Comprehensive Guide to the Central and Peripheral Nervous Systems

Basic Definitions in Neurology

• Grey Matter: The gray area of the central nervous system ($CNS$). It forms the outer layer of the brain and the inner layer of the spinal cord. It is primarily composed of cell bodies and unmyelinated nerve fibres.
• White Matter: The white area of the $CNS$. It forms the inner layer of the brain and the outer layer of the spinal cord. It contains myelinated nerve fibres.
• Tract: A collection of nerve fibres within the $CNS$ that share the same origin, termination, and function.
• Nerve: A white fibre or bundle of fibres located in the peripheral nervous system. It transmits sensory impulses to the brain or spinal cord and carries motor impulses from the $CNS$ back to muscles and organs.
• Ganglion: A collection of nerve cell bodies located outside of the $CNS$, specifically in the peripheral nervous system ($PNS$).
• Decussation: A crossed tract of nerve fibres that passes between different centres located on opposite sides of the nervous system.
• Cortex: The outer surface of an organ.
• Central Nervous System ($CNS$): The part of the nervous system consisting of the brain and spinal cord, where nerve fibres and impulses occur.
• Peripheral Nervous System ($PNS$): The part of the nervous system located outside the brain and spinal cord, consisting of nerves and ganglia.
• Autonomic Nervous System ($ANS$): An efferent division of the $PNS$ that innervates cardiac muscle, smooth muscles, and glands. It is considered an involuntary or visceral motor system.
• Somatic Nervous System: Composed of somatic nerve fibres that conduct impulses from the $CNS$ to skeletal muscles. It is the voluntary nervous system under conscious control.

Support Cells: The Neuroglia

Neuroglia, or glial cells, support the nervous system. There are $6$ types in total: $4$ found in the $CNS$ and $2$ found in the $PNS$.

• Neuroglia of the $CNS$:

  • Astrocytes: These are the most abundant and versatile glial cells. They facilitate exchanges between capillaries and neurons, help determine capillary permeability, guide the migration of young neurons, and assist in synapse formation. They are involved in information processing, recycling released neurotransmitters, and taking up leaked potassium ($K^+$) ions.
  • Microglia: These cells monitor the health of nearby neurons through physical contact. If they sense danger, they migrate toward the site. In the presence of dead neurons or invading microorganisms, they transform into macrophages to phagocytize the threat. They act as immune protection for the $CNS$.
  • Ependymal Cells: These cells line the ventricles and are responsible for the production and secretion of cerebrospinal fluid ($CSF$). Their cilia assist in the circulation of $CSF$, which cushions the brain and spinal cord.
  • Oligodendrocytes: These cells produce the insulating coverings known as myelin sheaths within the $CNS$.

• Neuroglia of the $PNS$:

  • Satellite Cells: These are thought to share the same functions as astrocytes.
  • Schwann Cells: These surround larger nerve fibres in the $PNS$ to form myelin sheaths. They perform a role similar to oligodendrocytes and are vital for the regeneration of damaged $PNS$ nerve fibres.

Neuronal Anatomy and Classification

• Components of a Neuron:

  • Cell Body: Contains the nucleus and organelles such as the Golgi apparatus and ribosomes. It is the site of protein production for the dendrites, axons, and synaptic terminals and provides energy for the cell.
  • Dendrites: The input/receptive regions for stimulus. They receive signals from other neurons and afferents, conveying messages toward the cell body. These signals are graded potentials rather than action potentials.
  • Axons: Arising from the axon hillock, these are the conducting components of the neuron. They generate and transmit nerve impulses away from the cell body to an effector. The terminals, or boutons, produce neurotransmitters.
  • Axon Hillock: The cone-shaped area where the axon originates from the cell body.
  • Nodes of Ranvier: Gaps found between Schwann cells where nerve impulses travel at a faster rate via saltatory conduction ("jumping").

• Structural Classification:

  • Multipolar Neurons: Feature $3$ or more processes, typically one axon and multiple dendrites. These are the most common type in humans and the major neuron type in the $CNS$.
  • Bipolar Neurons: Feature $2$ processes extending from opposite sides of the cell body (one axon and one dendrite). These are rare, found in special sense organs like the retina.
  • Unipolar Neurons: Feature a single short process that divides into proximal and distal branches. The distal (peripheral) process is associated with sensory receptors, while the proximal (central) process enters the $CNS$. These are found primarily in the ganglia of the $PNS$.

• Functional Classification:

  • Sensory (Afferent) Neurons: Transmit impulses from sensory receptors in the skin or internal organs toward the $CNS$. Most are unipolar, with cell bodies in sensory ganglia outside the $CNS$.
  • Motor (Efferent) Neurons: Carry impulses away from the $CNS$ toward effector organs. They are multipolar, and their cell bodies are located in the $CNS$ (except for those in the $ANS$).
  • Interneurons (Association Neurons): Located between sensory and motor neurons in neural pathways. They shuttle signals through $CNS$ pathways for integration. Most are confined to the $CNS$, are multipolar, and vary significantly in size and branching.

The Process of Myelination

• Myelination begins when an axon lies in a groove on the surface of a Schwann cell.
• The Schwann cell surrounds the axon, defining two domains: the adaxonal plasma-membrane (periaxomal) domain and the abaxonal plasma-membrane domain (exposed to the external environment/endoneurium).
• The mesaxon plasma membrane links these domains and initiates myelination by surrounding the embedded axon.
• A sheet-like extension of the mesaxon membrane wraps repeatedly around the axon.
• During the wrapping process, the cytoplasm is forced out from between the two opposing membranes, which then compact to form myelin.
• The outer mesaxon is the invaginated plasma membrane extending from the abaxonal surface to the myelin. The inner mesaxon extends from the adaxonal surface (facing the axon) to the myelin.

The Ventricular System and Cerebrospinal Fluid ($CSF$)

• System Overview: The ventricles arise from the embryonic neural tube lumen. They are continuous with one another and the central canal of the spinal cord. They are filled with $CSF$ and lined by ependymal cells.

• Structure:

  • Paired Lateral Ventricles: C-shaped chambers located deep within each cerebral hemisphere, separated by the septum pellucidum.
  • Third Ventricle: A narrow space in the diencephalon. It connects to the lateral ventricles via the interventricular foramen.
  • Fourth Ventricle: Connected to the third ventricle via the cerebral aqueduct in the midbrain. It sits in the hindbrain, dorsal to the pons and superior to the medulla. It features $3$ openings: $2$ lateral apertures and $1$ median aperture in the roof, allowing $CSF$ to flow into the subarachnoid space.

• $CSF$ Formation, Circulation, and Absorption:

  • Formation: Produced by choroid plexuses hanging from the roof of each ventricle.
  • Circulation: Moves through ventricles and enters the subarachnoid space via the fourth ventricle apertures. Flow is aided by ependymal cell cilia.
  • Path: Choroid plexus in lateral ventricles → Interventricular foramen → Third ventricle → Cerebral aqueduct → Fourth ventricle → Subarachnoid space (via apertures) → Superior sagittal sinus → Straight sinus → Confluence of sinuses.
  • Absorption: Returns to venous blood in the dural sinuses via arachnoid villi.

Cerebral Topography: Sulci, Gyri, and Fissures

• Sulci:

  • Central Sulcus: Separates the frontal lobe from the parietal lobe.
  • Parieto-occipital Sulcus: Separates the occipital lobe from the parietal lobe.
  • Lateral Sulcus: Separates the temporal lobe from the parietal and frontal lobes.
  • Insula: A lobe buried deep within the lateral sulcus, covered by the temporal, parietal, and frontal lobes.

• Gyri:

  • Precentral Gyrus: Borders the central sulcus anteriorly.
  • Postcentral Gyrus: Borders the central sulcus posteriorly.

• Fissures:

  • Median Longitudinal Fissure: Separates the two cerebral hemispheres.
  • Transverse Fissure: Separates the cerebrum from the cerebellum below.

Topographical and Functional Organization of the Cerebral Cortex

• Motor Homunculus (Medial to Lateral):

  • Medial (Lower body): Knee, hip, trunk, toes.
  • Superior (Mid-body): Arm, elbow, wrist, hand, fingers, thumb.
  • Lateral (Upper body): Neck, brow, eyes, face, lips, jaw, tongue.

• Sensory Homunculus (Medial to Lateral):

  • Medial (Lower body): Leg, hip, trunk, genitals, head, neck.
  • Superior (Mid-body): Arm, elbow, forearm, hand, fingers, thumb.
  • Lateral (Upper body): Eye, nose, face, lips, teeth, gums, jaw, tongue, pharynx, intra-abdominal.

• Functional Motor Areas:

  • Primary Motor Cortex (Precentral Gyrus): Controls precise or skilled voluntary skeletal muscle movements.
  • Premotor Cortex (Anterior to Precentral Gyrus): Coordinates simultaneous movement of multiple muscle groups and plans learned motor skills.
  • Broca's Area (Inferior Region of Premotor Area): Usually in the left hemisphere; directs muscles for speech production (tongue, throat, lips).
  • Frontal Eye Field (Superior to Broca's): Controls voluntary eye movements.

• Functional Sensory Areas:

  • Primary Somatosensory Cortex (Postcentral Gyrus): Receives input from skin and muscle receptors; identifies the specific part of the body being stimulated (spatial discrimination).
  • Somatosensory Association Area: Integrates sensory inputs to produce an understanding of the object being felt.
  • Visual Areas: Primary visual cortex at the posterior tip of the occipital lobe; receives information from the retina.
  • Primary Auditory Cortex (Superior Temporal Lobe): Interprets pitch, rhythm, and loudness from cochlear impulses.
  • Olfactory Cortex (Frontal/Temporal Lobes): Responsible for conscious awareness of odors.
  • Gustatory Cortex (Parietal Lobe): Perceives taste stimuli.
  • Vestibular Cortex (Posterior Insula): Conscious awareness of balance and head position.
  • Prefrontal Cortex (Anterior Frontal Lobes): Involved in intellect, recall, personality, and complex learning.
  • Wernicke's Area: Located posterior to the lateral sulcus; involved in understanding spoken and written language.

Subdivisions of the Diencephalon

• Thalamus: Functions as the central relay station of the nervous system where information is sorted and edited. It relays sensory impulses to the cortex, motor inputs from subcortical nuclei/cerebellum to the motor cortex, and impulses between lower centers and association cortices.
• Hypothalamus: A primary $ANS$ control system. It regulates cardiac and smooth muscle, gland secretion, water balance, thirst, eating behavior, gastrointestinal activity, body temperature, and the anterior pituitary. It also regulates sleep-wake cycles and is part of the limbic system (pleasure, fear, rage).
• Epithalamus: Contains the pineal gland, which secretes melatonin to regulate the sleep-wake cycle.

The Brainstem: Midbrain, Pons, and Medulla Oblongata

• Midbrain: Situated between the diencephalon and the pons. It includes:

  • Cerebral Peduncles: Pillars holding up the cerebrum; house pyramidal fiber tracts.
  • Corpora Quadrigemina: Contains $2$ superior colliculi (visual reflex) and $2$ inferior colliculi (auditory reflex).
  • Red Nucleus: Subcortical motor center.
  • Substantia Nigra: Source of the neurotransmitter dopamine.
  • Periaqueductal Gray Matter: Involved in pain suppression; contains motor nuclei for cranial nerves $III$ and $IV$.

• Pons: Located between the midbrain and the medulla. It contributes to the fourth ventricle dorsally. It serves as a conduction area with nuclei that regulate respiration and host cranial nerves $V$ through $VII$.

• Medulla Oblongata: The most inferior part of the brainstem, blending into the spinal cord at the foramen magnum. It contains:

  • Pyramids: Descending corticospinal tracts that decussate before entering the spinal cord.
  • Nuclei Cuneatus and Gracilis: Synapse points for ascending sensory pathways from the skin and proprioceptors.
  • Visceral Nuclei: Control heart rate, blood vessel diameter, respiratory rate, and vomiting.
  • Inferior Olivary Nuclei: Sensory relay to the cerebellum.

The Cerebellum

• Structure: Bilaterally symmetrical with $2$ hemispheres connected by the vermis. The surface is convoluted with folia (pleat-like gyri). It is subdivided into anterior, posterior, and flocculonodular lobes by fissures.
• Connectivity: Attached to the brainstem by the superior, middle, and inferior peduncles.
• Function: Coordinates motor activity by processing impulses from the motor cortex and sensory pathways to ensure smooth, well-timed movements. It also has roles in cognition.

Meningeal Layers and the Blood-Brain Barrier

• Meninges:

  • Dura Mater: The strongest, two-layered fibrous connective tissue. It forms internal partitions:
    • Falx Cerebri: Dips into the longitudinal fissure.
    • Falx Cerebelli: Runs along the cerebellar vermis.
    • Tentorium Cerebelli: Dips into the transverse fissure.
  • Arachnoid Mater: The middle meninx. It forms a loose covering and is separated from the dura by the subdural space. The subarachnoid space lies beneath it.
  • Pia Mater: Delicate tissue filled with tiny blood vessels. It is the only meninx that clings tightly to the brain surface.

• Dural Venous Sinuses: Areas where the two dural layers separate to collect venous blood from the brain and direct it into the internal jugular veins.

• Blood-Brain Barrier ($BBB$): Represents the relative impermeability of brain capillary epithelium. It allows water, respiratory gases, fat-soluble molecules, and essential nutrients to enter but prevents passage of many water-soluble, harmful substances.

Spinal Cord Anatomy: Cross-Sectional and Longitudinal

• Internal Gray Matter: H-shaped. Ventral horns contain somatic motor neurons; lateral horns contain visceral (autonomic) motor neurons; dorsal horns contain interneurons.

• Roots: Axons from the lateral and ventral horns exit via ventral roots. Sensory axons enter via dorsal roots. These combine to form spinal nerves.

• White Matter: Organized into dorsal, lateral, and ventral columns (funiculi). Each contains specific tracts, most of which are paired and decussate.

• Ascending (Sensory) Tracts:

  • Fasciculi Gracilis and Cuneatus: Part of the dorsal column-medial lemniscal pathway, providing precise transmission of sensory modalities.
  • Spinothalamic Tracts: Anterolateral pathways for brainstem processing of ascending impulses.
  • Spinocerebellar Tracts: Convey information about muscle or tendon stretch to the cerebellum; serve muscle sense rather than conscious perception.

• Descending (Motor) Tracts:

  • Pyramidal Tracts: Direct system from the motor cortex (ventral and lateral corticospinal tracts).
  • Subcortical Motor Nuclei: Origin of indirect motor tracts.

The 12 Cranial Nerves

1. $I$. Olfactory: Sensory; sense of smell.
2. $II$. Optic: Sensory; visual impulses from the retina to the thalamus.
3. $III$. Oculomotor: Motor; serves $4$ extrinsic eye muscles, ciliary muscle, and sphincter pupillae. Also carries proprioceptive impulses.
4. $IV$. Trochlear: Motor; serves the superior oblique muscle of the eyeball.
5. $V$. Trigeminal: Mixed; major sensory nerve of the face. Divisions: Ophthalmic, Maxillary, Mandibular (serves mastication muscles).
6. $VI$. Abducens: Motor; serves the lateral rectus muscle of the eye.
7. $VII$. Facial: Mixed; major motor nerve of the face (muscles of expression, stapedius, stylohyoid). Sensory for taste (anterior $2/3$ of tongue). Innervates submandibular, sublingual, and lacrimal glands.
8. $VIII$. Vestibulocochlear: Sensory; hearing and equilibrium receptors.
9. $IX$. Glossopharyngeal: Mixed; sensory for posterior taste and pharynx. Motor for parotid gland and stylopharyngeus muscle.
10. $X$. Vagus: Mixed; parasympathetic motor to heart, lungs, and abdominal viscera. Sensory from the same organs and the pharynx/larynx.
11. $XI$. Accessory: Motor; supplies the trapezius and sternocleidomastoid muscles.
12. $XII$. Hypoglossal: Motor; serves the tongue and strap muscles.

Innervation and Plexuses of the Peripheral Nervous System

• Facial Innervation: The face displays contralateral innervation for the lower half. The upper half features bilateral innervation. Motor branches of the facial nerve follow $5$ courses: Temporal, zygomatic, buccal, mandibular, and cervical.

• Spinal Nerves: $31$ pairs issue from the spinal cord. They are formed by dorsal and ventral roots. Branches include dorsal rami (posterior trunk), ventral rami (limbs and anterior trunk), and meningeal branches.

• Major Plexuses:

  • Cervical ($C_1 - C_5$): Occipital (neck), Auricular (ear/parotid), Cutaneous (neck), Supraclavicular (shoulder).
  • Brachial ($C_5 - C_8, T_1$): Axillary (deltoid), Median (forearm/arm), Ulnar (wrist/hand), Radial (triceps/extensors), Subscapular (teres major), Suprascapular (shoulder), Pectoral (chest).
  • Lumbar ($L_1 - L_4$): Femoral (thigh/leg), Obturator (motor to leg), Iliolingual (genitalia), Genitofemoral (genitalia).
  • Sacral ($L_4 - L_5, S_1 - S_4$): Sciatic (tibia), Tibial (leg/foot/hamstrings), Fibular (leg/foot/biceps femoris), Gluteal (gluteus maximus), Pudendal (skin/muscles).

• Functional Concepts:

  • Dermatome: Area of skin innervated by a single spinal nerve (all except $C_1$).
  • Myotome: Muscles innervated by a specific spinal nerve.
  • Hilton’s Law: any nerve serving a muscle producing movement at a joint also innervates that joint and the overlying skin.

Sensory Receptor Classification

• Free Nerve Endings: Epidermis; detect pain, temperature, and pressure.
• Root Hair Plexus: Surrounding hair follicles; detect pressure, vibration, and stretch.
• Meissner’s Corpuscles: Hairless skin (nipples, eyelids, soles); mechanoreceptors for discriminative touch.
• Krause’s End Bulbs: Mucosa and hairless skin; mechanoreceptors.
• Pacinian Corpuscles: Subcutaneous tissue; mechanoreceptors for deep pressure.
• Ruffini’s Corpuscles: Deep dermis and joints; mechanoreceptors for stretch.
• Golgi Tendon Organs: Tendons; detect tendon stretch.
• Muscle Spindles: Skeletal muscle; detect muscle stretch.

Comparison of Somatic and Autonomic Nervous Systems

• Effectors: Somatic innervates skeletal muscle; $ANS$ innervates cardiac/smooth muscle and glands.
• Efferent Pathways: Somatic utilizes a single motor neuron (thick, heavily methylated $\text{Type A}$ fibres). $ANS$ utilizes a two-neuron chain consisting of a pre-ganglionic neuron (in the $CNS$) and a ganglionic neuron (in an autonomic ganglion).
• Neurotransmitters: Somatic release Acetylcholine ($Ach$) which is always excitatory. $ANS$ uses $Ach$ (parasympathetic) and Noradrenaline ($NA$ - sympathetic) which can be excitatory or inhibitory.

Divisions of the Autonomic Nervous System

• Parasympathetic ($PNS$) Division (Craniosacral):

  • Preganglionic neurons located in cranial nerves $III, VII, IX, X$ and spinal levels $S_2 - S_4$.
  • Cranial axons synapse in terminal ganglia near organs. Sacral axons form pelvic splanchnic nerves.
  • Structure: Long pre-ganglionic and short post-ganglionic fibres.

• Sympathetic ($SNS$) Division (Thoracolumbar):

  • Preganglionic neurons located in the gray matter from $T_1$ through $L_2$.
  • Axons exit the ventral root and enter the sympathetic chain ganglion via a ramus communicans.
  • Axons may synapse in the chain at the same or different level, or pass through to form splanchnic nerves and synapse in collateral ganglia (celiac, superior mesenteric, inferior mesenteric).
  • Structure: Short pre-ganglionic and long post-ganglionic fibres.