Present by: M.Nafly

HND in Biomedical scientist

Present by: M.Nafly

HND in Biomedical scientist

* Nafly Hussain *

Neurons The human body is made up of trillions of cells.

Cells of the nervous system, called nerve cells or neurons.

The human brain has approximately 100 billion neurons, more in the spinal cord, peripheral nervous system and sensory organs.

These system’s basic building blocks, are specialized to carry "messages" through an electrochemical process.

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Neurons come in many different shapes and sizes, can be quite large such as corticospinal neurons (from motor cortex to spinal cord) or primary afferent neurons (neurons that extend from the skin into the spinal cord and up to the brain stem), can be several feet long

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Neurons are similar to other cells in the body because,,

• Neurons are surrounded by a cell membrane.

• Neurons have a nucleus that contains genes.

• Neurons contain cytoplasm, mitochondria and other organelles.

• Neurons carry out basic cellular processes such as protein synthesis and energy production.

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Neurons are different from other cells in the body because,,

• Neurons have specialized extensions called dendrites and axons. Dendrites bring information to the cell body and axons take information away from the cell body.

• Neurons communicate with each other through an electrochemical process.

• Neurons contain some specialized structures (for example, synapses) and chemicals (for example, neurotransmitters).

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STRUCTURE OF A NEURON

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Axon of anotherneuron

Axon of anotherneuron

Cell BodyCell BodyDendritesDendrites

AxonAxon

MyelinSheathMyelinSheath

Dendrites of another neuron

Dendrites of another neuron

The NeuronNode of RanvierNode of Ranvier

Synaptic knobs

Synaptic knobs

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• Synapse- space between the axon of one neuron and the dendrite of another cell (they do not touch)

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• Dendrites are short branching processes from the cell body

• Axon is a projection from the cell body and it is few millimeters to one meter in length & covered

with the myelin sheath

• Myelin have an insulating sheath

Supports, insulates & nourishes the axon and helps maintain chemical balance

• Nodes of Ranvier allow passage of the electrical signal through ion channels which

helps the transmission very fast. • The cell body is responsible for the nutrition

and maintenance of the entire cell. * Nafly Hussain *

Inside of a neuron!

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CLASSIFICATION OF NEURONS

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• On of the basis of structural On of the basis of structural differences, neurons are classified differences, neurons are classified into the following three major into the following three major groups:groups:

1.1. Bipolar NeuronsBipolar Neurons

2.2. Unipolar NeuronsUnipolar Neurons

3.3. Multipolar NeuronsMultipolar Neurons* Nafly Hussain *

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• Bipolar

– oval shaped soma

– 2 processes: dendrite (info in) & axon (info out)

– most sensory neurons are bipolar

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Bipolar neurons have two processes extending from the cell body(examples: retinal cells, olfactory epithelium cells).

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• Unipolar– Sensory cells of touch, pressure, pain are special types of unipolar

– 1st developed as bipolar; the 2nd processes fuse to form another axon

– axon splits at the cell body– one goes to spinal cord, other to periphery (skin, joints, muscles)

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Pseudounipolar cells (example: dorsal root ganglion cells). Actually, these cells have 2 axons rather than an axon and dendrite. One axon extends centrally toward the spinal cord, the other axon extends toward the skin or muscle.* Nafly Hussain *

• Multipolar– most common type in vertebrates

– one axon and one or more dendrite

– vary in size and shape

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Multipolar neurons have many processes that extend from the cell body. However, each neuron has only one axon (examples: spinal motor neurons, pyramidal neurons, Purkinje cells). * Nafly Hussain *

Classification Based on Dendrites

• Pyramidal cells/stellate cells• Spiny cells/aspinous cells

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Basis of functional differences, Basis of functional differences, neurons are classified as follows:neurons are classified as follows:

1.1. Sensory neuronsSensory neurons

2.2. Inter neuronsInter neurons

3.3. Motor NeuronsMotor Neurons

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• Sensory (or afferent) neurons: send information from sensory receptors (e.g., in skin, eyes, nose, tongue, ears) TOWARD the central nervous system.

• Motor (or efferent) neurons: send information AWAY from the central nervous system to muscles or glands.

• Inter neurons: send information BETWEEN sensory neurons and motor neurons. Most inter neurons are located in the central nervous system.

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The neuron's role as the primary functional unit of the nervous system was first discovered by the Spanish anatomist Santiago Ramón y Cajal in

the early 20th century.

The neuron's role as the primary functional unit of the nervous system was first discovered by the Spanish anatomist Santiago Ramón y Cajal in

the early 20th century. * Nafly Hussain *

FUNCTION OF A NEURON

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How Neurons Carry the Message

• Only neurons are involved in transmission of electrical signals.

• Within a nerve cell, message is an electrical signal = action potential– Cascading membrane depolarization

creates the movement of the action potential as a nerve impulse

– rapid, all or none impulses– in myelinated neurons, gaps at regular

intervals allow regeneration of the action potential

• Between nerve cells the message is carried chemically

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• Body’s master controlling and Body’s master controlling and communicating systemcommunicating system

• Three functionsThree functions– Sensory inputSensory input

• Gathers information Gathers information from sensory receptorsfrom sensory receptors

– IntegrationIntegration• Processes and interprets Processes and interprets

sensory inputsensory input– Motor outputMotor output

• Activates effector organs Activates effector organs to cause a responseto cause a response

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Direction of neural impulse

DENDRITES -----> CELL BODY (with nucleus) -------> AXON* Nafly Hussain *

The direction of a neural impulse is always from dendrites to cell body to axon. All of the life functions of the neuron take place in the cell body.

Here nutrients are assimilated, broken down and used to maintain the existence of the entire cell. Part of the material in the cell body makes up the nucleus, which directs the life functions of the neuron.

The dendrites are sensitive to external stimuli & possess chemically regulated ion gates which respond to stimulation by neurotransmitters. Then transfer it as a neural impulse through the cell body to the axon

The axon has voltage regulated ion gates and therefore is responsible for carrying an impulse to another neuron or effectors.

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At the end of the axon, the axon terminus, is the secretory region where the neurotransmitters are released into the synapse.

The function of the axon is to propagate the neural impulse along its entire length. The destination may be a muscle, a gland, or, more commonly, the dendrites of another neuron.

The neural impulses sometimes travel as fast as 390 feet per second and are seldom as slow as 10 feet per second as they travel to the hair like end of the axon.

Neurons are the oldest and longest cells in the body. You have many of the same neurons for your whole life. Although other cells die and are replaced, many neurons are never replaced when they die. * Nafly Hussain *

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