Homeostasis Recap

PARTS OF THE NERVOUS SYSTEM

• central nervous system (CNS): in mammals, the CNS is connected to the body by sensory and motor neurons; in vertebrates, it consists of only the brain and spinal chord
• sensory neurons: the neurons that carry information as electrical impulses from the receptors to the CNS
• motor neurons: the neurons that carry electrical impulses from the CNS to effectors
• effectors: all the muscles and glands, which respond to nervous impulses

- Receptors and effectors can form parts of complex organs

• receptors are the cells that detect stimuli and they come in varying forms, such as taste receptors and sound receptors; they can form part of larger organs, such as the retina of the eye, which is covered in light receptor cells
• effectors respond to nervous impulses and bring about change; muscles and glands are known as effectors and respond differently; muscles contract, whereas glands secrete hormones

- Synapses connect neurons; the nerve signal is transferred by chemicals which diffuse across the gap and then trigger a new electrical signal in the next neuron.

- Reflexes are rapid, automatic responses to certain stimuli that help prevent injury without involving the conscious part of the brain.

- The passage from receptor to effector is called a reflex arc; the neurons in reflex arcs go through the spinal chord or an unconscious part of the brain.

STIMULUS --> RECEPTORS --> SENSORY NEURONS --> RELAY NEURONS --> MOTOR NEURONS --> EFFECTORS --> RESPONSE

• sclera: tough, supporting wall of the eye
• cornea: transparent outer layer at the front of the eye which refracts light
• iris: contains muscles that allow it to control the diameter of the pupil, therefore how much light enters the eye
• lens: focuses the light on to the retina
• retina: contains receptor cells, which are sensitive to light intensity and colour
• ciliary muscles and suspensory ligaments: control the shape of the lens
• optic nerve: carries impulses from the receptors on the retina to the brain

- When light receptors detect very bright light, the circular muscles in the iris contract and the radial muscles relax, making the pupil smaller and reducing the amount of light that can enter the eye.

- When light receptors detect dim light, the radial muscles contract and the circular muscles relax, making the pupil wider and allowing more light into the eye.

- When looking at near objects, the ciliary muscles contract, slackening the suspensory ligaments, therefore making the lens fat and increasing the amount by which it refracts light.

- When looking at distant objects, the ciliary muscles relax and the suspensory ligaments pull tight, making the lens go thin and causing it to refract less light.

- Long-sighted people are unable to focus on near objects as their lens is the wrong shape and doesn't refract light enough, or their eyeball is too short.

• the images of near objects are brought into focus behind the retina
• glasses with convex lenses (that curve outwards) can correct it, refracting light rays so they focus on the retina
• the medical term for long-sightedness is hyperopia

- Short-sighted people are unable to focus on distant objects as their lens is the wrong shape and refracts too much light, or their eyeball is too long.

• the image of distant objects is brought into focus in front of the retina
• glasses with concave lenses (that curve inwards) can correct it so that the light rays focus on the retina
• the medical term for short-sightedness is myopia

- The kidneys filter substances out of the blood as they pass through; waste products are filtered out and make urine, whereas useful substances (glucose, ions and water) are absorbed back into the blood through a process called selective reabsorption.

• UREA: proteins (and their amino acids) can't be stored by the body so any excess is converted into fats and carbohydrates, which can be stored- this process is deamination; ammonia is a waste product of the process and is toxic, so it's converted to urea by the liver- the urea is transported to the kidneys, where it's filtered out of the blood and excreted in urine.
• IONS: sodium and other ions are taken into the body for food, then absorbed into the blood; if the ion or water content is wrong, the balance is disrupted, which may damage the cells, meaning they don't work properly; the right balance must be maintained by the kidneys- the right amount of ions is reabsorbed into the blood after filtration and the rest is excreted.
• WATER: the body has to constantly balance the water coming in against the water going out; we lose water from the skin and lungs when sweating and breathing out; we can't control the amount lost in these ways so the amount of water is balanced by the amount we consume and the amount removed in urine by the kidneys.

STAGE 1: menstruation starts and the uterus lining breaks down for about 4 days

STAGE 2: the uterus lining build up again from day 4 to 14, into a thick spongy layer of blood vessels, ready to receive a fertilised egg

STAGE 3: an egg develops and is released at day 14- ovulation

STAGE 4: the wall is maintained for around 14 days until day 28; if no fertilised egg has implanted in the uterus wall the spongy layer starts to break down and the cycle starts again

• FSH is produced in the pituitary gland and causes the egg to mature in one of the ovaries- it stimulates the production of oestrogen
• OESTROGEN is produced in the ovaries and causes the uterus lining to grow- it stimulates the release of LH and inhibits the release of FSH
• LH is produced in the pituitary gland and it stimulates the release of an egg at day 14
• PROGESTERONE is produced in the ovaries and maintains the uterus lining during the second half of the cycle (when progesterone levels fall, the lining breaks down)- it inhibits the release of LH and FSH