The body has 75 trillion cells whose functions and responses must be
coordinated. This is accomplished by two systems of the body.
a) Nervous System--swift but brief responses to stimuli
b) Endocrine system--slower but effects last longer (affects things
such as growth, sexual development, pregnancy, etc.)
The nervous system has 3 functions: sensory (receiving information from
the outside world), motor (responding to the outside world by initiating
muscle contractions or glandular secretions) and integrative (processing
information through analysis & storage so that the optimal response
can be chosen).
We can divide the nervous system in several ways:
--CNS: central nervous system consisting of brain & spinal cord
--PNS: peripheral nervous system consisting of cranial & spinal
nerves (depending on their place of origin) and ganglia
--Sensory (affarent) Division: brings sensory info to the CNS
--Motor (efferent) Division: carries motor commands to the muscles &
--SNS: somatic nervous system that controls skeletal muscle
--ANS: autonomic nervous system that involuntarily controls cardiac
and smooth muscle & glands
Nervous tissue (such as the images of the human brain below)has 2 primary
types of cells, neurons & neuroglia.
1) Neuroglia--cells which support & protect the neurons and actually
Neurons are excitable cells which may vary form less than 1 mm to more
than 5 feet in length (or more). Most neurons have 3 general regions:
a) dendrites-Dendrites are highly branched processes that extend from
the cell body involved in monitoring the surrounding area for local
chemical, mechanical, or electrical information.
b) the cell body or soma-The soma contains the nucleus & organelles.
Clusters of soma in the CNS are called nuclei while clusters outside
the CNS are called ganglia.
c) axon: Axons are present on the vast majority of neurons and can be
quite long. They transmit electrical impulses.
A neuron's electrical message (the action potential) can speed along
an axon at 3'/sec (2 mph). This speed can be increased 20x by a coating
of myelin over the axon secreted by oligodendrocytes or Schwann cells.
Fat, myelinated axons transmit messages at up to 280 mph. Why can't
all neurons be fat & myelinated? First of all, not all messages
are urgent. Secondly, this would make the spinal cord the width of a
garbage can & a spinal nerve as thick as a garden hose.
Eventually, a neuron's action potential reaches the synaptic terminals.
Although the action potential is electrical, most intercellular communication
is chemical. The synaptic knobs may have millions of vesicles of the
neurotransmitter. For example, neuromuscular synapses (where neurons
contact muscle cells). The neurotransmitter ACh (acetyl choline) is
packaged into vesicles, each with thousands of ACh molecules.
The communication between neurons and other cells (muscles, glands,
and other neurons) is essential to the body. Many drugs and poisons
effect the body through the interaction of neurotransmitters with receptors
at synapses or through altering the neuron's ability to conduct electricity.
Here are some examples of drugs and poisons which affect ACh and other
EFFECTS OF DRUGS AND POISONS ON ACh
1. BOTULINUS TOXIN--blocks ACh release,paralyzes voluntary muscles
2. BARBITUATES--decreases ACH release, muscular weakness
3. INSECTICIDES, NERVE GAS--prevents ACh inactivation by cholinesterase;
sustained muscle contraction
4. CURARE--prevents ACh binding to receptors; paralysis of voluntary
5. NICOTINE--binds to ACh receptors, facilitiating them
EFFECTS OF DRUGS AND POISONS ON OTHER NEUROTRANSMITTERS
1. ANTI-PSYCHOTIC DRUGS(thorazine)--block dopamine receptors; long term
reduction of dopamine in muscle control regions leads to motor problems
2. MINOR TRANQUILIZERS (valium, librium)--bind to GABA receptors, enhancing
its inhibitory effect
3. ANTIDEPRESSANTS (Elavil)--prevents the reuptake of NE and serotonin
form the synaptic cleft; prolongs elevating effects
4. ANTIHYPERTENSIVE--act in PNS, block NE's stimulation of smooth muscle
by binding to NE receptors
5. LSD--binds to serotonin receptors, blocking the inhibition of some
pathways; sensory information is no longer filtered--overload
6. AMPHETAMINES--resemble NE and dopamine, used at pleasure center;
body begins to make less NE and dopamine
7. COCAINE--blocks reabsorption of dopamine (the most responsible for
the feeling of euphoria), NE, and serotonin and their effects are felt
for longer periods; eventually the neurotransmitters are washed away
and degraded--since they weren't reabsorbed, the brain's neurotransmitter
supply decreases; it becomes harder to feel pleasure without cocaine
and may eventually become impossible
8. STRYCHNINE--blocks glycine (an inhibitor) receptors of motor neurons
resulting in muscle spasms
9. CAPSAICIN (from jalapeno and Hungarian peppers)--causes massive release
of substance P from pain receptors in tongue and mouth
10. NOVOCAIN--reduces membrane permeability to sodium; prevents stimulation
of sensory neurons
11. ALCOHOL--alters cell membrane conduction of impulses; as blood alcohol
levels increase more and more brain centers are affected (which is why
the effect of alcohol varies with the amount consumed)
THE SPINAL CORD
The spinal cord is not just a highway to the brain; it initiates many
important processes on its own. It makes many simple but urgent decisions,
saving time; your hand can be moving away from a hot stove before your
brain realizes you've touched it. The spinal cord is about 18"
long and only reaches the lower back from which point the final spinal
nerves form the cauda equina (the "horse's tail).
There are three layers around both the brain and spinal cord to provide
a) dura mater: the tough outer layer
b) arachnoid: a procedure known are a spinal tap removes 5-10 ml of
cerebrospinal fluid from the space between the arachnoid and the pia
mater known as the subarachnoid space
c) pia mater: contains blood vessels, and is fused to the brain or spinal
Below is a picture of the meninges around a sheep's brain.
Between the meninges, spaces are filled with cerebrospinal fluid that
functions as a shock absorber, transport of nutrients, wastes and gases.
The tissue of the spinal cord can be divided into gray and white matter.
The gray matter contains the cell bodies of neurons organized into groups
called nuclei. Sensory nuclei in the posterior horns receive & relay
sensory info from receptors. Motor nuclei in the anterior horns issue
motor commands to the periphery.
The white matter contains axons organized into tracts; the axons of
a tract are all moving in the same direction and are of the same kind
(sensory or motor).
--Ascending tracts send sensory information to the brain:
--the posterior column carries touch information
--the spinothalamic tract carries pain and temperature information
--the spinocerebellar tract carries information about body position
--Descending tracts carry motor commands from the brain:
--the corticospinal tract carries voluntary commands to skeletal muscle
--the reticulospinal tract carries involuntary commands to skeletal
From the spinal cord, spinal nerves reach the various regions of the
body. These spinal nerves carry sensory information from these body
regions to the CNS and carry commands from the CNS to these regions.
Each spinal nerve supplies an area of the skin referred to as its dermatome.
Reflexes are rapid adjustments to preserve the normal conditions of
the body. They are fixed "wiring" and are not conscious or
voluntary. Each stimulus results in the same response. In general, there
are 5 steps in a reflex.
1) A receptor is activated. Receptors are constantly monitoring the
internal conditions in the body and the external environment. Too much
stress on a muscle, a bright light, and food in the mouth will be detected
by receptors and result in reflex responses to these stimuli.
2) A sensory neuron is activated which conducts an impulse to the CNS.
3) The neurotransmitters of the synaptic knob may stimulate a motor
neuron (in a monosynaptic reflex) or an interneuron (in a polysynaptic
reflex). Interneurons are neurons which analyze incoming sensory info
& coordinate motor outputs. Thousands of interneurons may be organized
into pools and their connections can be obviously complex.
4) Activation of a motor neuron which carries message to muscles or
5) The response of a peripheral effector (e.g. muscle or gland). The
response typically counteracts the original stimulus as part of a negative
REFLEXES CLASSIFIED BY:
A) Innate reflexes are genetically determined. They may be simple (blinking,
withdrawal from pain) or complex (suckling, tracking objects with eyes).
B) Acquired reflexes can be more complex, such as skiing or driving.
2) Nature of Motor Response
A) Somatic reflexes involve skeletal muscle
B) Visceral reflexes other systems, such as the digestive system, respiratory
system, cardiovasculary system, and reproductive system.
3) Processing Site
A) Cranial reflexes are processed in the brain.
B) Spinal reflexes are processed in the spinal cord.
EXAMPLES OF COMMON REFLEXES
a) In a stretch reflex, the stimulus is the stretch of a muscle detected
by receptors known as spindle fibers and the response is the contraction
of that muscle. We not only use stretch reflexes when a doctor hits
us with a reflex hammer, they are continually making adjustments to
b) Golgi tendon reflexes prevent damage to muscle through excessive
stress. If one tries to create so much tension in a muscle that it might
damage the muscle, this reflex can tell the muscle to relax. For example,
an arm might suddenly "give out" in an arm-wrestling match
or when trying to bench press too much weight in a weight room.
c) Withdrawl reflexes are triggered by painful stimuli. If a limb is
involved, many body parts must be involved to maintain balance.