* initially bipolar during embryonic dev’t. and the two processes fuse during later dev’t and form one process.
blood-brain barrier, which restricts the movement of molecules from the blood into the interstitium of the CNS. -In general, the blood–cerebrospinal fluid and blood-brain barriers are highly permeable to water, carbon dioxide, oxygen, and most lipid-soluble substances such as alcohol and anesthetics; slightly permeable to electrolytes such as sodium, chloride, and potassium; and almost totally impermeable to plasma proteins and most non–lipid-soluble large organic molecules. Therefore, the blood-cerebrospinal fluid and blood-brain barriers often make it impossible to achieve effective concentrations of therapeutic drugs, such as protein antibodies and non–lipid-soluble drugs, in the cerebrospinal fluid or parenchyma of the brain. * If these metabolic chemicals are not quickly removed from these sites, they can interfere with neuronal functions.
Respond to internal and external stimuli.
Transmit nerve impulses to and away from CNS.
Interpret nerve impulses at the cerebral cortex.
Assimilate experiences in memory and learning.
Initiate glandular secretion and muscular contraction.
Program instinctual behavior.
Central nervous system (CNS)
Includes Brain and Spinal cord (SC)
Peripheral nervous system (PNS)
composed of nerve fibers and nerve ganglia out
side the CNS
Nerve fibers / nerves – cordlike extension of
neurons from the brain and spinal cord
Ganglia - small aggregates of nerve cells; nerve
Designed to deliver rapid and precise communication
between different parts of the body by the action of
specialized nerve cells called neurons.
Neurons operate by generating electrical signals that
move from one part of the cell to another part of the
same cell or to neighboring cells.
Highly cellular – 80% (Neurons and supporting cells)
Extracellular space - 20% (in the brain and spinal cord)
CNS: Brain and spinal cord
Do not have connective tissue component within.
Only the meninges (covering) has CT component.
Fresh CNS tissue have very soft, “jelly-like” consistency.
Covered by meninges and protected by bony structure (skull
and vertebral column)
The structural and functional unit of the nervous
highly specialized for irritability, conductivity, and
synthesis of neuroactive substances such as
neurotransmitters and neurohormones.
After a mechanical or chemical stimulus:
these neurons react (irritability) to the stimulus
and transmit (conductivity) the information via
axons to other neurons in different regions of the
Strong stimuli create a wave of excitation, or nerve
impulse (action potential), that is then propagated
along the entire length of the axon (nerve fiber).
Nerve Cell Body / Soma
1) Nucleus – centrally located in the soma of most
neurons; euchromatin and dark nucleolus (“owl-eyed
2) Cytoplasmic organelles and inclusions:
Nissl bodies – dark clumps in the cytoplasm made
of polysomes and rough ER
Cytoskeletons: neurofilaments (regulates neuronal
axon diameter); microtubules (in cytoplasm);
microfilaments (in plasma membrane)
In general, the cell bodies of all neurons are
located in the central nervous system
Motor neurons, integrating neurons
Except: the cell bodies of most primary sensory
neurons and the terminal effector neurons of the
autonomic nervous system
(the cell bodies lie in aggregations called ganglia
in peripheral sites).
Dendrites – a series of highly branched outgrowths
of the cell body.
major sites of information input into the neuron.
with abundant mitochondria
receive stimuli (signals) from adjacent sensory
cells, or from other neuron and convert the
stimuli into electrical impulse.
Processes that either become:
specialised sensory receptors (as in primary
• visual receptors in the eyes
• auditory receptors in the ears
• tactile receptors on the surface of the body
form synapses with neighbouring neurons
from which they receive stimuli (from an axons
or other neurons).
* Break in the axon indicates the length of up to 1 meter..
Skin Sensory Receptors
Merkel’s disks – free nerve endings (unencapsulated),
unmyelinated nerve fibers (skin: stratum basale)
Meissner’s corpuscle – sensitive to light touch (skin
Pacinian corpuscle – deep pressure or coarse touch
End bulb of Krause – sensitive to cold (oropharynx
lining & eye conjunctiva)
Ruffinian corpuscle – sensitive to heat (soles of feet)
Axon – is a single long process that
extends from the cell body and carries
output to its target cells.
also “nerve fibers”.
conduct impulses away from the soma and
sends it to effector/target organs or adjacent
Parts of the axon:
Axon hillock – specialized area of soma with
high neurofilaments; initial segment.
Axoplasm (axon cytoplasm) – contains sER,
rER and long mitochondria
Axolemma – plasma membrane of axon.
Axon collaterals - branches that form from the
main axon along its course.
Axon terminals – ends of axon;
with small swellings called terminal boutons.
responsible for releasing neurotransmitters
The axons of most but not all neurons are covered by
myelin, which consists of 20 to 200 layers of highly
modified plasma membrane wrapped around the axon by
a nearby supporting cell that produced it.
Myelin – an insulating sheath of fatlike lipid that wraps
around the axon.
Myelin sheath – formed by oligodendrocyte
(CNS) or Schwann cells (PNS).
Schwann cell covering only a segment of the axon. Between the Schwann cells
there are short intervals at which the axon is not covered by a myelin sheath;
these points are known as nodes of Ranvier.
Basic types of neurons:
- three main patterns based on the number of dendrites and axons that
originate from the cell body.
one dendrite and one
numerous dendrites and
one process that
branches into two.
Types of neurons (based on structure):
Bi-polar neurons – a single dendrite and a single axon
are associated with the cell body.
are purely sensory neurons.
Multipolar neurons - numerous branched dendrites
arise from the cell body with a single axon on the
all motor neurons and interneurons of the brain,
cerebellum, and spinal cord.
Pseudo-unipolar neurons* –exhibit only one process leaving
the cell body.
Most neurons in the adult organism - most other primary
Example of a neuron: Multipolar neuron
Functional classes of neurons
Sensory neurons or Afferent neurons
Transmit impulses to the CNS.
Somatic sensory – carry impulses from the skin, bones, and
Visceral sensory – carry impulses from the visceral organs.
Motor or Efferent neurons
Transmit impulses away from the CNS.
Somatic motor – innervate skeletal muscles.
Visceral motor (autonomic motor) – innervate cardiac, smooth
muscles and glands.
Associated neurons or Interneurons
connect neurons within the CNS
Conduct impulses from sensory to motor neurons
Characteristics of three classes of neurons
I. Sensory or Afferent neurons
Transmit information into the CNS from receptors at their
Cell body and the long peripheral process of the axon are in
the PNS; only the short central process of the axon enters
Have no dendrites (do not receive inputs from other
II. Motor or Efferent neurons
Transmit information out of the central nervous system to
effector cells, particularly muscles, glands, or other neurons
Cell body, dendrites, and a small segment of the axon are in
the central nervous system; most of the axon is in the
peripheral nervous system
Function as integrators and
Integrate groups of afferent
and efferent neurons into
Lie entirely within the
central nervous system
Account for 99 percent of
Supporting cells of CNS
Small-sized and numerous than neuron; origin: neural
Support, nourishes and protect neurons
Types: astrocytes, oligodendrocytes, microglia,
Schwann cells - in the PNS
Astrocytes – largest neuroglial cell
contain many processes forming pedicels (vascular feet)
around a blood capillary forming part of the “blood-brain
control the chemical environment around neurons
clearing intercellular spaces of increased K ions* and released
neurotransmitters, such as glutamate*
support metabolic exchange between neurons and the
capillaries of the CNS.
contain reserves of glycogen, from which they release
as glucose for energy metabolism of the CNS.
small, phagocytotic neuroglial cell that arised from
monocytes in the bone marrow;
they become APCs (antigen-presenting cell) in the
CNS tissue when activated by trapped foreign
epithelial cells that line the neural tube and
ventricles of the brain
they have cilia that aids in moving the CSF
cells of the choroid plexus (CSF production)
Peripheral Nervous System
Branches from the brain and spinal cord
Cranial nerves and spinal nerves
Ganglion, peripheral nerves
Nerve fiber – individual axons enveloped by myelin sheath
Nerves – bundles of nerve fibers; appear whitish due to (+)
myelin; CT wrappings
Ganglia – encapsulated aggregations of soma or cell bodies
not continuous, solid sheets along the axon
nodes of Ranvier - punctuations / gaps / intervals
significantly accelerate the conduction of nerve impulses
(action potentials) along the axons.
Saltatory conduction - impulse propagation in
myelinated axons where the nerve impulse or action
potential jumps from node to node, resulting in a more
efficient and faster conduction of the impulse.
Discrete aggregations of neuron cell bodies
located outside the CNS.
The spinal ganglia lie on the posterior nerve roots
of the spinal cord as they pass through the
They contain the cell bodies of primary sensory
neurones which are of the pseudo-unipolar form.
Connective tissue wrappings of
Epineurium – covers the whole nerve bundle that forms
external coat of nerves formed by dense CT(fascia).
Perineurium – surrounds each nerve bundle (fascicle);
flattened cells are joined by tight junctions (z. occludens)
prohibiting passage of macromolecules.
Endoneurium – forms a thin layer of reticular fibers produced
by Schwann cells that surrounds individual nerve fibers
Dura mater – connected to the endosteum of the skull
Arachnoid mater – sublayer between endosteum and
brain, contains network (cobweb) of blood vessels.
Pia mater – covers the brain parenchyma
Da Lamarcke’s theory of use and disuse can be associated
with microcephaly. Such condition may happen if you don’t
exercise your brain!