Transcript Additional Nervous System Notes

Additional Nervous System Notes
Important neurotransmitters – “classical”
neurotransmitters that have been
recognized for many years:
1. Acetylcholine
– secreted at neuromuscular junctions, by
autonomic nervous system, and central nervous
system
– only neurotransmitter released at synapses
between neurons and muscles (always
excitatory)
– may be excitatory or inhibitory at other
synapses
2. Noradrenaline (also called
Norepinephrine)
– secreted by autonomic NS, and CNS
– chemically very similar to the hormone
adrenaline (also called epinephrine)
– prepares body for stressful situations
3. Dopamine
• secreted by CNS
• thought to affect motor function
• may be involved in causing schizophrenia
• degeneration of neurons that produce dopamine
in a specific brain region causes Parkinson’s
disease
– characterized by difficulty in initiating conscious
movements, uncontrolled tremors, shuffling gait, and
muscle weakness
– without dopamine, impulses cannot be transmitted
properly
– the drug levodopa (L-dopa) can be used by unharmed
neurons in the brain to synthesize dopamine - reduces
symptoms
Classifying synapses in the peripheral
nervous system:
• Cholinergic synapses use acetylcholine
– most synapses in the parasympathetic NS are
cholinergic
– neuromuscular junctions (between neurons and
muscle fibers) are cholinergic
• Adrenergic synapses use noradrenaline
– most synapses of sympathetic NS are
adrenergic
• Central NS uses much wider range of
neurotransmitters
Pain reception
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pain receptors are located in the skin and other
organs
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consist of free nerve endings which perceive
mechanical, thermal or chemical stimuli
pain signals are sent along nerve fibers to spinal
cord
signals pass across synapses to neurons that
carry them to the brain stem or thalamus of brain
signals may also pass to other neurons in
sensory areas of cerebral cortex causing
conscious pain sensation
two types of nerve fibers carry impulses from
nerve endings to brain – fast and slow
–
painful stimulus causes an initial sharp pain sensation,
followed by slow, burning pain
Pain Withdrawal Reflex
Pain Withdrawal Reflex
• Natural pain killers – enkephalins and endorphins
• small polypeptide chains that act by inhibiting
association neurons (interneurons) that transmit
pain to brain
• enkephalins
– pain control pathways in brain lead to neurons that carry
impulses down a descending tract of the spinal cord
– these neurons release enkephalins at synapses where
pain signals are passed to neurons that carry them to
brain
– enkephalins block calcium channels in membrane of
pre-synaptic neurons and block synaptic transmission to
brain
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Pituitary gland releases endorphins to
control pain
– endorphins are carried in blood to brain and
other organs
– they bind to receptors in membranes of
neurons that send pain signals and block the
release of neurotransmitters that transmit
pain to brain
The Retina
• Contains two types of cells called rods and cones (both
synapse with a bipolar neuron)
– Rods
• Most numerous
• Distributed evenly throughout retina
• Rods detect dim light
• Contain rhodopsin – visual pigment made up of protein
(opsin) and retinal (made from vitamin A)
– Light falling on rhodopsin causes reversible change in
shape – called bleaching
– This generates an action potential that is carried to
visual cortex of brain via optic nerve
• Groups of rods may pass impulses to the same sensory
neuron – not as sharp an image as created by cone cells
– Cones
• Distinguish colors
• Concentrated in fovea
• Work in a similar way to rods except visual
pigment is iodopsin
• Require much more light to be stimulated
than rods
• There are three different types of cone cells
–Each absorb different wavelengths (colors)
of white light
• Many cone cells have their own sensory
neuron so image is sharper than rods
Processing Visual Stimuli
• Light passes through the pupil and is
focused by the cornea, lens and humours
(fluids in eye)
• Image is focused on retina upside down
• Photoreceptors of retina are stimulated
(rods/cones)
• Impulse is sent to bipolar neuron
• Impulse is then sent to ganglion cells of
optic nerve
• Axons from ganglion cells travel to visual
cortex of brain
Structure and Function of Retina
Contralateral Processing
• Right and left optic nerves meet at the optic
chiasma
• Image information coming from the right half
of each visual field converge at the optic
chasma and pass to the left side of the brain
• Image information coming from the left half
of each visual field passes to right half of
brain
• Brain interprets information so we see entire
field of vision
Optic Chiasma
Structure of the Human Ear
How sound is perceived
• Outer ear catches sound waves
• Sound waves cause eardrum (tympanic
membrane) to vibrate
• Eardrum causes ear bones to vibrate (malleus,
incus, stapes) – bones multiply the vibrations
• Stapes strikes oval window causing it to vibrate
• Vibration causes fluid in cochlea to move
• Fluid movement causes hair cells (receptors)
attached to basilar membrane to move to rub
against the tectorial membrane
• Basilar membrane generates an impulse that
travels to brain via auditory nerve
Structure of Cochlea and Basilar Membrane
Cochlear
duct
Bone
Auditory
nerve
Vestibular
canal
Tympanic
canal
Hair Cells
Basilar Membrane
Fig. 50-8c
Tectorial
Hair cells membrane
Basilar
membrane
Axons of
sensory neurons
To auditory
nerve
Structure of the Brain
• Cerebral hemispheres act as integrating center for high
complex functions (learning, memory, emotions)
• Hypothalamus maintains homeostasis, coordinates the
nervous and endocrine systems, regulates pituitary gland
• Cerebellum coordinates unconscious functions such as
movement and balance
• Medulla oblongata controls automatic and homeostatic
activities (swallowing, digestion, vomiting, breathing, heart
activity)
• Pituitary gland – posterior lobe stores and releases
hormones made by hypothalamus, anterior produces
hormones that regulate many body functions
3-D Brain
Psychoactive Drugs
Affect brain and personality
– increase or decrease synaptic transmission
– may bind to receptor site on postsynaptic
membranes and mimic the usual
neurotransmitter or block the binding of the
usual neurotransmitter
– can also reduce the effect of the enzyme
which normally breaks down the
neurotransmitter substance, causing an
increase in the effect of the neurotransmitter
Behavioral effects of excitatory psychoactive drugs
1. nicotine – causes release of adrenaline from the
adrenal glands, increases blood pressure and heart
beat – affects mood, acts like a stimulant and causes
feeling of euphoria
2. caffeine – increases heart rate and urine production –
causes some mood elevation and increases alertness
3. cocaine – raises heart rate, body temperature, and
dilates pupils – increases energy, alertness, and
talkativeness – also give intense feeling of euphoria
• Stimulates transmission at adrenergic synapses
• Causes dopamine to be released and blocks
removal of dopamine so postsynaptic neuron is
overstimulated
• “crack” – smokable form of cocaine that absorbed
very rapidly and gives very intense effects (causes
greater addiction and overdose problems than
other forms of cocaine)
4.amphetamines – causes increase in heart
function, respiration, and blood pressure –
increases alertness (hyperactivity), reduces
appetite
– “ectasy” – derivative of amphetamines that
causes hyperactivity – can lead to dangerous
levels of overheating of body
• has some unusual behavioral effects:
• causes feelings of empathy, openness and
caring, lowers feelings of aggression and
increases sexual behavior – causes longterm damage to neurons
Behavioral effects of inhibitory psychoactive drugs:
1. benzodiazepines – Valium, Temazepam,
Librium – relax muscles, decrease circulation,
respiration, and blood pressure – reduce anxiety
and elevate mood
2. Tetrahydrocannabinol (THC) – main
psychoactive chemical in marijuana
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Mimics the neurotransmitter, anandamide (scientists
are not sure what anandamide does but may play a
role in memory functions)
THC acts on cannabinoid receptors (found in
cerebellum, hippocampus and cerebral hemispheres)
Causes short-term memory impairment, loss of
coordination, and stimulation of appetite
3. alcohol – acts as an inhibitor – in small
quantities, reduces inhibitions, impairs
reaction times and fine muscle
coordination – in large quantities can
cause loss of memory, slurred speech,
loss of balance and poor muscle
coordination
Causes of Addiction
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Addiction is a chemical dependency on drugs –
the drug has “rewired” the brain and has
become an essential biochemical in the body
Body often develops a tolerance and needs
more of the drug to produce the same result
Three factors increase the levels of addiction:
1. Dopamine secretion – many addictive drugs stimulate
transmission in synapses that use dopamine
- these synapses are part of the “reward
pathway” that leads to feelings of well-being
- withdrawal of the drug leads to anxiety,
depression and craving
2. Genetic predisposition – there appears to be a
genetic link to addiction (i.e. alcoholism may
run in families)
- may be the result of genetically determined
deficiency of dopamine receptors
3. Social Factors – cultural traditions, peer
pressure, family addiction, family parenting
skills, poverty and social deprivation,
traumatic life experiences and metal health
problems can all be factors increasing the
chances of addiction