual images with other senses.

• Insula: Converts sensory information into emotional responses.

Motor Areas of the Cortex

• Primary Motor Areas (Motor Cortex): In frontal lobe (precentral gyrus), controls voluntary muscles by initiating motor signals.

• Pyramidal Cells: Neurons with large triangular bodies responsible for voluntary movement signals.

• Broca’s Area: Controls muscles for speech, found anterior to primary motor cortex in the left hemisphere.

• Frontal Eye Field: Above Broca’s area, controls voluntary eye and eyelid movements.

Speech Functions

• Coordination between Broca’s (motor component) and Wernicke’s (understanding component) areas is necessary for speech production and comprehension.

Table 11.3: Functions of Cerebral Lobes

Hemisphere Dominance

• Dominant Hemisphere: Typically the left side, controlling language skills and analytical abilities.

• Nondominant Hemisphere: Controls nonverbal tasks and emotional processing.

Memory

• Types of Memory:

  • Short-term (Working) Memory:

  • Neurons connected in a circuit, can be temporary unless consolidated into long-term memory.

  • Long-term Memory:

  • Involves structural changes in neuron connections lasting a lifetime.

  • Long-term potentiation: Increased neurotransmitter release and synaptic transmission efficiency from repeated stimulation.

  • Involvement of the Hippocampus: Critical for converting short-term memories into long-term.

  • Amygdala's Role: Assigns emotional value to memories (pleasant/traumatic).

Basal Nuclei

• Also known as basal ganglia; groups of gray matter masses deep within the cerebral hemispheres, comprising:

  • Caudate nucleus

  • Putamen

  • Globus pallidus

• Functions to control voluntary movement and produce dopamine.

Clinical Applications

Clinical Application 11.3: Parkinson's Disease (PD)

• PD results from neuron degeneration in the substantia nigra, affecting dopamine production, leading to motor dysfunctions (tremors, rigidity).

• Treatment options include Levodopa (dopamine precursor) and deep brain stimulation.

Section 11.5: Spinal Cord

• Structure of the Spinal Cord: Slender column of nervous tissue continuing from brain and brainstem, extending down the vertebral canal.

• CNS Segmentation: 31 segments connecting to pairs of spinal nerves, grouped based on the vertebrae levels.

Spinal Nerves Segments:

• Cervical (C1-C8) - 8 pairs

• Thoracic (T1-T12) - 12 pairs

• Lumbar (L1-L5) - 5 pairs

• Sacral (S1-S5) - 5 pairs

• Coccygeal (Co) - 1 pair

Structure of the Spinal Cord:

• Longitudinal Section:

  • Cervical enlargement: Supplies upper limb nerves.

  • Lumbar enlargement: Supplies lower limb nerves.

  • Conus medullaris: Tapered section at the end.

  • Filum terminale: Connective tissue anchoring the spinal cord to the coccyx.

  • Cauda equina: Bundle of lumbar and sacral nerves extending from the conus medullaris.

Cross-Sectional Structure:

• Composed of:

  • Anterior median fissure and posterior median sulcus (longitudinal grooves).

  • White matter surrounds gray matter core arranged in horns (mostly unmyelinated interneurons).

  • Gray Commissure: Surrounds the central canal.

Main Functions of the Spinal Cord:

1. Center for spinal reflexes.

2. Conduit for impulses traveling to and from the brain.

Reflex Overview:

• Definition of Reflex: Automatic, subconscious reaction to a stimulus, helping maintain homeostasis.

• Reflex Arc: Neural pathway that includes sensory receptor, neurons, and effector.

Types of Reflexes:

1. Monosynaptic Reflex: Involves two neurons (e.g., knee-jerk reflex).

2. Withdrawal Reflex: Protects from pain, involves more neurons.

3. Crossed Extensor Reflex: Balances body weight during withdrawal by engaging opposite limb muscles.

Clinical Applications:

Clinical Application 11.5: Uses of Reflexes

• Utilized to assess nervous system condition; examples include the knee-jerk and plantar reflex tests.

Ascending and Descending Tracts:

• Ascending Tracts: Conduct sensory impulses to the brain (e.g., fasciculus gracilis, spinothalamic).

• Descending Tracts: Conduct motor impulses from the brain (e.g., corticospinal tracts).

Clinical Application 11.6: Amyotrophic Lateral Sclerosis (ALS)

• ALS: Degeneration of motor neurons; leads to difficulty with muscle control and coordination; often fatal within few years.

Clinical Application 11.7: Spinal Cord Injuries

• Results from trauma (accidents, pressure from tumors), impairing sensations and movements based on damage severity.

Section 11.6: Peripheral Nervous System (PNS)

• PNS Composition: Includes nerves connecting CNS to other body parts, such as cranial and spinal nerves.

• Subdivisions of PNS:

  1. Somatic Nervous System: Connects CNS to skin and skeletal muscles (voluntary).

  2. Autonomic Nervous System: Connects to viscera, uncontrolled by conscious effort.

Structure of Peripheral Nerves:

• Nerves are bundles of axons.

• Classification:

  • By Function: Sensory, motor, and mixed nerves, with most nerves being mixed.

  • By Origination: Cranial nerves (from brain) and spinal nerves (from spinal cord).

Cranial Nerves:

• 12 pairs: Most are mixed nerves with sensory/motor functions.

• Cranial Nerves I & II:

  • Olfactory nerve (I): Sensory for smell.

  • Optic nerve (II): Sensory for vision.

Other Important Cranial Nerves:

• Cranial Nerves III - VII: Includes oculomotor, trochlear, trigeminal (mixed), abducens (motor), and facial (mixed).

• Cranial Nerves VIII-XII: Include vestibulocochlear (VIII, sensory), glossopharyngeal (IX, mixed), vagus (X, mixed), accessory (XI, motor), and hypoglossal (XII, motor).

Table 11.9: Functions of Cranial Nerves

Spinal Nerves:

• All mixed, except for the first pair.

• 31 pairs of spinal nerves classified as:

  • Cervical: 8 pairs

  • Thoracic: 12 pairs

  • Lumbar: 5 pairs

  • Sacral: 5 pairs

  • Coccygeal: 1 pair.

Spinal Nerve Branches:

• Roots: Each spinal nerve splits into anterior (motor) and posterior (sensory) roots.

• Dorsal Root Ganglion: Contains cell bodies of sensory neurons.

• Branches of Spinal Nerves Include:

  • Meningeal branch (innervates meninges)

  • Posterior branch (innervates skin of back)

  • Anterior branch (forms peripheral nerves)

  • Visceral branch (only in thoracic/lumbar).

Nerve Plexuses:

• Complex networks formed by spinal nerves, absent in T2-T12; includes cervical, brachial, and lumbosacral plexuses.

Section 11.7: Autonomic Nervous System (ANS)

• Definition: Efferent part of PNS that functions without conscious effort, regulating visceral activities and maintaininghomeostasis.

• Two Main Divisions:

  • Sympathetic Division: Prepares body for "fight or flight" situations.

  • Parasympathetic Division: Prepares body for "rest and digest" activities.

Structure of Sympathetic Division:

• Thoracolumbar division, consisting of preganglionic fibers originating from T1 to L2.

• Preganglionic fibers connect to sympathetic chain ganglia.

• Can synapse at the same ganglia, skip to other ganglia or collateral ganglia.

Structure of Parasympathetic Division:

• Craniosacral division where preganglionic neurons arise from brainstem and S2-S4.

• Preganglionic fibers extend to terminal ganglia close to their target organs.

Autonomic Neurotransmitters:

• Cholinergic Neurons: Release acetylcholine; include preganglionic sympathetic and parasympathetic neurons.

• Adrenergic Neurons: Release norepinephrine (noradrenaline); mainly postganglionic sympathetic fibers.

Section 11.8: Life-Span Changes of the Nervous System

• Brain cell death begins before birth due to apoptosis.

• Reduction in brain size by ~10% occurs over a lifetime, with a greater loss of gray matter.

• Aged 90: Up to 50% neuron loss in the frontal cortex.

• Notable decreases in dendritic connections and neurotransmitter levels.

• Action potential propagation slows by 5-10%.

• Increased memory fading and response time, leading to more risks of fainting and falling ■ Change in sleep patterns, resulting in fewer sleeping hours.