AP EXAM 4 Review

Nervous System Function

• Sensory: Responsible for detecting environmental stimuli and internal body conditions. Involves specialized receptors that transduce various forms of energy (chemical, mechanical, thermal, etc.) into neural signals.

• Integration/Interpretation: The process by which the CNS interprets sensory information. It involves higher brain functions to evaluate sensory inputs, apply past experiences, and determine appropriate responses.

• Response: Actions executed by effectors (muscles and glands) in response to interpreted signals from the CNS. This can include voluntary movements or involuntary responses like reflex actions.

• Homeostasis: The maintenance of a stable internal environment. The nervous system works in conjunction with the endocrine system to regulate body functions such as temperature, pH balance, and fluid levels.

Central Nervous System (CNS)

• Components: The CNS comprises the brain and spinal cord. These structures serve as the control center for processing information, integrating signals, and generating responses. The brain is the center of thought, emotion, and sensory processing, while the spinal cord is the pathway for signals between the brain and the rest of the body.

• Protection: Surrounded by three protective membranes known as the meninges:

  • Dura Mater: The outermost layer, tough and protective. Contains the dural sinus where blood collects.

  • Arachnoid Mater: The middle layer, web-like and cushioning.

  • Pia Mater: Directly adheres to the brain and spinal cord. Highly vascularized to supply nutrients.

• Cerebrospinal Fluid (CSF): A clear fluid that circulates around the brain and spinal cord, providing cushioning, nutrient transport, and waste removal. It maintains intracranial pressure and offers buoyancy to the brain.

Peripheral Nervous System (PNS)

• Function: Connects the CNS to limbs and organs. It is crucial for transmitting sensory information to the CNS and executing motor commands from the CNS.

• Divisions:

  • Sensory Division (Afferent Pathways):

    • Transmits sensory information from receptors to the CNS. Contains sensory neurons that can be categorized as:

      • General Sensory Neurons: Transmit sensations such as pain, touch, temperature, and proprioception. Involve unipolar neurons, typically found in dorsal root ganglia.

      • Special Sensory Neurons: Associated with special senses (sight, hearing, taste, smell, balance). Often involve bipolar neurons located in sensory organs.

  • Motor Division (Efferent Pathways):

    • Carries motor commands from the CNS to effectors, which include muscles and glands. Divided into:

      • Somatic Nervous System: Controls voluntary movements and is under conscious control (e.g., skeletal muscle contraction).

      • Autonomic Nervous System: Regulates involuntary functions and controls smooth/cardiac muscle and controls glandular secretion. (e.g., heart rate, digestion).

        • Sympathetic Branch: Promotes 'fight or flight' responses during stress, accelerating heart rate and redirecting blood flow.

        • Parasympathetic Branch: Encourages 'rest and digest' responses, slowing the heart rate and stimulating digestive processes.

Neurons

Structure of Neurons:

• Axon: A long projection that transmits neural impulses to anothr cell. Each neuron has one axon, which can branch to communicate with multiple target cells. Axons are surrounded by myelin sheaths produced by neuroglia cells, enhancing signal transmission speed.

• Cell Body (Soma): Contains the nucleus, organelles, and synapses. Responsible for the overall metabolic functions of the neuron, and can recieve neural impulses from another cell.

• Dendrites: Short, branched processes that receive signals from other cells. Each neuron can have multiple dendrites, increasing its ability to receive information from various sources.

Types of Neurons:

• Multipolar Neurons: Characterized by one axon and multiple dendrites, the most common type in the CNS, involved in motor and interneuron functions.

• Bipolar Neurons: Possess one axon and one dendrite, primarily found in sensory organs such as the retina and olfactory mucosa.

• Unipolar Neurons: Have a single process that branches into two parts, found in sensory pathways for touch and pain.

Action Potentials (AP)

Mechanism of AP Generation:

• Ion Distribution: The inside of the neuron has a high concentration of potassium ions (K+) and negatively charged proteins, while the outside has a higher concentration of sodium (Na+) and calcium ions (Ca2+). This unequal distribution contributes to electric potential (voltage) across the membrane.

• Resting Potential: Typically around -70 millivolts. Maintained by the sodium-potassium pump, which actively transports Na+ out and K+ into the cell, preserving the negative interior charge.

• Depolarization and Repolarization:

  • When a stimulus reaches threshold potential (approximately -55 millivolts), voltage-gated sodium channels open, allowing Na+ to rush into the cell, causing depolarization (the interior becomes positively charged).

  • As the voltage reaches +30 to +40 millivolts, sodium channels close and potassium channels open, allowing K+ to exit, leading to repolarization (restoration of a negative interior). The membrane briefly becomes hyperpolarized before returning to resting potential.

Summative Nature of Neural Signaling

Neural signaling is often described as summative, meaning that multiple signals can influence whether a neuron will fire an action potential. This process is primarily observed in the integration of excitatory and inhibitory postsynaptic potentials.

Threshold Potential: The summation of all incoming signals determines whether the membrane potential reaches the threshold (around -55 millivolts). If the threshold is reached, an action potential is initiated and propagated along the axon.

Sequence of Events at the Synapse:

1. Action potential stimulates the the exocytosis of neurotransmitters.

2. Neurotransmitter binds to ligand -gated channel

3. Ligand-gated ion channels opens.

4. Ions diffuse which this can lead to either:

   • Excitatory Signals: Depolarization and potential generation of an AP in the postsynaptic cell.

   • Inhibitory Signals: Hyperpolarization, making it less likely for an AP to occur.

Neurotransmitters

GABA (Gamma-Aminobutyric Acid)

• Function: GABA is the primary inhibitory neurotransmitter in the central nervous system. It works to reduce neuronal excitability and prevent overstimulation, helping to maintain a balance in brain activity.

• Mechanism: GABA binds to GABA receptors, which are ligand-gated ion channels. When GABA binds, it typically allows chloride ions (Cl-) to flow into the neuron, leading to hyperpolarization and making it less likely to fire an action potential.

• Roles in the Body: GABA plays a key role in regulating muscle tone, sleep cycles, and anxiety. It's also involved in the processes that govern learning and memory.

• Clinical Significance: Abnormal levels of GABA may be linked with various neurological disorders, including epilepsy, anxiety disorders, and schizophrenia. Drugs that enhance GABA activity (such as benzodiazepines) are commonly prescribed for anxiety and sleep disorders.

Acetylcholine (ACh)

• Function: Acetylcholine is a neurotransmitter that plays roles in both the peripheral and central nervous systems. It is crucial for muscle activation, the regulation of heart rate, and has functions in attention, memory, and learning within the brain.

• Mechanism: Acetylcholine acts on two types of receptors: nicotinic (ionotropic) and muscarinic (metabotropic), each mediating different physiological responses. In muscle cells, ACh binding opens ion channels, leading to muscle contraction. In the CNS, it modulates neuronal excitability and synaptic plasticity.

• Roles in the Body: In the peripheral nervous system, acetylcholine activates skeletal muscles and manages parasympathetic functions (e.g., slowing heart rate). In the central nervous system, it's involved in attention, memory, and cognitive functions.

• Clinical Significance: Dysfunction in acetylcholine signaling is associated with disorders such as Alzheimer’s disease, myasthenia gravis, and certain types of paralysis. Drugs that inhibit acetylcholinesterase, an enzyme that breaks down acetylcholine, are used to

Sequence of Events at the Axon

1. Influx of ions at the dendrites/ body of the neuron

2. Ions diffuse to the axon (trigger zone)

3. If the threshold potential (-55 millivolts) is reached due to sufficient stimulation. The voltage-gated sodium channels open and Sodium Ions flow into the axon.

4. Membrane fully depolarizes and AP is generated.

5. AP propagates down the axon as voltage-gated ion channels are succesively opened.

6. Axon repolarizes behind the AP as voltage-gated potassium channels open, allowing potassium ions to flow out of the axon, restoring the negative membrane potential.

7. Ultimately, the AP reaches the presynaptic terminal, where it triggers the release of neurotransmitters into the synaptic cleft, facilitating communication with the postsynaptic neuron.

8. Exocytosis of the neurotransmitter is stimulated.

Neural Halting

Neural halting refers to mechanisms that inhibit neuronal firing, crucial for maintaining balance between excitation and inhibition in neural networks.

Key Aspects:

1. Mechanisms of Inhibition:

   • Neurotransmitter Release: GABA, the primary inhibitory neurotransmitter, hyperpolarizes neurons, reducing the likelihood of action potentials.

   • Feedback Inhibition: Activated neurons inhibit their own firing or that of nearby neurons, contributing to the halting process.

2. Importance:

   • Prevention of Overstimulation: Halting mechanisms protect against excessive neuronal activity and excitotoxicity.

   • Regulation of Brain Activity: They help maintain homeostasis, impacting mood, cognition, and motor control.

   • Role in Disorders: Dysfunctional halting can lead to conditions like anxiety and epilepsy.

Saltatory Conduction

• Myelin Sheath: Composed of lipids, acts as insulation for the axon, decreasing leakage and speeds up electrical impulse propagation by reducing the number of voltage-gated channels (found only in the nodes of ranvier) required for signaling propagation.The action potential propagates by jumping from one Node of Ranvier (gaps in the myelin sheath) to another, speeding up transmission compared to unmyelinated fibers.

Neuroglia

• Support cells that provide structural support, nourishment, and protection for neurons. Key types include:

  • PNS:

    • Schwann Cells: Responsible for myelination of peripheral axons and forming the neurilemma, essential for nerve regeneration.

  • CNS:

    • Astrocytes: Maintain the blood-brain barrier, regulating the passage of substances between blood and neurons, provide structural support, and supply nutrients. Feet wrap around capillaries

    • Ependymal Cells: Epithelia like and line ventricles of the brain and spinal cord, involved in the production and circulation of cerebrospinal fluid (CSF). Part of the blood-brain barrier.

    • Oligodendrocytes: No neurilemmia. Produces the myelin sheath in the CNS and can myelinate multiple axons simultaneously, unlike Schwann cells.

CNS Structure

Central Nervous System

• Components: The central nervous system (CNS) includes the brain and spinal cord, functioning as the primary control center for processing information and coordinating responses.

• Meninges: The CNS is protected by three layers of protective membranes:

  • Dura Mater: The tough outer layer that houses the dural sinus for blood collection.

  • Arachnoid Mater: The middle web-like layer, containing the subarachnoid space filled with cerebrospinal fluid (CSF) that cushions the brain.

  • Pia Mater: The delicate inner layer that adheres closely to the brain and spinal cord and is highly vascularized to supply nutrients.

• Ventricles: These are fluid-filled spaces within the brain that include two lateral ventricles, a third ventricle, a cerebral aqueduct, and a fourth ventricle.

  • They produce Cerebrospinal Fluid (CSF) through blood-based ependymal cells in the choroid plexuses, which circulate nutrients and waste throughout the CNS.

• CSF Circulation: CSF drains from the ventricles into the central canal of the spine and the subarachnoid space, filtering through arachnoid granulations into the dural sinuses, ultimately returning to the circulatory system.

• Spinal Cord: Commencing just below the foramen magnum and typically terminating between the L1 and L2 vertebrae, the spinal cord houses:

  • White Matter: Composed of myelinated axons that facilitate signal transmission.

  • Gray Matter: Contains unmyelinated axons and neuron cell bodies

  • Funiculi: Longitudinal tracts of white matter that run along the length of the spinal cord, organized into three main columns (dorsal, lateral, and ventral) which carry sensory and motor information.

  • Horns: The gray matter of the spinal cord is arranged into anterior (ventral), posterior (dorsal), and lateral horns.

  • Posterior Median Sulcus: A shallow groove on the posterior side of the spinal cord that divides the right and left posterior columns.

  • Anterior Medial Fissure: A deep groove located on the anterior side of the spinal cord that separates the right and left anterior columns.

  • Dorsal Root: Has the Dorsal Root Ganglion contains the cell bodies of sensory neurons that transmit information from the periphery to the spinal cord. Sensory information is through the dorsal root.

  • Ventral Root: Has Motor Neurons that carry signals from the spinal cord to the muscles, facilitating voluntary movement and reflex actions. Motor response is through the ventral root.

• Reflex Arc: These pathways enable quick responses:

  • Simple spinal reflexes involve only the spinal cord, such as the patellar reflex, which requires one sensory and one motor neuron.

  • More complex reflexes may involve brain areas but are characterized by the absence of conscious thought.

  • Withdrawal Reflex: Involves interneurons in the spinal cord and acts as a relay providing a rapid response mechanism that can be overridden by conscious decision-making.
    

Brain:

• Cerebrum: Divided into left and right hemispheres, responsible for higher cognitive functions, sensory processing, and voluntary motor actions.

• Structure: The cerebrum consists of two hemispheres separated by the longitudinal fissure, which accommodates the meninges, including the falx cerebri which isa double layer of dura mater that helps anchor the brain within the skull and provides support for the cerebral hemispheres.

• Gray Matter: Located superficially, this layer contains neuron cell bodies, unmyelinated axons, and constitutes the cerebral cortex, which is involved in higher cognitive functions, sensory processing, and voluntary motor actions.

• White Matter: Situated deeper within the cerebrum, this layer comprises myelinated axons that facilitate communication between different brain regions.

• Surface Anatomy: The cerebrum features gyri (ridges) and sulci (grooves) that increase surface area and enhance functional capacity.

  Cerebrum Includes areas such as:

  • Frontal Lobe: Involved in decision-making, executive functions, and motor control. Central Sulcus seperates the Frontal from Parietal

  • Parietal Lobe: Processes sensory information such as touch, temperature, and pain.

  • Temporal Lobe: Involved in auditory perception, memory, and language comprehension. Lateral Sulcus seperates the Temporal from Frontal.

  • Occipital Lobe: Primarily responsible for visual processing.

  • Insular Lobe: Involved in the perception of bodily sensations and emotional processing.

Functions of the Nervous System

• Voluntary Motor Control: The nervous system coordinates and regulates voluntary movements through the primary motor cortex (precentral gyrus).

• Receive and Interpret Sensory Information: It processes sensory data through the primary somatosensory cortex (postcentral gyrus), enabling perception of touch, temperature, pain, and proprioception.

• Higher Order Functions: It supports complex cognitive functions, including language, decision-making, and social behavior.

Functional Regions of the Brain

• Primary Motor Cortex (Precentral Gyrus): Responsible for voluntary movement control.

• Primary Somatosensory Cortex (Postcentral Gyrus): Processes tactile sensory information from the body.

• Primary Visual Cortex (Occipital Lobe): Responsible for processing visual stimuli.

• Primary Auditory Cortex (Temporal Lobe): Processes auditory information, including sound perception.

• Broca’s Area: Involved in speech production, typically located in the left hemisphere.

• Wernicke’s Area: Critical for language comprehension, usually found in the left hemisphere.

• Association and Integration Areas: Involved in integrating sensory information and higher cognitive functions.

• Prefrontal Cortex: Engaged in complex behaviors such as planning, decision-making, and social interactions.

• Diencephalon:

  • Thalamus: Relays and routes most sensory to the cerebral cortex and is essential for regulating sleep, alertness, and consciousness like Circadian rhythm and Endocrine System.

  • Hypothalamus: Has a major role in many homeostatic controls and bridges endorcine through the pituatary gland and cerebrum through the fornix.

• Brainstem: Comprises the midbrain, pons, and medulla oblongata. Involved in autonomic functions like heart rate and respiration, and acts as a relay between the brain and spinal cord.

• Medulla Oblongata:

  • Cranial Nerves: 8 – 12

  • Functions:

    • Regulates heart rate

    • Controls vasomotion

    • Manages breathing

• Pons:

  • Functions:

    • Acts as a relay station for signals between the cerebrum and cerebellum

    • Involved in breathing regulation

  • Cranial Nerves: 5 - 8

• Midbrain:

  • Cranial Nerves: 3 - 4

  • Functions:

    • Responsible for awareness and wakefulness

  • Corpora Quadrigemina:

    • Involved in auditory and visual reflexes

    • Composed of 4 bumps on the posterior midbrain:

      • Superior Colliculi: Processes visual reflexes

      • Inferior Colliculi: Processes auditory reflexes.

• Cerebellum

  • Separation: Separated from the cerebrum via the transverse fissure (meninges fold inward, known as tentorium cerebelli).

  • Hemispheres: Divided into left and right hemispheres (meninges fold inward, known as falx cerebelli).

  • Structure: Includes the cerebellar cortex (gray matter) and arbor vitae (white matter).

  • Functions:

    • Fine motor control.

    • Development of muscle memory (procedural memory).

Spinal Cord:

• Segments: Divided into cervical, thoracic, lumbar, sacral, and coccygeal regions, each containing pairs of spinal nerves.

• Gray Matter: Centrally located, shaped like a butterfly, contains neuron cell bodies, interneurons, and glial cells. Divisions include dorsal (sensory), ventral (motor), and lateral horns.

• White Matter: Surrounding gray matter, composed of myelinated axons grouped into tracts that carry signals up and down the spinal cord.

• Reflex Arc: The pathway that mediates reflex actions. It can involve sensory neurons, interneurons (in some cases), and motor neurons that bypass the brain for rapid responses.

Peripheral Nervous System

Connective Tissue Surrounding Nerves

• Epineurium: Surrounds the entire nerve.

• Perineurium: Surrounds a fascicle (bundle of nerve fibers).

• Endoneurium: Surrounds individual axons and Schwann cells.

Nerve Fibers

• Types of Nerves:

  • Sensory Nerves: Carry sensory information to the CNS.

  • Motor Nerves: Transmit motor commands from the CNS to muscles.

  • Mixed Nerves: Contain both sensory and motor fibers.

    • Spinal Nerves tend to be mixed nerves.

• Cranial Nerves: Know which cranial nerves are sensory, motor, or mixed.

Afferent vs. Efferent Nerve Fibers

• Afferent Fibers: Carry sensory information to the CNS.

• Efferent Fibers: Transmit motor commands from the CNS to effectors.

Cranial Nerves

1. Olfactory Nerve (I)

   • Function: Sense of smell

   • Type: Sensory

2. Optic Nerve (II)

   • Function: Vision

   • Type: Sensory

3. Oculomotor Nerve (III)

   • Function: Eye movement, pupil constriction, lens shape

   • Type: Motor

4. Trochlear Nerve (IV)

   • Function: Eye movement (superior oblique muscle)

   • Type: Motor

5. Trigeminal Nerve (V)

   • Function: Sensation from the face; chewing

   • Type: Mixed

6. Abducens Nerve (VI)

   • Function: Eye movement (lateral rectus muscle)

   • Type: Motor

7. Facial Nerve (VII)

   • Function: Facial expressions, taste (anterior two-thirds of the tongue), secretion of saliva and tears

   • Type: Mixed

8. Vestibulocochlear Nerve (VIII)

   • Function: Hearing and balance

   • Type: Sensory

9. Glossopharyngeal Nerve (IX)

   • Function: Taste (posterior one-third of the tongue), swallowing, secretion of saliva, monitoring of carotid body and sinus

   • Type: Mixed

10. Vagus Nerve (X)

• Function: Control of heart, lungs, and digestive tract; taste from the epiglottis; swallowing

• Type: Mixed

11. Accessory Nerve (XI)

• Function: Shoulder elevation and head rotation (sternocleidomastoid and trapezius muscles)

• Type: Motor

12. Hypoglossal Nerve (XII)

• Function: Tongue movement

• Type: Motor

Spinal Nerves

• Dermatomes: Areas of skin supplied by sensory fibers of a single spinal nerve root.

• Plexuses:

  • Brachial Plexus: Supplies the upper limb.

  • Cervical Plexus: Supplies the head and neck.

  • Lumbosacral Plexus: Supplies lower limbs.

  • Thoracic Spinal Nerves: No major plexuses near their origin.

• Ventral and Dorsal Rami: Branches of spinal nerves; dorsal rami innervate the back, while ventral rami innervate the front and limbs.

Somatic vs. Autonomic Pathways

• Somatic Pathways: Often use a single motor neuron to reach target tissues.

• Autonomic Pathways: Typically use at least two neurons:

  • Preganglionic Neurons: From the CNS to a ganglion.

  • Postganglionic Neurons: From the ganglion to the target tissue.

Sympathetic vs. Parasympathetic Divisions

• Sympathetic Pathway (Fight or Flight): Prepares the body for stressful situations.

• Parasympathetic Pathway (Rest and Digest): Promotes rest and conservation of energy.

• Both divisions often innervate the same target organs but have antagonistic effects:

  • Sympathetic: Inhibits digestion, dilates pupils.

  • Parasympathetic: Stimulates digestion, constricts pupils.

Sensory

• Process of Sensation: The sequence begins with a stimulus, which is detected by a receptor cell. This information is transmitted through the Peripheral Nervous System (PNS) to the Central Nervous System (CNS), where it is processed and leads to the conscious experience of sensation/perception.

• Types of Stimuli:

  • Chemicals: These include substances that can evoke taste or smell when detected by corresponding receptors.

  • Energy:

    • Mechanical: Forces that cause perception of touch, pressure, or vibration (e.g., sound waves).

    • Light: Electromagnetic radiation that is perceived by photoreceptors in the visual system.

    • Heat: Variations in temperature detected by thermoreceptors.

• Receptor Cells:

  • Chemoreceptors: Specialized receptors that detect chemical stimuli; essential for the senses of taste (gustation) and olfaction (smell).

  • Photoreceptors: Cells that respond to light stimuli, primarily located in the retina, enabling the sense of sight.

  • Mechanoreceptors: These receptors respond to mechanical forces and are involved in various sensations, including auditory (hearing) and equilibrium (balance), as well as many general sensations like pressure and touch.

  • Thermoreceptors: Types of receptors that sense hot and cold temperatures, allowing for thermal perception.

  • Nociceptors: Receptors that can be classified as chemoreceptors, thermoreceptors, or mechanoreceptors; they specialize in sensing pain and are crucial for signaling potential harm to the body.

• Action Potentials: When stimuli are detected, they generate an action potential (AP) in the receptor cell or the corresponding innervating neuron. The pathway that the AP takes within the CNS determines how and where the sensation is interpreted, thus influencing the conscious experience of that sensation.

• Adaptation: Adaptation refers to the gradual reduction in sensitivity to stimuli after prolonged exposure:

  • Taste and olfaction: These senses adapt quickly, often adjusting rapidly to new scents or flavors, making it easier to detect changes.

  • Nociception: Pain sensations do not exhibit adaptation; consistent pain signaling remains elevated until the source is removed or treated.

• Projection: The brain processes sensory information and projects it back to the appropriate region, determining where in the body the sensation is perceived.

• General vs. Special Senses: General senses include widespread perceptions such as touch, pressure, temperature, and pain which involve widespread receptors. In contrast, special senses are more complex modalities like vision, hearing, taste, and smell, each with specialized receptors and neural pathways dedicated to processing that specific type of sensory information.

General Senses (Mostly Associated with the Integument):

• Light (Tactile Corpuscle) and Heavy Pressure (Lamellated Corpuscle):

  • Mechanoreception:

    • Tactile corpuscles (Meissner's corpuscles) are responsible for detecting light touch and vibrations, primarily located in the dermal papillae.

    • Lamellated corpuscles (Pacinian corpuscles) are deep pressure receptors found in the deeper layers of the dermis and subcutaneous tissue, responding to heavy pressure and vibrations.

• Warm and Cold Receptors:

  • Thermoreception:

    • These receptors are responsible for detecting temperature changes in the environment.

    • Warm receptors respond to mild increases in temperature, while cold receptors respond to mild decreases; both types contribute to our ability to sense temperature without causing pain.

    • It is important to note that these receptors do not perceive thermal pain, which is detected by nociceptors.

• Nociception:

  • Pain:

    • Nociceptive receptors can be activated by potentially harmful stimuli, classified as chemical (e.g., inflammation), thermal (extreme temperatures), or mechanical (sharp objects).

    • This type of sensation is crucial in protecting the body from harm.

    • Nociceptors exhibit little to no adaptation, meaning they continue to signal pain persistently even under constant stimulus.

• Proprioception:

  • Mechanoreception:

    • This system senses the position and movement of the body, particularly through muscle and tendon tension receptors called muscle spindles and Golgi tendon organs.

    • Proprioception allows for awareness of body position and movement in space, contributing to balance, coordination, and overall motor control.

Vision Overview

Eye Muscles and Innervating Cranial Nerves

• Palpebra (Eyelid Muscles):

  • Orbicularis Oculi (CN VII): Responsible for closing the eyelids and helps with blinking, protecting the eye from excessive light and foreign bodies.

  • Levator Palpebrae Superioris (CN III): Elevates the upper eyelid, allowing for a wide opening of the eye for better vision.

• Extrinsic Eye Muscles: Control the movements of the eyeball, allowing it to follow objects.

  • Superior Rectus (CN III): Elevates the eye and assists in adduction (moving the eye towards the nose) and medial rotation.

  • Inferior Rectus (CN III): Depresses the eye and assists in adduction and lateral rotation.

  • Medial Rectus (CN III): Primarily responsible for moving the eye towards the nose (adduction).

  • Lateral Rectus (CN VI): Moves the eye away from the nose (abduction).

  • Superior Oblique (CN IV): Helps in rotating the eye downward and laterally, facilitating complex movements.

  • Inferior Oblique (CN III): Assists in elevating the eye and rotating it laterally.

Lacrimal Apparatus

• Components:

  • Lacrimal Gland: Produces tears that lubricate the surface of the eye, providing moisture and protection against infection.

  • Lacrimal Sac: Collects tears before they drain into the nasolacrimal duct.

  • Nasolacrimal Duct: Drains tears from the lacrimal sac into the nasal cavity, explaining why emotions can cause a runny nose.

• Tears: Form a protective layer on the eye, washing away debris and providing oxygen to the cornea.

Conjunctiva

• Mucous Membrane: Lines the inner eyelids and covers the sclera, providing lubrication and protection to the eye surface.

Meibomian (Tarsal) Glands

• Function: Secretes oily substance (meibum) that prevents tear evaporation, contributing to eye lubrication and comfort.

Cornea

• Structure: Transparent connective tissue that refracts light.

• Function: Focuses light rays onto the lens, initiating the process of vision.

Sclera

• Protective Layer: A dense connective tissue that provides structure and protection to the eyeball; continuous with the cornea and ensures the integrity of the eye.

Anterior Cavity

• Divided into:

  • Anterior Chamber: Located anterior to the iris; filled with aqueous humor.

  • Posterior Chamber: Located around the lens; also filled with aqueous humor, which maintains intraocular pressure and provides nutrients.

• Aqueous Humor Flow: Circulates from the posterior chamber through the pupil into the anterior chamber, eventually draining through Schlemm’s canal into the systemic circulation.

Iris

• Composition: Pigmented muscular layer responsible for controlling the size of the pupil.

• Function: Regulates the amount of light entering the eye by contracting (CN III) to constrict the pupil or relaxing (sympathetic response) to dilate it.

Lens

• Function: Focuses light on the fovea centralis of the retina; its shape can change to focus on objects at varying distances (accommodation).

Ciliary Body

• Role: Produces aqueous humor and contains the ciliary muscle, which controls lens shape for focusing (CN III).

Posterior Cavity

• Choroid Coat: Highly vascularized layer that provides nourishment to the retina and absorbs excess light not captured by the retina.

Retina

• Structure: Contains photoreceptors (rods and cones) which convert light into neural signals.

• Fovea Centralis: Area of the retina with the highest concentration of cones and responsible for sharp central vision.

• Macula Lutea: Surrounds the fovea, containing yellowish pigments that protect against excess light.

• Optic Disc: The point where the optic nerve enters the retina; lacks photoreceptors, creating a blind spot.

Photoreceptors

• Rods: Sensory receptors responsive to low light levels; responsible for peripheral and night vision; do not perceive color.

• Cones: Responsible for color vision and function best in bright light. There are three types: red, blue, and green cones that detect different wavelengths of light.

Pathway of Light

1. Light enters through the cornea.

2. Passes into the anterior chamber.

3. Travels through the pupil.

4. Enters the posterior chamber.

5. Passes through the lens, which refracts and focuses light.

6. Enters the posterior cavity and is absorbed by the retina.

7. Excess light is absorbed by the choroid to prevent scatter.

Eye Conditions

• Hyperopia (Farsightedness): Difficulty in seeing nearby objects clearly due to the eyeball being too short or the lens being too flat.

• Myopia (Nearsightedness): Difficulty in seeing distant objects clearly due to the eyeball being too long or the lens being too curved.

• Astigmatism: Distorted vision occurring due to an irregularly shaped cornea or lens, leading to a blurred image.

Hearing Anatomy and Mechanism

Mechanoreception

• The process involved in detecting sound through mechanical changes in the environment.

External (Outer) Ear

• Auricle: The visible part of the ear that collects sound waves.

• External Acoustic Meatus: The canal that transports sound waves to the tympanic membrane.

• Tympanic Membrane: Vibrates in response to sound, marking the boundary between the outer ear and middle ear.

Middle Ear (Tympanic Cavity)

• Air-filled cavity containing:

  • Auditory Ossicles:

    • Malleus (Hammer): Connects to the tympanic membrane.

    • Incus (Anvil): Intermediary bone.

    • Stapes (Stirrup): Connects to the oval window.

  • Oval Window: Amplifies sound by transferring acoustic waves to the inner ear due to its smaller size compared to the tympanic membrane.

  • Tympanic Reflex:

    • Muscles:

      • Tensor Tympani: Attaches to the malleus.

      • Stapedius: Attaches to the stapes.

    • Function: Stiffen the auditory ossicles to dampen loud sounds.

  • Round Window: Allows propagation and exit of acoustic waves from the inner ear.

  • Eustachian Tube: Equalizes air pressure between the external and middle ear; drains into the pharynx.

Inner Ear

• Osseous (Bony) Labyrinth: Filled with perilymph.

• Membranous Labyrinth: Contains endolymph and is located inside the osseous labyrinth.

• Cochlea: Responsible for hearing.

  • Scala Media (Cochlear Duct): Part of the membranous labyrinth, separates the cochlea into two halves (scala tympani and scala vestibuli) using:

    • Vestibular Membrane

    • Basilar Membrane

  • Organ of Corti (Spiral Organ): Located within the scala media and consists of:

    • Basilar Membrane

    • Hair Cells: Sensory receptors for sound.

    • Tectorial Membrane: Overlying layer that works with hair cells.

Pathway for Acoustic Waves (Sound Waves)

1. Sound waves enter the external acoustic meatus.

2. Vibrate the tympanic membrane.

3. Vibrate the malleus, incus, and stapes in succession.

4. Vibrate the oval window, allowing waves to enter the scala vestibuli and propagate through the perilymph.

5. Waves wrap around and return through the scala tympani.

6. Vibrate the basilar membrane, compressing hair cells against the tectorial membrane.

7. Generate an action potential in the cochlear branch of CNVIII (Vestibulocochlear Nerve).

8. Acoustic waves ultimately exit through the round window.

Equilibrium (Balance)

Mechanoreception

• The process of detecting changes in position and motion to maintain balance.

Static Equilibrium

• Vestibule of Inner Ear:

  • Senses the position of the head.

  • Involved in detecting the head's orientation relative to gravity.

  • When the head tilts due to gravity, it causes the movement of otoliths (small calcium carbonate crystals) which bend the hairs of hair cells, generating an action potential (AP).

Dynamic Equilibrium

• Semicircular Canals:

  • Senses the acceleration and rotational movements of the head.

  • When the head rotates, the relative motion of the fluid (endolymph) within the canals (due to inertia) causes the fluid to bend the hairs of hair cells, generating an action potential (AP).

• This helps the body maintain balance during movements.

Olfaction (Smell)

• Chemoreception: The process through which the olfactory system detects chemical stimuli in the environment, allowing for the perception of smell.

• Adapts Quickly: The olfactory system has a high adaptability; one can become desensitized to persistent odors after a short period of exposure, a phenomenon known as olfactory adaptation.

• Olfactory Nerves: These nerves originate from the olfactory bulb located at the base of the brain and extend into the nasal cavity.

• Entry Point: The olfactory nerves enter the nasal cavity through the foramina of the cribriform plate of the ethmoid bone, which allows for the passage of sensory information from the nasal mucosa to the brain.

Gustation (Taste)

• Chemoreception: The process by which the gustatory system detects chemicals in food, leading to the perception of taste.

• Adapts Quickly: The taste system can adjust to prolonged exposure to a certain taste, often reducing sensitivity.

• Papillae and Taste Buds: Papillae are small bumps on the tongue that contain taste buds, which house the sensory cells responsible for taste.

• Taste Cells and Taste Hairs: Taste cells within the taste buds have hair-like structures (taste hairs) that interact with dissolved chemicals in food, leading to taste perception.

• Five Taste Sensations:

  • Sweet: Detects carbohydrates and sugars, providing energy sources.

  • Sour: Associated with acidic foods, typically signaling the presence of acid.

  • Salty: Usually detects the presence of salt cations, important for electrolyte balance.

  • Bitter: Often associated with many organic compounds, serving a protective function against the ingestion of potentially toxic substances.

  • Umami: Often referred to as savory taste, is associated with amino acids, especially glutamate, enhancing flavors in many foods.