HL2 Biology: E.4a Neurotransmitter & Synapses


Excitatory post-synaptic potentials (EPSP)

  • post-synaptic neurons have receptor proteins specific to excitatory neurotransmitters
  • binding neurotransmitter makes post-synaptic membrane permeable to Na+, which moves across post-synaptic membrane
  • causing depolarization of the post-synaptic membrane
  • enzymes catabolize neurotransmitters
  • monoamine oxidase catabolizes norepinephrine
  • acetylcholine esterase catabolizes acetylcholine
  • examples
    • epinephrine
    • dopamine
    • serotonin

 Inhibitory post-synaptic potentials (IPSP)

  • post-synaptic neurons have receptor proteins specific to inhibitory neurotransmitters
  • that make the post-synaptic membrane less permeable to Na+
  • or allow K+ to diffuse out of the post-synaptic membrane
  • causing hyperpolarization of the post-synaptic membrane
  • enzymes catabolize neurotransmitters
  • examples 
    • glycine
    • gamma-aminobutyric acid (GABA)
    • acetylcholine

HL2 Biology: E.5a Brain Structure

The Human Brain

Parts of the brain

  • Medulla oblongata: controls automatic, homeostatic responses (e.g. swallowing, vomiting, digestion, breathing, heart)
  • Cerebellum: controls unconscious functions (e.g. balance, movements, hand-eye coordination)
  • Hypothalamus: homeostasis maintenance through the nervous/endocrine systems (i.e. hormone production/secretion)
  • Pituitary gland: consists of posterior lobe (stores and secretes hormones  produced by the hypothalamus) and anterior lobe (produces, secretes hormones)
  • Cerebral hemisphere: sensory input from the eye, ear, nose, and tongue is sent here; integrates complex functions (e.g. learning, memory, emotions, consciousness)

Examining the brain

  • Methods
    • Animal experiments: brain is examined through surgeries on living primates and other animals; controversial
    • Lesions: brain damage through accidents, strokes, tumors lead to sequela (aftereffects), and their effects can be used to determine the functions of the damaged part of the brain (e.g. Broca’s area => dysphasia, insula => craving lost)
    • Functional magnetic resonance imaging

HL2 Biology: E.2c Ears

Perception of sound

  • Mechanoreceptor
    • used to perceive sounds through the ear
    • senses vibrations from the surrounding environment and transmits them to our brain through auditory nerves (sensory neurons)
  • Hearing process
    1. sound waves reach our eardrum, a thin layer of tissue dividing the outer and inner ear
    2. eardrums respond to the sounds by vibrating rapidly
    3. these vibrations are then transmitted to the bones of the middle ear, which has functions to control the volume of the sound (amplification by 20 times, damping using surrounding muscles)
    4. bones then transfer the vibrations to another thin layer of tissue called oval window
    5. movements are passed on to the cochlea, a coiled tube filled with liquid
      • cochlea contain membranes attached to sensory neurons and hairs that resonate to specific frequencies of sound waves, allowing us to sense different sounds.
      • for the vibrations to be transmitted from the oval window to the cochlea, the round window, another thin membrane, is necessary because it functions to contain the cochlear fluids in the given space by moving inwards or outwards depending on the movements of the oval window.

HL2 Biology: E.2b Vision: Retina

Perception of light

  • Photoreceptors
    • found in retina of eye
    • two types
      • rod cells
        • more sensitive to light
        • function better in dim light, become bleached in bright light
        • can absorb all wavelengths of visible light = monochrome vision
        • 200 cells pass impulse to same sensory neuron of optic nerve = less accuracy
        • more widely dispersed = wider field of vision
      • cone cells
        • function better bright light
        • three types of cone cells, only red/green/blue light is  absorbed = color vision
        • many cone cells have their own neuron used to communicate to brain = greater accuracy
        • very concentrated in fovea = one acute field of vision
      • both absorb light and transmit messages to brain via optic nerve
  • Processing of visual stimuli
    • Convergence
      • bipolar cells in retina combine impulses from rod/cone cells and transfer them to ganglion cells
    • Edge enhancement
      • two types of ganglion cells that respond differently to light stimulation in the receptive field in the retina
        • type 1: stimulated when light hits the center of the receptive field, de-stimulated when light hits periphery at the same time
        • type 2: stimulated when light hits the periphery of the receptive field, de-stimulated when light hits center at the same time
      • more stimulation occurs in both ganglion cells if light/dark edge is in the receptive field
      • example: Herman grid illusion
        • lateral inhibition
        • optical illusion
    • Contralateral processing
      • optic chiasma: cross over point for left and right nasal optic nerves
      • hence, left optic nerve carries impulse from right field of vision and right optic nerve carries impulse from left field of vision
      • this process helps us tell distances and sizes

HL2 Biology: E.2a Perception of Stimuli

Stimulus and response

  1. Stimulus: change in the environment (internal/external) detected by receptor, resulting in a response
  2. Response: change in organism, produced by stimulus
  3. Reflex: rapid unconscious response to stimulus
    • reflex arc
      • neuron pathway and at least 3 synapses involved in reflex
      • receptors (sensory cells or sensory neuron nerve endings) are used to detect stimulus
        • mechanoreceptors (e.g. hair cells in cochlea of ear, pressure receptor cells in skin) detect stimuli in the form of mechanical energy,  including sound waves and pressure/gravity
        • chemoreceptors (e.g. receptor cells in tongue, nerve endings in nose) detect stimuli in the form of dissolved or vaporized chemical substances
        • thermoreceptors (e.g. nerve endings in skin) detect stimuli in the form of temperature
        • photoreceptors (e.g. rod and cone cells in eye) detect stimuli in the form of electromagnetic radiation, usually light
      • sensory neurons are used to receive messages from receptors and transmit them to the central nervous system (spinal chord/brain) via synapses, which lead to perception
      • relay neurons are used to receive messages from sensory neurons and transmit them them to motor neurons via synapses, which lead to responses
      • effectors are needed to carry out the appropriate response ordered by the message from the motor neuron (e.g. muscles, glands)

HL2 Biology: 11.2b Muscles & Movement

DBQ p.267
  1. A transverse section of muscle is perpendicular to a longitudinal section, which is cut along the length of the muscle, whereas the transfer section is cut along the width.
  2. The small dots represent the light band, because it consists of only actin.
  3. The first diagram has a mixture of both small dots and large dots (size is in terms of diameter), while the second only has large dots and the third only small dots. The small dots found in diagrams one and three both are around the same sizes and quantities. Similarly, the large dots found in diagrams one and two are around the same sizes and quantities as well. The first diagram appears to be a combination of the second and third diagrams.
  4. If the small dots represent actin fibers and the large dots myosin fibers, the first diagram is a combination of small dots and large dots, meaning it is likely that it is a cross section of the A band, in which both actin and myosin are present. The second diagram, on the other hand consists only of large dots, and hence myosin fibers, meaning it may be representing the H-zone that only has myosin. Lastly, the third diagram only has small dots, hence only actin is present, meaning that it is a representation of the I-bands only consisting of actin fibers.