Life Sciences: Human Reproduction, Nervous System, and Plant Responses

Spermatogenesis and Oogenesis Processes

• Spermatogenesis

  • This process occurs under the influence of the hormone testosterone.
  • The germinal epithelium located in the seminiferous tubules of the testes undergoes meiosis.
  • Each individual cell that undergoes meiosis produces $4$ haploid spermatids.
  • Each of these spermatids then matures to form a spermatozoon (sperm cell).

• Oogenesis

  • This process occurs under the influence of Follicle Stimulating Hormone (FSH).
  • The germinal epithelium of the ovary undergoes mitosis to form numerous follicles.
  • One specific cell inside a follicle enlarges and then undergoes meiosis.
  • Of the $4$ cells that are formed during this meiotic division, only $1$ survives to form a mature ovum.

The Ovarian and Uterine Cycles

• Ovarian Cycle

  • Regulated by the Follicle Stimulating Hormone (FSH) produced by the pituitary gland.
  • The follicles in the ovary develop into a Graafian follicle which contains a mature ovum.
  • The Graafian follicle produces the hormone oestrogen, which initiates the preparation of the uterus for the attachment of a fertilised ovum.
  • Approximately every $4$ weeks, the Graafian follicle ruptures to release an ovum in a process known as ovulation.
  • The released ovum is collected by the funnels of the fallopian tube.
  • Luteinising Hormone (LH), also produced by the pituitary gland, helps convert the empty Graafian follicle into a structure called the corpus luteum.
  • The corpus luteum secretes the hormone progesterone, which is responsible for maintaining pregnancy.
  • If fertilisation does not take place, the corpus luteum degenerates, causing progesterone production to drop.
  • The unfertilised ovum then passes down the fallopian tube into the uterus and eventually leaves the body.

• Uterine Cycle

  • The Graafian follicle produces oestrogen, which starts preparing the uterus for the attachment of the fertilised ovum by making the endometrium thicker, more vascular, and more glandular.
  • This preparation occurs so the uterus is ready for the attachment of the fertilised ovum in case fertilisation occurs.
  • After ovulation, the corpus luteum continues to secrete progesterone to maintain the uterine preparation.
  • If the ovum is fertilised by a sperm cell, the corpus luteum continues secreting progesterone to ensure the embryo remains attached to the uterine wall.
  • If the ovum is not fertilised, the corpus luteum is destroyed, progesterone levels drop, and menstruation occurs.

Fertilisation and Embryonic Development

• Fertilisation

  • This process takes place in the fallopian tube.
  • A haploid sperm cell makes contact with a haploid ovum.
  • Enzymes contained within the acrosome of the sperm penetrate the egg cell membrane.
  • Only the nucleus of the sperm cell enters the ovum.
  • The sperm nucleus fuses with the egg cell nucleus to form a diploid zygote.

• Development of the Zygote into a Foetus

  • The zygote divides by the process of mitosis.
  • It forms a solid ball of cells called the morula.
  • Further mitotic divisions occur within the morula to form a hollow ball of cells called a blastocyst.
  • The blastocyst attaches to the endometrial lining of the uterus.
  • The outer wall of the blastocyst is known as the chorion.
  • The chorion develops projections called villi, which embed or implant into the uterine wall.
  • The cells of the embryo continue to divide and differentiate to form different organs and limbs.
  • At this stage of development, the organism is called a foetus.

Protection, Gas Exchange, and Nourishment of the Foetus

• Protection of the Embryo

  • The chorionic villi and the endometrium together form the placenta.
  • In the placenta, the blood of the foetus and the mother run close to each other without mixing, allowing nutrients to diffuse into the foetal blood.
  • The umbilical vein carries the absorbed nutrients from the mother to the foetus.
  • The foetus is enclosed in a sac called the amnion, which is filled with amniotic fluid.
  • Amniotic Fluid Functions:
    • Protects the foetus against temperature fluctuations.
    • Protects the foetus against dehydration.
    • Protects the foetus against mechanical injuries by acting as a shock absorber.

• Gas Exchange and Nourishment

  • These processes occur within the placenta.
  • Maternal blood comes into close contact with foetal blood.
  • Oxygen and nutrients diffuse from the mother's blood into the foetal blood via the umbilical veins.
  • This nutrient-rich blood is transported to the foetus through the umbilical cord.
  • Carbon dioxide diffuses from the foetal blood through the umbilical artery and into the maternal blood.

The Autonomic Nervous System and Reflex Actions

• Location and Function of the Autonomic Nervous System (ANS)

  • The ANS is a collection of motor neurons located in the neck, head, thorax, abdomen, and pelvis.
  • Every organ and gland is controlled by two sets of nerves, a concept known as double innervation.
  • These two sets of nerves act antagonistically to control involuntary events and maintain homeostasis.
  • Sympathetic Nerves: Stimulate a response and prepare body systems for "fight or flight" (e.g., increasing heartbeat speed).
  • Parasympathetic Nerves: Inhibit a response and bring about states associated with rest and relaxation, thereby conserving energy.

• Reflex Action Mechanism

  • The receptor receives a stimulus and converts it into an impulse.
  • The sensory neuron carries the impulse from the receptor to the interneuron (connector neuron).
  • The interneuron transmits the impulse to the motor neuron.
  • The motor neuron carries the impulse to the effector (the muscle) to trigger movement away from the stimulus.

• Specific Reflex Path (Spinal Cord Focus)

  • The receptor converts the stimulus to a nerve impulse.
  • The sensory neuron conducts the impulse through the dorsal root of the spinal nerve into the spinal cord.
  • Inside the spinal cord, the sensory neuron makes synaptic contact with a connector neuron.
  • The connector neuron makes synaptic contact with a motor neuron.
  • The motor neuron conducts the impulse out of the spinal cord toward the muscles (effectors).
  • The muscles contract to pull the affected limb (hand or foot) away.

Physiology of Hearing and Balance

• The Process of Hearing

  • The pinna of the ear traps sound waves.
  • The auditory canal directs these sound waves to the tympanic membrane (eardrum).
  • Sound waves cause the tympanic membrane to vibrate.
  • Vibrations are passed from the tympanic membrane to the ossicles, which also vibrate.
  • The ossicles pass the vibrations to the oval window.
  • Vibrations at the oval window cause pressure waves in the inner ear fluids (perilymph or endolymph).
  • These pressure waves stimulate the organ of Corti.
  • The organ of Corti converts the mechanical stimuli into nerve impulses.
  • The auditory nerve transmits these impulses to the cerebrum of the brain for interpretation.

• Role of Semi-circular Canals in Balance

  • A change in the speed or direction of movement stimulates the cristae within the semi-circular canals.
  • The stimulus is converted into a nerve impulse.
  • The impulse is transmitted via the auditory nerve to the cerebellum.
  • The cerebellum sends impulses to the muscles to restore and maintain body balance.

Visual Accommodation and Pupillary Mechanisms

• Accommodation for Near Vision

  • The ciliary muscles contract.
  • The suspensory ligaments become slack.
  • Tension on the lens decreases.
  • The lens becomes more convex.
  • The refractive power of the lens increases.
  • A clear image of the near object is formed on the retina.

• Accommodation for Distance Vision

  • The ciliary muscles relax.
  • The suspensory ligaments become taut.
  • Tension on the lens increases.
  • The lens becomes less convex (it flattens).
  • The refractive power of the lens decreases.
  • A clear image of the distant object is formed on the retina.

• Pupillary Mechanism in Bright Light

  • The circular muscles of the iris contract.
  • The radial muscles of the iris relax.
  • The pupil constricts (becomes smaller).
  • The amount of light entering the eye is reduced to prevent damage.
  • Mnemonic: $C2$ (circular muscles contract), $R2$ (radial muscles relax), $PC$ (pupil constricts).

• Pupillary Mechanism in Dim Light

  • The radial muscles of the iris contract ($RC$).
  • The circular muscles of the iris relax ($CR$).
  • The pupil dilates ($PD$).
  • The amount of light entering the eye is increased to improve visibility.

Osmoregulation and the Role of ADH

• Accumulation of Excess Water

  • Caused by low temperatures, inactivity, or high fluid intake.
  • When blood water volume increases, osmoregulators in the hypothalamus are stimulated.
  • A message is sent to the pituitary gland, which secretes less Antidiuretic Hormone (ADH) into the blood.
  • Due to reduced ADH levels, the walls of the distal convoluted tubules and collecting tubules become less permeable to water.
  • Less water leaves the tubules by osmosis to enter the medulla.
  • More water remains in the tubules, forming dilute urine which is excreted.
  • Less water is re-absorbed into the capillaries, and blood water levels decrease back to normal.

• Water Deficiency (Low Water Levels)

  • Caused by high temperatures, strenuous activity, or insufficient fluid intake.
  • When blood water volume decreases, osmoregulators in the hypothalamus are stimulated.
  • A message is sent to the pituitary gland, which secretes more ADH into the blood.
  • Increased ADH levels make the walls of the distal convoluted tubule and collecting tubule more permeable to water.
  • More water leaves the tubule by osmosis and enters the medulla.
  • This water in the medulla is re-absorbed at a faster rate by the blood capillaries.
  • Blood water volume increases, and the tubules produce very concentrated urine for excretion.

Thermoregulation by the Skin

• Response to Cold Days

  • Cold receptors in the skin are stimulated.
  • Stimuli are converted to impulses and transmitted to the hypothalamus.
  • Low blood temperature also stimulates the hypothalamus directly.
  • Impulses are sent to the muscles of the blood vessels (arterioles) in the skin.
  • The blood vessels constrict (vasoconstriction).
  • Less blood is sent to the skin surface, reducing heat loss.
  • Less blood is sent to the sweat glands.
  • Less sweat is formed, reducing heat loss by evaporation.
  • Body temperature rises back to normal.

• Response to Hot Days

  • Heat receptors in the skin are stimulated.
  • Stimuli are converted to impulses and transmitted to the hypothalamus.
  • High blood temperature also stimulates the hypothalamus directly.
  • Impulses are sent to the muscles of the skin blood vessels.
  • The blood vessels dilate (vasodilation).
  • More blood is sent to the skin surface, increasing heat loss.
  • More blood is sent to the sweat glands.
  • More sweat is formed, and heat is lost via evaporation.
  • Body temperature drops back to normal.

Plant Responses and the Role of Auxins

• Auxins in Phototropism

  • Auxins are produced at the tip of the stem and move downwards evenly.
  • Even distribution leads to equal growth on all sides, and the stem grows straight up.
  • Under unilateral light (light from one side), the brightly lit side lacks auxins because they are destroyed by light or move to the darker shaded side.
  • High auxin concentration in stems promotes growth.
  • The darker side grows faster due to higher auxin levels.
  • This uneven growth causes the stem to bend toward the light (positive phototropism).

• Auxins in Geotropism

  • Auxins are produced at the tip of the root and move upwards evenly when vertical.
  • If a root is placed horizontally, auxins accumulate on the lower side due to gravity.
  • In roots, a high concentration of auxins inhibits growth.
  • The upper side of the root (lower auxin concentration) grows faster than the lower side.
  • This causes the root to bend downwards (positive geotropism).