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.