1 The Department of Human anatomy The Nervous System
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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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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!