Ch. 8

Cell Membrane and Resting Potential

  • The cell membrane (neural membrane) of a neuron has an unequal distribution of ions and electric charges.

  • Charge Distribution:

    • Positive charge outside the membrane.

    • Negative charge inside the membrane.

  • This charge difference is known as resting potential, measured in millivolts.

  • Resting potential arises due to differences in concentrations of:

    • Positively Charged Ions: Sodium (Na+) ions concentrated outside.

    • Negatively Charged Ions: More abundant in cytoplasm.

  • The sodium-potassium pump maintains this ion imbalance by active transport against concentration gradients.

Action Potential

  • Rapid depolarization of the cell membrane results in an action potential (a temporary reversal of the electric potential).

  • An action potential lasts less than a millisecond.

  • Mechanism:

    1. Sodium gates open, allowing Na+ ions to enter, causing the membrane to become positively charged.

    2. Sodium gates close at peak potential, potassium (K+) channels open.

    3. K+ ions leave the cell, reestablishing resting potential.

  • The sodium-potassium pump resets ion distributions continuously.

  • Action potentials propagate along the membrane, starting at a single point and spreading to adjacent areas.

Refractory Period

  • Post-action potential, there is a brief refractory period where the membrane cannot be stimulated, preventing backward transmission of impulses.

Neurotransmitters

  • To transmit impulses across a synapse, neurotransmitters (chemical agents) are involved.

  • Mechanism:

    • Neurotransmitters are discharged with action potential arrival.

    • They diffuse across the synapse and bind to receptors on the next cell’s membrane.

    • This causes ion channels to open or close in the second cell, affecting its excitability.

Types of Neurotransmitters

  • Excitatory Neurotransmitters:

    • Acetylcholine

    • Norepinephrine

    • Increase likelihood of action potential in subsequent cell.

  • Inhibitory Neurotransmitters:

    • Dopamine

    • Serotonin

    • Decrease likelihood of action potential.

  • Neurotransmitter removal from synapse can occur via:

    • Enzymatic destruction.

    • Diffusion away.

    • Reabsorption by the neuron.

Neurological Diseases

  • Imbalances in neurotransmitters linked to various neurologic diseases:

    • Parkinson’s Disease: Associated with dopamine deficiency.

    • Huntington’s Disease: Linked to loss of inhibitory neurotransmitters, affecting neurons and memory.

    • Alzheimer’s Disease: Depression linked to low levels of excitatory neurotransmitters.

  • Cocaine: Blocks norepinephrine uptake while stimulating dopamine uptake, affecting neurotransmitter balance.

Endorphins and Anesthesia

  • Endorphins: Natural opioids that alleviate pain and can produce feelings of euphoria.

  • Local Anesthetics: Like lidocaine mimic inhibitory neurotransmitters, reducing sensory neuron action potentials for localized anesthesia.

Central Nervous System (CNS)

  • Major division includes brain and spinal cord.

  • Protection:

    • Encased in bone (skull/vertebrae).

    • Surrounded by meninges: dura mater, arachnoid mater, pia mater.

  • Cerebrum: Largest brain division, coordinates sensory data, motor functions, intelligence, learning, and memory.

  • Cerebellum: Produces muscle coordination, maintains muscle tone, posture, and balance.

  • Brainstem: Includes medulla (regulates heartbeat, breathing), pons (connects brainstem to cerebellum), and midbrain (relay stations).

  • Diencephalon: Contains thalamus (relay point for nerve impulses) and hypothalamus (regulates homeostasis: thirst, hunger, temperature).

Spinal Cord

  • Runs along the dorsal body, linking brain to body.

  • Consists of gray and white matter:

    • Gray Matter: Contains unmyelinated cell bodies, dendrites.

    • White Matter: Composed of myelinated axon tracts (ascending for brain-messages and descending from brain).

Peripheral Nervous System (PNS)

  • Comprises all nerves creating pathways among the CNS and other body regions.

  • Divided into:

    • Afferent Nervous System (Sensory): Carries info from receptors to CNS.

    • Efferent Nervous System (Motor): Carries info from CNS to muscles/glands.

Somatic Nervous System

  • Involves both afferent and efferent nerves.

  • Controls voluntary muscles; sensory input from external sense organs processed by the CNS, responses sent via PNS to organs.

Autonomic Nervous System (ANS)

  • Operates without conscious control; responsible for automatic functions.

  • Contains two subdivisions: Sympathetic (fight-or-flight) and Parasympathetic (rest-and-digest).

Sympathetic and Parasympathetic Functions

  • Sympathetic Nervous System:

    • Involved in reducing salivary secretion, increasing heart rate, etc.

  • Parasympathetic Nervous System:

    • Opposes sympathetic functions, enhances salivary gland secretions.

Cranial Nerves

  • Overview:

    • 12 pairs connected to the brain, innervate structures of the head and neck.

    • Categorized as afferent/efferent or mixed types.

    • Numbered I to XII based on location in the brain.

Cranial Nerve I - Olfactory Nerve

  • Transmits smell from nasal mucosa to brain (afferent).

  • Enters skull via cribriform plate of ethmoid bone.

Cranial Nerve II - Optic Nerve

  • Transmits sight from retina to brain (afferent).

  • Enters via optic canal of sphenoid.

Cranial Nerve III - Oculomotor Nerve

  • Efferent nerve for eye muscles, includes parasympathetic fibers.

  • Exits skull through superior orbital fissure.

Cranial Nerve IV - Trochlear Nerve

  • Efferent nerve for eye muscle, no parasympathetic fibers.

  • Exits via superior orbital fissure.

Cranial Nerve V - Trigeminal Nerve

  • Largest cranial nerve; has afferent (skin of face) and efferent (muscles of mastication) components.

  • Subdivided into three divisions:

    • Ophthalmic: Sensation for upper face.

    • Maxillary: Sensation for middle face.

    • Mandibular: Sensation and motor for lower face.

Cranial Nerve VI - Abducens Nerve

  • Efferent nerve for eye muscle, exits through superior orbital fissure.

Cranial Nerve VII - Facial Nerve

  • Mixed: efferent for facial muscles, afferent for taste (anterior 2/3 of tongue).

  • Exits skull via stylomastoid foramen.

  • Important for dental professionals; innervates glands and muscle tissue.

Cranial Nerve VIII - Vestibulocochlear Nerve

  • Afferent nerve responsible for hearing and balance from the inner ear.

Cranial Nerve IX - Glossopharyngeal Nerve

  • Carries efferent component for pharyngeal muscles and afferent component for oropharynx taste and sensation.

Cranial Nerve X - Vagus Nerve

  • Affects soft palate and pharynx muscles; parasympathetic fibers to thorax and abdomen organs.

Cranial Nerve XI - Accessory Nerve

  • Efferent for trapezius and sternocleidomastoid muscles.

Cranial Nerve XII - Hypoglossal Nerve

  • Efferent nerve for intrinsic and extrinsic tongue muscles.

Trigeminal Nerve Introduction

  • Dental professionals must understand the trigeminal nerve anatomy.

  • Divisions:

    • Sensory (afferent) and motor (efferent) roots.

    • Main trunk divides into three divisions: ophthalmic, maxillary, mandibular.

Ophthalmic Nerve (V1)

  • Afferent for structures like the conjunctiva and forehead.

Maxillary Nerve (V2)

  • Afferent for maxilla, palate, nose.

Mandibular Nerve (V3)

  • A mixed nerve supplying muscles of mastication and innervating teeth.

Conclusion

  • Understanding the anatomy and functions of cranial nerves is essential for effective dental practice, particularly in administering anesthesia.