Additional Nervous System NotesImportant 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• pain receptors are located in the skin and other

organs– 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

• 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

Basilar Membrane

Cochlearduct

Vestibularcanal

Bone

Tympaniccanal

Auditorynerve

Hair Cells

Fig. 50-8c

Tectorialmembrane

To auditorynerve

Axons ofsensory neurons

Basilarmembrane

Hair cells

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 DrugsAffect 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 drugs1. 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 long-term 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 • 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

• 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