The spinal cord
The lowest level of the central nervous system (CNS), anatomically and
functionally, is the spinal cord. Continuous with the brainstem, it exits the
skull through the foramen magnum. The spinal cord then passes through
the vertebral canal of the vertebral column to the level of the first or
second lumbar vertebrae. The spinal cord is divided into four anatomical
regions: cervical, thoracic, lumbar, and sacral.
These regions are named according to the vertebrae adjacent to them
during embryonic development. Each region is subdivided into functional
segments. A pair of spinal nerves extends from each segment (one nerve
from the left side of the spinal cord and one nerve from the right ) and
exits the CNS through the intervertebral foramina, or openings between
adjacent vertebrae. There are a total of 31 pairs of spinal nerves:
• 8 Cervical
• 12 Thoracic
• 5 Lumbar
• 5 Sacral
• 1 Coccygeal
The spinal nerves associate with the spinal cord by way of two branches,
or roots:
• Dorsal root
• Ventral root
The dorsal root contains afferent, or sensory, neurons. Impulses in these
neurons travel from peripheral tissues toward the spinal cord. The
ventral root contains efferent, or motor, neurons. Impulses in these
neurons travel away from the spinal cord toward the peripheral tissues.
At this point, it is important to note that a nerve is defined as a bundle
of neuronal axons; some are afferent and some are efferent. A nerve does
not consist of entire neurons, only their axons. Furthermore, nerves are
found only in the peripheral nervous system. Bundles of neurons with
similar functions located within the CNS are referred to as tracts.
Therefore, technically speaking, no nerves are within the brain or the
spinal cord.
Functions of the spinal cord
The spinal cord is responsible for two vital CNS functions. The cord:
• Conducts nerve impulses to and from the brain
• Processes sensory input from the skin, joints, and muscles of the trunk
and limbs and initiates reflex responses to this input
Composition of the cord
The spinal cord consists of:
• Gray matter
• White matter
The gray matter is composed of nerve cell bodies and unmyelinated
interneuron fibers. The location of the gray matter in the spinal cord is
opposite to that of the brain. In the brain, the gray matter of the cerebrum
and the cerebellum is found externally forming a cortex, or covering, over
the internally located white matter. In the spinal cord, the gray matter is
found internally and is surrounded by the white matter.
The white matter is composed of myelinated axons of neurons. These
axons are grouped together according to function to form tracts. Neurons
transmitting impulses toward the brain in the ascending tracts carry
sensory information. Those transmitting impulses away from the brain in
the descending tracts carry motor information.
Gray matter
A cross-sectional view of the spinal cord reveals that the gray matter has
a butterfly or “H” shape. As such, on each side of the spinal cord the gray
matter is divided into three regions:
• Dorsal horn (posterior, toward the back)
• Ventral horn (anterior)
• Lateral horn
Each spinal segment contains millions of neurons within the gray matter.
Functionally, four types of neurons exist:
• Second-order sensory neurons
• Somatic motor neurons
• Visceral motor neurons
• Interneurons
The cell bodies of second-order sensory neurons are found in the dorsal
horn. These neurons receive input from afferent neurons (first-order
sensory neurons) entering the CNS from the periphery of the body
through the dorsal root of the spinal nerve. The function of the secondorder sensory neuron is to transmit nerve impulses to higher levels in the
CNS. The axons of these neurons leave the gray matter and travel upward
in the appropriate ascending tracts of the white matter. The cell bodies of
somatic motor neurons are found in the ventral horn.
The axons of these neurons exit the CNS through the ventral root of the
spinal nerve and innervate skeletal muscles.
The cell bodies of visceral motor neurons are found in the lateral horn.
The axons of these neurons form efferent nerve fibers of the autonomic
nervous system (ANS). The ANS innervates cardiac muscle, smooth
muscle and glands. The axons of these neurons exit the spinal cord by
way of the ventral root.
Interneurons are found in all areas of the spinal cord gray matter. These
neurons are quite numerous, small, and highly excitable; they have many
interconnections. They receive input from higher levels of the CNS as
well as from sensory neurons entering the CNS through the spinal nerves.
Many interneurons in the spinal cord synapse with motor neurons in the
ventral horn. These interconnections are responsible for the integrative
functions of the spinal cord including reflexes.
White matter
The white matter of the spinal cord consists of myelinated axons of
neurons.
These axons may travel up the spinal cord to a higher spinal segment or
to the brain. On the other hand, they may travel down the spinal cord to a
lower spinal segment. The axons of neurons that carry similar types of
impulses are bundled together to form tracts.
Ascending tracts carry sensory information from the spinal cord toward
the brain.
Descending tracts carry motor impulses from the brain toward the motor
neurons in the lateral or ventral horns of the spinal cord gray matter.
Autonomic Nervous System (ANS)
The nervous system is divided into
1- Central nervous system
2- Peripheral nervous system
The central nervous system (CNS) consists of : Brain + Spinal cord
The peripheral nervous system (PNS) consists of : 12 pairs of cranial
nerves + 31 pairs of spinal nerves
The (PNS) consists of two divisions : afferent + efferent
The efferent division is divided into : the somatic nervous system (SNS)
+ the autonomic nervous system (ANS)
There are three functional classes of neurons
Afferents: lie predominantly in the PNS . Each has a sensory receptor
activated by a particular type of stimulus, a cell body located adjacent to
the spinal cord, and an axon. The peripheral axon extends from the
receptor to the cell body and the central axon continues from the cell
body into the spinal cord.
Efferents: also lie predominantly in the PNS. In this case, the cell bodies
are found in the CNS in the spinal cord or brainstem and the axons extend
out into the periphery of the body where they innervate the effector
tissues.
Interneurons: which lie entirely within the CNS. Because the human
brain and spinal cord contain well over 100 billion neurons, interneurons
account for approximately 99% of all the neurons in the body taken
together. Interneurons lie between afferent and efferent neurons and are
responsible for integrating sensory input and coordinating a motor
response.
The autonomic nervous system
It is also known as the visceral nervous system or involuntary nervous
system. ANS functions below the level of consciousness. This system
innervates cardiac muscle, smooth muscle, and various endocrine and
exocrine glands. Regulation of blood pressure; gastrointestinal responses
to food; contraction of the urinary bladder; focusing of the eyes; and
thermoregulation are just a few of the many homeostatic functions
regulated the ANS.
Regulation of autonomic nervous system activity
The ANS is regulate by several regions in the central nervous system:
1- Hypothalamus and brainstem
2- Cerebral cortex and limbic system
3- Spinal cord
The hypothalamus and the brainstem contain many homeostatic control
centers. These regions control cardiovascular , respiratory, and digestive
activity.
The cerebral cortex and limbic system influence ANS activities
associated with emotional responses. Blushing during an embarrassing
moment, a response most likely originating in the frontal association
cortex, involves vasodilation (vasodilatation) of blood vessels to the face.
Many autonomic reflexes, such as the micturation reflex(urination), are
mediated at the level of the spinal cord.
Efferent pathways of autonomic nervous system
The efferent pathways of the ANS consist of two neurons;
1- The preganglionic neuron which originates in the CNS with its
cell body in the lateral horn of the gray matter of the spinal cord or
in the brainstem. The axon of this neuron travels to an autonomic
ganglion located outside the CNS.
2- The postganglionic neuron. This neuron synapses with the
preganglionic neuron at an autonomic ganglion, the extends to
innervates the effector tissue.
Synapses between the autonomic postganglionic neuron and effector
tissue (neuroeffector junction) differ greatly from the neuron-to-neuron
synapses. The postganglionic fibers in the ANS do not terminate in a
single swelling like the synaptic knob, nor do they synapse directly with
the cells of a tissue. Instead, the axon terminals branch and contain
multiple swellings called varicosities that lie across the surface of the
varicosities release neurotransmitter over a large surface area of the
effector tissue. The neurotransmitter affects many tissue cells
simultaneously. Furthermore, cardiac muscle and most smooth muscle
have gap junction between cells. These specialized intercellular
communications allow for spread of electrical activity from one cell to
the next. As a result, the discharge of a single autonomic nerve fiber to an
effector tissue may alter activity of the entire tissue.
Division of autonomic nervous system
The ANS is composed of two anatomically and functionally distinct
divisions: the sympathetic system and the parasympathetic system. Two
important features of these divisions include:
1- Tonic activity
2- Dual innervations
both systems are tonically active. That means, they provide some degree
of input to a given tissue all times. This characteristic of CNS improves
its ability to regulate a tissue's function more precisely. Without tonic
activity, nervous input to a tissue can only increase.
Many tissues are innervated by both systems. These systems typically
have opposing effects on a given tissue.
Each system is dominant under certain conditions. The sympathetic
system predominates during emergency "fight-or-flight" reactions and
during exercise. It prepares the body for strenuous physical activity.
The parasympathetic system predominates during quiet to conserve and
store energy and to regulate basic body functions such as digestion and
urination.
The sympathetic division
The preganglionic neurons of this system arise from the thoracic and
lumber regions of the spinal cord (segment T1 through L2). Most of these
preganglionic axons are short and synapse with postganglionic neurons
within ganglia found in sympathetic ganglion chains. Running parallel
immediately along either side of the spinal cord, each of these chains
consists of 22 ganglia. The neuron may synapses in a ganglion at the
same spinal cord level from which it arises. This neuron may also travel
more rostrally or caudally (upward or downward) in the ganglion chain to
synapse with postganglionic neurons in ganglia at other levels .in fact, a
single preganglionic neuron may synapse with several postganglionic
neurons in many different ganglia. Overall, the ratio of preganglionic
fibers to postganglionic fibers is 1:20 results in mass sympathetic
discharge. The long postganglionic neurons then travel outward and
terminate on the effector tissues.
Other preganglionic neurons exist the spinal cord and pass through the
ganglion chain without synapsing with a postganglionic neuron. Instead,
they synapse in one of the sympathetic collateral ganglia. These ganglia
are located about halfway between the CNS and effector tissue.
The preganglionic neuron may travel to the adrenal medulla and synapse
directly with this glandular tissue. Adrenal medulla function as modified
postganglionic neurons. The secretory products of the adrenal medulla
are picked up by the blood and travel throughout the body to all of the
effector tissues of the sympathetic system.
The postganglionic neurons of the sympathetic system travel with each of
the 31 pairs of spinal nerves.
Sympathetic nerve fibers innervate skin (including blood vessels and
sweat glands), blood vessels in the entire body( primarily arterioles and
veins which receive only sympathetic nerve fibers), structures of the head
(eye, salivary glands, mucus membranes of the nasal cavity), thoracic
viscera (heart, lung), and viscera of the abdominal and pelvic cavities
(e.g., stomach, intestines, pancreas, spleen, adrenal medulla, urinary
bladder).
Parasympathetic division
Preganglionic neurons of this system arise from several nuclei of the
brain and from the sacral region of the spinal cord (segment S2 to S4).
The axons of the preganglionic neurons synapse with postganglionic
neurons within terminal ganglia that are closed or embedded within the
effector tissues.
The preganglionic neurons that arise from the brain exit the CNS through
cranial nerves.
1234-
The occulomotor nerve (III)
The facial nerve (VIII)
The glossopharyngeal nerve (IX)
The vagus nerve (X) . the fact that 75% of all parasympathetic
fibers are in the vagus nerve.
The preganglionic neurons that arise from the sacral region of the spinal
cord exit the CNS and join together to form the pelvic nerves. These
nerves innervate the viscera of the pelvic cavity (e.g., urinary bladder,
colon).
In many organs, the ratio of preganglionic fibers to postganglionic fibers
is 1:1. Therefore, the ratio of parasympathetic system tend to be more
discrete and localized.
Neurotransmitters of autonomic nervous system
The neurotransmitters of the ANS are acetylcholine (Ach) and
norepinephrine (NE). Nerve fibers that release acetylcholine are referred
to as cholinergic fibers and include all preganglionic fibers of the ANS
– sympathetic and parasympathetic system; all postganglionic fibers of
the parasympathetic system; and sympathetic postganglionic fibers
innervating sweat glands.
Nerve fibers that release norepinephrine are referred to as adrenergic
fibers. Most sympathetic postganglionic fibers release norepinephrine.
Adrenal medulla releases hormones into blood. Approximately 20% of
the hormonal secretion of the adrenal medulla is norepinephrine, and
80% is epinephrine (adrenalin). The two hormone are collectively
referred to as the catecholamines.
Termination of neurotransmitter activity
Neurotransmitter activity may be terminated by three mechanisms:
1- Diffusion out of the synapse
2- Enzymatic degradation
3- Reuptake into the neuron
The primary mechanism used by cholinergic synapses is enzymatic
degradation.
Acetylcholine is hydrolyzed by acetylcholinesterase into choline and
acetate.
Norepinephrine is removed from the neuroeffector junction by reuptake
it into the synaptic neuron that released it. Norepinephrine may then be
metabolized intraneuronally by monoamine oxidase (MAO). The
circulating catecholamines are inactivated by catechol-O-methyltrasferase
(COMT) in the liver.
Receptors for autonomic neurotransmitters
All the effects of the ANS are carried out by only three substances:
acetylcholine, norepinephrine, and epinephrine. Furthermore, each of
these substances may stimulate activity in some tissues and inhibit
activity in others.
The effects of any of these substances is determined by receptors
distributed in a particular tissue and biochemical properties of the cells
in that tissue (the second messenger and enzyme systems present in the
cell).
The neurotransmitters of the ANS and the circulating catecholamines
bind to specific receptors on the cell membranes of effector tissue. Each
receptor is coupled to a G protein also embedded within the plasma
membrane. Receptor stimulation causes activation of G protein and
formation of an intracellular chemical, the second messenger. (the
neurotransmitter molecule, which cannot enter the cell, is the first
messenger.) the function of intracellular second messenger molecules is
to elicit tissue-specific biochemical events within the cell that alter the
cell's activity.
Acetylcholine binds to two types of cholinergic receptors:
1- Nicotinic receptors
2- Muscarinic receptors
Nicotinic receptors are found on the cell bodies of all sympathetic
and parasympathetic postganglionic neurons in the ganglia of the ANS.
Acetylcholine causes a rapid increase in the cellular permeability to Na+
and Ca++ ions.
Muscarinic receptors are found on cell membranes of effector tissues
and are linked to G proteins and second messenger systems that carry out
the intracellular effects.
Acetylcholine released from all parasympathetic postganglionic neurons,
and some sympathetic postganglionic neurons traveling to sweat glands.
Muscarinic receptors may be inhibitory or excitatory, depending on the
tissue upon which they are found.
The two classes of adrenergic receptors for nor norepinephrine and
epinephrine are:
1- Alpha (α)
2- Beta (ᵦ)
Furthermore each class has at least two subtypes of receptors: α1, α2,
ᵦ1,and ᵦ2. All of these receptors are linked to G proteins and second
messenger systems.
Alpha receptors are the most abundant of the adrenergic receptors. α1receptors are more widely distributed; these receptors tend to be
excitatory. For example, they causes vasoconstriction.
Functions of the autonomic nervous system
The two divisions of the ANS are dominant under different conditions: as
stated previously, the sympathetic system is activated during emergency
“fight-or-flight” reactions and during exercise, and the parasympathetic
system is predominant during quiet resting conditions. As such, the
physiological effects caused by each system are quite predictable. In
other words, all the changes in organ and tissue function induced by the
sympathetic system work together to support strenuous physical activity
and changes induced by the parasympathetic system are appropriate when
the body is resting.
The “fight-or-flight” reaction elicited by the sympathetic system is
essentially a whole-body response. Changes in organ and tissue function
throughout the body are coordinated so that delivery of well-oxygenated,
nutrient- rich blood to the working skeletal muscles increases. Heart rate
and myocardial contractility are increased so that the heart pumps more
blood per minute. Sympathetic stimulation of vascular smooth muscle
causes widespread vasoconstriction, particularly in organs of the
gastrointestinal system and in the kidneys. This vasoconstriction serves to
“redirect” or redistribute the blood away from these metabolically
inactive tissues and toward the contracting muscles. Bronchodilation in
the lungs facilitates the movement of air in and out of the lungs so that
uptake of oxygen from the atmosphere and elimination of carbon dioxide
from the body are maximized. An enhanced rate of glycogenolysis
(breakdown of glycogen into its component glucose molecules) and
gluconeogenesis (formation of new glucose from noncarbohydrate
sources) in the liver increases the concentration of glucose molecules in
the blood. This is necessary for the brain because glucose is the
only nutrient molecule that it can utilize to form metabolic energy. An
enhanced rate of lipolysis in adipose tissue increases the concentration of
fatty acid molecules in the blood. Skeletal muscles then utilize these fatty
acids to form metabolic energy for contraction. Generalized sweating
elicited by the sympathetic system enables the individual to
thermoregulate during conditions of increased physical activity and heat
production. Finally, the eye is adjusted so that the pupil dilates, letting
more light in toward the retina (mydriasis) and the lens adapts for
distance vision.
The parasympathetic system decreases heart rate, which helps to conserve
energy under resting conditions. Salivary secretion is enhanced to
facilitate swallowing food. Gastric motility and secretion are stimulated
to begin processing of ingested food. Intestinal motility and secretion are
also stimulated to continue processing and to facilitate absorption of these
nutrients.
Exocrine and endocrine secretion from the pancreas is promoted.
Enzymes released from the exocrine glands of the pancreas contribute to
the chemical breakdown of the food in the intestine, and insulin released
from the pancreatic islets promotes storage of nutrient molecules within
the tissues once they are absorbed into the body. Another bodily
maintenance type of function caused by the parasympathetic system is
contraction of the urinary bladder, which results in urination. Finally, the
eye is adjusted so that the pupil contracts (miosis) and the lens adapts for
near vision.