Deals with voluntary movement; has striations (striped appearance). Muscle cells are very long cells called fibers.
Multinucleate: Has many nuclei around the membrane.
Sarcolemma: The membrane surrounding the muscle fiber.
Musculoskeletal system: Composed of muscle fibers with connective tissue wrappings.
Epimysium: Dense fibrous connective tissue surrounding the entire muscle.
Muscle fibers are the cells themselves; when discussing nerves, the term "fiber" refers to axons.
Skeletal muscle cells are very long and therefore called fibers.
Connective tissue is continuous with the tendons and periosteum of the bone.
Tendons: Connect muscle to bone.
Ligaments: Connect bone to bone.
Isometric contraction: Contraction without movement.
Paramecium: Thin projections of connective tissue that extend from the epimysium into the muscle to surround bundles of muscle fibers known as fascicles.
Endomycium: Loose connective tissue within each fascicle that separates individual muscle fibers.
Muscle Fiber Structure
Single, elongated cell surrounded by a plasma membrane (sarcolemma).
Multiple nuclei located just beneath the sarcolemma.
Myofibrils: Thread-like structures that extend the length of the fiber and dominate its interior.
Myofilaments: Protein filaments within myofibrils.
Actin: Thin myofilaments.
Myosin: Thick myofilaments.
Sarcomere
Contractile unit of the muscle.
Formed by the orderly arrangement of actin and myosin myofilaments.
Sliding of filaments shortens the sarcomere, leading to muscle contraction.
Muscle Contraction
In a relaxed muscle, actin and myosin lie side by side; H zones and I band are at maximum width.
During contraction, actin slides toward the center of each myosin myofilament, shortening the sarcomere.
In a fully contracted muscle, actin myofilaments overlap, H zones disappear, and the I band becomes very narrow.
Mechanism of Muscle Contraction
Action potential is introduced at the neuromuscular junction and propagates along the sarcolemma.
Action potential propagates along the T-tubule membrane into the interior of the muscle cell near the sarcoplasmic reticulum.
Depolarization of the T-tubule membrane causes voltage-gated calcium ion channels on the sarcoplasmic reticulum to open.
Calcium ions diffuse from the sarcoplasmic reticulum into the sarcoplasm.
Tropomyosin proteins cover myosin binding sites on actin myofilaments.
Troponin proteins are attached to tropomyosin.
Calcium ions bind to troponin, causing it to change conformation and move tropomyosin.
Uncovering myosin binding sites on actin allows myosin heads to bind and form cross-bridges.
Cross-bridges play a role in muscle contraction.
Meiosis
Myopia cells reproduce by mitosis, producing two identical daughter cells, except in the reproductive system.
Meiosis: An extra step is needed to reduce the chromosome number to haploid.
Diploid number: 46 chromosomes in a normal human cell.
Haploid number: 23 chromosomes in reproductive cells, allowing each parent to contribute half the genetic code.
Meiosis results in the formation of sperm cells and egg cells.
Germline cells: Diploid cells in the testes and ovaries that undergo meiosis.
Meiosis produces haploid cells with one set of chromosomes.
Fertilization: Haploid cells fuse to form a diploid offspring.
Meiosis is preceded by interphase, during which DNA replicates, and each chromosome becomes doubled.
Meiosis involves two divisions:
Meiosis I: Halves the number of chromosomes.
Meiosis II: Splits the sister chromatids.
End result: Four haploid cells.
Meiosis I
Prophase I: Individual replicated chromosomes become visible; each chromosome consists of identical sister chromatids joined at their centromeres. Spindle fibers start to form the nuclear envelope disintegrates. Homologous chromosomes line up next to each other.
Crossing over: Homologous chromosomes exchange DNA between adjacent homologous chromatids.
Metaphase I: Homologous chromosomes line up along the equator of the cell. The orientation on the equator is random (independent assortment).
Anaphase I: Chromosomes move apart from one another along the spindle fiber to the opposite ends of the cell. Each chromosome is still double-stranded.
Telophase I: The spindle fiber disintegrates, and cytokinesis begins. Two cells are created, each with half the number of chromosomes compared to the original cell.
Meiosis II
Prophase II: Spindle fibers form at the poles of the cell.
Metaphase II: Chromosomes line up along the equator (different from metaphase I, where homologous chromosomes were lined up).
Anaphase II: Sister chromatids move away from each other along the spindle fiber.
Telophase II: Four genetically different haploid cells are formed, along with cytokinesis.
Female Reproductive Cycle
Initiation: Begins at puberty, usually between the ages of 10 and 14.
Cycle Length: Averages 28 days.
Hormonal Control: Gonadotropin-releasing hormone (GnRH) from the hypothalamus stimulates the anterior pituitary to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
Target Organ: Ovary.
Two Components:
Ovarian cycle
Uterine cycle
Ovarian Cycle
Follicular phase: The 14 days prior to ovulation.
Ovulation: Occurs on day 14 of a 28-day cycle.
Luteal phase: The 14 days following ovulation.
Uterine Cycle
Menstrual phase: Days 1-5 (sloughing of the endometrial lining).
Proliferative phase: Days 6-14 (endometrium begins to reform).
Secretory phase: The 14 days after ovulation (further thickening of the endometrium).
Follicular Phase Events
Increased GnRH stimulates the anterior pituitary to increase production of FSH and LH.
Primordial follicles in the ovary mature into primary follicles, each containing a diploid primary oocyte arrested in prophase I of meiosis I.
Decreasing levels of progesterone lead to sloughing of the endometrial lining (menstrual phase).
Primary follicles form secondary follicles.
About 16 hours before ovulation, FSH and LH surge, causing final maturation of the follicle (mature or Grafian follicle).
Primary oocyte completes meiosis I to form a haploid secondary oocyte.
The endometrium begins to reform under the influence of estrogen produced by the ovarian follicular cells (proliferative phase).
Ovulation
The level of LH peaks, causing proteolytic enzymes to weaken and rupture the follicle wall.
Release of the secondary oocyte from the mature follicle.
Luteal Phase Events
Remaining ovarian follicular cells form a corpus luteum, which produces progesterone and estrogen.
Progesterone and estrogen stimulate further thickening of the endometrium (secretory phase).
Fertilization and Implantation
Cells at the implantation site produce human chorionic gonadotropin (HCG).
HCG prevents degeneration of the corpus luteum, maintaining elevated progesterone and estrogen levels.
After 8-12 weeks, the placenta replaces the corpus luteum in producing progesterone and estrogen.
No Fertilization
The corpus luteum becomes a corpus albicans.
Decreased progesterone and estrogen levels lead to menstruation.
Vision
Structures involved: Eye, optic nerve, and portions of the brain.
Cornea: Transparent connective tissue that covers the anterior one-sixth of the eye; refracts light.
Pupil: Opening in the colored portion of the eye.
Lens: Biconvex lens that focuses light onto the retina.
Retina: Lines the interior surface of the eye.
Retinal pigment epithelium: Prevents light scatter within the eye.
Neural retina: Multiple cell types arranged in three cell layers.
Outer layer:
Rod photoreceptor cells: Used in poor lighting conditions, low spatial resolution, no color vision.
Cone photoreceptor cells : Used for high spatial resolution and color vision.
Inner nuclear layer: Processes signals from rods and cones.
Ganglionic layer: Transmits signals to the optic nerve.
Optic Nerve
Formed by axons of retinal ganglion cells.
Attaches to the ventral aspect of the brain at the optic chiasm.
Optic Tracts
Extend around the brainstem to the lateral geniculate nucleus of the thalamus.
Lateral Geniculate Nucleus
Most ganglion cell axons from the retina end at synapses here.
Optic Radiations
Visual information leaves the thalamus in the optic radiations.
Primary Visual Cortex
Located in the cerebral occipital lobe.
Where visual information is perceived as an image.
Hearing
Sound waves strike the tympanic membrane (eardrum), causing it to vibrate.
Tympanic Membrane: Separates the outer ear from the middle ear.
Auditory Ossicles (Malleus, Incus, Stapes)
Three bones of the middle ear that vibrate.
The footplate of the stapes vibrates in the oval window.
Vibration of the footplate causes perilymph in the scala vestibuli to vibrate, displacing the basilar membrane.
Basilar Membrane
Short wavelengths (high-pitched sounds) cause displacement near the oval window.
Long wavelengths (low-pitched sounds) cause displacement far from the oval window.
Movement is detected by hair cells of the spiral organ.
Vibrations reach the perilymph in the scala tympani and travel to the round window, where they are dampened.
Meninges
Three membranes that protect the brain and spinal cord.
Meningitis: Inflammation of the meninges.
Layers (superficial to deep):
Dura mater
Arachnoid mater
Pia mater
Dura Mater (Cranial Cavity)
Two sublayers:
Periosteal layer: Tightly adherent to the inner surface of the skull.
Meningeal layer: Inner portion of the dura.
Dural venous sinuses: Formed where the periosteal and meningeal layers split; collect venous blood from the brain.
Arachnoid Mater
Thin transparent membrane in direct contact with the overlying dura.
Includes web-like extensions to the pia mater.
Subarachnoid Space
Between the arachnoid and pia mater.
Filled with cerebrospinal fluid.
Pia Mater
Delicate layer of connective tissue.
Located directly on the surface of the brain and spinal cord.