HL2 Biology: E.4c Neurotransmitters & Drugs

Addiction to psychoactive drugs

• Factors contributing to addiction:

1. Dopamine secretion: drugs that are addictive stimulate synapses involving dopamine, which makes the user feel pleasure because dopamine activates the reward pathway.
2. Genetic predisposition: although specific genes are still unknown, addiction to psychoactive drugs is thought to be hereditary, like with alcoholism
3. Social factors: cultural tradition, peer pressure, traumatic circumstances/experiences, deprivation, mental health issues can encourage and even enhance addiction to psychoactive drugs but it can also prevent it

• Examples
  • Cocaine
    • Excitatory psychoactive drug
    • Highly addictive
    • Mechanism
      • once in the body, it binds to membrane proteins involved in reuptake of dopamine
      • dopamine cannot be absorbed by the presynaptic neuron anymore, resulting in an increased concentration of dopamine in the synapse
      • hence, cocaine users’ reward pathway is artificially stimulated, leading to constant euphoria
      • cocaine-induced depression can occur because of increased tolerance and adaptation by the body, which decreases the secretion of dopamine by the brain
    • Crack: vaporous so easy uptake into body and stronger effects

• THC (tetrahydrocannabinol)
  • Chemical in cannabis that causes most of the psychoactive effects
  • Receptors are in many places in the brain
    • cerebellum, hippocampus, cerebral hemisphere
  • Mechanism
    • it binds to cannabinoid receptors, which is at a synapse where signaling chemicals are released by postsynaptic neuron to bind to presynaptic neuron
    • THC blocks release of excitatory neurotransmitters
    • hence, THC is an inhibitory psychoactive drug
  • General effects
    • psychomotor behavior, short-term memory is disrupted
    • appetite stimulated
    • other effects noted by users

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HL2 Biology: E.4b Neurotransmitters & Personality

Psychoactive drugs

Drugs in general

• chemical substances ingested/injected/inhaled by a person 
• changes function of body

Psychoactive drugs

• change function of brain
• synapses are altered
  • excitatory drugs: promotes excitatory and inhibits inhibitory neurotransmitter transmission at synapses
  • inhibitory drugs: inhibits excitatory and promotes inhibitory neurotransmitter transmission at synapses
• how synapses are altered
  • some drugs have similar chemical structure as the neurotransmitter
    • can bind to neurotransmitter receptors on the post-synaptic membrane and acts as a competitive inhibitor
    • usual effect is hindered as well
  • some drugs have similar chemical structure AND effect
    • can bind to neurotransmitter receptors on the post-synaptic membrane and acts as a competitive inhibitor
    • same effect is achieved but cannot be broken down so effect lasts longer
  • some drugs interfere with the recycling of neurotransmitters at a synapse
    • hinders breakdown and/or reabsorption into the pre-synaptic membrane
    • neurotransmitter’s effect is extended
• examples
  • excitatory
    • nicotine
    • cocaine
    • amphetamines
  • inhibitory
    • benzodiazepines
    • alcohol
    • THC

HL2 Biology: E.4a Neurotransmitter & Synapses

Neurotransmitters

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.5b Brain Function

Brain and behavior

• Unconscious coordination
  • autonomic nervous system
    • nerve impulses are sent to two systems:
      • parasympathetic system
      • sympathetic system

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