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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!