The Nervous System
Nervous System
The nervous system has
three functions
Sensory input-
Sensory receptors present in skin and organs respond to external and internal
stimuli by generating nerve impulses that travel to the brain and spinal cord.
Integration-
The brain and spinal cord sum up the data received from all over the body and
send out nerve impulses.
Motor output-
The nerve impulses from the brain and spinal cord go to the effectors, which
are muscles and glands. Muscle contractions and gland secretions are responses
to stimuli received by sensory receptors.
Divisions of the
Nervous System
The nervous system has
two major divisions:
The
Central nervous system (CNS) is composed
of the brain and spinal cord. The CNS is surrounded by bone-skull and
vertebrae. Fluid and tissue also insulate the brain and spinal cord. The brain is
composed of three parts: the cerebrum (seat
of consciousness), the cerebellum,
and the medulla oblongata (these latter two are "part of the unconscious brain").
The medulla oblongata is
closest to the spinal cord, and is involved with the regulation of heartbeat,
breathing, vasoconstriction (blood pressure), and reflex centers for vomiting,
coughing, sneezing, swallowing, and hiccuping. The hypothalamus
regulates homeostasis. It has regulatory areas for thirst, hunger, body
temperature, water balance, and blood pressure, and links the Nervous System
to the Endocrine System.
The midbrain and pons are also
part of the unconscious brain. The thalamus
serves as a central relay point for incoming nervous messages.
The Peripheral nervous system (PNS) contains only nerves and connects
the brain and spinal cord (CNS) to the rest of the body. The axons and
dendrites are surrounded by a white myelin sheath. Cell bodies are in the
central nervous system (CNS) or ganglia. Ganglia are collections of nerve cell bodies. Cranial nerves in
the PNS take impulses to and from the brain (CNS). Spinal nerves take impulses to and away from the spinal cord. There are two major subdivisions of the PNS motor pathways: the
somatic and the autonomic.
There are two main components of the PNS
1. sensory (afferent) pathways that provide input from the body
into the CNS.
2. motor (efferent) pathways that carry signals to muscles and
glands (effectors).
Most sensory input carried in the PNS remains below the level of
conscious awareness. Input that does reach the conscious level contributes to
perception of our external environment.
The Neuron Structure
The neuron
is the functional unit of the nervous system. Humans have about 100 billion
neurons in their brain alone! While variable in size and shape, all neurons
have three parts. Dendrites
receive information from another cell and transmit the message to the cell
body. The cell body
contains the nucleus, mitochondria and other organelles typical of eukaryotic
cells. The axon
conducts messages away from the cell body.
Three types of neurons occur. Sensory neurons
typically have a long dendrite and short axon, and carry messages from sensory
receptors to the central nervous
system. Motor neurons
have a long axon and short dendrites and transmit messages from the central
nervous system to the muscles (or to glands). Interneurons
are found only in the central nervous system where they connect neuron to
neuron.
Some axons are wrapped in a myelin sheath
formed from the plasma membranes of specialized glial cells known as Schwann cells.
Schwann cells serve as supportive, nutritive, and service facilities for
neurons. The gap between Schwann cells is known as the node of Ranvier,
and serves as points along the neuron for generating a signal. Signals jumping
from node to node travel hundreds of times faster than signals traveling along
the surface of the axon. This allows your brain to communicate with your toes
in a few thousandths of a second.
Nerve Impulses
When axons are resting, they are not conducting nerve impulses.
When they are active, axons are conducting nerve impulses, also called action
potentials.
Resting Potential
When an axon is resting, its membrane is polarized; that is, the
outside is positive compared to the inside, which is negative. A protein
carrier in the membrane called the sodium potassium pump, pumps sodium out of
the axon and potassium into the axon. Another factor that causes the inside of
the axon to be negative compared to the outside is the presence of large,
negatively charged protein ions inside an axon. The polarity across an axon
that is not conducting nerve impulses is called the resting potential.
Action Potential
When the nerve fiber is conducting a nerve impulse, a change in
polarity occurs across the axon's membrane. First, the inside of an axon
becomes positive compared to the outside, and then the inside becomes negative
again. An action potential requires two types of channels in the membrane: One
channel can allow NA ions to pass through the membrane, and the other can allow
K ions move to the inside of the axon, and during repolarization, K ions move
to the outside.
The Brain
During embryonic development, the brain first forms as a tube, the
anterior end of which enlarges into three hollow swellings that form the brain,
and the posterior of which develops into the spinal cord. Some parts of the
brain have changed little during vertebrate evolutionary history.
The Brain Stem and Midbrain:
The brain stem
is the smallest and from an evolutionary viewpoint, the oldest and most
primitive part of the brain. The brain stem is continuous with the spinal cord,
and is composed of the parts of the hindbrain and midbrain. The medulla
oblongata and pons control heart rate, constriction of blood vessels, digestion
and respiration. The midbrain consists of connections between the hindbrain and
forebrain. Mammals use this part of the brain only for eye reflexes.
Four Major Parts of the Brain
The cortex in each hemisphere of the cerebrum is between 1 and 4
mm thick. Folds divide the cortex into four lobes: occipital,
temporal,
parietal,
and frontal.
No region of the brain functions alone, although major functions of various
parts of the lobes have been determined.
The Brain and Drugs
Some neurotransmitters are excitory, such as acetylcholine,
norepinephrine, serotonin, and dopamine. Some are associated with relaxation,
such as dopamine and serotonin. Dopamine release seems related to sensations of
pleasure. Endorphins are natural opioids that produce elation and reduction of
pain, as do artificial chemicals such as opium and heroin. Neurological
diseases, for example Parkinson's
disease and Huntington's disease, are due to imbalances of neurotransmitters.
Parkinson's is due to a dopamine deficiency. Huntington's disease is thought to
be cause by malfunctioning of an inhibitory neurotransmitter. Alzheimer's disease is associated
with protein plaques in the brain.
Effects of aging
Marijuana, material from the Indian hemp
plant (Cannabis sativa), has a potent chemical THC
(tetrahydracannibinol) that in low, concentrations causes a euphoric high (if
inhaled, the most common form of action is smoke inhalation). High dosages may
cause severe effects such as hallucinations, anxiety, depression, and psychotic
symptoms.
Cocaine is derives from the plant Erthoxylon
coca. Inhaled, smoked or injected. Cocaine users report a
"rush" of euphoria following use. Following the rush is a short (5-30
minute) period of arousal followed by a depression. Repeated cycle of use
terminate in a "crash" when the cocaine is gone. Prolonged used
causes production of less dopamine, causing the user to need more of the drug.
Heroin is a derivative of morphine, which in
turn is obtained from opium, the milky secretions obtained from the opium
poppy, Papaver somniferum. Heroin is usually injected
intravenously, although snorting and smoking serve as alternative delivery
methods. Heroin binds to ophioid receptors in the brain, where the natural
chemical endorphins are involved in the cessation pain. Heroin is physically
addictive, and prolonged use causes less endorphin production. Once this
happens, the euphoria is no longer felt, only dependence and delay of
withdrawal symptoms.