Human Physiology Part-2

1 Introduction to the Human Reproductive System—Puberty and Sex Hormones

Reproduction is the process by which an organism produces offspring of its own kind. Humans exhibit sexual reproduction, which is dioecious—that is, males and females exist in separate individuals. Fertilization of the male gamete (sperm) and the female gamete (ovum) results in the formation of a zygote, which subsequently develops into a new organism.

Puberty

The development of germ cells responsible for sexual reproduction occurs during a specific period called puberty. During this stage, sexual development becomes visible and reproductive maturity occurs—usually at the age of 12–14 years for girls and 13–15 years for boys. Sexual maturity is complete at around the age of 18–19 years.

At the root of all these changes is the secretion of various hormones. Testosterone in human males and estrogen and progesterone in women are the main sex hormones. During this period, significant changes also occur in human emotions and their intellectual and mental levels.

2 Male reproductive system – structure of primary and secondary organs

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🖼️ Male reproductive system – structures from the testes to the urethra

The male reproductive organs are mainly differentiated into two parts – primary and secondary sex organs.

Primary Reproductive Organs

The organs that produce reproductive cells or gametes (sperm) are called primary organs – these are called gonads.

Testes — Humans have a pair of testes, located outside the abdominal cavity in the scrotum. The testes have two main functions: Leydig cells secrete testosterone, and spermatogenic cells produce sperm; Sertoli cells provide protection, support, and nutrition to sperm.

Secondary Reproductive Organs

Any organs other than the testes that support the reproductive system are called secondary sex organs – their main function is to ensure the transport, nutrition, and maturation of sperm.

• Scrotum—Holds the testicles anchored outside the body. Spermatogenesis requires a temperature about 2-3°C lower than normal body temperature, so it acts as a thermoregulator.
• Epididymis – Nourishes sperm and stores them for about a month.
• Vas deferens—carries sperm to the seminal vesicle; joins with the seminal vesicle to form the ejaculatory duct.
• Seminal Vesicles — Produce a thick, viscous fluid that makes up about 60% of semen; this fluid provides sperm with energy and movement through fructose.
• Prostate gland—located just below the bladder; secretes a milky, alkaline fluid (about 30% of semen) that activates sperm and protects the urinary tract by neutralizing its acidity.
• Urethra – forms the urogenital tube and carries both urine and semen out through the penis.
• Penis – Erectile organ, which carries semen to the female genitalia.

3 Female Reproductive System – Structure of Primary and Secondary Organs

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🖼️ Female reproductive system – structures from the ovaries to the vagina

Primary Reproductive Organs

Females have a pair of ovaries as their primary sex organs. The ovaries have two primary functions: egg production (the production of female germ cells) and hormone secretion (as an endocrine gland, producing estrogen and progesterone). The ovaries are located in the abdominal cavity, below the kidneys, in the pelvic region, on either side of the uterus. Each ovary contains numerous ovarian follicles, which produce eggs.

Secondary Reproductive Organs

• Fallopian Tube — A tube about 10–12 cm long; finger-like structures at the mouth called fimbriae capture the egg released from the ovary. Fertilization occurs in the ampulla of this tube—its main function is to transport the egg to the uterus.
• Uterus — An inverted, pear-shaped muscular organ where the fertilized egg implants, the placenta develops, and the baby develops for nine months. During delivery, its walls contract and help push the baby out.
• Vagina—a flexible, muscular tube; Lactobacillus bacteria convert glycogen into lactic acid, keeping the vaginal tract acidic (pH approximately 3.5–4.5), reducing the risk of infection. It is also called the birth canal because childbirth takes place through it.

External genitalia – The vulva includes the mons pubis, labia majora, labia minora, clitoris, and Bartholin's glands (alkaline secretions that facilitate sexual activity).

Breast

The mammary glands are made up of milk ducts, similar to bunches of grapes, that nourish the newborn. The growth and function of the mammary glands are controlled by the hormones somatotropin, prolactin, estrogen, and progesterone.

4 Menstrual Cycle and Ovarian Cells

After puberty, a woman's body undergoes a cyclical process each month designed to prepare her for a potential pregnancy—this is called the menstrual cycle. This cycle typically lasts 28 days and is divided into four phases:

Menstruation involves the inner lining of the uterus (endometrium), which has thickened to accept a potential fertilized egg, shedding blood and tissue if fertilization doesn't occur. Ovulation occurs around day 14—a mature egg is released from the ovary and travels into the fallopian tube. If fertilization doesn't occur, the cycle begins again with menstruation.

Main cells of the ovary and their functions

5 Gametogenesis, fertilization, embryo implantation and parturition

The process of reproduction in humans mainly takes place in the following stages:

(a) Gametogenesis

The process of formation of haploid gametes in the testis and ovary is called gametogenesis. The process of formation of sperms in the testis of male is called spermatogenesis and the process of formation of ova in the ovary of female is called oogenesis.

(b) Fertilization

When sperm released by the male during copulation come into contact with the female's egg, their fusion results in the formation of a zygote—a process called fertilization, which usually occurs in the fallopian tube.

(c) Cleavage and Embryo Implantation

After fertilization, the zygote undergoes mitosis, a process called cleavage. This results in the formation of a multicellular structure called a blastula/blastocyst. This blastocyst then implants itself in the endometrium of the uterus—a process called embryo implantation.

(d) Parturition/Delivery

After implantation, the embryo goes through various developmental stages (such as gastrulation). When the fetus is fully developed (about 9 months), the baby is born—this process is called parturition.

6 Introduction and classification of the nervous system

The various organs of the human body function in cooperation and coordination with each other. To maintain this coordination between organs, two main systems operate within the body—the nervous system and the endocrine system. Since the nervous system cannot directly control the functions of all cells, the ductless glands of the endocrine system act as messengers by secreting hormones.

Functions of the nervous system

• providing information about the outside world through the senses
• Enabling memory, thinking, and reasoning
• Control of all voluntary muscle movements (such as running, speaking)
• Regulating many involuntary functions (respiration, heartbeat, movement of food through the digestive tract)

Classification of the human nervous system

The human nervous system is mainly divided into two parts:

• Central Nervous System (CNS) — Consists of the brain and spinal cord; it is the main center for information processing.
• Peripheral Nervous System (PNS) — The group of nerves that originate from the brain and spinal cord and carry messages to and from the CNS. It has two subdivisions: the somatic nervous system (voluntary actions) and the autonomic nervous system (involuntary actions, further divided into sympathetic and parasympathetic).

Basic terminology: Stimulus – a sudden change in the external or internal environment; Impulse – a wave of electrical disturbance traveling through a nerve cell; Response – a change in the activity of an organism due to a stimulus; Receptors – cells that receive a stimulus and send a signal to the CNS; Effectors – muscles or glands that contract or secrete upon receiving an impulse.

7 Neuron – Structure, Types and Synapses

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🖼️ Structure of a neuron – cell body, dendrites, axon and myelin sheath

The neuron is the structural and functional unit of the nervous system. Nervous tissue contains two types of cells—neurons (information transmitters) and neuroglia (supporting cells).

Structure of a neuron

• Cell body (cyton or soma) – contains the nucleus, organelles such as mitochondria, and Nissl granules (dark-colored particles made of RNA and protein). The cytoplasm is called neuroplasm.
• Dendrites — Small, branched projections that extend from the cyton and transmit stimuli to the cell body. A single neuron can have up to 200 dendrites!
• Axon — A long, cylindrical fiber (1-20 microns thick) that extends from the cyton and carries the impulse to its terminal end. The terminal end contains synaptic buttons that connect to the dendron of another neuron.
• Myelin Sheath – A fatty sheath covering some axons that speeds saltatory transmission; it is broken down at the nodes of Ranvier.

Synapse

The point of communication between one nerve cell and another, or between a nerve cell and a muscle, is called a synapse. As an impulse travels from the dendrite to the axon, it reaches the synaptic site, where acetylcholine is released—it acts as a neurotransmitter (chemical messenger) that transmits the impulse to the next neuron.

Types of Neurons (Based on Structure)

• Unipolar neuron
• Bipolar neuron
• Multipolar neuron
• Apolar neuron

8 Nerve impulse transmission—from polarization to saltatory transmission

The conduction of a nerve impulse in neurons is an electrochemical phenomenon—the impulse travels along the fiber as an electrochemical wave. This is not simply a flow of electrons along a wire, but a self-propagating wave of depolarization.

Resting Potential

The nerve is in a polarized state—the axon is positively charged on the outside and negatively charged on the inside due to an excess of Na+ on its outer surface. This state is maintained by the sodium-potassium pump, which continuously pumps Na+ ions out through active transport, expending ATP energy—a mechanism similar to the continuous charging of a battery.

Depolarization – Action Potential

When a stimulus arrives, the membrane at that location becomes more permeable to Na+. The influx of Na+ exceeds the outflow of K+, causing the neuron to become negatively charged on the outside and positively charged on the inside—the potential generated during this process is called the action potential. This potential acts as a stimulus for other parts of the brain, causing the wave to propagate.

Repolarization

As the depolarization wave progresses forward along the neuron, the effect of stimulation in the previous regions ends and the neuron returns to the resting state – in this process K+ is released and Na+ is sent back out, this is called repolarization.

Under normal circumstances, nerve impulses are transmitted at a speed of about 45 m/s; this speed is much faster in myelin-sheathed fibers, because the impulse is transmitted by 'jumping' from one node of Ranvier to another—this is called saltatory conduction.

9 Brain Part 1 – Forebrain and Midbrain

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🖼️ Vertical section of the human brain — forebrain, midbrain and hindbrain

The brain is a delicate organ located in the cranium, protected by three meninges—the outer tough duramater, the middle thin meshwork arachnoid, and the innermost blood vessel-rich piamater. Cerebrospinal fluid (CSF) is filled between these membranes, which protects the brain from shock. The brain has three main parts—the forebrain, the midbrain, and the hindbrain.

Forebrain

(i) Cerebrum

The largest part of the brain is divided into two cerebral hemispheres. Each hemisphere is divided into four lobes—the frontal lobe, parietal lobe, temporal lobe, and occipital lobe. The outer part of the cerebrum is made up of gray matter (cerebral cortex), and the inner part is made up of white matter (cerebral medulla).

Functions: Sensory functions (vision—in the occipital lobe, hearing—in the temporal lobe, touch-heat-pain—in the parietal lobe), motor functions (the motor center located near the central fissure of the frontal lobe regulates the movements of all voluntary muscles), and coordination functions (the centers of higher mental functions such as intelligence, memory, consciousness, reasoning, and speech are located here).

(ii) Diencephalon

It has three parts – epithalamus (pineal gland is located here), thalamus (center for transmission and interpretation of sensory information like vision, taste, hearing, touch, temperature, pressure), and hypothalamus (bridge between nervous system and endocrine system; higher center of autonomic functions like hunger-thirst-sleep-anger-love; controls body temperature; controls the secretion of pituitary gland; also secretes vasopressin and oxytocin).

Midbrain

The smallest part of the brain, divided into two parts—the cerebral peduncles and the corpora quadrigemina (the quadrigemina—two peduncles for visual reflexes, two peduncles for auditory reflexes). This part controls and regulates reflex actions and provides a pathway for impulses to and from the brain.

10 Brain Part 2 – Hindbrain and Spinal Cord

Hindbrain

Cerebellum damage impairs a person's balance and coordination—walking, writing, and normal movements become difficult, highlighting how important this area is. The medulla oblongata is the part of the brain that directly controls life-sustaining involuntary functions (breathing, heartbeat)—so severe damage to this area can be fatal.

Spinal Cord

The spinal cord begins at the medulla and extends almost the entire length of the spine. Like the brain, it is also enclosed in three sheaths, between which is a layer of CSF. An important difference is that the arrangement of gray matter and white matter in the spinal cord is reversed—the white matter is on the outside and the gray matter is on the inside (whereas in the brain, the opposite is true).

The main functions of the spinal cord are: carrying out reflexes from the neck down, transmitting sensory impulses from the skin and muscles to the brain, and transmitting motor responses from the brain to the trunk and limbs.

11 Peripheral and Autonomic Nervous System

The peripheral nervous system (PNS) is composed of two types of nerves—sensory nerves, which carry stimuli from the organs to the CNS, and motor nerves, which carry regulatory stimuli from the CNS to the relevant organs. Twelve pairs of cranial nerves arise from the brain, and 31 pairs of spinal nerves (cervical 8, thoracic 12, lumbar 5, sacral 5, and caudal 1) arise from the spinal cord.

Somatic vs. Autonomic Nervous System

The autonomic nervous system (ANS) maintains the body's internal environment and regulates vital bodily functions. It is divided into two parts: the sympathetic (active during times of pain, anger, fear, and stress) and the parasympathetic (active during times of pleasure, happiness, and satisfaction).

Effects of the sympathetic versus parasympathetic systems

12 Reflex Action and Disorders of the Nervous System

The process of carrying the stimuli received by the sensory organs to the spinal cord through sensory nerves and immediately sending the response to the muscles, tissues or organs through motor nerves and stimulating them is called reflex action. The innate or unconscious response to a stimulus or sudden change in the external environment is called simple reflex.

Examples of reflexive verbs

• shivering when cold
• Opening the mouth on hearing a sudden loud noise
• Closing of the eyes in front of bright light
• Blinking when an object suddenly appears before the eyes
• If a pin/needle pricks any part of the body, remove it immediately.
• Saliva in the mouth at the sight of good food

Types of reflexive verbs

• Unconditioned Reflex – Innate reflexes that occur automatically without learning or practice; e.g., immediately withdrawing the hand upon touching a hot object.
• Conditioned Reflexes — Learned reflexes that develop through practice and experience. A famous example is Ivan Pavlov's experiment on dogs, in which the sound of a bell caused the dog to salivate. Riding a bicycle is also an example of a learned reflex.

Major disorders of the nervous system