Drugs and the Nervous System

Chapter 3

The nervous system is an electro-chemical communication system that regulates all physiological systems

Psychotropic drugs exert effects through the alteration of nervous system activity

Understanding drugs means understanding the nervous system

The Basis of the Nervous System •Neurons – Single cell unit of the nervous

system▫10 billion in the brain alone▫Receives, processes and transmits information

Each neuron in the brain received signals from thousands of other neurons.

•Synapses – Small gaps that separate neurons▫The site of neurotransmission

•Neurotransmission

The Neuron• Parts

▫ Soma – Cell body▫ Dendrites – Branches that

receive messages from other neurons

▫ Axon – Trunk of neuron that sends messages to other neurons

▫ Myelin Sheath – Fatty layer protects and speeds conduction

▫ Axon terminals – Buds at end of axon from which chemical messages are sent

• Types▫ Sensory▫ Motor▫ Interneurons – All CNS

neurons

SynapseSynapse

Synapse(Gk., to clasp or join)

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2 4

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7 3

6.5

Multiple types of synapsesMultiple types of synapses

Multiple patterns of connectivity▫Axodendritic▫Axoaxonic▫Axosomatic▫Dendrodendritic

6.1

Neurotransmission

• Transmission Involves Multiple Steps▫ Mediated by

neurotransmitters• Functions of

Neurotransmitters▫ Substances that enhance,

inhibit or modulate neuron firing

▫ Most drugs enhance or inhibit neurotransmission

• Main Types and Functions of Neurotransmitters ▫ Acetylcholine▫ Dopamine ▫ Norepinephrine▫ Serotonin (5HT) ▫ Endorphins▫ Gamma aminobutyric acid

(GABA)

Steps in Synaptic TransmissionSteps in Synaptic TransmissionSteps in Synaptic TransmissionSteps in Synaptic Transmission

• Synthesis▫Occurs in soma

• Transport▫Transmitter moves down

neuron• Storage

▫Vesicles in axon terminals• Release

▫Stimulated by action potential• Inactivation

▫Reuptake▫Degradation

Cholinergic system (Acetylcholine)

ACETYLCHOLINE (ACh)•Acetylcholine (ACh) is a neurotransmitter substance that is found in both the CNS and in the PNS.•In the PNS, it is the NT released at synapses on skeletal muscles and is also found in the cell bodies of the autonomic nervous system.•In the brain, it appears to be involved in learning/memory, attention as well as sleeping and dreaming.

Dopamine

DOPAMINE (DA)

•Dopamine (DA) is an inhibitory neurotransmitter. •It is implicated in movement control, attention, and learning.

▫Parkinson’s Disease •Dopamine excess may be involved in Schizophrenia.•Most importantly, it is involved in the “reward system of the brain.”

Noradrenergic system (Norepinephrine)

NOREPINEPHERINE (NE)•NE is not synthesized in the cell body, but the dopamine synthesized there is converted into NE inside the synaptic vesicles.•Like ACh, NE is found in the autonomic nervous system. Here it has an excitatory role.

▫Leads to release of adrenalin (HR & BP)•In the brain, NE is inhibitory and is primarily involved in control of alertness/vigilance. •Also involved in the control of eating (stimulates eating).

Serotonergic System (Serotonin)

SEROTONIN•At most synapses, serotonin (5HT) is an inhibitory neurotransmitter.•Facilitates and regulates motor behavior•It plays a role in the regulation of mood, producing sedation or relaxation.•It also has a role in the control of eating, sleep and arousal. In addition, it can regulate pain.

Endorphins•Generally inhibitory•Modulate the experience of pain•Controls breathing and heart rate•Controlling cough reflex as well as nausea

and vomiting • Involved in feelings of euphoria and reward•Sensory motor integration•Olfaction•Endocrine functions (eating, temp control)

…

GABA• Most prevalent inhibitory

neurotransmitter in the brain

• GABA secreted by “local” interneurons all over the brain.▫ Works as an off switch.

• Implicated in relaxation/anti-anxiety

• Many drugs target this system

Glutamate

•The “workhorse” of the neurotransmitters•Most prevalent excitatory NT.•Involved in everything, but especially

important in the formation of memories.•Can be highly toxic when out of control

Putting it Together

• PRESYNAPTIC neuron releases neurotransmitter into the synapse

• POSTSYNAPTIC neuron dendrites receive signal through interaction with RECEPTORS

• Cell body sums signal. If enough “signal” is generated then neuron depolarizes

• ACTION POTENTIAL then propagates down axon. Sodium potassium pump.

• At the axon terminal VESICLES move and fuse at the end and neurotransmitter is released into the synapse

• The process continues

The Nervous System

Peripheral Nervous System • Somatic Branch of PNS

▫ Controls voluntary muscles and movement

• Autonomic Branch of the PNS▫ Sympathetic and

parasympathetic branches of the ANS

▫ Regulates cardiovascular system & body temperature

▫ Also regulates the endocrine system and aids in digestion

▫ Sympathetic NS = Fight/Flight

The Central Nervous System

• The CNS is the BRAIN and the SPINAL CHORD

• It is the site of all psychoactive drug action

• Made up of internuerons• The BRAIN is divided into

numerous divisions▫ Subcortical▫ Cortical

Divisions of the Brain• Hindbrain

▫ Medulla – Heart rate, blood pressure, respiration

▫ Pons – Regulates sleep ▫ Cerebellum – Involved in

physical coordination• Midbrain

▫ Coordinates movement with sensory input

▫ Inferior and superior colliculi

▫ Substantia nigra• Forebrain (Cerebral cortex)

▫ Location of most sensory, emotional, and cognitive processing

▫ Two specialized hemispheres (left and right) joined by the corpus callosum

Neuroscience and the Divisions of the Brain (cont.)

Cerebral Cortex• Lobes of cerebral cortex

▫ Frontal – Thinking and reasoning abilities, memory, executive function

▫ Parietal – Touch recognition

▫ Occipital – Integrates visual input

▫ Temporal – Recognition of sights and sounds and long-term memory storage

Limbic System• Limbic System

▫ Cortical and subcortical structures that are in part responsible for emotional arousal Hippocampus Amygdala Hypothalamus