Lesson B1 - PHYSIOLOGICAL AND PHARMACOLOGICAL ASPECTS OF THE CENTRAL AND PERIPHERAL NERVOUS SYSTEM Flashcards
The nervous system can be divided into two main components
the central nervous system (the brain and spinal cord) and the peripheral nervous system
afferent or sensory nerve fibres which carry messages to the brain, and efferent nerve fibres which carry messages
from the brain and spinal cord to tissues.
The efferent system is divided into motor nerves which innervate skeletal muscle, and
the autonomic nervous system.
The central nervous system (CNS) controls
all bodily functions
It consists of a central part, the
brain and spinal cord, linked to
a peripheral part, nerve fibres
The sensory nerve fibres carry
messages from tissues to the brain or spinal cord, and the motor nerve fibres carry messages
from the brain or spinal cord to the tissues.
Three main parts of the brain
- Forebrain
- Midbrain
- Hindbrain
The forebrain: cerebral cortex =
This is the largest part of the brain which is very rich in nerve cells. It is composed of grey matter (outside) and white matter (inside); it is divided
into lobes or regions, each with specific functions
The forebrain: Thalamus:
A relay centre; from here impulses are relayed to the cerebral cortex
The forebrain:Hypothalamus:
A very important area; consists of various specialized regions of nuclei
located near the base of the skull. The functions are to control the involuntary functions of
the body that are necessary for living, e.g. regulation of heart, blood pressure, body
temperature, and metabolism
The forebrain: Pituitary
A small gland located at the base of the brain which secretes hormones that
control growth, behaviour and metabolism of the body through the action of these
hormones on peripheral tissues
The Midbrain
The midbrain is the area that links the forebrain with the hindbrain.
The Hindbrain
Medulla (the bulb): This is the site of origin of many cranial nerves. It is where
regulation of respiration (breathing centre) and regulation of heart and blood pressure
occurs
Cerebellum: The cerebellum is a large, highly convoluted structure connected to the brain
stem by large fibre tracts. It is responsible for coordination and posture
The functional unit of the brain
is the neuron
each nerve cell or neuron has three
parts:
The cell body or soma contains a nucleus and surrounding cytoplasm which is packed with
rough endoplasmic reticulum, a network of smooth endoplasmic reticulum, and abundant
vesicles which can be secreted.
The dendrites function as the receiving antennae for incoming information, are usually
short, and can have highly complex branching patterns.
The axon, a single fibre that extends from the cell body and ends at a synapse. The axon
carries signals away from the cell body.
In order for the brain to function properly, the nerve cells (neurons)
must communicate with each
other.
The junction between two neurons is called the
synapse
The synapse is commonly formed by contact of the
axon belonging to one neuron with a dendrite or
the cell body of another neuron.
The passage of a signal from one neuron to
another neuron is called
synaptic transmission
Synaptic transmission is usually
chemical in nature
Substances mediating synaptic transmission are
synaptic transmitters
In chemical transmission, the
release of a transmitter substance is required in order to
activate the other cell or pass on the
message.
The nerve impulse (electrical activity) passes down a nerve axon and releases
a chemical substance into the synaptic cleft.
The postsynaptic membrane contains binding sites for the
chemical transmitter. These binding sites are called
receptors
The binding of the chemical
transmitter to the receptor usually provokes a change in the permeability of the membrane and
ions (calcium) move across the membrane, causing
a change in electrical activity of the
membrane and this electrical activity is passed along to the next cell
. To
prevent the synapses from becoming non-functional, the chemical transmitter is
removed by one
of two major mechanisms: broken down by enzymes or taken back up into the presynaptic
structure.
, after release into the synaptic cleft, the neurochemical transmitter or messenger
binds to specific molecules known as
receptors
Receptors are
proteins synthesized in the rough
endoplasmic reticulum, transported to different parts of the cell and inserted into the cell
membrane of the cell body, dendrites and axons.
each endogenous transmitter usually has its own
specific receptor
When the transmitter binds to
the receptor, it
elicits a specific response.
Drugs can either stimulate a receptor
called agonist, or inhibit action on a receptor (called antagonists)
Acetylcholine:
. Cholinergic receptors have two broad classifications.
Those that are stimulated by nicotine are called nicotinic receptors. Nicotinic receptors are
found in all autonomic ganglia, at the neuromuscular junction, and in certain regions of the brain.
Muscarinic receptors are stimulated by the alkaloid, muscarine, and are found in a wide array of
the regions of the brain.
Serotonin:
Serotonin and its receptors are found in the upper brain stem, with significant
amounts in the pons and medulla, hypothalamus, hippocampus, and cerebral cortex
Dopaminergic pathways occur predominantly in three areas:
hypothalamus, basal ganglia and
brain stem, and midbrain
Dopaminergic pathways are involved in control of some hormonal
systems (hypothalamus), motor coordination (basal ganglia), and motivation and
reward
There are several subclasses of dopamine receptors. The two most important are D1 and D2. D1
receptors, when activated by dopamine, are excitatory and D2
receptors are inhibitory.
Norepinephrine pathways originate in the brain stem and send projections to the cerebral cortex,
hypothalamus, limbic system, and the
cerebellum.
There are a large number of receptor types for
norepinephrine. The two main classes are α and β. Activation of these receptors usually leads to
excitation of the cell, but one of the subclasses of these receptors, when activated, is inhibitory.
Glutamate: Glutamate or glutamic acid is one of the more important
amino acid
neurotransmitters in the brain.
Gamma-aminobutyric acid (GABA): GABA is the main inhibitory neurotransmitter in the
CNS
GABAergic neurons and receptors are found in high concentrations in the cerebral cortex,
hippocampus and
cerebellum.
A number of CNS depressants (e.g. barbiturates and
benzodiazepines) bind to the
GABA receptor.
Opioid peptides: There are three main classes of opioid peptides:
enkephalins, endorphins, and
dynorphins.
They have varying degrees of selectivity for one of the opioid receptors
mu (μ), delta (δ) or kappa (κ).
The μ opioid receptor subtype is most abundant in the cerebral cortex, hypothalamus, brain stem,
and part of the spinal cord. This distribution is consistent with their involvement
in pain regulation.
The δ opioid receptor is concentrated in the olfactory system and various limbic
structures where they play an important role in
olfaction, motor integration (coordination),
reward, and cognitive (thinking) functions
The κ receptors are abundant in the caudate-putamen and hypothalamic sites and are involved in regulation of food intake, water balance, pain
perception, and
control of the endocrine system.
The efferent component of the peripheral nervous system consists of the motor nerves and
the autonomic nervous system.
The autonomic nervous system (ANS) is involved in maintaining a
stable internal environment
the autonomic nervous
system is often called the
involuntary nervous system
The Autonomic Nervous system has two distinct parts.
- Sympathetic (system)
2. Parasympathetic (system)
The parasympathetic and sympathetic systems normally act in a
balanced and opposite fashion
Parasympathetic system: General stimulation of this system promotes or increases
the vegetative functions of the body.
At rest, the parasympathetic type of activity is the predominant
activity. The sympathetic activity is largely
inhibited.
Sympathetic system: General stimulation of this system results in the mobilization of resources
to prepare the body to
meet emergencies.
The mass sympathetic discharge results in increased
activity of many functions of the body, including
increased heart rate, blood pressure, blood
supply to the tissues, rate of cell metabolism, and blood glucose
As it is stress
that usually excites the sympathetic system, it is frequently said that the sympathetic system
provides extra energy to the body for a state of stress, and this is often called the
alarm reaction or stress reaction
Acetylcholine is the
transmitter at all autonomic ganglia and the receptors are designated as nicotinic
The transmitter at the postganglionic parasympathetic nerve ending is acetylcholine and the synapse
is cholinergic
The transmitter at the postganglionic sympathetic nerve ending is norepinephrine and these
receptors have been
designated adrenergic
It is prudent to consider three of these types of adrenergic receptors. Alpha
(α) receptors are located predominantly on
smooth muscle, e.g. blood vessels, gastrointestinal
muscle, and uterus.
Beta (β) receptors can be sub-divided into β1 and β2. β1 receptors are found in the heart,
and when activated, increase the force and rate of contraction of the heart. β2 receptors are found
in the lungs, blood vessels, gastrointestinal muscle and uterus, and activation of these receptors
leads to
muscle relaxation
