homeostasis and response

• homeostasis is the regulation of the internal environment of an individual cell or organism to maintain constant optimum conditions in response to internal and external changes
• the internal environment of a cell or organisms includes:

  1. the cytoplasm of the cell
  2. tissue fluid (surrounds all cells)
  3. blood plasma
• thermoregulation is the homeostatic control of body temperature. This is important as temperature affects how enzymes work, and therefore metabolism. the body temp of humans is 37 degrees celcius and is controlled by the hypothalamus of the brain
• negative feedback loop (for rising temp):

  optimum → temp rises (stimulus) → thermoreceptors (receptor) → brain (coordinator) → effector (sweat glands, muscles in hands) → temp reduces (response)
• when your body is too warm, vasodilation allows more blood to the skin surface so more heat is lost by radiation and convection. sweat glands release sweat to lose heat through evaporation. liver metabolism slows down so less heat is produced by respiration.
• when your body is too cold, vasoconstriction lets less blood to the skin's surface and so less heat loss. sweat glands stop releasing sweat. shivering means that the muscles contract to generate heat. liver metabolism speeds up so more heat is produced by respiration. hairs stand on end which traps a layer of air against the skin to insulate the body.
• other conditions that need to be kept constant:

  • concentration of water and ions as these have an effect on the osmotic effect on cells
  • concentration of glucose as if there is too little, cell cannot respire, too much and there is osmotic damage to cells
  • pH as it affects enzymes and metabolism
• the nervous system is an organ system containing a network of highly specialised cells called neurones. it has 3 roles; collects info about the internal/external environment, integrates the information, and acts upon the information by coordinating the actions of an organism.

  the CNS consists of the brain and spinal chord. The PNS consists of the nerves connecting body to the CNS.
• sense organs are a group of highly specialised sensory cells with receptors which detect a particular stimulus. most are arranged near the outer surface of the body, particularly the head area.
• balance - sense organs are semi-circular canals in inner ear. the receptor cells are receptors in the semi-circular canals. stimulus is the movement and position of the head.
• taste - sense organ is the mouth. receptor cells are the taste buds. stimulus is the chemicals in food.
• smell - sense organ is the nose. receptor cells are olfactory receptors. stimulus is chemicals in the air.
• touch - sense organ is the skin. receptor cells are pain, temp, pressure or touch receptors. stimulus is pain, temp, pressure or touch.
• sight - sense organ is the eyes. receptor cells are rods and cones in the retina. stimulus is light.
• hearing - sense organ is inner ear. receptor cells are auditory hairs in cochlea. stimulus is sounds or vibrations.
• neurones are specialised cells with usual cell structures. neurone axon is up to 1m long and thin and impulses can travel long distances. dendrites allow contact between neurones. an insulating myelin sheath speeds up the impulse and prevents interference from other adjacent neurones.
• sensory neurone - sends electrical impulses from the receptor to a relay neurone located in the CNS

  A) to the CNS

  B) dendrites ending in CNS

  C) fatty myelin sheath

  D) sensory receptors ending in a sense organ

  E) cell body

  F) axon

  G) nucleus

  
• relay neurone - only found in the CNS, connect sensory neurones and motor neurones

  A) dendrites

  B) cell body

  C) nucleus

  D) axon

  E) synapses

  
• motor neurone - receives impulses from the relay neurones, then carries them from the CNS to the effectors

  A) dendrites

  B) dendrites synapsing onto a muscle or gland

  C) axon

  D) myelin sheath

  E) cell body

  
• how the nervous system connects:

  receptors in sense organ are stimulated → electrical impulse sent along sensory neurone → if the impulse is strong enough it passes across a synapse in the CNS → impulses from the CNS travel along motor neurones in nerve fibres to effectors → reaches responding organ
• synapses block non-essential stimuli and ensure electrical impulses only travel in one direction.

  A) electrical impulse

  B) pre-synaptic neurone

  C) synaptic vesicles

  D) neurotransmitter

  E) receptor molecules on surface

  F) post-synaptic neurone

  G) synaptic gap (about 20 nanometres wide)

  
• connections between neurones:

  1. electrical impulse reaches the synapse
  2. cannot cross the gap
  3. chemical neurotransmitter is released from vesicles in the pre-synaptic neurone
  4. the neurotransmitter diffuses across the gap and binds to receptors on the post-synaptic neurone on the other side
  5. this causes an electrical impulse to start in this next neurone
  6. the neurotransmitter is either reabsorbed back into the vesicles in the presynaptic neurone to be used again, or broken down by enzymes
• voluntary actions use the brain (cerebrum). they are conscious and less rapid in creating a response, eg walking or writing
• reflex (involuntary) actions - unconscious and automatic, innate, rapid responses. play an important role in survival, eg blinking or withdrawal reflex.
• reflex arc

  A) sensory neurone

  B) CNS

  C) synapses

  D) relay neurone

  E) motor neurone

  F) sharp object

  G) muscle cells (effector)

  
• the brain is made up of billions of inter-connected neurones forming a neural network. coordinates and controls all our activities and behaviours, including breathing, walking, writing, digestion and falling in love

  A) cerebellum

  B) cerebrum

  C) pituitary gland

  D) medulla oblongata

  
• the cerebrum/cerebral cortex is the outer layer of the brain, made up of a large sheet of tissue, folded to fit inside the skull. coordinates voluntary/skeletal muscle movement. contains centres for memory, learning, personality, emotions, speech, etc. also has centres controlling sight, hearing, touch, taste and smell.
• cerebellum coordinates balance and posture (involuntary muscle movement). controls precise movements involving writing and other hand work. modifies muscle movement involved in talking.
• medulla oblongata links spinal chord and brain. control of involuntary muscle movement such as swallowing, vomiting, coughing, sneezing - simple, innate reflex actions. involved in homeostatic control of heart rate, breathing and blood pressure.
• brain disorders are difficult to treat to due to complexity and delicacy of the brain. we have learnt about the brain through studies of brain damage, electrical stimulation, and MRI scans. a functional MRI shows which parts of the brain are most active when a patient does different tasks.
• the eye is as sense organ containing receptors. sensitive to light intensity and colour. two functions - adaptation to light intensity and accommodation so focus on near or distant objects.

  A) sclera

  B) choroid

  C) retina

  D) fovea

  E) blind spot

  F) optic nerve

  G) suspensory ligament

  H) lens

  I) cornea

  J) pupil

  K) iris

  L) ciliary body

  
• in low light intensity, light receptors on retina send messages along optic nerve via sensory neurones to the brain. a message is sent back to the radial iris muscles of the eye via motor neurones. these contract, increasing the size of the pupil and increasing light intensity of light entering the eye. this helps to see in dim light.

  A) radial muscles contracted

  B) circular muscles relaxed

  C) dilated pupil

  
• in high light intensity, light receptors on retina send messages along optic nerve via sensory neurones to the brain. a message is sent back to the circular iris muscles of the eye via motor neurones. these contract, decreasing the size of the pupil and cutting down intensity of light entering the eye. this helps to prevent damage to the retina.

  A) radial muscles relaxed

  B) circular muscles contracted

  C) contracted pupil

  
• accommodation is the process of changing the shape of the lens to focus on near or distant objects. light from an object needs to be focused on the retina so the light has to be refracted. most of this refraction occurs as the light passes through the cornea. the lens makes the final adjustment to focus the image onto the retina.
• for distant objects - ciliary muscles relax and the suspensory ligaments are pulled tight. the lens is pulled thin and only slightly refracts light rays.
• for near objects, ciliary muscles contract and the suspensory ligaments slacken. the lens is rounded and becomes thicker and refracts light rays.
• eye defects

  • myopia - the focal point is in front of the retina, distant objects appear blurry, corrected with concave lens
  • hyperopia - the focal point is behind the retina, close up objects appear blurry, corrected with convex lens

  can be treated with spectacle lenses, hard and soft lenses, laser surgery to change shape of cornea, or change lens.
• the hormonal (endocrine) system coordinates the body's organs so that they work together for a combined effect. based on the production of hormones. they are produced by glands all over the body and are made in small quantities. they are secreted into and transported in the blood plasma.
• endocrine glands are ductless and they secrete hormones directly into the bloodstream. for example, the pituitary gland (master gland), thyroid gland (near neck), adrenal glands, pancreas, ovaries and testes.
• hormones have a specific shape so they can only affect cells with the correct receptor on the cell surface. the receptor will have a complimentary shape