The Nervous System | Nervous Tissue | Anatomy ppt

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The Nervous System | Nervous Tissue | Anatomy ppt

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The Nervous System | Nervous Tissue | Anatomy ppt

Human Anatomy, First Edition
McKinley & O’Loughlin

Chapter 14 :

Nervous Tissue

The Nervous System

The body’s primary communication and control system.

Can be divided according to:

Structural categories

Functional categories.

 

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)

Nervous System: Functional Organization

Functional divisions of the nervous system:

Sensory afferent division:

receives sensory information (input) from receptors

transmits this information to the CNS.

Motor efferent division:

transmits motor impulses (output) from the CNS

to muscles or glands (effector organs).

 

Sensory Division: two components

Somatic sensory components:

General somatic senses:

touch

pain

pressure

vibration,

temperature

proprioception.

Special senses:

Taste

Vision

Hearing

Balance

smell

 

Sensory Division: two components

 

Visceral sensory components

transmit nerve impulses from blood vessels and viscera to the CNS

visceral senses primarily include:

temperature

stretch (of the organ wall).

Motor Division: two components

The somatic motor component (somatic nervous system; SNS):

conducts nerve impulses from the CNS to skeletal muscles

also known as the voluntary nervous system

The autonomic motor component (autonomic nervous system; ANS): internal organs, regulates smooth muscle, cardiac muscle, and glands.

Innervates

Internal organs

Regulates smooth muscle

Regulates cardiac muscle

Regulates glands

also known as the visceral motor system or involuntary nervous system

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

Characteristics of Neurons

Neurons have a high metabolic rate.

Neurons have extreme longevity.

Neurons typically are non-mitotic.

 

Neuron Structure

Neurons come in all shapes and sizes

All neurons share certain basic structural features.

typical neuron:

Cell body (soma, perikaryon)

Dendrites

Axon

Collaterals: branches

axon terminals or telodendria

Synaptic knobs

 

Neuron Structure – Cell Body

The cell body (perikaryon, soma)

the neuron’s control center

responsible for:

receiving

integrating

sending nerve impulses.

Consists of:

Plasma membrane

Cytoplasm

Nucleus with prominent nucleolus

Chromatophobic substance (Nissil bodies): RER

Free ribosomes

 

 

 

Neuron Structure – Dendrites

Shorter, smaller processes

Branch off the cell body.

Some neurons have only one dendrite, while others have many.

Dendrites conduct nerve impulses toward the cell body

they receive input

transfer input to the cell body for processing.

The more dendrites a neuron has, the more nerve impulses that neuron can receive from other cells.

 

Neuron Structure – Axon

larger, typically longer nerve cell process

Extend from the cell body

Axon hillock

also called a nerve fiber

Most neurons have only one axon.

Anaxonic

 

Neuron Structure – Axon

Structures

Collaterals

Telodendria (axon terminals)

Synaptic knobs (terminal boutons)

The axon transmits a nerve impulse away from the cell body toward another cell.

 

 

Neuron Structure

Cytoskeleton

Neurotubules

microtubules

Neurofilaments

Intermediate fibers

Neurofibrils

Bundles of neurofibrils

In both dendrites and axons

Provide strength

 

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

 

 

Functional Classification

Sensory afferent neurons: receptor to CNS

Motor efferent neurons: CNS to effector

Interneurons (association neurons): facilitate communication between sensory and motor neurons.

Interneurons

Interneurons, or association neurons

lie entirely within the CNS

multipolar.

They receive nerve impulses from many other neurons

They carry out the integrative function of the nervous system.

Interneurons facilitate communication between sensory and motor neurons.

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.

Glial Cells of the CNS: astrocytes

Exhibit a starlike shape due to projections from their surface.

The most abundant glial cells in the CNS

constitute over 90% of the tissue in some areas of the brain.

Help form the blood-brain barrier (BBB):

strictly controls substances entering the nervous tissue in the brain from the bloodstream.

Regulate tissue fluid composition.

Provide structural support

Replace damaged neurons

Assist neuronal development

 

 

Glial Cells of the CNS: ependymal cells

Cuboid ET

Cilia on apical surface

Circulates CSF.

Line internal cavities

Processes make contact with other glial cells

Help form the choroid plexus

CSF: cerebral spinal fluid

 

Glial Cells of the CNS: microglia

Smallest % of CNS glial cells.

Phagocytic

Move through the tissue in response to infection

Remove debris.

Like macrophages

 

Glial Cells of the CNS: oligodendrocytes

Large, with big body and processes.

Processes form myelin sheaths

Speeds up transmission

 

Glial Cells of the PNS

Satellite cells:

Flattened cells

Cover somas in ganglia

Separate soma from surrounding tissue fluid

Regulate exchange.

Neurolemmocytes (Schwann cells)

Myelination in the PNS

 

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

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

 

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.

Regeneration of PNS Axons

PNS axons are vulnerable to cuts and trauma.

A damaged axon can regenerate

if some neurilemma remains.

PNS axon regeneration depends upon three factors.

amount of damage

neurolemmocyte secretion of nerve growth factors

stimulates outgrowth of severed axons

distance between the site of the damaged axon and the effector organ

Regeneration of PNS Axons

Wallerian degeneration.

Axon damaged

Proximal end seals, and swells.

Distal end degenerates, macrophages clean up

Distal neurolemmocytes survive

Neurolemmocytes form regeneration tube (with endoneurinum)

Axon regenerates, remyelinates

Axon reestablishes contact with effector

 

 

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

 

 

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.

 

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.

 

Types of synapses: based on contacts

axodendritic

axosomatic

axoaxonic

Main types of synapses

Electrical synapses

Gap junctions

Chemical synapses

Use neurotransmitters

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.

 

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

 

Neurotransmitters

Released from the plasma membrane of the presynaptic cell.

Then binds to receptor proteins on the plasma membrane of the postsynaptic cell.

A unidirectional flow of information and communication

Two factors influence the rate of conduction of the impulse:

axon’s diameter

presence (or absence) of a myelin sheath.

 

Neuronal Pools (or Neuronal Circuits or Pathways)

Billions of interneurons within the CNS are grouped in complex patterns called neuronal pools (or neuronal circuits or pathways).

Neuronal pools are defined based upon function, not anatomy, into four types of circuits:

converging

diverging

reverberating

parallel-after-discharge

A pool may be localized, or its neurons may be distributed in several different regions of the CNS.

 

 

 The Nervous System | Nervous Tissue | Anatomy ppt

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