Ch. 11: The Nervous System 11/20/2013
The nervous system is the 4 organ system and is divided into 2 divisions, the central nervous system
and the peripheral nervous system.
Central nervous system (CNS): located in the dorsal body cavity, covered by the meninges (includes
the brain and spinal cord)
Nucleus: a cluster of cell bodies in the CNS (plural: nuclei)
Tract: a bundle of axons in the CNS
Peripheral nervous system (PNS): consists of all the neural structures outside of the CNS,
including the cranial nerves, spinal nerves, and sensory receptors. It includes the sensory (afferent)
division, which sends information to the CNS, and the motor (efferent) division, which carries
information away from the CNS.
Ganglion: a bundle of cell bodies in the PNS (plural: ganglia)
Nerve: a bundle of axons in the PNS
The nervous system is composed mainly of nervous tissue, with connective tissue and blood vessels also
present. There are 2 types of cells in nervous tissue – neurons and supporting cells.
Neurons: nerve cells that have cell bodies (biosynthetic regions), dendrites (receptive regions), and
axons (conducting regions).
Axons (conducting regions): each neuron can only have one axon, but the axon branches at the end into
telodendria, which end in bulbous structures called axon terminals, which are known as the
secretory regions of the neuron. Only the axon of a neuron can be myelinated by either Schwann cells (in
the PNS) or oligodendrocytes (in the CNS). Axons generate action potentials, which are conducted
away from the cell body (graded potentials lose intensity over distance, but action potentials maintain
intensity over distance).
Cell body (biosynthetic regions): contains the nucleus and all other cytoplasmic organelles except for the
centrioles, making neurons amitotic. It has a welldeveloped nucleoli and a rough ER (called a nissl body or
chromatophilic substance), indicating that neurons are secretory cells that secrete proteins that can be
neurotransmitters. It also contains intermediate filaments called neurofilaments.
Dendrites (receptive regions): tapered processes that are not myelinated, which receive and convey
graded potentials towards the cell body
Classification of neurons (structural and functional)
Structural classification: multipolar, bipolar, and unipolar neurons
Multipolar neuron: has at least 3 processes – 1 axon and at least 2 dendrites; the most abundant type
of neuron in the human body
Bipolar neuron: has 2 processes – 1 axon and 1 dendrite
Unipolar neuron: has one short process extending from the cell body that bifurcates into a central
process and a peripheral process Functional classification: efferent/motor neurons, afferent/sensory neurons, and association neurons
Motor/efferent neurons: transmit impulses away from the CNS to effector organs
Sensory/afferent neurons: transmit impulses from sensory receptors towards the CNS
Association neurons (interneurons): located in the CNS between the motor and sensory
neurons. Most neurons (99%) in the body are association neurons, so most association neurons must be
multipolar neurons by deduction.
Supporting cells, called neuroglia, which literally means glue of the neurons: nonconducting cells
4 types in the CNS
Astrocytes: the most abundant type of supporting cell. Tight junctions form between astrocytes to form
the bloodbrain barrier, which is a selective barrier that allows lipidsoluble substances to cross into the
vicinity of the neurons in the CNS, which means lipidsoluble substances can affect the function of neurons
in the CNS (and most drugs are lipidbased). Hence, astrocytes regulate brain function.
Microglia: since the specific immune system does not have access to the CNS, the microglia act as
macrophages to engulf/destroy pathogens and cell debris.
Ependymal cells: ciliated columnar cells that line the ventricles, which are cavities in the brain that
contain cerebrospinal fluid (CSF). Cilia beat to create currents that circulate the CSF.
Oligodendrocytes: cells that myenilate axons in the CNS
2 types in the PNS
Schwann cells: also neurolemmocytes; myenilate axons in the PNS
Satellite cells: surround the cell bodies of neurons and control their chemical environment
Myelination of axons
In the PNS: each Schwann cell wraps around a segment of an axon (external to the axolemma), and
squeezes around the segment of axon, wrapping concentric rings of its plasma membrane (called the
myelin sheath) around the axon. The cytoplasm and the nucleus of the Schwann cell squeezed outside
the myelin sheath is called the neurilemma. The spaces between adjacent myelin sheaths are called
nodes of Ranvier.
In the CNS: the axons in the CNS are myelinated by extensions from the oligodendrocytes, hence, the
neurilemma is absent.
The functions of the myelin sheath are protection, electrical insulation, and increase in the rate of impulse
The structure of a tract/nerve The plasma membrane of an axon is called an axolemma. Each axon is wrapped in a delicate CT
membrane called the endoneurium, which is external to the myelin sheath (or to the axolemma in an
unmyelinated axon). A bundle of endoneuriumcovered axons is a fascicle. Each fascicle is covered by a
coarse CT membrane called the perineurium. A bundle of perineuriumcovered fascicles form the nerve
or tract, which is covered in the tough CT membrane called the epineurium.
Severed axons in the PNS can regenerate, but severed axons in the CNS can not.
Severed axons in the PNS can regenerate because when the axon is severed, cells of the immune system
clean up the damaged area of cell debris, a process known as debridement, which sets the stage for
regeneration. The neurilemma of the Schwann cell forms a regeneration tube that guides the
regeneration of the severed axon.
Severed axons in the CNS cannot regenerate because the microglia do a poor job of cleaning up the
damaged area, so debridement is incomplete. Also, there is no neurilemma to form a regeneration tube to
guide the growth of the severed axon. Finally, the presence of growthinhibiting proteins in the
CNS inhibit regeneration of a severed axon.
The generation of an action potential
Resting membrane potential: the axolemma is partial to potassium efflux, which moves from inside
the axoplasm in the axon to the exterior, down its concentration gradient. The axolemma restricts sodium
ion influx; sodium moves from the exterior into the axoplasm, down its concentration gradient. Due to the
partial nature of the axolemma, there is a separation of charges; the cytoplasmic face of the axolemma is
negative compared to the external face of the axolemma.
Phases of action potential
Depolarization phase: a stimulus that excites a neuron will also allow a sodium influx (entry of
sodium ions) as sodium enters the axoplasm, making the membrane potential less and less negative until a
threshold potential is reached. At the threshold potential, more sodium channels (called voltage
sensitive sodium channels) in the axolemma open to allow increase in sodium influx, and eventually drives
the membrane potential from negative, to zero, and then to a positive number, until it reaches critical
potential (+30 mV). Then, the sodium channels close, ending sodium influx, and thus ending the
The tracing from the threshold potential to critical potential is referred to as the upshoot/spike of the action
potential. The amplitude of +30 mV is attained by any stimulus that can activate an axon to generate action
Repolarization phase: when sodium influx stops, potassium channels open, causing potassium ions
to rush out of the axon (potassium efflux), bringing the membrane potential back down towards the RMP. Hyperpolarization phase: the potassium channels are sluggish and do not close when RMP is
reached, allowing for more potassium efflux, driving the membrane potential below the RMP. This is also
called the undershoot phase. The sodium/potassium pump actively transports 3 calcium out/2 potassium in
to reestablish the RMP.
There are 2 refractory periods during an action potential
Absolute refractory period: the point during the depolarization phase when the sodium channels
are already open and another action potential cannot be generated
Relative refractory period: the point during the repolarization phase when sodium channels are
closed and can be reopened to initiate another action potential by overriding the repolarization phase by an
exceptionally strong stimulus
Characteristics of action potentials
It is an allornone phenomenon: an action potential will be generated if depolarization reaches a threshold
It is selfpropagating: once generated by the axon, it is propagated down the axon to the axonal terminals
as an impulse.
The difference between a stronger stimulus and a weaker stimulus that causes the generation of an action
potential is that the stronger stimulus causes the impulse to be generated at a higher frequency than the
weaker stimulus. Stronger stimuli will generate more action potentials than weaker stimuli because the
strong stimuli can generate another action potential during the relative refractory period.
Factors affecting the transmission of action potentials: the rate of action potential transmission is referred to
as the conduction velocity.
Size (diameter) of the nerve fiber/axon: larger axons transmit impulses faster than smaller axons because
the larger axons have larger diameters and therefore less resistance when conducting impulses. The
resistance level is higher in smaller axons, which impedes transmission.
Degree of myelination: myelinated axons transmit impulses at a faster rate than unmyelinated axons.
Myelinated axons use saltatory conduction, where action potentials are generated only at the nodes
of Ranvier. Hence, the impulse jumps from node to node down the axon. Unmyelinated axons use
continuous conduction, where action potentials are developed stepwise across the entire
There are 3 types of nerve fibers based on diameter and degree of myelination:
Group A fibers: largest diameter (least resistance), heavily myelinated (saltatory conduction); fastest
Group B fibers: intermediate diameter (some resistance), lightly myelinated
Group C fibers: smallest diameters (most resistance), unmyelinated (continuous conduction);
slowest Ch. 12: The Central Nervous System 11/20/2013
The brain has 4 protective structures:
Cranium (cranial vault): bony helmet composed of the 8 cranial bones
Cerebrospinal fluid (CSF): filtered from blood; located in the ventricles and the subarachnoid space,
hence inside and outside of the brain as a liquid cushion that provides buoyancy to the brain, provides
nutrients, and removes metabolic wastes
Bloodbrain barrier (BBB): selective barrier that prevents harmful substances in blood from crossing
into the brain
Meninges: the dura mater, arachnoid mater, and pia mater
Differences between the meninges surrounding the brain and the meninges surrounding the spinal cord
The dura mater surrounding the brain is doublelayered (outer periosteal layer that lines the internal
surface of the cranial bones, and inner meningeal layer). In the spinal cord, the dura mater is singlelayered
and called the spinodural sheath, which does not line the internal surface of the vertebrae forming
the vertebral column that protects the spinal cord. A space exists between the spinodural sheath and the
vertebrae, called the epidural space.
The pia mater clings to the surface of the brain to allow for the passage of blood vessels, which are
covered by astrocytes to form the BBB. The pia mater clings to the surface of the spinal cord, but astrocytes
do not form a barrier. In addition, the pia extends laterally in the vertebral column to form structures called
the denticulate ligaments, which anchor the spinal cord laterally in the vertebral column. In addition,
the pia mater forms part of a structure called the filum terminae, which anchors the spinal cord
vertically in the vertebral column by attaching the spinal cord to the coccyx.
The brain has 4 ventricles, or cavities, that contain CSF. Each cerebral hemisphere contains a lateral
ventricle; the 2 lateral ventricles are connected by a median membrane called the septum
pellucidum, and connected to the 3 ventricle by a channel called the interventricular foramen.
The third ventricle is located in the diencephalon, and is connected to the 4 ventricle below via the
cerebral aqueduct. The fourth ventricle is located in the brain stem. The brain weighs about 3.5
lbs. there are 4 major regions in the postnatal (adult) brain.
Cerebrum: the largest region of the brain; a highly convoluted space including ridges (gyri), grooves
(sulci), and deeper sulci (fissures). A median fissure called the longitudinal fissure divides the
cerebrum into right and left cerebral hemispheres, which are held together medially by the corpus callosum.
Each cerebral hemisphere has 5 lobes, named for their overlying cranial bones: frontal lobe, parietal lobe,
temporal lobe, occipital lobe, and the insula. The insula is located deep to the lateral sulcus, which
separates the frontal and parietal lobes from the temporal lobe.
The central sulcus separates the frontal lobe from the parietal lobe. The gyrus in front of the central
sulcus, called the precentral sulcus, houses large neurons called pyramidal cells.
Each cerebral hemisphere has 3 regions Ch. 12: The Central Nervous System 11/20/2013
Outer cerebral cortex: highly convoluted and 24 mm thick; accounts for 40% of brain mass, composed
of gray matter (cell bodies and dendrites), which is the location of our conscious mind. There are 3
functional areas located in the cerebral cortex:
Motor areas, which control voluntary movements. There are 4 motor areas, all located in the frontal
Primary motor cortex: located in the precentral gyrus in each cerebral hemisphere, where the
pyramidal cells are located, the primary motor cortex controls the voluntary movements of skeletal muscles.
The axons of the pyramidal cells bundle to form the pyramidal or corticospinal tracts, which
decussate (cross over) on the ventral side of the medulla oblongata. This explains the contralateral
control of the voluntary movements of skeletal muscles by the cerebral hemisphere – voluntary
movements on the left side of the body are controlled by the right cerebral hemisphere, likewise for the
Hemiplegia: damage to the left precentral gyrus results in paralysis on the right side of the body, and
Premotor cortex: controls learned motor skills that are patterned or repetitive, like typing. Damage to
the premotor cortex causes one to have to relearn skills, such as walking.
Broca’s area: controls the skeletal muscles involved in speech production, hence, it is referred to as the
motor speech area. It is located only in the left cerebral hemisphere, so damage to the frontal lobe in the
left cerebral hemisphere can result in right side paralysis and loss of speech. It is connected to
Wernicke’s area by the actuate fasciculate. Wernicke’s area is responsible for language
acquisition. Damage to Wernicke’s area results in “word salad,” also known as Wernicke’s aphasia.
The Broca’s area can act to cause speech, but not comprehensive language. Damage to the Broca’s area
results in loss of speech, or Broca’s aphasia.
Frontal eye field: controls voluntary movements of the skeletal muscles that position the eyes. Damage
to the frontal eye field causes inability to move the eyes.
Sensory areas: for the conscious awareness of sensation; each input will have a different primary
sensory cortex in a different lobe (all 5 lobes are involved). Ch. 12: The Central Nervous System 11/20/2013
Somatosensory cortex in the parietal lobe: located in the postcentral gyrus in each cerebral
hemisphere; determines spatial discrimination. The sensory input nerves in the PNS carry impulses into the
CNS, and synapse with nuclei in the thalamus and then get relayed by tracts to the postcentral gyrus. The
somatosensory input from the left side of the body will be relayed to the right postcentral gyrus (which is in
the right cerebral hemisphere). Hence, damage to the left postcentral gyrus will result in loss of sensation
on the right side of the body.
Association areas: integrate and interpret sensory inputs from the sensory areas hence, each primary
sensory area above has an association area.
Inner cerebral white matter: composed of myelinated axons; inner/deep to the cerebral cortex. The
myelinated axons are bundled into 3 types of tracts, based on direction:
Commissural tracts: commissures connect corresponding areas in the 2 cerebral hemispheres. The
corpus callosum is a commissure.
Projection tracts: connect the cerebrum to the lower brain areas and spinal cord; 2 types:
Descending tracts: motor information from the cerebral cortex to lower brain regions/spinal cord (ex:
Ascending tracts: sensory input into the thalamus and then relayed to specific areas in the cerebral
cortex (ex: spinothalamic tract)
Association tracts: connect areas within the same cerebral hemisphere, like the actuate fasciculate.
Basal nuclei: islands of clusters of neuronal cell bodies located deep in the cerebral white matter. There
are 3 major basal nuclei: caudate nucleus, putamen, and the globus pallidus. All 3 basal nuclei
are referred to as the corpus striatum (literally means striated body), and just the putamen + the
globus pallidus is referred to as the lentiform nucleus.
Basal nuclei initiate and stop movements