Nervous system histology and anatomy chapters 12, 13, 14

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

2. Integration

• Interpretation of sensory input (CNS)

3. 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 SystemThe central nervous system(CNS) consists of the brain andspinal cord and is responsiblefor integrating, processing, andcoordinating sensory data andmotor commands.

Information processingincludes the integration anddistribution of information inthe CNS.

Peripheral NervousSystem

The peripheralnervous system(PNS) includes all theneural tissue outsidethe CNS.

The motor division of thePNS carries motor commandsfrom the CNS to peripheraltissues and systems.

includes

The autonomicnervous system(ANS) providesautomatic regulationof smooth muscle,cardiac muscle,glands, and adiposetissue.

The somaticnervoussystem(SNS)controlsskeletalmusclecontractions.

The sensory division of the PNSbrings information to the CNSfrom receptors in peripheraltissues and organs.

Somatic sensoryreceptors provideposition, touch,pressure, pain, andtemperature sensations.

Special sensoryreceptors providesensations ofsmell, taste,vision, balance,and hearing.

Visceral sensory receptorsmonitor internal organs.

Receptors are sensory structures that detectchanges in the internal or externalenvironment.

Skeletalmuscle

Effectors are target organs whoseactivities change in response to neuralcommands.

• 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


Neuroglia in the CNS: Astrocytes functions

• 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


Neuroglia in the CNS: Astrocytes functions

• 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

Nervefibers

Myelin sheathProcess ofoligodendrocyte


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 (whichform myelin) surround neurons in the PNS.

Schwann cells(forming myelin sheath)

Cell body of neuronSatellitecells

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 cordPons

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)

http://www.bioon.com/book/biology/whole/image/1/1-6.tif.jpg


Layers of the Cerebral Cortex

Figure 9.12a–b


Premotor cortex(coordinatesvoluntarymovements)

Primary somatosensorycortex (somesthetic sensationsand proprioception)

Sensory associationareas (integration ofsensory information)

Primary motor cortex(voluntary movement)

Central sulcus

Prefrontalassociationareas (idea andplan for voluntarymovement, thoughts,personality)

Broca’s area(speech formation)

Limbic associationcortex (emotions,learning, and memory)

Olfactory cortex(smell)

Visual associationareas (higher visionprocessing)

Wernicke’s area(languagecomprehension)

Auditoryassociationareas

Primary auditorycortex (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

Somaticsensoryneuron

Dorsal root (sensory)

Dorsal root ganglion

Visceralsensory neuron

Somaticmotor neuron

Spinal nerve

Ventral root(motor)

Ventral horn(motor neurons)

Dorsal horn (interneurons)

Visceralmotorneuron

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

Nervous system histology and anatomy chapters 12, 13, 14

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