A & P 240: Overview of the Human Nervous System (Generalized Outlines for Ch. 12 – 17)
CHAPTER 12: Nervous Tissue
Organization
1. The N.S. helps control and integrate all body activities by sensing changes (sensory), interpreting them
(integrative), and responding to them (motor).
2. The N.S. has two principal Divisions: the Central N.S. (CNS) and the Peripheral N.S. (PNS).
3. The CNS consists of the Brain and Spinal Cord. Within the CNS, incoming sensory information is
integrated and correlated, thoughts and emotions are generated, and muscles are stimulated to contract
and glands to secrete via outgoing nerve impulses.
4. The PNS consists of 12 pairs of Cranial Nerves emerging from the Brain and 31 pairs of Spinal Nerves
emerging from the Spinal Cord. These nerves carry afferent impulses inward from receptors to the CNS and
efferent impulses outward from the CNS to effectors (muscles and glands). In other words, there is “twoway traffic” of impulses utilizing these 12 pairs of Cranial Nerves and 31 pairs of Spinal Nerves.
5. Based upon the origin of incoming sensory information and the part of the body that responds, the PNS is
subdivided into a Somatic Nervous System (SNS); an Autonomic Nervous System (ANS); and an
Enteric Nervous System (ENS).
6. The SNS consists of sensory neurons that convey information from cutaneous, proprioceptive, and special
sensory receptors primarily in the head, body wall, and extremities to the CNS and motor neurons that
conduct impulses to skeletal muscle effectors only. The SNS is voluntary.
7. The ANS consists of sensory neurons that convey homeostatic information from receptors primarily in
the viscera to the CNS and motor neurons from the CNS that conduct impulses to smooth muscle, cardiac
muscle, and glands. The motor part of the ANS has two subdivisions: Sympathetic and Parasympathetic and is involuntary. The Sympathetic NS is referred to as Thoracolumbar (because its impulses are generated between cord levels T1-L2); the Parasympathetic NS is referred to as Craniosacral
(because its impulses are generated from Cranial Nerves III, VII, IX, X and Spinal Nerves S2-S4). (For the
most part, effectors are innervated by both subdivisions, which then have opposing actions on that given effector.)
8. The ENS (“brain of the gut”), once considered part of the ANS, consists of approx. 100 million neurons
in the various enteric plexuses of the GI Tract which function somewhat independently of the ANS and
CNS. Enteric sensory neurons monitor chemical changes and stretch activity; enteric motor neurons
govern smooth muscle contraction, secretion of GI tract organs and GI tract endocrine cells.
Histology
Neurons: 1. Neurons (functional nerve cells) consist of three parts: a cell body; dendrite(s) that receive stimuli,
and a single axon that sends impulses to another neuron or to an effector (muscle or gland).
2. Myelin is formed by neurolemmocytes (Schwann cells) in the PNS; and oligodendrocytes in the CNS.
3. The three parts of neurons are grouped in various, specific ways in different parts of the N.S. to form
nerves, ganglia, tracts, nuclei, horns and columns.
4. On the basis of structure, neurons are unipolar, bipolar, or multipolar.
5. On the basis of function, neurons are sensory (afferent) – conduct impulses to the CNS; motor (efferent)
–conduct impulses to effectors; or interneurons (association/integrative) – conduct impulses to other neurons.
Neuroglial Cells: 1. Neuroglia are specialized connective tissue cells that support neurons; attach neurons to
blood vessels; produce the myelin sheath around CNS neuron processes; phagocytose; or assist with
filtration of blood to form CSF in the ventricles of the brain.
2. Neuroglia include 4 types in the CNS: astrocytes, oligodendrocytes, microglia, ependymal cells; and
2 types in the PNS: neurolemmocytes (Schwann cells), and satellite cells.
Functions
1. The nerve impulse (Nerve Action Potential or NAP) is the body’s quickest way of communicating change.
2. Plasmalemma contain membrane proteins that contain channels; primarily channels for ions.
3. The plasmalemma of a nonconducting (resting) neuron is positive outside and negative inside, owing to
the differing concentrations of K+ and Na+; large, negatively-charged proteins inside; differential
permeability of the plasmalemma to Na+ and K+; and operation of the sodium-potassium pump. Such a
resting membrane is said to be POLARIZED. (The difference in electrical charges between inside the
cell and outside the cell is termed the membrane potential.)
4. When a stimulus causes the inside of the plasmalemma to become positive and the outside negative, the
it is now said to have generated a nerve action potential- NAP - which travels along the plasmalemma.
The traveling (propagating) NAP is the nerve impulse. The ability of a neuron to respond to a stimulus
and convert that stimulul into a nerve impulse is called excitability.
5 Restoration of the resting membrane potential is called REPOLARIZATION. The period of time during
which the membrane recovers and cannot initiate another action potential is called the refractory period.
6. According to the all-or-none principle, if a stimulus is strong enough to generate an action potential, the
impulse travels at a constant and maximum strength. A stronger stimulus will not cause a larger impulse.
(Stimuli that can trigger a NAP are termed: Threshold (aka: Liminal); Stimuli that are not strong enough to trigger
a NAP are termed: Subthreshold (aka: Subliminal.)
7. Nerve impulse conduction may occur in two fashions: (1) as a step-by-step process called continuous conduction; or (2) by “jumping” from node (of Ranvier) to node called saltatory conduction. Saltatory conduction is much more rapid and can only take place on myelinated neuron processes (axons/dendrites.)
8. Processes with larger diameters conduct impulses faster than those with smaller diameters; myelinated
processes conduct impulses faster than unmyelinated; and “warmed up” processes conduct impulses faster
than cool processes.
Conduction Across Synapses
1. Nerve impulse conduction can occur from one neuron to another or from a neuron to an effector.
2. The junction between neurons, neurons and myofibers, or neurons and glandular cells is called a SYNAPSE.
3. Therefore, SYNAPSES include NNJ’s; NMJ’s; and NGJ’s.
4. At a synapse, there is only one-way nerve impulse conduction from a presynaptic axon to a postsynaptic
unit (neuron dendrite, cell body, axon, or effector.)
5. If the postsynaptic unit is a neuron, it is an integrator. It receives signals, integrates them, and then
responds accordingly. If the postsynaptic unit is an effector, it will respond directly.
6. At a synapse, there may be facilitation (a state of near-threshold excitation, so that additional stimuli can
generate an impulse more easily), impulse generation, or impulse inhibition.
7. Synaptic conduction may be altered by disease, drugs, and pressure.
CHAPTER 13: The Spinal Cord and Spinal Nerves
CNS – Spinal Cord
Protection and Coverings
1. The spinal cord is protected by four components: (1) the boney vertebral canal, (2) 3 meninges,
(3) Cerbrospinal Fluid (CSF), and (4) the vertebral ligaments.
2. The meninges are three connective tissue coverings that run continuously around the spinal cord and brain.
They are the: dura mater; arachnoid mater; and pia mater. There is a subdural space and a subarachnoid
space (where CSF circulates), but there is NO sub-pial space. (The inferior-most rolled edges of pia mater
form the filum terminale; the lateral-going rolled edges of pia mater form the denticulate ligaments.)
3. Removal of CSF from the subarachnoid space is called a spinal puncture or spinal tap. The procedure is
used to diagnose pathologies and to introduce antibiotics or contrast media.
General Features
1. The spinal cord begins as a continuation of the medulla oblongata, exists the skull through the foramen
magnum and terminates via its conus medullaris at the level of the second lumbar vertebra (L2).
2. It contains cervical and lumbar enlargements that serve as points of origin for nerves to the extremities.
3. Nerves arising from the lowest portion of the cord are called the cauda equina.
4. The gray matter in the spinal cord is divided into horns (posterior, lateral, anterior); the white matter in
the spinal cord is divided into columns (posterior, lateral, anterior).
5. The white columns contain ascending (sensory) tracts and descending (motor) tracts.
6. In the center of the spinal cord is the central canal, running the length of the spinal cord and containing CSF.
Structure in Cross Section
1. Parts of the spinal cord observed in cross section are the central canal; anterior/posterior/lateral
gray horns; anterior/posterior/lateral white columns, and ascending and descending tracts;
anterior median fissure and posterior medium sulcus.
2. The spinal cord conveys sensory and motor information by way of the ascending and descending
tracts respectively.
Functions
1. One major function of the spinal cord is to convey sensory impulses from the periphery to the brain and to
conduct motor impulses from the brain to the periphery.
2. The other major function is to serve as a REFLEX CENTER. The posterior root, posterior root ganglion,
and anterior root (aka: the dorsal root, dorsal root ganglion, and ventral root) are involved in the route that
impulse travels.
3. A REFLEX is a rapid response to a change (stimulus) in the internal or external environment that attempts
to restore homeostasis; a REFLEX ARC is the functional unit of the N.S.; that is, it is the most basic conduction pathway through the nervous system, connecting a receptor and an effector and consisting of five
components in a SNS reflex arc: (1) a receptor; (2) a sensory neuron; (3) an integrating center (in the CNS);
(4) a motor neuron,; and (5) an effector. In an ANS reflex arc, there are 6 components: (1) a receptor,
(2) a sensory neuron; (3) an integrating center (in the CNS); (4) a pre-synaptic motor neuron; (5) a postsynaptic motor neuron; and (6) an effector.
PNS – Spinal Nerves
Names
1. The 31 pairs of spinal nerves are named and numbered according to the region and level of the
spinal cord from which they emerge.
2. There are 8 pairs of cervical spinal nerves, named for the vertebral level ABOVE which they exit.
3.
4.
5.
6.
(C1 spinal nerve exits the cord ABOVE the Atlas; C2 spinal nerve exits the cord ABOVE the Axis; C3
spinal nerve…ABOVE C3 vertebra, etc. down to C7 spinal nerve exits ABOVE C7 vertebra and C8 spinal
nerve exits the cord ABOVE T1 vertebra.)
There are 12 pairs of thoracic spinal nerves, named for the vertebral level BELOW which they exit.
There are 5 pairs of lumbar spinal nerves, named for the vertebral level BELOW which they exit.
There are 5 pairs of sacral spinal nerves, named for the vertebral level BELOW which they exit.
There is only one pair of coccygeal spinal nerves, named for the first coccygeal vertebra.
Composition and Coverings
1. Spinal nerves are attached to the spinal cord by means of a posterior root and an anterior
root. All spinal nerves are mixed (both sensory and motor).
2. Individual processes, bundles of processes (fascicles), and the entire nerve are sheathed in
connective tissue “wrappings” known as endoneurium, perineurium, and epineurium.
Distribution
1. Branches of a spinal nerve include the dorsal ramus, ventral ramus, meningeal branch and rami
communicantes (aka rami communicans).
2. The ventral rami of spinal nerves, except for T2 – T12, form networks of nerves called plexuses.
3. The principal plexuses are: Cervical Plexus (C1 – C5); Brachial Plexus (C5 – T1), Lumbar Plexus
(L1 – L4), and Sacral Plexus (L4 – S4). Within these plexuses, the ventral rami converge and diverge many times into Trunks, Divisions, Cords, and, ultimately into terminal branches. These
terminal nerves bear the name of the structure (or region) innervated by that nerve.