Cns histology and anatomy

Copyright © 2010 Pearson Education, Inc.
Nervous system histology and
anatomy
chapters 12, 13, 14
http://www.eb.tuebingen.mpg.de/core-facilities/microscopy-unit/sem_pictures/axon.jpg

Functions of the Nervous System
1. Sensory input
• Information gathered by sensory receptors about
internal and external changes
3. Integration
• Interpretation of sensory input (CNS)
5. Motor output
• Activation of effector organs (muscles and
glands) produces a response

Anatomical Organization of the Nervous System
• Central nervous system (CNS)
• Brain and spinal cord
• Integration and command center
• Peripheral nervous system (PNS)
• Paired spinal and cranial nerves
• Carries messages to and from the spinal cord and
brain

Copyright © 2010 Pearson Education, Inc. Figure 11 Section 1
The major components and functions of the nervous system
Central Nervous System
The central nervous system
(CNS) consists of the brain and
spinal cord and is responsible
for integrating, processing, and
coordinating sensory data and
motor commands.
Information processing
includes the integration and
distribution of information in
the CNS.
Peripheral Nervous
System
The peripheral
nervous system
(PNS) includes all the
neural tissue outside
the CNS.
The motor division of the
PNS carries motor commands
from the CNS to peripheral
tissues and systems.
includes
The autonomic
nervous system
(ANS) provides
automatic regulation
of smooth muscle,
cardiac muscle,
glands, and adipose
tissue.
The somatic
nervous
system
(SNS)
controls
skeletal
muscle
contractions.
The sensory division of the PNS
brings information to the CNS
from receptors in peripheral
tissues and organs.
Somatic sensory
receptors provide
position, touch,
pressure, pain, and
temperature sensations.
Special sensory
receptors provide
sensations of
smell, taste,
vision, balance,
and hearing.
Visceral sensory receptors
monitor internal organs.
Receptors are sensory structures that detect
changes in the internal or external
environment.
Skeletal
muscle
Effectors are target organs whose
activities change in response to neural
commands.
• Smooth muscle
• Cardiac muscle
• Glands
• Adipose tissue

Histology of Nerve Tissue
• The two cell types of the nervous system are:
• Neurons – excitable cells that transmit electrical
signals
• Supporting cells – cells that surround and wrap
neurons

Neurons (Nerve Cells)
• Structural units of the nervous system
• Composed of a body, axon, and dendrites
• Long-lived (over 100 years)
• Amitotic – once achieve there roles in the system
they loose the ability to divide
• High metabolic rate – require high supply of
oxygen and glucose
• Their plasma membrane function in Electrical signaling

Neuron Cell Body (Perikaryon or Soma)
• Posses all the components needed to produce membrane parts,
synthesize enzymes and neurotransmitters:
• Contains the nucleus and a nucleolus
• rough endoplasmic reticulum/Nissle bodies for protein synthesis
• Golgi apparatus which packages materials into vesicles for
transport
• Numerous mitochondria
• Cytoskeletal elements
• Has no centrioles (hence its amitotic nature)
• Neural stem cells are still found in the adult but are not active
• Cells in the hippocampus (part that is involves memory) can still
dividing

Dendrites
• Short branched processes
• They are the receptive, or input,
regions of the neuron
• Supply big surface for receiving
signals
• Convey incoming messages
towards the cell body.
• Electrical signals are conveyed as
graded potentials (not action
potentials)

Axons: Structure
• The axon contains the same organelles as the cell body with the
exception of Nissle bodies and Golgi apparatus (protein
synthesis and packaging)
• Depends on the cell body to supply proteins
• Each neuron has single axon of uniform diameter
• Initiate in an enlarged area called the axon hillock
• Numerous terminal branches (telodendria)
• Knoblike axon terminals (synaptic knobs or boutons)
• Secretory region of neuron
• Release neurotransmitters to excite or inhibit other cells
• Long axons are called nerve fibers

Axons: Function
• Generate and transmit action potentials – away from the cell
• The signal starts at the junction between the axon hillock and
axon – trigger zone
• Secrete neurotransmitters from the axonal terminals in response
to the impulse arriving along the axon
• Movement along axons (not the signal movement) occurs in
two ways
• Anterograde — toward axonal terminal (mitochondria,
cytoskeletal elements, membrane components for membrane
renewal, enzymes Examples: mitochondria, membrane
components, enzymes
• Retrograde — away from axonal terminal (organelles to be
degraded, signal molecules, viruses, and bacterial toxins like
polio, rabies, herpes simplex)

Supporting Cells: Neuroglia
• Two groups
• In the CNS – astrocytes, microglia, ependymal
cells and oligodendrocytes
• In the PNS – satellite and Schwann cells

Neuroglia in the CNS: Astrocytes
• Most abundant, versatile, and highly branched glial cells
• In some areas of the brain they account for 90% of the
tissue
• They cling to neurons and their synaptic endings, and cover
capillaries
http://www.ucl.ac.uk/biology/images/astrocytes.gifFigure 11.3a
Capillary
Neuron
Astrocyte

Neuroglia in the CNS: Astrocytes functions
• Maintaining blood-brain barrier – preventing free access of
circulating compounds to the CNS.
• The extensions of the astrocytes end in expanded portion
that wraps capillaries

Why are the capillaries in the BBB less permeable?
• Endothelial cells form tight junctions that prevent
solutes movement between cells
• Astrocytes
• Selective transport properties of the endothelial cells
• The BBB
• Helps maintain a stable environment for the brain
• Separates neurons from some bloodborne substances

Blood-Brain Barrier: Functions
• Selective barrier that allows nutrients to pass freely
• Is ineffective against substances that can diffuse through plasma
membranes (ex. Ethanol, caffeine)
• Absent in some areas:
• Ex. - hormones generally do not penetrate the brain from the
blood, so in order to control the rate of hormone secretion
effectively, there are specialized sites where neurons can
"sample" the composition of the circulating blood. At these
sites, the blood-brain barrier is 'leaky‘ (pituitary gland)
• Capillaries of the choroid plexus
• The BBB can break down under certain conditions:
• hypertension, radiation, infection and brain trauma

Example
• In Parkinson’s disease there is a lack in the NT
dopamine (neurons that produce it are either damaged
or dead)
• Dopamine can not be given in a pill or injection
because it can’t cross the BBB
• Instead, a precursor - L-dopa – is given. This is being
transported across the BBB and is being used by
neurons

• Creating a framework for the CNS with microfilaments
that extend from the astrocytes
• Repairing damaged neural tissue- limited structural
repairs that stabilize and prevent further injury.
• In the embryonic brain, the astrocyte appear to be
involve in directing the growth and interconnection of
developing neurons.
• Secrete nerve growth factors that promote neuron
growth and synapse formation

• Control the chemical environment
• Regulating the concentration of sodium, potassium
and carbon dioxide ions
• Providing system for transporting nutrients and
dissolved gasses between capillaries and neurons
• absorb neurotransmitters to prevent excessive
levels in tissue fluid; control ion concentration in
the interstitial fluid

Oligodendrocytes
• Branched cells
• Processes wrap CNS nerve fibers, forming insulating
myelin sheaths
Nerve
fibers
Myelin sheath
Process of
oligodendrocyte

Neuroglia cells in the PNS
• Schwann cells (neurolemmocytes) – surround fibers of
the PNS
• Similar function as the oligodendrocytes
• Satellite cells surround neuron cell bodies in the PNS
• similar function to the astrocytes in the CNS –
controlling the chemical environment

Copyright © 2010 Pearson Education, Inc. Figure 11.3e
(e) Satellite cells and Schwann cells (which
form myelin) surround neurons in the PNS.
Schwann cells
(forming myelin sheath)
Cell body of neuron
Satellite
cells
Nerve fiber

Myelin Sheath: Formation
• Formed by Schwann cells in the PNS
• Schwann cells wraps many times around the axon
• Myelin sheath — concentric layers of Schwann cell
membrane
• Neurilemma—peripheral bulge of Schwann cell
cytoplasm
• Myelin sheath functions :
• Protect the axon
• Electrically insulate fibers from one another
• Increase the speed of nerve impulse transmission

Nodes of Ranvier (Neurofibral Nodes)
• Gaps in the myelin sheath
between adjacent Schwann
cells
• They are the sites where
axon collaterals can emerge
http://www.jdaross.cwc.net/introd13.jpg

Unmyelinated Axons
• Thin nerve fibers are unmyelinated
• One Schwann cell may incompletely enclose 15 or
more unmyelinated axons

Unmyelinated Axons
• A Schwann cell surrounds nerve fibers but coiling does not take place
• Schwann cells partially enclose 15 or more axons
http://www.bu.edu/histology/p/21201loa.htm

Axons of the CNS
• Both myelinated and
unmyelinated fibers are present
• Myelin sheaths are formed by
oligodendrocytes
• Nodes of Ranvier are widely
spaced

Regions of the Brain and Spinal Cord
• White matter – dense collections of myelinated fibers
• Gray matter – mostly soma (cell bodies) and
unmyelinated fibers

CNS: Gray and White Matter
Figure 9.5a

Neurons Cell body locations
• Groups of cell bodies are named according to their
location:
• Most cell bodies are in the CNS in clusters called
nuclei
• Some are in the PNS in clusters called ganglia

Neurons’ Processes location
• The groups of axons are named according to their
locations:
• In the CNS the extensions called tracts
• In the PNS the extensions (axons) are called nerves

Neuron Classification - Structural
• Multipolar — three or more processes
• Many extensions; many dendrites lead toward cell body, one axon leads
away from cell body.
• Most neurons in the CNS and those whose axons carry impulses away
from the CNS are multipolar.
• Bipolar — two processes (axon and dendrite)
• Two extensions; one fused dendrite leads toward cell body, one axon
leads away from cell body
• These are rare and can be found as part of the receptor apparatus in the
eye, ear and olfactory mucosa.
• Unipolar — single, short process
• One very short process from cell body that divides into central and
peripheral processes. Only the distal ends of the peripheral process are
dendrites and the rest act as an axon along with the central process.
• Nearly all neurons that conduct impulses towards the CNS are unipolar.

Neuron Classification - Functional
• Sensory (afferent) - transmit impulses toward the CNS
• Most are unipolar neurons with their bodies in ganglia in
the PNS.
• Location – sensory receptors in the internal organs, skin,
skeletal muscles and joints. They sense changes in the
immediate environment
• Motor (efferent) - carry impulses away from the CNS
• Carry activating impulses from CNS and to the viscera,
body muscles and glands;
• They are multipolar and their cell bodies are in the CNS.

Neuron Classification - Functional
• Interneurons (association neurons) - shuttle signals
through CNS pathways
• Link other neurons together (i.e. sensory neuron to
interneuron to motor neuron).
• All bodies are in CNS and they are multipolar.

Central nervous system
http://www.courses.vcu.edu/DANC291-003/unit%201.htm

3 Major levels of the CNS
• Spinal cord
• involved in walking movement, reflexes of both skeletal
muscles and the autonomic nervous system effectors
• Lower brain/subcortical level
• Subconscious activities of both the autonomic and
somatic nervous system. Involved in several emotional
patterns
• Higher brain/cortical level
• Very large memory storage, place of higher functions
like thoughts, awareness

CNS protection
• Bony protection by the skeleton – brain by skull and
spinal cord by vertebrae
• 3 layers of membrane collectively called meninges
that are found between the bone and the CNS tissue
• Cerebro-spinal fluid – CSF - found in the ventricles,
central canal and subarachnoid space
• Blood brain barrier - BBB

CNS: Physical Support
Figure 9.2a

CNS: Physical Support
Figure 9.2b

Ventricles
• Filled with CSF (cerebrospinal fluid)
• Ventricles continuous w/each other + central canal
of spinal cord
• Lateral Ventricle (#1+2)
• Cerebral Hemisphere
• Third Ventricle
• Diencephalon
• Fourth Ventricle
• Cerebral Aqueduct: connects 3rd and 4th
ventricles
• Connects to central canal of spinal cord &
medulla
3
4
lateral

Choroid Plexuses and CSF
• Clusters of capillaries and the ependymal cells
http://www.sci.uidaho.edu/med532/choroid.htm

CSF production
• The choroid plexus forms tissue fluid filters
• Have ion pumps that selectively pump ions from
the plasma into the ventricles.
• The ions pumps create osmotic pressure that
draws water to the CSF
• the choroid plexus of the lateral ventricles producing the
most.
• The rate of formation is approximately 0.35 ml/min or 500
ml/day; a rate which replaces the total volume of CSF
approximately 2-3 times over in 24 hours.

CSF – cerebrospinal fluid functions
• Liquid cushion for brain and spinal cord
• Nourishes brain
• Removes waste
• Conducts chemical signals between parts of CNS

Copyright © 2010 Pearson Education, Inc. Figure 9.11c
Midbrain
Spinal cord
Pons
Medulla oblongata
Forebrain
Cerebrum
Thalamus
Hypothalamus
Pituitary gland
Brainstem
Diencephalon
Cerebellum
Corpus
callosum
(c) Midsagittal section
Brain: Midsagittal View

http://www.bioon.com/book/biology/whole/image/1/1-6.tif.jpg
Basal nuclei
Cerebrum functional regions
• The cerebrum has three basic regions: cortex
(gray matter), white matter, and basal nuclei
(gray matter)

Layers of the Cerebral Cortex
Figure 9.12a–b

Premotor cortex
(coordinates
voluntary
movements)
Primary somatosensory
cortex (somesthetic sensations
and proprioception)
Sensory association
areas (integration of
sensory information)
Primary motor cortex
(voluntary movement)
Central sulcus
Prefrontal
association
areas (idea and
plan for voluntary
movement, thoughts,
personality)
Broca’s area
(speech formation)
Limbic association
cortex (emotions,
learning, and memory)
Olfactory cortex
(smell)
Visual association
areas (higher vision
processing)
Wernicke’s area
(language
comprehension)
Auditory
association
areas
Primary auditory
cortex (hearing)
Primary visual cortex
(vision)
Figure 9.14
Functional Areas of Cerebrum

Beneath the cerebrum – basal nuclei
• Are masses of gray matter that are embedded in white
matter of cerebrum
• Function in the subconscious control of skeletal
muscle tone
• The coordination of learned movement patterns
(walking, lifting)

White Matter of the Cerebrum
• Association fibers
• Connections within one hemisphere
• Commissural fibers connecting two hemispheres:
●
corpus callosum
●
anterior commissure
• Projection fibers link cerebral cortex with:
●
diencephalon, brain stem, cerebellum, and
spinal cord

Diencephalon
• Consists of three structures
• thalamus
• Hypothalamus
• epithalamus
• Encloses the third ventricle

Epithalamus
• Most dorsal portion of the diencephalon; forms roof of
the third ventricle
• Pineal gland – extends from the posterior border and
secretes melatonin
• Melatonin – a hormone involved with sleep
regulation, sleep-wake cycles, and mood
• Choroid plexus – a structure that secretes cerebral
spinal fluid (CSF)

Brain Stem
• Consists of three regions –
• midbrain,
• Pons
• medulla oblongata
• Controls automatic behaviors necessary for survival
(breathing, digestion, heart rate, blood pressure)
• Provides the pathway for tracts between higher and lower
brain centers
• Associated with 10 of the 12 pairs of cranial nerves

Cerebellar Processing
• Cerebellum receives impulses of the intent to initiate
voluntary muscle contraction
• Proprioceptors and visual signals “inform” the
cerebellum of the body’s condition
• Cerebellar cortex calculates the best way to perform a
movement
• A “blueprint” of coordinated movement is sent to the
cerebral motor cortex

Spinal cord

Copyright © 2010 Pearson Education, Inc. Figure 12.32
Somatic
sensory
neuron
Dorsal root (sensory)
Dorsal root ganglion
Visceral
sensory
neuron
Somatic
motor neuron
Spinal nerve
Ventral root
(motor)
Ventral horn
(motor neurons)
Dorsal horn (interneurons)
Visceral
motor
neuron
Interneurons receiving input from somatic sensory neurons
Interneurons receiving input from visceral sensory neurons
Visceral motor (autonomic) neurons
Somatic motor neurons
Gray Matter: Organization

White Matter in the Spinal Cord
• Composed of both myelinated and unmyelinated fibers that run
in three directions
• Ascending – to higher centers; sensory
• Descending – to lower levels from brain or higher places in
the cord; motor
• Transversely – from one side of the cord to the other
• Divided into three funiculi (columns) – posterior, lateral, and
anterior
• Each funiculus contains several fiber tracks with similar
destination and function
• Fiber tract names reveal their origin and destination
• Fiber tracts are composed of axons with similar functions

Peripheral nervous system

Peripheral nervous system components
• Includes nerves and ganglia
• Most nerves are mixtures of afferent and efferent fibers
and somatic and autonomic (visceral) fibers
• Pure sensory (afferent) or motor (efferent) nerves are rare
• Peripheral nerves classified as cranial or spinal nerves
• Ganglia contain neuron cell bodies associated with
nerves
• Dorsal root ganglia (sensory, somatic) (Chapter 12)
• Autonomic ganglia (motor, visceral) (Chapter 14)

Structure of a Nerve
• Nerve – cordlike organ of the
PNS consisting of peripheral
axons enclosed by connective
tissue
• Connective tissue coverings
include:
• Endoneurium – loose
connective tissue that
surrounds axons
• Perineurium – coarse
connective tissue that
bundles fibers into fascicles
• Epineurium – tough fibrous
sheath around a nerve

Spinal Nerves: Rami
• The short spinal nerves branch into three or four mixed,
distal rami
• Dorsal ramus – innervate skin of the back and deep
back muscle
• Ventral ramus – form plexuses. The ventral rami of
T2-T12 do not form plexuses and innervate
intercostal spaces and muscles of the trunk
• Tiny meningeal branch – innervate the meninges and
blood vessels within the vertebral canal
• Rami communicantes at the base of the ventral rami
in the thoracic region that contain autonomic nerve
fibers

Nerve Plexuses
• All ventral rami except T2-T12 form nerve networks
called plexuses
• Plexuses are found in the cervical, brachial, lumbar, and
sacral regions
• Each resulting branch of a plexus contains fibers from
several spinal nerves
• Fibers travel to the periphery via several different routes
• Each muscle receives a nerve supply from more than one
spinal nerve
• Damage to one spinal segment cannot completely
paralyze a muscle

Plexus Main spinal nerves Regions innervated Major nerves
Cervical C1-C5 Skin and muscles of head &
neck. Superior chest and
shoulder
Phrenic (diaphragm)
Brachial C5-C8, T1 Shoulder and upper limbs Axillary
Musculocutaneous
Radial
Median
Ulnar
Lumbar L1-L4 Antero-lateral abdominal
wall, external genitalia, part
of lower limbs
Femoral
Sacral L4-L5, S1-S4 Buttocks, perineum, lower
limbs
Sciatic

• Neurons in the nervous system link together to form
circuits with specific functions.
http://www.thefullwiki.org/Nervous_System

Nervous system functions
• Motor functions
• Control contraction of skeletal muscles – somatic nervous
system
• Control contractions of smooth and cardiac muscle –
autonomic nervous system
• Control of glands function – autonomic nervous system
• Sensory and integrative functions
• Process incoming information to ensure appropriate motor
response
●
More than 99% of the incoming information is classified
as irrelevant and unimportant

The nervous system - the study approach in this class
• Our approach will be to cover functional systems:
• Sensory pathways - Somatic and autonomic
• Special senses
• Motor pathways – somatic and autonomic
• All system will cover both the central and
peripheral nervous systems

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