Stimuli

A stimulus is a physical or chemical change that takes place outside or inside the body and triggers a response in it

Receptors

Internal receptors perceive changes inside the body and are spread all over the body

External receptors perceive stimuli in the external environment and can be:

• Thermocereptors perceive changes in temperature

• Chemoreceptors detect chemical substances

• Nociceptors perceive pain

• Photoreceptors are light-sensitive

Senses and their receptors

Smell

Olfactory cells are chemoreceptors that are sensitive to volatile substances in the air

Touch

Thermoreceptors (heat and cold); mechanoreceptors (contact and pressure); and nociceptors (pain)

Taste

Gustatory cells are chemoreceptors sensitive to chemical substances from food dissolved in saliva

Hearing

Mechanoreceptors called auditory cells are sensitive to vibrations and balance cells are sensitive to movement

Sight

Photoreceptors are sensitive to variations in light intensity and enable us to see

CNS (Central Nervous System)

• The CNS coordinates all the body’s functions. It’s formed by the encephalon and the spinal cord

• The encephalon is protected by the skull (or cranium) and by three membranes called the meninges, between which cerebrospinal fluid circulates. The three layers are:

  1. Pia mater

  2. Arachnoid

  3. Dura mater

• The encephalon is divided into three parts:

  • The brain is divided into two hemispheres, which are separated by a fissure. Its surface, called the cerebral cortex, is rough and full of folds known as circumvoltions.

  • The cerebellum controls balance and voluntary movements such as walking

  • The brainstem links the encephalon to the spinal cord and controls involuntary functions

  • The medulla oblongata controls our breathing and heart rate

• The spinal cord is a long thin bundle of nerves protected by the spine, and links the encephalon to the rest of the body.

  • It serves as a conduit for nerve impulses from the receptors to the encephalon and from the encephalon to the effectors

  • It also coordinates simple responses called reflex actions

PNS (Peripheral Nervous System)

• The PNS links the CNS to the body’s organs

• It’s formed by 31 pairs of nerves which stem from the spinal cord (spinal nerves) and 12 nerves which originate from the encephalon (cranial nerves)

• Nerves are divided into:

  • Sensory nerves carry information from the receptors to the CNS

  • Motor nerves convey orders from the CNS to the effectors

• The somatic PNS consists of the sensory nerves and motor nerves. It controls voluntary movements

• The autonomic PNS consists of motor neurons which regulate the activity of the internal organs and controls involuntary movements. It’s divided in two parts:

  1. The sympathetic nervous system reacts to feelings like stress and fear

  2. The parasympathetic NS causes the body to relax

Involuntary actions

• Reflex actions are fast, involuntary automatic responses that are controlled by the spinal cord

• Reflex arc involves a receptor, sensory nerve, interneuron, motor nerve and effector organ

Endocrine system

The endocrine system consists of various glands that produce hormones

Pituitary gland

The pituitary gland produces:

• Growth hormone (GH), which stimulates the growth of muscles and bones

• Antidiuretic hormone (ADH), which regulates the amount of water in the urine

• Gonadotropins (FSH and LH), which stimulates activity in the gonads

• Thyroid-stimulating hormone (TSH) stimulates activity in the thyroid

• Oxytocin induces contractions of the uterus during labour

• Prolactin stimulates milk secretion in the breasts

Thyroid

• Thyroxine, which activates cell metabolism and influences growth and development

Parathyroid

• Prolactin, which regulates the amount of calcium and phosphate in the blood

Adrenal glands

The adrenal glands produce:

• Adrenaline, which prepares the body for a dangerous situation or for action

• Cortisol, which regulates carbohydrate metabolism

Pancreas

The pancreas produces:

• Insuline, which decreases the amount of glucose in the blood

• Glucagon, which increases the amount of glucose in the blood

Ovaries

• Oestrogen and progesterone, which control the development of secondary sex characteristics, regulate the menstrual cycle and ovulation, and prepare the body for a pregnancy

Testicles

• Testosterone, which controls the development of secondary sex characteristics and regulates the production of sperm

Hormones

Hormones are a regulatory substance that stimulate specific cells or tissues into action

Skeletal system

The skeletal system is formed by bones and joints. Its functions are:

• Internal framework for our bodies

• Protect organ and internal tissues

• Support for muscles

• Manufacture blood cells

• Storage of calcium and phosphorous

Muscular system

Antagonistic pairs of muscles work when one of the muscles is contracted and the other one is relaxed

Sex, sexuality and gender

• Sex refers to the different characteristics of males and females, such as reproductive organs, chromosomes, hormones, etc

• Sexuality is a person’s identity in relation to the gender to which they are typically attracted to (sexual orientation)

• Gender refers to the socially constructed characteristics of women and men

Adolescence

• Adolescence is a stage that lasts more or less from the age of 11 to 18

• It starts with puberty, which marks the beginning of an individual’s reproductive capacity

• There are some changes, which are due to the secretion of sex hormones into the bloodstream

• The reproductive organs develop and become fully functional

  • For girls this is menstruation

  • For boys it’s the first ejaculation

• Secondary sex characteristics develop:

  • Girls: sex organs grow, breasts develop, hips become wider, waist becomes thinner and hair grows in pubic area and underarms

  • Boys: penis and testes grow larger, shoulders become broader, muscle mass is gained, voice drops, hair grows in pubic area, underarms, face and chest

• Sexual attraction is also developed

Male reproductive system

It’s made up of the testes, the epididymis, the vasa deferentia, the accessory glands and the penis:

• The testes are two glands situated inside the scrotum. They are filled with tiny tubes called seminiferous tubules and produce both testosterone and spermatozoa

• The epididymis stores spermatozoa whilst they mature and become fertile

• The vasa deferentia connects the epididymis to the urethra, which leads to the outside of the body

• The accesory glands secrete substances that joined with spermatozoa will produce semen. They are:

  • The seminal vesicles, which produce substances that are an energy source for spermatozoa

  • The prostate gland secretes substances that activate the mobility of the spermatozoa

  • Cowper’s gland, which produces a lubricating secretion that neutralises acidity in the urethra and vagina

• The penis’ function is to deposit the spermatozoa in the female reproductive system. Its head is protected by a fold of skin called the foreskin

Female reproductive system

It’s made up of the ovaries, the genital tracts and the external genitalia:

• The ovaries are a pair of glands located at the bottom of the abdominal cavity on either side of the uterus. They produce ova and female sex hormones

• The genital tracts are:

  • The fallopian tubes, two tubes that connect the ovaries to the uterus. Here is where fertilization takes place

  • The uterus is a hollow, muscular organ where the embryo develops. The lower part (cervix) connects with the vagina

  • The vagina is an elastic tube which connects the uterus to the outside of the body. Here is where spermatozoa are deposited

• The external genitalia (vulva), is formed by the labia, folds of skin and the clitoris

Formation of spermatozoa and ova

• The formation of spermatozoa is called spermatogenesis and takes place in the seminiferous tubules, inside the testes

• The formation of ova is called oogenesis, it takes place in the ovaries and has two stages:

  1. Stage one: Females develop immature ova when they are embryos. These ova start developing when the female reaches puberty

  2. Stage two: After the onset of puberty, it’s a cycle that repeats every 28 days approximately.

Ovarian and uterine cycle

• The ovarian cycle lasts 28 days approximately and is divided into two stages:

  1. The follicle grows for 14 days. When ovulation occurs, the follicle breaks and releases ova into the fallopian tubes

  2. Formation of the corpus luteum, where the follicle becomes a mass of cells. If fertilisation doesn’t happen, it disintegrates

• The uterine cycle is the thickening of the internal wall of the uterus (the endometrium) as the ovum matures in the ovary. It also lasts approximately 28 days and has three phases:

  1. Menstruation sets the beginning of the cycle. The thick endometrium is shed and expelled through the vagina along with blood

  2. The endometrium’s lining regrows

  3. The endometrium thickens again so that if the ovum is fertilised, the embryo has somewhere to develop

Hormones that regulate the ovarian and uterine cycles:

• FSH and LH are secreted by the pituitary gland. They stimulate growth of the follicle and trigger ovulation

• After ovulation, the corpus luteum secretes oestrogen and progesterone which cause the endometrium to thicken

• If fertilisation doesn’t take place, the corpus luteum disintegrates and stops secreting hormones. As a result, the endometrium is expelled and menstruation occurs. The cycle then starts again

Fertlisation and embryonic development

• It lasts approximately nine months (40 weeks)

• Fertilisation is the process in which two gametes join together and create a new cell, the zygote

• The zygote first divides whilst it travels to the uterus. The zygote turns into an embryo as it divides

• Implantation happens when the embryo arrives in the uterus and embeds in the endometrium

• After implantation, the placenta and amniotic sac develop

  • The placenta is an organ that links the embryo to the mother through the umbilical cord to exchange nutrients, oxygen and waste

  • The amniotic sac is filled with amniotic fluid where the embryo develops

• During the first trimester, the majority of the organs have formed but not developed. The zygote becomes a foetus

• During the second trimester, the foetus’ skeleton ossifies and it starts moving. It begins to hear around 18 weeks

• During the third trimester, the foetus grows and the organs finish developing. Hair grows and the lungs are completely functional

• After 40 weeks of gestation, the mother’s pituitary gland releases oxytocin. It causes contractions in the uterus, which is the start of labour, and also stimulates milk secretion. Labour has three phases:

  1. Dilation of the walls of the uterus

  2. Expulsion of the foetus by involuntary contractions of the uterus

  3. Afterbirth, expulsion of the placenta and fetal membranes

Sterility

• Sterility is the inability to have children

• It affects both men and women and may be caused by a number of factors

• Assisted reproduction are different artificial techniques used to facilitate fertilisation. The most common are:

  • Artificial fertilisation is when the sperm is washed and injected into the uterus, where it fertilises the egg naturally

  • In vitro fertilisation, when specialists harvest and fertilise the eggs in a lab

Contraception methods

• Barrier methods prevent semen from entering the uterus. The condom and diaphragm are the most common. Plus, condoms avoid STIs

• Hormonal or chemical methods are drugs that prevent ovulation or kill spermatozoa. Birth-control pills and spermicides

• Surgical methods are sterilisation techniques

  • In women it’s called tubal ligation, when the fallopian tubes are cut and tied

  • In men, it’s called vasectomy, when the vasa deferentia is cut