Development Of Nervous SystemPrepared by:Mohammed Abdul GaderAdamAzhari

CONTENT

• Introduction• Central Nervous

System• Peripheral Nervous

System• Conclusion

How does this happen ?o Many mechanisms of human brain development remain

hidden, buto Neuroscientists are beginning to uncover some of

these complex steps through studies of o the roundworm, fruit fly, frog, zebrafish, mouse, rat,

chicken, cat and monkey.o Many initial steps in brain development are similar

across species, while later steps are different.o By studying these similarities and differences, we can

o learn how the human brain develops and o how brain abnormalities, such as mental retardation

and other brain disorders, can be prevented or treated.

Introduction• Nervous system (NS) is the most complex but by body weight is one of the smallest organ systems.

• Neural development is one of the earliest systems to begin and the last to be completed after birth

Introduction• Along with the endocrine system the NS

controls and adjusts activities of all other systems

• Shared characteristics:- chemical communication with target tissues- NS has swift but brief responses- endocrine system is slower but responses may last much longer.

The Nervous System

Development from Embryo

• The embryo has three primary layers that undergo many interactions in order to evolve into organ, bone, muscle, skin or neural tissue. – The outside layer is the ectoderm

• (skin, neural tissue),

– the middle layer is the mesoderm • (skeleton, cardiac) and

– inner layer is the endoderm • (digestion, respiratory).

• The nervous system develops from embryonic tissue called the ectoderm

• The first sign of the developing nervous The first sign of the developing nervous system is the neural plate that can be system is the neural plate that can be seen at about the 16th day of developmentseen at about the 16th day of development

Early development• release of special chemicals, which induce ectorderm to release of special chemicals, which induce ectorderm to

divide more rapidly, forming a thickened mass called the divide more rapidly, forming a thickened mass called the neural plateneural plate

• Over the next few days, a “channel” is formed in the Over the next few days, a “channel” is formed in the neural plate - this creates a neural groove.neural plate - this creates a neural groove.

The neural groove continues to The neural groove continues to deependeepen

By the 21st day of development, By the 21st day of development, a neural tube is formed when a neural tube is formed when the edges of the neural groove the edges of the neural groove meet.meet.

Early development

• The top of the tube thickens into three bulges that form the hindbrain, midbrain and forebrain.

• The first signs of the eyes and then the hemispheres of the brain appear later.

Neural Tube formation

development

• During brain development, 250,000 neurons are added every minute

• At birth, almost all the neurons that the brain will ever have are present.

• The brain continues to grow for a few years after birth. By the age of 2 years old, the brain is about 80% of the adult size.

Neural cell categories• After the ectodermal tissue has acquired its

neural fate,• another series of signaling interactions

determine the type of neural cell to which it gives rise.

• The mature nervous system contains a vast array of cell types, which can be divided into two main categories: – the neurons, primarily responsible for signaling, – and supporting cells called glial cells.

A Review of Neuron Structure

Relationship of the 4 parts of a neuron (dendrites, cell body, axon, and synaptic terminals); - the functional activities of each part - the normal direction of action potential conduction

Neuron Classification

Anaxonic neurons

Bipolar neurons

Pseudounipolar neurons

Multipolar neurons

Structural Classification of Neurons

Neurodevelopmental processes• can be broadly divided into two classes:1. activity-independent mechanisms 2. activity-dependent mechanisms• Activity-independent mechanisms are

generally believed to occur as hardwired processes determined by genetic programs played out within individual neurons. These include differentiation, migration and axon guidance to their initial target areas.

• These processes are thought of as being independent of neural activity and sensory experience. Once axons reach their target areas, activity-dependent mechanisms come into play.

• Neural activity and sensory experience will mediate formation of new synapses, as well as synaptic plasticity.

Central nervous system Peripheral nervous system

•brain and spinal cord.•integrating, processing, and coordinating.

•neural tissue outside the CNS.•provides sensory information to the CNS.•carries motor commands to peripheral tissues.

• NS includes all of the neural tissue in the body.

Two anatomical subdivisions: CNS and PNS

Peripheral Nervous System

The PNS is subdivided into 2 divisions:

• Afferent division brings sensory information to the CNS

• Efferent division carries motor commands to muscles and glands- further divided into the somatic nervous system (SNS) and the autonomic nervous system (ANS)

Afferent Division

• Begins at receptors that monitor specific characteristics of the environment and include:- dendrites, sensory processes of a neuron- a specialized cell or cluster of cells- complex sense organs – eye, nose, tongue, and ear

• Carries information from somatic and visceral sensory receptors – somatic include skeletal muscle, joints, and the skin- visceral include smooth and cardiac muscle and glands

The Efferent Division

• Begins inside the CNS and ends at an effector- a muscle cell, gland cell, or other specialized cell

• Efferent division includes the SNS and ANS

• SNS – controls skeletal muscle contraction- voluntary under conscious control- involuntary directed outside your control

• ANS or visceral motor system, regulates smooth and cardiac muscle, and glandular activity

Neuron

• Representative neuron has 4 main parts:- cell body or soma with a perikaryon, region around the nucleus- dendrites, branch from the soma and each branch has fine processes called dendritic spines- axon, elongated process attached to the soma- synaptic terminals, end of the axon that communicates with another cell

• The soma contains the organelles- energy production and biosynthesis of organic molecules

Neuroglia• Have many functions, including:

- provide framework for the neural tissue- maintain the intercellular environment- act as phagocytes

• 4 types: astrocytes, oligodendrocytes, microglia, and ependymal cells- distinguished on basis of size, intracellular organization, and presence of specific cytoplasmic processess

• 100 billion neuroglia, or glial cells- about 5 times the number of neurons

Fig 13.4 The Classification of Neuroglia

Astrocytes

• Largest and most numerous glial cells

• Variety of functions include:- Controlling the interstitial environment- Maintaining the blood-brain barrier- Creating a 3-dimensional framework for the CNS- Performing repairs in damaged neural tissue- Guiding neuron development

Fig 13.5 Histology of Neural Tissue in the Spinal Cord of the CNS

Neuroglia of the PNS• PNS neuron cell bodies are usually clustered

together in masses called ganglia (sing. ganglion)

• Axons are bundled together and wrapped in CT, forming peripheral nerves, or nerves

• Neuron cell bodies and axons are insulated from their surroundings by processes of glial cells:- satellite cells surround cell bodies in peripheral ganglia - every peripheral axon (unmyelinated or myelinated) is covered by Schwann cells or neurolemmocytes - plasmalemma of an axon is called axolemma and the superficial covering by Schwann cells is the neurilemma

a) a single Schwann cell forms the myelin sheath around a portion of a single axon

b) a single Schwann cell can encircle several unmyelinated axons

Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig 13.8

The Nerve Impulse• Excitability is the ability of a plasmalemma to

conduct electrical impulses

• An electrical impulse, or action potential, develops after the plasmalemma is stimulated to its threshold

• Nerve impulse is an action potential traveling along an axon

• The rate of impulse conduction depends on the properties of the axon, specifically:- presence or absence of myelin sheath- the diameter

Synaptic Communication

Conclusion• The brain is the part of the CNS which

regulate ,control and monitoring all functions of the body.

• CN(12-13) emerge directly from the brain stem.

• Spinal cord with its 31pairs of spinal nerves serves 2 important functions

A- link between the brain and most of the body.

B- it is involved in the spinal reflex actions , both somatic and visceral .

• the cranial nerves serve the sensory and motor nerve system of the head and neck region.

• the CNS is a mirror to reflect many diseases that occur in the body.

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