The nervous system is composed of the central nervous system, which contains the brain and spinal cord, and the peripheral nervous system, which contains the nerves and sensory organs.
The brain is the control center of the nervous system and contains roughly 100 billion neurons. The brain controls both high mental functions, like memory and consciousness, as well as low body functions like digestion and respiration. The spinal cord consists of a bundle of neurons which extends through the spin from the medulla oblongata to the lumbar region.
The nervous tissue is composed of two kinds of cells―neurons or ‘nerve cells’, which transmit electrochemical signals to the body, and neuroglia which surround and help to maintain the neurons. Neurons cells are composed of bundles of axons called fascicles, which are wrapped in connective tissue called the perineurium. There are 3 kinds of neurons:
Afferent neurons send sensory signals to the central nervous system.
Efferent neurons send signals from the central nervous system to the muscular system.
Interneurons form the network that transmits information from afferent neurons to efferent neurons.
The nerve cells interact with other cells through junctions called synapses. There are two kinds of synapses: chemical synapses and electrical synapses. Chemical synapses cause neurotransmitters to release chemicals by exocytosis, which then stimulate other cells into action. Electrical synapses connect two neurons together and allow them to pass an electric current from one to the other.
The male reproductive system is composed of the testes, glands, sperm ducts, urethra, and penis. The testes are contained within the scrotum and produce sperm (the male sex cells) and male sex hormones. The sperm exits the testes and travels through the sperm ducts where it is mixed with the nutrient-filled fluid produced by the glands, becoming semen. During sexual intercourse the semen travels through the penis, in a tube called the urethra.
The primary male sex hormone is testosterone. Testosterone is secreted in males after puberty, stimulating the production of sperm, increasing body hair growth, causing enlargement of the larynx and thickening of the vocal cords, increasing muscle mass, and thickening the skin and bones.
The female reproductive system is composed of the ovaries, fallopian tubes, uterus, cervix, and vagina and includes a 28-day cycle of events, known as the menstrual cycle. The ovaries contain hundreds of eggs or ova (the female sex cell). Hormones are released by the ovaries, which cause the lining of the uterus to develop and cause an egg to mature. The mature egg is released from the ovary and travels down the fallopian tube into the uterus. If the egg is not fertilized, the lining of the uterus breaks down, resulting in menstruation. However, if the egg is fertilized by a sperm cell, the cells start to divide, forming an embryo that attaches to the lining of the uterus. The cervix is a ring of muscle between the uterus and vagina, which dilates when the fetus is mature, enabling passage through the birth canal.
The main female sex hormones are estrogen, progesterone, luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These hormones regulate the menstrual cycle and estrogen in particular, stimulates the changes which occur during puberty.