Nervous System – Part 1

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The Department of Human anatomy
The Nervous
System

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The Nervous System
 The body’s primary communication and
control system.
 Can be divided according to:
 Structural categories
 Functional categories.

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Nervous System: Structural
Organization
Structural subdivisions of the nervous system:
 Central nervous system (CNS)
 brain and spinal cord
 Peripheral nervous system (PNS)
 cranial nerves (nerves that extend from the brain)
 spinal nerves (nerves that extend from the spinal cord)
 ganglia (clusters of neuron cell bodies (somas) located
outside the CNS)

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5. Sponsored
Medical Lecture Notes – All Subjects
USMLE Exam (America) – Practice

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Nerve Cells
 Nervous Tissue
 Two distinct cell types

Neurons
 excitable cells
 initiate and transmit nerve impulses

Glial cells
 nonexcitable cells
 support and protect the neurons

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Characteristics of Neurons
 Neurons have a high metabolic rate.
 Neurons have extreme longevity.
 Neurons typically are non-mitotic.

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Neuron Structure
 Neurons come in all shapes and sizes
 All neurons share certain basic structural features.
 typical neuron:
 Cell body (soma, perikaryon)
 Dendrites
 Axon

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Classifications of Neurons
 Neurons vary widely in morphology and
location.
 classified based on

structure

function.
 Structural classification: number of processes
extending from the cell body.
 unipolar neuron has a single process
 bipolar neurons have two processes
 multipolar neurons have three or more processes

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Functional Classification
 Sensory afferent neurons: receptor to CNS
 Motor efferent neurons: CNS to effector
 Interneurons (association neurons): facilitate
communication between sensory and motor neurons.

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Glial Cells
 Also called neuroglia
 Occur within both the CNS and the PNS.
 are smaller than neurons
 are capable of mitosis.
 do not transmit nerve impulses.
 Glial cells
 physically protect neurons
 help nourish neurons
 provide a supporting framework for all the nervous tissue.
 Glial cells far outnumber neurons.
 Glial cells account for about half the volume of the
nervous system.

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Myelination
 Process by which part of an axon is
wrapped with a myelin sheath
 Forms a protective fatty coating
 Has a glossy-white appearance.
 The myelin sheath:
 supports the axon
 protects the axon
 insulates an axon

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Myelination
 No change in voltage can occur across the
membrane in the insulated portion of an axon.
 Voltage change occurs at the nodes
 Neurolemmocytes: form myelin sheaths in PNS
 Oligodendrocytes: form myelin sheaths in the CNS

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Mylenated vs. Unmylenated
Axons
 myelinated axon
 nerve impulse “jumps” from neurofibril node to neurofibril
node
 known as saltatory conduction
 requires less energy (ATP) than does an unmyelinated axon
 unmyelinated axon
 nerve impulse must travel the entire length of the axon
 known as continuous conduction
 nerve impulse takes longer to reach the end of the axon
 Using continuous conduction, unmyelinated axons conduct
nerve impulses from pain stimuli
 A myelinated axon produces a faster nerve impulse.

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Structure of a Nerve
 A nerve is a cable-like bundle of parallel axons.
 three connective tissue wrappings
 Endoneurium

delicate layer of loose connective tissue
 Perineurium

a cellular and fibrous connective tissue layer

wraps groups of axons into fascicles
 Epineurium - a superficial connective tissue covering

This thick layer of dense irregular fibrous connective tissue

encloses entire nerve

provides support and protection

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Nerves
 Nerves are organs of the PNS.
 Sensory (afferent) nerves convey sensory information
to the CNS.
 Motor (efferent) nerves convey motor impulses from
the CNS to the muscles and glands.
 Mixed nerves: both sensory and motor
 Axons terminate as they contact other neurons,
muscle cells, or gland cells.
 An axon transmits a nerve impulse at a specialized
junction with another neuron called synapse.

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Synapses
 Presynaptic neurons
 transmit nerve impulses toward a synapse.
 Postsynaptic neurons
 conduct nerve impulses away from the synapse.
 Axons may establish synaptic contacts with
any portion of the surface of another neuron
 except those regions that are myelinated.

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Types of synapses: based on contacts
 axodendritic
 axosomatic
 axoaxonic

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Main types of synapses
 Electrical synapses
 Gap junctions
 Chemical synapses
 Use neurotransmitters

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Electrical Synapses
 Electrical synapses are not very common in
mammals.
 In humans, these synapses occur primarily between
smooth muscle cells where quick, uniform innervation
is essential.
 Electrical synapses are also located in cardiac
muscle.

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Chemical Synapses
 Most numerous type of synapse
 Facilitates interactions
 between neurons
 between neurons and effectors.
 These are cell junctions
 Presynaptic membrane:
 releases a signaling molecule called a neurotransmitter,
such as acetylcholine (ACh).
 Other types of neurons use other neurotransmitters.
 Postsynaptic membrane:
 Contains receptors for neurotransmitters

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The Spinal Cord
 Link between the brain and the body.
 Exhibits some functional independence from
the brain.
 The spinal cord and spinal nerves serve two
functions:
 pathway for sensory and motor impulses
 responsible for reflexes

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Structure of the Spinal Cord
 Typical adult spinal cord
 ranges between 42 and 45 centimeters (cm) (16 to
18 inches) in length.
 In cross section
 roughly cylindrical
 slightly flattened both posteriorly and anteriorly.
 External surface has two longitudinal
depressions:
 the posterior (dorsal) median sulcus
 the anterior (ventral) median fissure

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Regions of the Spinal Cord
 The cervical region
 continuous with the medulla oblongata
 contains neurons whose axons form the cervical spinal
nerves (8)
 The thoracic region
 attached to this region are the thoracic spinal nerves (12)
 The lumbar region
 contains the neurons for the lumbar spinal nerves (5)
 The sacral region
 contains the neurons for the sacral spinal nerves (5)
 The coccygeal region
 one pair of coccygeal spinal nerves arises from this region

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Structure of the Spinal Cord
 The spinal cord is shorter than the vertebral canal
that houses it.
 Conus medullaris:
 tapered inferior end of the spinal cord
 marks the official “end” of the spinal cord proper.
 Cauda equina
 Inferior to conus medularis
 nerve roots (groups of axons) that project inferiorly from the
spinal cord.
 Filum terminale
 Within the cauda equina
 thin strand of pia mater
 helps anchor the conus medullaris to the coccyx.

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Structure of the Spinal Cord
 The spinal cord is associated with 31 pairs of spinal
nerves
 Connect the CNS to:
 receptors
 effectors (muscle and glands)
 Each side of the spinal cord contains:
 8 cervical nerves (called C1–C8)
 12 thoracic nerves (T1–T12)
 5 lumbar nerves (L1–L5)
 5 sacral nerves (S1–S5)
 1 coccygeal nerve (Co1)

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Arrangement and Functions of
the Spinal Meninges
 Are continuous with the cranial meninges.
 Structures that encircle the spinal cord, listed from
superficial to deep are:
 vertebra
 epidural space
 dura mater
 subdural space
 arachnoid
 subarachnoid space
 pia mater

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Location and Distribution of
Gray Matter
 In the spinal cord, it is centrally located.
 Its shape resembles a letter H or a butterfly.
 The gray matter may be subdivided into the following
components:
 anterior horns
 lateral horns
 posterior horns
 the gray commissure

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Location and Distribution of
White Matter
 The white matter of the spinal cord is
external to the gray matter.
 Three regions.
 Composed of tracts

Ascending

Descending
 A posterior funiculus:

lies between the posterior gray horns and the
posterior median sulcus.

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Location and Distribution of
White Matter
 Lateral funiculus.
 Anterior funiculus
 between the anterior gray horns and the
anterior median fissure.
 The anterior funiculi are interconnected
by the white commissure.

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 Schematic representation of
transverse section of spinal
cord.
 I. Cornu posterius.
 II.Cornu laterale.
 III Cornu anterius.
 1. Zоna spongiosa.
 2. Substantia gelatinosa.
 3. Nucleus proprius.
 4.Nucleus thoracicus (Stilling-
Clarce)
 5. Nucleus intermediomedialis.
 6. Nucleus intermediolateralis.
 7. Nucleus posterolateralis.
 8. Nucleus anterolateralis.
 9. Nucleus centralis.
 10. Nucleus anteromedialis.
 11. Nucleus posteromedialis.

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Spinal Cord Development
 The central nervous system forms from the
embryonic neural tube.
 Cranial and spinal nerves form from neural crest cells
that have split off from the developing neural tube.
 The cranial (superior) part of the neural tube expands
and develops into the brain.
 The caudal (inferior) part of the neural tube forms the
spinal cord.

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Reflexes
 A reflex is a response:
 Rapid, automatic
 involuntary reactions of effectors to a stimulus.
 Properties.
 a stimulus

required to initiate a response to sensory input
 a rapid response

requires that few neurons be involved

synaptic delay be minimal
 an automatic response occurs the same way every time
 An involuntary response requires no intent or pre-awareness of
the reflex activity.
 Reflexes usually can not be suppressed.
 Awareness of the stimulus occurs after the reflex action
 in time to correct or avoid a potentially dangerous situation.

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 Schematic
representation of a
simple reflex arch.
 1. Cornu posterius.
 2. Cornu anterius.
 3. Radix posterior.
 4. Radix anterior.
 5. Ganglion spinale.
 6. Nervus spinale.
 7. Corpus neurale
afferentes.
 8. Dendritum.
 9. Axon neurale
afferentes.
 10. Interneuron.
 11. Corpus neurale
efferentes.
 12. Axon neurale
efferentes.
 13. Receptor.
 14. Musculus.

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Components of a Reflex Arc
 The neural “wiring” of a single reflex.
 Always begins at a receptor in the PNS
 Sensory afferent
 Communicates with the CNS.
 May involve interneurons
 Ends at a peripheral effector (muscle or
gland)
 Motor efferent

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Ipsilateral and Contralateral
Reflex Arcs
 Ipsilateral:
 both the receptor and effector organs of the reflex
are on the same side of the spinal cord.
 Contralateral
 the sensory impulses from a receptor organ cross
over through the spinal cord to activate effector
organs in the opposite side

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Monosynaptic Reflexes
 The simplest of all reflexes.
 No interneurons.
 The patellar (knee-jerk) reflex is a
monosynaptic reflex
 physicians use to assess the functioning of the
spinal cord.
 tap the patellar ligament with a reflex hammer
 muscle spindles in the quadriceps muscles are
stretched.
 Produces a noticeable kick of the leg.

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Polysynaptic Reflexes
 Have more complex neural pathways
 exhibit a number of synapses
 involve interneurons within the reflex arc.
 Has more components
 more prolonged delay between stimulus and response.

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Stretch Reflexes
 Monosynaptic reflex that monitors and regulates
skeletal muscle length.
 When a stimulus results in the stretching of a muscle,
that muscle reflexively contracts.
 The patellar (knee-jerk) reflex is an example of a
stretch reflex.
 The stimulus (the tap on the patellar tendon) initiates
contraction of the quadriceps femoris muscle and
extension of the knee joint.

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Golgi Tendon Reflex
 Prevents skeletal muscles from tensing excessively.
 Golgi tendon organs are nerve endings located within
tendons near a muscle–tendon junction.
 activation of the Golgi tendon organ signal interneurons in
the spinal cord, which in turn inhibit the actions of the motor
neurons
 The associated muscle is allowed to relax, thus
protecting the muscle and tendon from excessive
tension damage.

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Reflex Testing in a Clinical
Setting
 Reflexes can be used to test specific muscle groups
and specific spinal nerves or segments of the spinal
cord.
 Consistently abnormal reflex response may indicate
damage to the nervous system or muscles.
 A reflex response may be normal, hypoactive, or
hyperactive.

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Thank you for attention!