Organization and nerve cells

NEUROSCIENCE NEUROSCIENCE

CHAPTER 1CHAPTER 1ORGANIZATION OF NERVE CELLSORGANIZATION OF NERVE CELLS

By Hermizan Bin HalihanafiahBy Hermizan Bin Halihanafiah

The nervous system senses changes in our internal

& external environments, coordinates and integrates

this data, and initiates & transmits action potentials.

Together with endocrine system maintaining

homeostasis.

It must detect changes in & around the body.

The nervous system is organized into two (2) basic

component:

◦ Central nervous system (CNS) – Brain and Spinal

Cord

◦ Peripheral Nervous System (PNS) – 12 pairs

Cranial Nerves, 31 pairs of spinal nerves, ganglia

Functionally can be divided into:

1. Sensory nervous system

◦ Somatic nervous system

◦ Autonomic nervous system

2. Motor nervous system

◦ Somatic nervous system

◦ Autonomic nervous system

3. Enteric Nervous System

1. Sensory function Sensory receptor detect stimuli (internal and external stimuli) and

then carried in to the spinal cord and brain.

2. Integrative Function Integrates (processes) sensory information by analyzing and storing

the information. Important integrative function is perception. Perception is a

conscious awareness of sensory stimuli and occurs in the brain.

3. Motor function Once sensory information is integrated, the nervous system will elicit

an appropriate motor response by activating effectors (muscle and gland) through cranial and spinal nerves.

This stimulation causes the muscles contract and glands to secrete.

1. NEURON – main function unit

2. NEUROGLIA – supportive cell

Posses electrical excitability

Able to respond to a stimulus and convert it into an action potential

A stimulus initiate the conduction of action potential (impulses).

Action potential is an electrical signal that propagates along the

surface of the membrane of a neuron.

Consist :- Cell Body- Dendrite - Axon- Axon Hillock- Axon Terminal- Myelin Sheath- Nodes of Ranvier- Schwann cell

Neuron

Axon Hillock

Node of Ranvier

Contain nucleus surrounded by cytoplasm

Cell bodies within the central nervous system (CNS) are frequently

cluster into region called nuclei.

Cell bodies in the peripheral nervous system (PNS) occur in cluster

called ganglia.

Function: integrate nerve impulse, metabolic centre.

Processes emerging from the cell body. In many neurons its form a tree shaped array of

processing extending from the cell body. Function: Receive information from receptor or other

neurons and send it to the neuron cell body or soma.

Structure: Long, cylindrical and joint the cell body at a

cone shaped, extend from the axon hillock.

Nerve impulses arises from axon hillock and initial

segment, called trigger zone where the action

potential begin.

Function: Send impuls from cell body towards another

neuron, muscles fiber or a gland.

AXON HILLOCK Specialized part of the cell body (or soma) of a

neuron that connects to the axon. Function: initiate the propagation of the action

potential.

AXON TERMINAL (end bulb) Small knobs at the end of an axon (synaptic end

bulb) that release chemicals called neurotransmitters.

Function: conducts electrical impulses (called "action potentials") away from the neuron's cell body, to transmit those impulses to other neurons

MYELINE SHEATH Consists of lipid and protein – myeline sheath. Axon surrounded with fat – myelinated. Schwann cells and oligodendrocyte cells Axon without covering – unmyelinated. Function: insulator - Increase the speed of nerve

impuls conduction.

NODES OF RANVIER The gaps formed between myelin sheath cells long

the axons Function: facilitate the rapid conduction of nerve

impulses – saltatory conduction

Smaller than neuron

More numerous than neurons

Do not generate impulses

Function: support, protect furnish nutrients to

neuron.

Can be divided into:

◦Neuroglia of the CNS (4 types)

◦Neuroglia of the PNS (2 types)

4 types of cells: Astrocytes Oligodendrocytes Microglia Ependymal cells

Star shaped, have many processes and largest and most numerous neuroglia.

FUNCTIONS of Astrocytes1. Strengthen and support neuron2. Isolate neurons from harmful substances in

blood. (BBB)3. In the embryo, regulate the growth, migration

and interconnection among neurons in the brain.

4. Maintain appropriate chemical environment for generation of impulses. (K+ buffering).

Resembles as a astrocytes, but smaller

and contain a fewer processes.

FUNCTION: Responsible for forming and

maintainning the myelin sheath that

covering the axon.

Small cells, slender processes

FUNCTIONS:

As a phagocytes.

Remove cellular debris form during normal

development of the nervous system.

Phagocytize microbes and damage nervous

tissue.

Cuboidal to columnar cells arrange in a single

layer that posses microvilli and cilia.

These line the ventricles of the brain and central

canal of the spinal cord (spaces filled with CSF).

FUNCTION: produce, and assist in circulation of

CSF.

1. Schwann Cells: Encircle PNS axon. Myelinates a single

axon. Participate in axon

regeneration.

Flat cells Surrounds the cell

bodies of neuron of PNS ganglia

Providing structural support and regulate the exchanges of materials between neuronal cell bodies and interstitial fluid.

Classification of neuron divided into two:

1. Classification by structural

2. Classification by functional

1. Multipolar neurons

2. Bipolar neurons

3. Unipolar neurons

Have numerous processes.

Many dendrites

One axon

Part sensory neuron, Motor neuron &

interneuron are multipolar neurons

Have 2 processes

Peripheral processes (dendrite)

Central processes (axon).

Found in the special sense organs

Have only one process. Have dendrites and one axon. The dendrite small and attach to

the axon instead of the neuron cell body.

Sensory neuron are unipolar neuron.

Functions as a sensory receptors that detect a stimulus such as touch, pressure, pain or thermal stimuli.

Functional, neurons are classified to:

1. Sensory (afferent) neurons

2. Interneurons or association neurons

3. Motor (efferent) neurons

CLASSIFICATION OF NEURON BY FUNCTIONAL

Sensory Neuron Motor neuron

Interneuron

Function: Detect a stimulus from receptor and

travel along the axon to the spinal cord and

brain (CNS). It forms an action potential in its

axon and the action potential or nerve impuls.

Contain sensory receptors at their distal ends

(dendrites).

Most sensory neurons are unipolar.

Integrate (process) incoming sensory

information from sensory neurons and then elicit

a a motor response by activating the appropiate

motor neurons.

Located within the CNS between sensory and

motor neurons.

Most interneuorns are multipolar neuron.

Deliver action potentials away from the CNS

(brain and spinal cord) to effectors (muscles or

gland) in the PNS through cranial or spinal

nerves.

Most motor neurons are multipolar structure.

Physiology of neuron

Physiology of neuron

Neurons are electrically excitable. They communicate each other using two

types of electrical signals:

1. Graded potentials - used for short distance communication

(receptor potential, postsynaptic potential, end plate potential)

2. Action potentials - used for long distance communication

The production of graded potential and action potential is depends on basic features of plasma membrane of excitable cells:

1. Existence of resting membrane potentials

2. Presence of specific ion channels

* When action potential occurs in a neuron, it is called a nerve action potential (nerve impulse)

It is the cell membrane of a non-

conduction or in the resting state. The difference in charges on the two sides

of the resting membrane is called the

resting membrane potential. This potentials is about – 70 millivolts

(mV)

Transmission of the impulses and action

potential due to movement of ion across the

nervous cell membrane.

In the resting state the nerve cell membrane is

polarised due to different concentration of ion

across the plasma membrane.

This condition is called resting membrane

potential.

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Resting membrane potential:◦ Sodium the main extracellular cation.◦ Potassium the main intracellular

cation.

Conduction of nerve impulses

When stimulated, the permeability of the nerve cell membrane to this ion change.

Sodium flood into the neuron from extracellular flud (ECF) causing depolarisation, creating a nerve impuls @ action potential.

Depolarisation is very rapid.

Its passes from the point of stimulation in one direction only.(away from the point towards the area of resting membrane potential)

Conduction of nerve impulses

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During this process, potassium floods out of the neuron cell.

Depolarization results because inward diffusion of sodium is much greater than an outward diffusion of potassium

DEPOLARIZATIONDEPOLARIZATION

◦ Voltage gated Na+ channels are closed.

◦ Voltage gated channel K+ are open.

◦ Sodium ion diffusion into the cell stops and K+

diffuse out of the cell, causing repolarisation.

Resting membrane potential is reestablish after

the voltage gated K+ channels closed.

REPOLARIZATIONREPOLARIZATIONREPOLARIZATIONREPOLARIZATION

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Definition of Synapse◦The site of communication between 2 neurons

or between neuron and effector cells (muscles or glands).

The tips of some axon terminals swell into bulb shaped structures called synaptic end bulbs.

Synaptic end bulbs contain many tiny membrane-enclosed sacs called synaptic vesicles that store a chemical called neurontransmitter.

The site of communication between 2 neurons or between neuron and effector cells (muscles or glands).

Neuron sending the signal is called the presynaptic neuron.

Neuron receiving the message is called the postsynaptic neuron.

Tjere are 2 types of synapse:1. Electrical synapse

◦ Action potential conducts directly between adjacent cells through structures called gap junction.

2. Chemical synapse◦ neuron releases a neurotransmitter.◦ Presynaptic and postsynaptic neuron

separated by synaptic cleft.◦ Postsynaptic neuron receives the chemical

signal – postsynaptic potential.

TYPES OF SYNAPSE

Although the plasma membrane of presynaptic and postsynaptic neuron in a chemical synapse are close, they do not touch.

The synaptic cleft, a space of 20-50 nm that is filled with interstitial fluid, separated the two neurons.

The presynaptic neuron converts an electrical signal (nerve impuls) into a chemical signal (release neurontransmitter).

The postsynaptic neuron receives the chemical signal and generate an electrical signal (postsynatic potential).

A typical chemical synapse transmits a signal asfollows:1. A nerve impulse arrives at a synaptic end bulb of a

presynaptic axon.

2. The depolarizing phase of the nerve impulse opens voltage gated Ca2+ channels, which are present in the membrane of synaptic end bulbs.

3. Increase [Ca2+] inside the presynatic neuron is the triggers exocytosis of some of the synaptic vesicles. As vesicles membrane merge with the plasma membrane, neorontransmitter molecules released into the synaptic cleft.

4. The neurontrasmitter molecule diffuse across the synaptic cleft and bind to neurontransmitter receptor in the postsynaptic neuron plasma membrane.

5. Binding of neurontransmitter molecules to their receptor on ligand-gated channels opens the channels and allow particular ions to flow across the membrane.

SIGNAL TRANSMISSION AT SYNAPSES

SIGNAL TRANSMISSION AT SYNAPSES

6. As ions flow through the opened channels, the voltage across the membrane changes. This change in membrane voltage is a postsynaptic potential.

7. When a depolarizing postsynaptic potential reaches threshold, it triggers one or more nerve impulse.

Synapse between somatic motor neuron and skeletal muscles fiber.

Structures of the presynaptic neuron same with other neurons.

NT released in the synaptic cleft – Ach. The region of the sarcolemma opposite to the

synaptic end bulbs are called motor end plate. Within the MEP, there is many ACh receptors. Attachment of Ach on its receptor permits influx Na

ions inside muscles fiber, thus generate action potentials.