Nervous System

a. CNS
i. Brain
ii. Spinal Cord
iii. Meninges – connective tissue membranes surrounding the
brain and spinal cord
b. PNS
i. Cranial nerves – 12 paired sets of PNS nerves (some
sensory, some motor, some mixed) emerging from mostly
the brain stem of the brain
ii. Spinal nerves – 31 pairs of mixed (contain both sensory and
motor neurons) of nerves that emerge from the spinal cord
iii. Functional Division
1. Somatic – control of voluntary muscles (skeletal
muscles)
2. Autonomic / ENS – control of involuntary muscles,
glands, and adipose tissue (cardiac and smooth
muscles; exocrine and endocrine glands; adipocytes)
a. Parasympathetic – “rest and digest”; subdivision
of the ANS that maintains resting and digesting
conditions
b. Sympathetic – “Fight or Flight”; Stress
responses; subdivision of the ANS that mobilizes
resources for quick, sometimes lifesaving,
responses
2. Functions of the nervous system
a. Homeostasis – regulation and control of almost all bodily
functions
b. Sensory input – detection of changes in one’s environment;
detection of stimuli; afferent input from receptors of the PNS to
the CNS for integration and processing

c. Integration – processing of information sent to the CNS by the
PNS and sending output to effectors of the PNS (effectors
include muscle fibers, glands and adipose tissues)
d. Control – motor, efferent output to effectors via PNS motor
neurons
e. Mental activities – thinking, learning, memory, etc.
3. Receptors (sensory; afferent. Part of PNS)
a. Temp – thermoreceptors – detect changes in temperature
b. Pain – nociceptors – free nerve endings that detect potentially
harmful stimuli, usually extremes, pain, extreme temperature,
itch
i. Somatic nociceptors – relatively superficial and detect
painful stimuli at or near the surface of the body and
associated with the somatic nervous system
ii. Visceral nociceptors – deep; detect painful stimuli in
organs and deep parts of the body, i.e., the viscera
c. Pressure – type of mechanoreceptor, e.g., Pacinian corpuscles of
the deep dermis
d. Chemoreceptors – e.g., taste cells of taste buds
e. Photoreceptors – detect light; found in the retina of the eye ball
4. Nerves – bundles of axons (conductive portion of a neuron)
5. Ganglia – collections of neuron cell bodies in the PNS; called nuclei
in the CNS
6. Plexuses – networks dedicated to a specific area of the body, e.g., the
enteric nervous system (ENS) = the enteric gut plexus
7. Sensory and Motor neurons
a. Sensory neurons – usually unipolar neurons that detect stimuli
and synapse with interneurons (association neurons) in the CNS
b. Motor neurons – usually multipolar neurons that synapse with
effectors (muscle fibers, glands, adipose)
8. Interneurons – also called association neurons – make up part of the
integration center of the CNS

a. Serve as a relay between sensory neurons and motor neurons
9. Classification of neurons
a. Unipolar – one cytoplasmic process; two branches
i. One end of the branch is dendritic and receptive to stimuli
ii. The other end of the branch is axonic and conducts a nerve
impulse to an interneuron in the CNS
iii. E.g., sensory (afferent) neurons
b. Bipolar – two cytoplasmic processes coming off of the cell
body
i. One process is dendritic and receptive
ii. The other process is axonic and conductive
iii. E.g., many neurons of the special senses, such as neurons
that synapse with photoreceptors in the retina
c. Multipolar – several receptive dendritic processes; one
conductive axonic process that synapses with effectors
i. E.g., motor neurons

10. ANS – autonomic nervous system - the part of the nervous
system responsible for control of the bodily functions not consciously
directed, such as breathing, the heartbeat, and digestive processes
a. Subdivisions
i. Sympathetic – fight or flight
ii. Parasympathetic – rest and digest
1. ENS – Enteric Nervous System – plexus of neurons
dedicated to the functions of the digestive system.
Some autonomous integration and processing takes
place here, without direct innervation from the CNS
11. Anatomy of the ANS
a. Spinal cord – collection of myelinated (white matter) and
unmyelinated (gray matter) neurons
i. Part of the CNS
ii. Gray matter in the center (medulla) of the spinal cord;
contains the integration center of somatic reflex arcs
1. Consists of unmyelinated interneurons
2. Relay between afferent and efferent neurons
iii. White matter in the cortical regions of the spinal cord
1. Myelinated neuron axons making up ascending and
descending tracts of the spinal cord
a. Ascending tracts (bundles of axons in the CNS) –
afferent pathways to the brain

b. Descending tracts – efferent pathways to the PNS
motor neurons
b. Reflex arcs
i. Components of reflex arcs:
1. Sensory receptors – specialized cells that synapse with
sensory neurons, e.g., Merkel disks of the epidermis or
nociceptors in the dermal papillae
2. Afferent neurons – sensory neurons that generate
action potentials moving toward the CNS (input of
sensory information)
3. Interneurons (association neurons) – unmyelinated
neurons in the gray matter of the spinal cord; makes
up the integration center for somatic reflex arcs
4. Efferent neurons – Motor neurons that synapse with
effector cells
5. Effectors – muscle fibers, glands, adipose
ii. Diff. b/t Somatic and ANS reflex arcs
1. Somatic – the effectors of somatic reflex arcs are
skeletal muscle fibers
a. E.g., the patellar knee jerk reflex is a common
somatic reflex
b. Most somatic reflex arcs are locally processed in
the spinal cord
c. They are also usually ipsilateral – same-sided
2. ANS, Enteric and visceral reflex arcs synapse with the
following effectors

a. ANS – endocrine and many exocrine glands,
cardiac muscle, and many smooth muscles
b. Enteric (ENS) – many smooth muscles of the GI
tract and endo- / endo- crine glands of the GI;
also, adipose tissue
c. Visceral – adipose tissues in the hypodermis,
organs, and glands of the thoracic and abdominal
cavities, e.g., respiratory smooth muscle, mucus
glands; lymphatics
12. Neuroglial cells – supportive and protective non-conductive
cells of the nervous system.

a. CNS
i. Oligodendrocytes – produce the myelin sheath of CNS
neurons in white matter
ii. Astrocytes – most abundant glial cells of the CNS; connect
CNS neurons to each other and to blood vessels; provide
nutrients and play a supportive role in the CNS
iii. Microglial cells – phagocytic protective cells of the CNS;
digest microbial invaders and clean up cellular debris
iv. Ependymal cells – ciliated glial cells of the CNS that help
form CSF (cerebrospinal fluid – the interstitial fluid of the
CNS); cilia help circulate CSF
1. These cells line the ventricles of the brain and central
canal of the spinal cord
2. Choroid plexus – capillaries in the brain ventricles that
also help form CSF
a. produce cerebrospinal fluid (CSF) via the
ependymal cells that line the ventricles of the
brain.

b. Secondly, the choroid plexus serves as a barrier
in the brain separating the blood from the CSF,
known as the blood-CSF barrier
b. PNS
i. Schwann cells – produce the myelin sheath of white matter
axons of PNS neurons
ii. Satellite cells – protective and supportive glial cells
associated with the cell bodies of PNS neurons; very
abundant around the cell bodies of sensory neurons in PNS
ganglia
13. Nerve impulse generation and propagation
a. Myelin sheath – insulative lipoprotein covering white matter
axons in CNS and PNS
i. In the CNS, oligodendrocytes wrap their cell membrane
around the axons of multiple neurons like this:
1.
ii. In the PNS, Schwann cells grow on white matter axons and
wrap their cell membrane around the axon.
1. Also called neurolemma and look like:

2.
b. Nodes of Ranvier – (Ron-vee-ay) – gaps between myelin sheath
i. Allows for faster nerve impulse conduction called
saltatory conduction
ii. Sodium ion influx at nodes of Ranvier depolarizes the cell
membrane at the node and then “jumps” from one node to
another, much faster than non-myelinated axons in what is
called continuous conduction – much slower.
14. The Cranial Nerves of the PNS:
a. Some are motor (somatic and autonomic efferent pathways)
b. Some are sensory
c. Some are mixed or have both motor and sensory functions
Some say marry money, but my brother says big brains matter more.
d. Numbered I – XII
Oh, oh, oh, to touch and feel a very glossy Volkswagen…ah heaven!
i. Olfactory (I) nerve – smell; sensory only
ii. Optic (II) nerve – vision; sensory only
iii. Oculomotor (III) nerve – movement of eyeballs and eyelids; motor only;
motor (parasympathetic) – smooth and cardiac effectors and somatic
(skeletal)
iv. Trochlear (IV) nerve – eyeball movement; somatic motor; Trochlear
(“pulley”)
v. Trigeminal (V) nerve – three branches; both sensory and motor; innervation
of the face and mouth. Largest cranial nerve. The three branches are the
ophthalmic, maxillary, and mandibular branches
vi. Abducens (VI) nerve – lateral movement of the eyeballs; motor

vii. Facial (VII) nerve – 5 branches: temporal, zygomatic, buccal, mandibular,
cervical. Both. Innervation of the face and neck.
viii. Auditory / Vestibulocochlear (VIII) nerve – two branches: auditory (hearing)
and vestibular (equilibrium). Sensory.
ix. Glossopharyngeal (IX) nerve – innervation of the tongue and throat; both
sensory and motor.
x. Vagus (X) nerve – longest of the cranial nerves – extends into the thoracic
and part of the upper abdominal regions. Both sensory and motor.
1. Senses changes in blood chemistry and other metabolic conditions
related to sympathetic and parasympathetic conditions
2. Motor output to the heart, mostly parasympathetically to slow heart
rate
a. Over stimulation of the Vagus nerve can cause the heart to slow
too much or even stop.
xi. Accessory (spinal) (XI) nerve – located more posterior than the rest of the of
the cranial nerves, very close to the beginning of the of the spinal cord,
therefore, some “authorities” argue that the accessory nerve is actually a
spinal nerve rather than a cranial nerve. For this class, we will consider it to
be a cranial nerve.
1. Motor. Functions in innervating the sternocleidomastoid and
trapezius muscles
xii. Hypoglossal (XII) nerve – (“under the tongue”) innervation of the tongue
muscles.
CNS Components
 Brain – contained in the cranial cavity
o The CPU of the human body
o Parts
I. Brainstem – connects spinal cord to the brain
 Integration of survival reflexes
o Midbrain
o Pons (bridge)
o Medulla oblongata (M.O.)
II. Cerebellum – “little brain”
 Locomotion
 Balance
 Posture
III. Diencephalon – 2-brained; double brain; secondary brain
 Thalamus – chamber

 Epithalamus – on top of…
 Hypothalamus – master regulator of the ANS; also, much of the
endocrine system
o Temperature regulation
o Thirst center
IV. Cerebrum – conscious thought and control of most of the body
V. Cranial nerves are part of the PNS, but they originate directly from the brain:
 2 pairs of olfactory and optic (I and II) nerves; the olfactory ones
consist of free axons that emerge through the foramina of the
cribriform plate of the ethmoid bone and extend into the upper
epithelium of the nasal cavity.
o Both I and II cranial nerves emerge from the cerebrum.
 10 pairs of cranial nerves arise from the brain stem
o Development of the CNS
I. Neural plate – forms from ectoderm
 Flattens
 Thickens
o Neural groove and neural folds make up the neural plate
o Anterior to the neural plate is the notochord – rod of connective
tissue – primitive vertebral column.
 Neural folds – form as parallel lines and ridges along the length of the
embryo
o “peaks” as a neural crest containing individual crest cells
 Neural crests – cells of the neural crest make up a temporary group of
cells that give rise to:
o Melanocytes
o Cartilage
o Bone
o Smooth muscle
 Neural tube – the neural folds meet to form the neural tube that
ultimately becomes the spinal cord
o Notochord – becomes the nucleus pulposa of the intervertebral
discs.
o Ventricles of the brain
I. Lateral ventricles (2) – left and right
II. 3rd ventricle
III. Cerebral aqueduct – delivers cerebrospinal fluid (CSF) to the ventricles and
spinal cord central canal