Nervous System Functions and Organization

Functions of the Nervous System

• Sensory Input: Gathers information from both external (touch, temperature, sound) and internal (bladder distension) environments.
• Sensory nerves detect and transmit information throughout the body to other nerve cells.
• Integration: Combines all incoming sensory signals to form a comprehensive understanding of the environment.
• Integration occurs in the brain or spinal cord.
• Motor Output: The nervous system creates an appropriate response by sending motor output signals through motor nerves after sensory input is integrated.
• These signals are then communicated to the skeletal, cardiac, and smooth muscles in the body.

Organization of the Nervous System

• Central Nervous System (CNS): Includes the brain and spinal cord and acts as the body's control center.
• Peripheral Nervous System (PNS): Consists of all nerves located outside the brain and spinal cord.

Overview of the Nervous System

• PNS: Divided into Motor (Efferent) and Sensory (Afferent) divisions.
  • Motor (Efferent): Includes Autonomic (Parasympathetic and Sympathetic) and Somatic systems.
  • Sensory (Afferent)
• CNS: Includes the Spinal Cord and Brain.

Neurons

• Excitable cells can generate and transmit rapid electrical signals to other neurons.
• Nerves communicate through the transmission of electrical signals, which are unidirectional.
• Signal direction: Dendrites to Cell Body to Axon to Dendrites of the next neuron.
• Structure: Consists of a cell body and cytoplasmic processes (dendrites and axon).

Classification of Neurons

• Pseudounipolar (Unipolar): Features one cellular process that splits into a dendrite and an axon; general sensory nerves are an example.
• Bipolar: Has one dendrite and one axon extending directly from the cell body; found in special senses like sight and hearing.
• Multipolar: Has multiple dendrites and one axon; the most common type of neuron, exemplified by motor neurons.

Supporting Cells

• Neuroglia: Protect and support neurons and improve communication by aiding signal transmission between neurons.
• Neuroglia do not generate or transmit nerve impulses.
• Astrocytes: Star-shaped, communicate with neurons and blood vessels to maintain neural tissue integrity; most abundant glial cell in the CNS.
• Microglia: Destroy pathogens via phagocytosis and eliminate waste products and are the least common type of supporting cells.
• Ependymal: Cuboidal cells lining brain and spinal cord cavities; assist in the production, circulation, and monitoring of cerebrospinal fluid (CSF).
• Oligodendrocytes: Wrap around axons in the CNS to form the myelin sheath; one cell can myelinate up to 60 axons.

Myelination

• Myelin: Composed mainly of fat and is white in appearance.
• Myelin insulates the axon and increases the speed of electrical signal transmission.
• Nodes of Ranvier: Small unmyelinated regions of axons between myelinated regions.

Supporting Cells of the PNS

• Schwann cells are the primary neuroglial cell in the PNS, equivalent to oligodendrocytes.
• Form the myelin sheath around PNS nerve axons.
• A single Schwann cell myelinates a portion of one axon, unlike oligodendrocytes.

Cerebral Hemispheres

• Separated by the longitudinal fissure.

Cerebral Cortex

• Outer layer of the cerebrum, distinguished by its dark grey color.

Sulci and Gyri

• Fissures and grooves that increase the surface area of neural tissue, enhancing functional capacity.
• Precentral Sulcus: Anterior border of the motor cortex.
• Lateral Sulcus: Divides the temporal lobe from the frontal and parietal lobes.
• Central Sulcus: Divides the frontal and parietal lobes.
• Postcentral Sulcus: Posterior border of the sensory cortex.
• Parieto-occipital Sulcus: Divides the parietal and occipital lobes.

Gyri

• Precentral Gyrus (Motor Cortex): Sends motor output signals and is located anterior to the central sulcus.
• Postcentral Gyrus (Somatosensory Cortex): Receives sensory input related to touch and is located posteriorly to the central sulcus.

Lobes of the Cerebrum

• Frontal
• Parietal
• Temporal
• Occipital

Grey Matter of the Brain

• Consists of nerve cell bodies, short interneurons, and glial cells.
• Found in the cerebral cortex and inner areas of the brain.

White Matter of the Brain

• Consists of bundles of nerve fibres and axons.
• Located in the inner region of the cerebrum.
• Corpus callosum: A specialized region of white matter that connects the two hemispheres of the brain.

White Matter Tracts

• Association: Facilitate communication within one hemisphere.
• Commissural: Facilitate communication between the two hemispheres.
• Projection Fibres: Facilitate communication between different levels of the CNS.

Diencephalon of the Cerebrum

• Located deep within the cerebrum, it controls sensory/motor relay into and out of the brain and autonomic nervous system.
• Serves as the central core of the cerebrum, surrounded by the cerebral hemispheres.
• Thalamus: Relays sensory information (excluding smell) to other brain areas.
• Hypothalamus: Controls hormone release through the pituitary gland.

Brainstem

• Connects the brain to the spinal cord.
• Houses autonomic reflex centers and white matter tracts essential for human functioning.
• Midbrain: Most superior part of the brainstem, featuring cerebral peduncles and superior and inferior colliculi.
• Pons: Located in the middle of the brainstem, anterior to the 4th ventricle and cerebellum; its fibers facilitate communication.
• Medulla Oblongata (Medulla): The brainstem's most inferior structure, continuous with the spinal cord.

Cerebellum

• Attached to the back of the brainstem under the occipital lobe.
• Coordinates motor activity.

Meninges

• Three connective tissue membranes that protect the CNS.
• Dura Mater: The outermost, tough layer firmly attached to the skull.
  • Encloses the longitudinal fissure (falx cerebri), the transverse fissure (tentorium cerebelli), and the midline of the cerebellum (falx cerebelli).
• Arachnoid Mater: Loosely covers the brain, creating the subarachnoid space filled with cerebrospinal fluid and blood vessels.
• Pia Mater: Innermost layer, firmly attached to the cerebral cortex.

Ventricles

• Hollow spaces continuous with each other and the spinal cord, providing brain support and protection.
• Lateral Ventricles: Paired C-shaped spaces deep within the cerebral hemispheres.
• Third Ventricle: Located in the middle of the diencephalon between the two halves of the thalamus, connected by the interventricular foramina.
• Fourth Ventricle: Situated between the pons/medulla and the cerebellum, connected to the third ventricle by the cerebral aqueduct, and continuous with the central canal of the spinal cord and subarachnoid space.

Cerebrospinal Fluid (CSF)

• Produced primarily by the choroid plexus (ependymal cells and blood vessels) inside the ventricles.
• Functions of CSF:
  • Provides buoyancy to the brain and spinal cord, reducing their effective weight.
  • Acts as a shock absorber, protecting the CNS.
  • Serves as a medium for transporting nutrients, waste, dissolved gases, and other substances.
• Directional Flow of CSF: Lateral Ventricles to Interventricular Foramen to Third Ventricle to Cerebral Aqueduct to Fourth Ventricle to Subarachnoid Space.

Spinal Cord

• Located in the vertebral canal, supported by the vertebral column.
• Facilitates communication between the CNS and the PNS.
• Spinal nerves exit the spinal cord, providing sensory and motor innervation to nearby regions. Spinal nerves from upper cervical serve the head and neck, and those from the thoracic region serve the thorax.
• External Topography: Begins at the foramen magnum, extends to L2 (conus medullaris); axons inferior to L2 form the cauda equina, and the filum terminale anchors the spinal cord.

Support and Protection of the Spinal Cord

• Dura Mater: Consists of one layer with a fat-filled epidural space between its external surface and bone.
• Arachnoid Mater: Lies deep to the dura mater; the subarachnoid space (between the arachnoid and pia mater) contains cerebrospinal fluid and is continuous with the brain.
• Pia Mater: Deep to the arachnoid mater and directly attached to the spinal cord.

Internal Topography of the Spinal Cord

• Contains inner grey matter and outer white matter, opposite of the brain's arrangement.
• Grey matter is composed of dendrites, neuron cell bodies, glial cells, and unmyelinated axons.
• White matter is composed of myelinated axons.
• Grey Matter: Composed of a ventral horn (motor information), dorsal horn (sensory information), and lateral/intermediate horn (sympathetic nervous system information).
• White Matter: Organized into anterior, lateral, and posterior funiculi (bundles).

The Peripheral Nervous System (PNS)

• Composed of all nerves outside the brain and spinal cord; contains spinal and cranial nerves.

Divisions of the PNS

• Sensory (Afferent): Transmits sensory information from the periphery to the CNS.
• Motor (Efferent): Transmits motor commands from the CNS to the periphery.
  • Somatic: Voluntary motor control of skeletal muscles.
  • Autonomic: Involuntary motor control of smooth and cardiac muscle.
    • Sympathetic: Fight or flight response.
    • Parasympathetic: Rest and digest response.

Cranial Nerves

• Nerve 1 (Olfactory Nerve): Sensory; Smell
• Nerve 2 (Optic Nerve): Sensory; Vision
• Nerve 3 (Oculomotor Nerve): Motor; Movement of the eye
• Nerve 4 (Trochlear Nerve): Motor; Movement of the eye
• Nerve 5 (Trigeminal Nerve): Sensory; Forehead, eye, upper jaw, lower jaw. Motor; Muscles of mastication
• Nerve 6 (Abducens Nerve): Motor; Movement of the eye
• Nerve 7 (Facial Nerve): Sensory; Taste for anterior ⅔ of tongue. Motor; Muscles of facial expression, parasympathetic innervation of tear and salivary glands
• Nerve 8 (Vestibulocochlear Nerve): Sensory; Equilibrium and hearing
• Nerve 9 (Glossopharyngeal Nerve): Sensory; Innervation of the pharynx, taste from posterior ⅓ of tongue. Motor; Pharynx muscles, parasympathetic innervation of salivary glands
• Nerve 10 (Vagus Nerve): Sensory; Sensory innervation of the larynx. Motor; Pharynx and larynx muscles, parasympathetic innervation of thoracic and abdominal organs
• Nerve 11 (Accessory Nerve): Motor; Trapezius and sternocleidomastoid muscles
• Nerve 12 (Hypoglossal Nerve): Motor; Muscles of the tongue

Spinal Nerves

• Identified based on a letter and a number, similarly to vertebrae classification.
• "C8" is the exception as it exits between the C7 and T1 vertebrae (there is no C8 vertebra).

Nerve Plexuses

• Fibres from the anterior (motor) roots of spinal nerves combine and redistribute into plexuses, which mostly innervate limb muscles.
• Each plexus contains fibers from a combination of spinal nerves, which is important in case of injury.
• Main spinal nerve plexuses: Cervical and brachial plexuses (cervical enlargement); Lumbar and sacral plexuses (lumbosacral enlargement).

Autonomic Nervous System

• Responsible for transmitting involuntary signals from the CNS to smooth muscle, cardiac muscle, and glands.

Organization of the ANS

• Central Nervous System:
  • Preganglionic Fibre: Has its cell body within the CNS that then extends to the autonomic ganglion.
• Autonomic Ganglion:
  • The location where the preganglionic and postganglionic neurons meet and communicate within the PNS.
• Postganglionic Fibre:
  • Entirely within the PNS with the cell body in the autonomic ganglion extending to the target organ.
• Effector Organ:
  • Is targeted within the PNS by the postganglionic fibre from the autonomic ganglion.

Comparing the Sympathetic and Parasympathetic Nervous Systems

• Sympathetic Nervous System:
  • Origin: Spinal cord segments T1-L2 (thoracolumbar division).
  • Response: Fight or flight.
  • Effect: Widely distributed to many parts of the body.
• Parasympathetic Nervous System:
  • Origin: Brainstem and spinal cord segments S2-S4 (craniosacral division).
  • Response: Rest and digest.
  • Effect: More limited distribution to target organs, with localized effect.

Sympathetic Pathways

• Lateral Horn of Spinal Cord: Sympathetic preganglionic axons leave the lateral horn along with somatic motor axons to form the anterior root.
• Sympathetic Trunk: Preganglionic axons pass through the spinal nerve to the sympathetic trunk, located lateral to the spinal cord.
  • Provides a pathway for sympathetic fibers to travel through the body.
• Ganglion: Preganglionic neurons synapse with postganglionic neurons.
• Effector Organ: Postganglionic axons leave the sympathetic trunk to enter the spinal nerve and extend to the effector tissue/organ at approximately the vertebral level.

Parasympathetic Pathways

• Preganglionic neurons leave the brain or spinal cord at the level of the sacrum.
• Preganglionic neurons meet postganglionic neurons in ganglia close to viscera.
• Postganglionic neurons travel to effector organs.
  • CN 3, 7, and 9 supply the head.
  • CN 10 supplies the thorax to the descending colon.
  • S 2, 3, 4 supply the terminal gut and pelvic viscera (bladder and reproductive organs).

Accessory Structures of the Eyeball

• Protect and move the eyeball.
• Lacrimal Gland: Produces tears to keep the cornea moist and clean; tear drainage system is part of the lacrimal apparatus.
• Eyelid: Contains the levator palpebrae superioris and orbicularis oculi muscles, which control eyelid movement.
• Extrinsic Eye Muscles: Superior Oblique, Medial Rectus, Inferior Rectus, Superior Rectus, Lateral Rectus, Inferior Oblique.

Cavities of the Eye

• Anterior Cavity:
  • Divided into anterior and posterior chambers filled with aqueous humour (watery fluid).
• Lens:
  • Separates the anterior cavity from the posterior cavity.
• Posterior Cavity:
  • Filled with vitreous humour (gelatinous fluid).

Layers of the Eye

• Outer Layer (Fibrous):
  • Outermost layer with portions internal and external to the lens.
    • Sclera: The internal aspect of the fibrous tunic, located posteriorly to the lens, of dense connective tissue which acts as an attachment point for extrinsic eye muscles.
    • Cornea: The external aspect of the fibrous tunic, located anteriorly to the lens.
• Middle Layer (Vascular):
  • Choroid: Contains blood vessels that supply the eye.
    • Ciliary Body: Produces aqueous humour and contains ciliary muscles (intrinsic eye muscles) which control the shape of the lens.
    • Iris: Contains sphincter and dilator muscles (intrinsic eye muscles) which control the size of the pupil, and also contains pigments, which give the eye its colour.
• Deep Layer (Sensory or Retina):
  • Neural Layer: Contains photoreceptors (rods and cones) which are light-sensitive; these receptors send signals through smaller nerves to the axons of the ganglion cells of the globe to the brain to form CN 2.
    • Optic Disc: Where smaller nerve cell axons converge.
    • Optic Nerve (CN 2)
    • Macula: Central area of the retina, lateral to the optic disc where fine detailed vision occurs in the central depression (the fovea).

Flows of Aqueous Humour

• Aqueous humour is produced by ciliary processes in the posterior chamber.
• It flows through the pupil into the anterior chamber and exits at the anterior chamber angle (the junction of the iris, ciliary body, and cornea) through the trabecular meshwork into Schlemm’s canal and into episcleral venous channels, eventually draining into systemic circulation.

Special Sense - Hearing and Equilibrium

• Receptors in the inner ear are responsible for hearing and equilibrium (balance and spatial orientation).

Structure of the Ear

• External Ear: Touched structure which sound waves from external environment pass through the external auditory canal to reach the tympanic membrane (eardrum).
• Middle Ear: Located within the temporal bone between the tympanic membrane and oval window.
  • Tympanic Cavity: Chamber in the temporal bone.
  • Ossicles: Small bones in the middle ear (Malleus, Incus, and Stapes).
    • Malleus (Hammer): Most lateral ossicle, behind the tympanic membrane.
    • Incus (Anvil): Sits between the malleus and stapes.
    • Stapes (Stirrup): The smallest bone, sits in the oval window.
  • Round and Oval Windows: Areas that connect the middle ear to the inner ear.
  • Eustachian Tube: Connects the middle ear to the nasopharynx.
• Inner Ear: Located within the temporal bone.
  • Vestibular Apparatus: Connects the receptors for equilibrium.
  • Cochlea: Contains the receptors for hearing and resembles a seashell.

Labyrinth

• Composed of the cochlea and vestibular apparatus structured into two channels in the bone: membranous and bony labyrinths.
• Membranous labyrinth: Contains fluid endolymph
• Bony labyrinth: Contains fluid perilymph
• Vestibular Apparatus: Contains the receptors for equilibrium and is composed of three structures
  • Semicircular Canals: Detect rotational movement, or changes in the head’s position in space on an angular axis (side to side, up/down, tilting).
  • Utricle & Saccule: Detect positional movement from one point to another with respect to gravity/linear acceleration.
• Cochlea:
  • Contains the receptors for hearing.
  • Composed of three ducts (vestibular, tympanic, and cochlear ducts).
• Organ of Corti:
  • A specialized structure within the cochlea that is integral to hearing.
  • Made of specialized cochlear hair cells which fire an impulse when the membrane is moved by sound waves.

Endocrine Glands

• Endocrine: Secrete hormones directly into the interstitial fluid, picked up by blood capillaries
• Exocrine: Secrete cell products into a duct system.
• Endocrine glands release hormones that travel through the blood to affect target organs.
• Overview of Endocrine Glands:
  • Pituitary Gland: Releases hormones that control all other endocrine glands.
  • Thyroid Gland: Produces hormones for metabolism and calcium homeostasis.
  • Parathyroid Glands: Maintain calcium homeostasis.
  • Adrenal Glands: Produce corticosteroids for glucose metabolism and electrolyte balance.
  • Pancreas: Controls blood sugar levels.

The Pituitary Gland (Hypophysis)

• "Master gland" that releases hormones controlling other endocrine glands.
• Located inferior to the hypothalamus, in the sphenoid bone, which allows for communication between the nervous and endocrine systems.
• Structures of the Pituitary Gland:
  • Anterior Lobe (Adenohypophysis)
    • Thyroid Stimulating Hormone (TSH): Acts on the thyroid gland.
    • Prolactin (PRL): Induces the mammary glands to produce milk.
    • Adrenocorticotropic Hormone (ACTH): Acts on the adrenal glands.
    • Growth Hormone (GH): Stimulates growth in all cells.
    • Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH): Stimulate secretion of estrogen and progesterone, maturation of oocytes in ovaries, and secretion of testosterone and sperm in the testes.
    • Melanocyte Stimulating Hormone (MSH): Stimulates melanocytes in the skin to produce pigment.
  • Posterior Lobe (Neurohypophysis)

Posterior Lobe (Neurohypophysis)

• Composed of nervous tissue
• Releases 2 Hypothalamus-produced hormones
  • Oxytocin: Promotes uterine contractions during childbirth.
  • Antidiuretic Hormone (ADH): Promotes reabsorption of water at the kidneys.

Pituitary Gland Histology

• Anterior lobe: Glandular tissue made of small acini in groups surrounded by connective tissue that stains darker than the posterior lobe.
• Posterior lobe: Nervous tissue, lacks acini, and stains lighter than the anterior lobe.

The Thyroid Gland

• Located in the neck anterior to the trachea and inferior to the larynx.
• Structures of the Thyroid Gland:
  • Right lobe
  • Left lobe
  • Isthmus (connects the two)

Histology of the Thyroid Gland

• Composed of follicles lined with simple cuboidal follicular cells with a central lumen filled with colloid (protein-rich fluid).
• Parafollicular or 'C' cells are dispersed between the follicles.

Cells of The Thyroid Gland

• Follicular Cells:
  • Produce Triiodothyronine (T3) and Thyroxine (T4).
  • Increase the rate of metabolism and oxygen consumption
• C Cells:
  • Produce hormone Calcitonin.
  • Contribute to calcium homeostasis.
  • Lowers blood calcium concentration.

Parathyroid Glands

• Four paired structures on the posterior side of the thyroid (two superior and two inferior).

Cells of the Parathyroid Glands

• Principal (Chief) cells:
  • Produce parathyroid hormone (PTH) that increases calcium ion concentration in the blood.
  • Contribute to calcium homeostasis

Adrenal Glands

• Two pyramid-shaped structures superior to each kidney, each with an adrenal cortex and medulla.

Adrenal Cortex (Outermost Layer)

• Divided into three layers:
  • Produce over 24 corticosteroid hormones
    • Outer layer (mineralocorticoids)
    • Middle layer (glucocorticoids)
    • Inner layer (androgens/sex hormones)

Adrenal Medulla (Inner Core)

• Produces epinephrine and norepinephrine in response to sympathetic nervous system regulation resulting in fight or flight response

The Pancreas

• Long organ posterior to the stomach with exocrine and endocrine functions
• Endocrine Cells:
  • Beta Cells: Produce insulin to lower blood glucose levels (hypoglycemic hormone).
  • Alpha Cells: Produce glucagon to increase blood glucose levels (hyperglycemic hormone).