HRIDYA.A

I. MSc.

BIOTECHNOLOGY

NERVOUS SYSTEM

Major regulatory system

Highly organized network of billions of neurons

and neuroglia

Brain, cranial nerves, and their branches,

ganglia, enteric plexuses and sensory receptors

NERVOUS TISSUE

Consists of 2 types of cells: Neurons and

Neuroglia or glial cells

Sensing

Thinking

Remembering

Controlling Muscle activity

Regulating glandular secretions etc.,

Neuroglia support nourish and protect the

interstitial fluid that bathes them

NEURONS

Basic structural and functional unit of

Nervous system

CNS consists of large number of neurons in

brain and spinal cord

PNS includes all neurons outside the CNS

CHARACTERISTICS OF NEURON

Ability to respond to a stimulus and convert it into

an action potential

Stimulus: any change in the environment which

can initiate an action potential

Action potential(nerve impulse): an electrical

signal that propagate along the neuron

Some neurons are tiny and propagate impulses a

short distance & some are longest cells in the body.

Given good nutrition, neurons can function

optimally for a life time (over 100 years)

Do not divide

Can’t be replaced if destroyed

Exceptions: olfactory epithelium & some

hippocampal regions contain stem cells.

Require continuous and abundant supply

of oxygen and glucose

STRUCTURE OF A NEURON

Typical neuron contains 4 parts

1. Cell Body

2. Dendrites

3. Axon

4. Axon terminals

Processes

Also called Perikaryon or Soma.

5 to 140µm in diameter

Major biosynthetic centre of

neuron

Centrioles absent(amitotic)

CONSISTS OF:

Nucleus with a nucleolus

Nissl bodies or chromatophilic

substance:

Rough ER and clustered free

ribosomes (Named after Franz

Nissl, who invented the Nissl

staining method)

Network of Neurofibrils and microtubules important in

maintaining cell shape and integrity

Lipofuscin; golden brown pigment inclusion, harmless by-

product of lysosomal activity.

Aging pigment because accumulate in

neurons of elderly individuals

Most cell bodies are located in CNS

Cluster of cell bodies in

CNS – Nuclei

Ganglia in PNS

LIPOFUSCIN

Bundle of neuron processes

are called in CNS &

in PNS

Dendrites and axon differ in

the structure and function of

plasma membrane

Short, tapering diffusely branching extensions

All the organelles in cell body are present

Main receptive or input regions

Provide an enormous surface area for receiving

signals from other neurons

Dendritic spines: thorny appendages having

bulbous or spiky ends

Convey incoming messages towards cell body

Graded Potentials (short distance signals)

Axon hillock- initial cone shaped area

Any long axon is called a Nerve fibre

Branches – Axon Collaterals

Axon Terminals : Knob like distal endings of Terminal branches or Telodendria

10,000 or more

Synaptic knob or boutons

(= buttons)

Conducting region – generates and transmits

nerve impulses away from cell body

Impulse generated at axon hillock

Conduct to terminals

Neurotransmitters are released

Signal transmitted

Same organelles in cell body are seen

Lack Nissl bodies and membrane components

& Golgi apparatus

AXONAL

TRANSPORT

Cytoskeletal elements help

axonal transport

Anterograde – Mitochondria,

enzymes for synthesis of

neurotransmitters

Retrograde – Organelles for

degradation, neurotrophic

survival signal

ATP-dependant Motor proteins

aid the movt.

Whitish, fatty (protein-lipoid), segmented structure

Protects & electrically insulates axon and speed of transmission

Dendrites – Unmyelinated

In PNS, they are formed by Schwann cells

• Schwann cell envelops axon.

• It then rotates around the axon, enveloping the axon loosely in successive layers of its plasma membrane.

• Schwann cell cytoplasm is forced from between the membranes and comes to lie peripherally just beneath the exposed portion of the Schwann cell plasma membrane.

• The tight membrane wrappings surrounding the axon form the myelin sheath; the area of Schwann cell cytoplasm and its exposed membrane is referred to as neurilemma.

Neurilemma: Portion of exposed part of

the plasma membrane of Schwann cells

Contain nucleus & most of cytoplasm

Nodes of Ranveir or Neurofibril Nodes:

Gaps between 2 adjacent Schwann cells

Axon collaterals arise from here

Oligodendrocytes forms CNS myelin

sheath, each of which forms only 1

segment (internode) of myelin sheath

WHITE MATTER & GRAY MATTER

Regions of brain and spinal cord containing

dense collections of Myelinated fibers are

called White matter

Gray matter contains mostly nerve cell

bodies and Unmyelinated fibers

1. Structural classification

2. Functional classification

Multipolar

Bipolar

Unipolar

Have 3 or more processes

Major neuron type in CNS

An axon and others are dendrites

Have 2 processes- An axon &

a dendrite that extend from

cell body oppositely

Rare neurons

Found in special sense

organs (e.g., those in retina of

eye and in olfactory mucosa)

Do not generate action

potentials

Have a single short process that emerges from cell

body and divides T-like into proximal and distal

branches

Distal sensory

receptor

Proximal CNS

Pseudounipolar neuron

Found in ganglia in PNS

Sensory (afferent) neuron

Motor (efferent) neuron

Interneuron

Transmits impulse from sensory receptors into

CNS

Most of them are Unipolar

Cell bodies located in sensory ganglia outside

CNS

Act as impulse receptor sites

Peripheral processes are very long

E.g., Fibers carrying sensory impulses from skin of great toe

Carry impulses away from CNS to effector organs

Multipolar

o Association neurons

o Function as Integrators and signal changers

o Confined in CNS

o 99% of neurons in body

o Multipolar

Autoimmune disease

Antibodies and WBC attack

myelin causing inflammation &

injury to the sheath and nerves

Leads to leakage of K+ through

channels, Hyper polarization and

failure to conduct action

potentials

Muscle weakness, fatigue,

diminished coordination, slurred

speech, blurred vision, bladder

dysfunction etc.,

SYNAPSES

TYPES OF SYNAPSES

Chemical Synapse

Electrical Synapse

Gap junctions

Direct open channels conduct electrical signal

Required in quick responses

Bidirectional flow of materials

Neurotransmitters are released to synaptic cleft which

cause excitation or inhibition of the post synaptic

neuron.

Acetylcholine, norepinephrin, histamine, gamma-

amino butyric acid (GABA), glycine, serotonine, and

glutamate

Ganong’s Review of Medical Physiology

Kim E Barrett, Susan M Barman, et al

Vander’ et al’s Human Physiology

Eric P Widmair, Hershel Raff, Kevin T Strong

Human Anatomy and Physiology

Elaine N Marieb, Katja Hoehn

Textbook of Medical Physiology- 11th edition

Guyton and Hall