HB Week11

Our Lady of Fatima University

College of Nursing

NCM101 – Human Behavior

Introduction to Psychiatric Nursing

Mental illnesses are generally not caused by ultra-structural or anatomical defect. Any anatomical defect in the brain and the spinal cord can be referred as neurological disease and not a psychiatric disease. Psychiatric diseases are mostly caused by neurobiological disturbances in the brain that may cause interruption in man’s thinking.

Before we go to specific diseases, lets discuss first the basic anatomy and physiology. The central nervous system is composed of the brain and the spinal cord. The basic unit of the nervous system is the neuron.

Many highly specialized types of neurons exist, and these differ widely in appearance. Characteristically, neurons are highly asymmetric in shape. Neurons consist of:

The soma, or cell-body, the relatively large central part of the cell between the dendrites and the axon.
The axon, a much finer, cable-like projection which may extend tens, hundreds, or even tens of thousands of times the diameter of the soma in length. This is the structure which carries nerve signals away from the neuron. Each neuron has only one axon, but this axon may undergo extensive branching and thereby enable communication with many target cells.
The dendrite, a short, branching arbor of cellular extensions. Each neuron has very many dendrites with profuse dendritic branches. These structures form the main information receiving network for the neuron.

Axon and dendrites alike are typically only about a micrometer thick, while the soma is usually about 25 micrometers in diameter and not much larger than the cell nucleus it contains. The axon of a human neuron can be over a meter long, reaching from the base of the spine to the toes.

Neurons communicate with one another and to other cells through synapses, where the axon tip of one cell impinges upon a dendrite or soma of another, or less commonly to an axon.

Neurons communicate with one another across synapses. This communication is usually chemically mediated by rapid secretion of neurotransmitter molecules. Pre-synaptic neurons (i.e. the neurons which release the neurotransmitter) may produce in the post-synaptic neurons (i.e. the neurons being affected by the neurotransmitter) an electrical stimulation (an electrical excitation) which will spread to the axon hillock generating an action potential which then travels as a wave of electrical excitation along the axon. Arrival of an action potential at the tip of an axon triggers the release of neurotransmitter at a synaptic gap. Neurotransmitters can either stimulate or suppress (inhibit) the electrical excitability of a target cell. An action potential will only be triggered in the target cell if neurotransmitter molecules acting on their post-synaptic receptors cause the cell to reach its threshold potential.

The narrow cross-section of axons and dendrites lessens the metabolic expense of carrying action potentials, although thicker axons convey the impulses more rapidly, generally speaking.

Many neurons have insulating sheaths of myelin around their axons, which enable their action potentials to travel faster than in unmyelinated axons of the same diameter. Formed by glial cells in the central nervous system and Schwann cells in the peripheral nervous system. The myelin sheath in peripheral nerves normally runs along the axon in sections about 1 mm long, punctuated by unsheathed nodes of Ranvier.

Synapses are specialized junctions through which cells of the nervous system signal to one another and to non-neuronal cells such as muscles or glands.

Synapses form the circuits in which the neurons of the central nervous system interconnect. They are thus crucial to the biological computations that underlie perception and thought. They also provide the means through which the nervous system connects to and controls the other systems of the body.

Within the cells, small-molecule neurotransmitter molecules are packaged in vesicles. When an action potential travels to the synapse, the rapid depolarization causes calcium ion channels to open. Calcium then stimulates the transport of vesicles to the synaptic membrane: the vesicle and cell membrane fuse, leading to the release of the packaged neurotransmitter, a mechanism called exocytosis.

A neurotransmitter's effect is determined by its receptor. For example, GABA can act as a rapid or slow inhibitor, depending on whether an ionotropic or metabotropic receptor is the target of the molecule. Small molecule transmitters tend to have consistently inhibitory or excitatory action on their targets. Meanwhile, the same polypeptide may have inhibitory or excitatory effect on a cell, depending on the receptor.

Neurotransmitters may cause either excitatory or inhibitory post-synaptic potentials. That is, they may help the initiation of a nerve impulse in the receiving neuron, or they may discourage such an impulse, by modifying the local membrane voltage potential. In the central nervous system, combined input from several synapses is usually required to trigger an action potential. Glutamate is the most prominent of excitatory transmitters; GABA and glycine are well-known inhibitory neurotransmitters.

Many neurotransmitters are removed from the synaptic cleft by a process is called reuptake (or often simply uptake). Without reuptake, the molecules might continue to stimulate or inhibit the firing of the postsynaptic neuron. Another mechanism for removal of a neurotransmitter is digestion by an enzyme. For example, at cholinergic synapses (where acetylcholine is the neurotransmitter) the enzyme acetylcholinesterase breaks down the acetylcholine. Neuroactive peptides are usually removed from the cleft by diffusion.

While some neurotransmitters (glutamate, GABA, glycine) are used very generally throughout the central nervous system, others are only used in certain brain regions by particular classes of nerve cells. Serotonin is generally used as a neurotransmitter in cells involved in emotional regulation. Dopamine acts as the neurotransmitter of choice for cells in the hypothalamus which are effectively the brain's reward system, however it is also involved in the control of movement.

Neurotransmitters which have these types of specific actions are often targeted by drugs. Cocaine, for example, blocks the reuptake of dopamine, leaving these neurotransmitters in the synaptic gap longer. Prozac is a serotonin reuptake inhibitor, hence potentiating its effect. AMPT prevents the conversion of tyrosine to L-DOPA, the precursor to dopamine; reserpine prevents dopamine storage within vesicles; and deprenyl inhibits monoamine oxidase (MAO) B and thus increases dopamine levels.

Neurotransmitter	Abormal	Disease Condition
Dopamine	increased	Schizophrenia
GABA	Decreased	Anxiety
Serotonin	decreased	Depression

Course Outline: prelims | midterm | finals