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Nervous System

.IOverview.IIHistology

.IIIElectrical Signals.IVSignal Transmission at Synapses

.VNeurotransmitters.VINeural Circuits

.VIIRepairs.VIIIPathology

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Nervous Tissue• Controls and integrates

all body activities within limits that maintain life

• Three basic functions

1. sensing changes with sensory receptors

2. interpreting and remembering those changes

3. reacting to those changes with effectors

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Major Structures of the Nervous System

• Brain

• spinal cord

• cranial nerves

• spinal nerves

• ganglia

• enteric plexuses

• sensory receptors

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Subdivisions of the PNS

1. Central nervous system (CNS)

2. Peripheral nervous system (PNS)

a) Somatic (voluntary) nervous system (SNS)

b) Autonomic (involuntary) nervous systems (ANS)

c) Enteric nervous system (ENS)

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OrganizationIntegration MotorSensory

SNS(Sensory)

ANS(Sensory)

Brain

Spinal cord

SNS(Motor)

ANS(Motor)

ENS(Sensory)

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Neurons• Functional unit of nervous

system

1. Cell body

a) Nissl bodies (rER)

b) Neurofilaments

c) Microtubules

d) Lipofuscin pigment clumps

2. Cell processes a) Dendrites b) Axons

Electron MicroscopyLight-Stained Purkinje Neurons

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Gray and White Matter• White matter = myelinated processes (white in color)

• Gray matter = nerve cell bodies, dendrites, axon terminals, bundles of unmyelinated axons and neuroglia (gray color)

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Dendrites• Conducts Impulses

Towards The Cell Body

• Typically short, highly branched & unmyelinated

• Surfaces specialized for contact with other neurons

• Contains neurofibrils & Nissl bodies (rER)

impulse

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Axons• Conduct impulses Away From

Cell Body

• Long, thin cylindrical process of cell

• Arises at Axon Hillock

• Impulses arise from initial segment (Trigger Zone)

• Side branches (collaterals) end in fine processes called Axon Terminals

• Swollen tips called synaptic end bulbs contain vesicles filled with Neurotransmitters

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Structural Classification of Neurons• Based on number of processes

found on cell body

1. Multipolar = several dendrites & one axon

• most common cell type

2. Bipolar Neurons = one main dendrite & one axon

• found in retina, inner ear & olfactory

3. Unipolar Neurons = one process only(develops from a bipolar)

• are always sensory neurons

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Structural Classification of Neurons• Based on number of processes

found on cell body

1. multipolar = several dendrites & one axon

• most common cell type

2. bipolar neurons = one main dendrite & one axon

• found in retina, inner ear & olfactory

3. unipolar neurons = one process only(develops from a bipolar)

• are always sensory neurons

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Structural Classification of Neurons• Based on number of processes

found on cell body

1. Multipolar Neurons= several dendrites & one axon

• most common cell type; Motor Neuron

2. Bipolar Neurons = one main dendrite & one axon

• found in retina, inner ear & olfactory (Special Senses)

3. Unipolar Neurons = one process only(develops from a bipolar)

• are always Sensory Neurons

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Neuroglial Cells• Half of the volume of the CNS

• Smaller cells than neurons

• Cells Can Divide

– rapid mitosis in tumor formation (Gliomas)

• 4 cell types in CNS

– astrocytes, oligodendrocytes, microglia & ependymal

• 2 cell types in PNS

– schwann and satellite cells

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Neuroglial Cells (CNS): Astrocytes• Star-shaped cells

• Form Blood-brain BarrierForms Perivascular Feet by covering blood capillaries

• Metabolize neurotransmitters

• Regulate K+ balance

• Provide structural support

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Neuroglial Cells (CNS): Oligodendrocytes

• Most common glial cell type

• Each forms Myelin Sheatharound more than one axons in CNS

• Analogous to Schwann cells of PNS

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Neuroglial Cells (CNS): Microglia• Small cells found near

blood vessels

• Phagocytic Cell –

• Derived macrophages & monocytes

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Neuroglial Cells (CNS): Ependymalcells

• Form epithelial membrane lining cerebral cavities &

central canal

• Produce Cerebrospinal Fluid (CSF)

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Neuroglial Cells (PNS): Satellite Cells• Flat cells surrounding

neuronal cell bodies in peripheral ganglia

• Support Neurons In The PNS Ganglia

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Neuroglial Cells (PNS): Schwann Cell

• Cells encircling PNS axons

• Each cell produces part of the myelin sheath surrounding an axon in the PNS

Myelination

• Insulation of axon

• Increase speed of nerve impulse

Myelination: PNS• All axons surrounded by a

lipid & protein covering (Myelin Sheath) produced by Schwann cells

• Neurilemma is fused layers of membranes of Schwann cell

– gaps called nodes of Ranvier

• Myelinated fibers

• Unmyelinated fibers

Node of Ranvier

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Myelination: CNS• Oligodendrocytes

myelinate axons in the CNS

• Broad, flat cell processes wrap about CNS axons. One Oligo Can MyelinateUpto 50 Nerves.

• Schwan Cell: PNS

• Many Schwan Cells Myelinate One Peripheral Nerve

• End of one SchwanSegment Node of Ranvier

Myelination Process

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Electrical Signals in Neurons

• Neurons are electrically excitable due to the voltagedifference across their membrane

• Communicate with 2 types of electric signals

1. action potentials that can travel long distances

2. graded potentials that are local membrane changes only

• In living cells, a flow of ions occurs Through Ion Channels In The Cell Membrane

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Continuous versus Saltatory Conduction1. Continuous

conduction (unmyelinatedfibers)

2. Saltatory conduction (myelinated fibers)

A.P.Na

Na

Na

Na

Na

Na

Na

Na

Na

Na

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Saltatory Conduction• Nerve impulse conduction

in which the impulse Jumps (Salta) from node to node

A.P.Na

Na

Na

Na

Na

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Signal Transmission at Synapses

• 2 Types of synapses1. Electrical

• ionic current spreads to next cell through gap junctions

• faster, two-way transmission & capable of synchronizing groups of neurons

2. Chemical• one-way information transfer from a presynaptic

neuron to a postsynaptic neuron– axodendritic -- from axon to dendrite– axosomatic -- from axon to cell body– axoaxonic -- from axon to axon

Synapses

Spinal Ganglia

Cell bodies Lie on posterior Nerve Root of Spinal Cord.

• Have Sensory Neurons (Unipolar).

• Widely Apart

• Sympathetic Ganglia

• Multipolar Type: Hence Many Dendrites Hence Widely Apart.