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Homeostatic Systems
and Drugs

Chapter 4

Homeostasis

Internal and external changes in the environment

Body self-regulates via

nervous system and endocrine system

Equilibrium

Introduction to Nervous Systems

  • All nervous systems consist of specialized nerve cells called neurons and glia (supporting cells).
  • Neurons are responsible for conducting the homeostatic functions of the brain and other parts of the nervous system by receiving and sending information.
  • Sending and receiving information is an electrochemical process.

Transfer of Messages by Neurons

  • The receiving region of the neuron is affected by a chemical message that either excites or inhibits it.
  • Neuronal message:
  • Impulse moves from the receiving region of the neuron down the axon to the sending region (terminal).
  • Chemical messengers (neurotransmitters) are released.

Transfer of Messages by Neurons (continued)

  • Neurotransmitters travel and attach to receiving proteins called receptors on target cells.
  • Activation of receptors causes a change in the activity of the target cell; the target cells can be other neurons or cells that make up organs, muscles, or glands.

Sending Messages by Neurons

Figure 4.1: The process of sending messages by neurons.

Neurons and Neurotransmitters

  • Neurons can send discrete excitatory or inhibitory messages to their target cells.
  • Neurons are distinguished by the type of neurotransmitter they release.
  • Neurotransmitters represent a wide variety of chemical substances and functions.
  • Example: Dopamine activates the pleasure center.

Common Neurotransmitters

Neurotransmitter Type of Effect CNS Changes Drugs of Abuse
Dopamine Inhibitory-excitatory Euphoria Agitation Paranoia Altered Amphetamines, cocaine
GABA Inhibitory Cognition Sedation Relaxation Drowsiness Depression Alcohol, valium-type barbiturates

Common Neurotransmitters (continued)

Serotonin Inhibitory Sleep Relaxation Sedation LSD
Acetylcholine Excitatory- inhibitory Mild euphoria Excitation Insomnia Tobacco, nicotine
Endorphins Inhibitory Mild euphoria Block pain Narcotics

Common Neurotransmitters (continued)

Anandamide Inhibitory Relaxation Increase sense of well-being Tetrahydro-cannabinol (marijuana-like)

Neurons

  • Dendrites are the receiving regions of a neuron’s cell body.
  • Each neuron in the central nervous system is in close proximity with other neurons.
  • Although they are close, neurons never actually touch.
  • Synapse is the point of communication between one neuron and another.
  • Synaptic cleft is the gap between neurons at the synapse.

Neurons (continued)

Figure 4.2: Each neuron may have many synaptic connections.

Synapses

  • Excitatory synapse initiates an impulse in the receiving neuron when stimulated, causing release of neurotransmitters or increasing activity in target cell.
  • Inhibitory synapse diminishes likelihood of an impulse in the receiving neuron or reduces the activity in other target cells.

Synapses (continued)

  • A receiving neuron or target cell may have many synapses.
  • Final cellular activity is a summation of these many excitatory and inhibitory synaptic signals.

Drug Receptors

  • The chemical messengers from glands and neurons exert their effects by interacting with special protein regions in membranes called receptors.
  • Receptors only interact with molecules that have specific configurations.

Drug Receptors (continued)

Figure 4.3: Cell membranes consist of a double layer of phospholipids.

Drug Receptors (continued)

  • Agonists: Substances or drugs that activate receptors
  • Antagonists: Substances or drugs that attach to receptors and prevent them from being activated

Drug Receptors (continued)

Figure 4.4: Interaction of agonist and antagonist with membrane receptor.

Neurotransmitters

  • Many drugs affect the activity of neuro-transmitters by altering their synthesis, storage, release, or deactivation.
  • Neurotransmitters frequently altered by drugs of abuse:
  • Acetylcholine
  • Catecholamines
  • Serotonin
  • GABA
  • Endorphins
  • Anandamide

Figure 4.6: Synthetic pathway for catecholamine neurotransmitters.

Major Divisions of the Nervous System

  • Two major components of the nervous system
  • Central nervous system (CNS)
  • Peripheral nervous system (PNS)

Central Nervous System

  • CNS includes the brain and the spinal cord
  • CNS receives information from PNS, evaluates information, then regulates muscle and organ activity via PNSReticular activating system
  • Receives input from all the sensory systems and cerebral cortex
  • Controls the brain’s state of arousal (sleep vs. awake)

Central Nervous System (continued)

  • Basal ganglia
  • Controls motor activity
  • Limbic system
  • Regulates emotional activities, memory, reward, and endocrine activity
  • Includes the nucleus accumbens, such as reward center
  • Dopamine

Central Nervous System (continued)

  • The cerebral cortex
  • Helps interpret, process, and respond to information; selects appropriate behavior and suppresses inappropriate behavior
  • The hypothalamus
  • Controls endocrine and basic body functions

Central Nervous System Structure

Figure 4.7: Functional components of the central nervous system.

Peripheral Nervous System

  • Consists of input and output nerves
  • Input to brain and spinal cord
  • Conveys sensory info (pain, pressure, temperature)
  • Output: Two types
  • Somatic (control of voluntary muscles)
  • Autonomic (control of unconscious functions)

Autonomic Nervous System

  • Sympathetic and parasympathetic system
  • These systems work in an antagonistic fashion to control unconscious, visceral functions such as breathing and cardiovascular activity
  • Sympathetic system
  • Norepinephrine
  • Parasympathetic system
  • Acetylcholine

Autonomic Nervous System Structure

Figure 4.8

Introduction to the Endocrine System

  • The endocrine system consists of secreting glands (e.g., adrenal, thyroid, pituitary)
  • These glands produce substances called hormones (e.g., adrenaline, steroids, insulin, sex hormones)
  • These substances are information transferring molecules

Introduction to the Endocrine System (continued)

  • Hormones are secreted into the bloodstream and carried by the blood to all the organs and tissues of the body.
  • Hormones affect selected tissues that are designed to receive the information.
  • Hormones may be highly selective or very general with regard to the cells or organs they influence.

Introduction to the Endocrine System (continued)

Figure 4.9

The Abuse of Hormones:
Anabolic Steroids

  • Androgens
  • Produce growth of muscle mass
  • Increase body weight
  • Anabolic steroids
  • Are structurally related to the male hormone testosterone
  • Sometimes abused by athletes and body builders to improve strength and appearances

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