E-mail: a.al-nuaimi@sheffield.ac.uk E. mail: abdulameerh@yahoo.com

Prof. Abdulameer Al-Nuaimi

Microscopic structure of the Nervous System

Histology of Nervous Tissue

• Nervous tissue is a vascular tissue comprised of two types of cells: neurons and neuroglia.

1- Neurons (or nerve cells):

• They connect all regions of the body to the central nervous system

• Highly specialized cells

• Lost the ability to undergo mitotic division

2- Neuroglia:

• Smaller cells that greatly more in number than neurons

• Support, nourish, and protect neurons

• Continue to divide throughout an individual’s lifetime

Neurons

• Neurons or nerve cells possess electrical excitability.

• A nerve impulse travels rapidly and at a constant strength.

• Nerve impulses travel these great distances at speeds ranging from 0.5 to130 meters per second (1 to 280 mi/hr).

Copyright © 2014 John Wiley &

Sons, Inc. All rights reserved.

Parts of a Neuron

Cell body has

1- Cell membrane and protoplasm.

2- Protoplasm is composed of Cytoplasm and its components such as

Large nucleus, Nissl bodies, Lipofuscin (fine yellow-brown pigment granules composed of lipid arranged around the nucleus) and Neurofilaments

3- Nerve fibers

– Dendrites , Axon, and Axon hillock

Structure of a Typical Neuron

Neuron

Neuron

Neurons are classified according to shape and function. According to Shape There are four major types of neurons 1- Unipolar neurons are the most common neurons in invertebrates. These neurons are characterized by one primary projection that serves as both the axon and the dendrites. 2- bipolar neurons, each having an axon that transmits signals from the cell body going to the brain and the spinal cord, and dendrites that send signals from the body organs to the cell body.

3- Pseudo-unipolar neurons resemble unipolar neurons because each of them has an axon, but no true dendrites. the single axon attached to the cell body, giving rise to projections to two opposite directions – one towards the muscle, joints and skin, and the other towards the spinal cord. Pseudo-unipolar neurons are responsible for the sense of touch, pain and pressure. 4- Multipolar neurons are the dominating neurons in vertebrates in terms of number. Each neuron has a cell body, a long axon, and short dendrites.

Neuron types

According to Function 1- Sensory neurons: are the neurons that transmit information from the different sensory organs such as the eyes, nose, ears, tongue and skin to the spinal cord to the brain 2- motor neurons: transmit signals from the brain to the spinal cord to the muscles to initiate action 3- Interneurons: connect one neuron to another a- Projection interneurons: have long axons that join brain regions that are far from one another. b- Local interneurons: have shorter axons that create small circuits between near brain cells or regions.

Axoplasmic transport: is a cellular process responsible for movement of mitochondria, lipids, synaptic vesicle, proteins, and other cell parts (i.e. organelles) to and from a neuron's cell body, through the cytoplasm of its axon (the axoplasm). Axons, which can be 1,000 or 10,000 times the length of the cell body, were originally thought not to contain any ribosomes as the means of producing proteins, and so were thought to rely on axoplasmic transport for all their protein needs. Axonal transport is also responsible for moving molecules which need degradation from the axon back to the cell body, where they are broken down by lysosomes. Movement toward the cell body is called retrograde transport and movement toward the synapse is called anterograde transport

The vast majority of axonal proteins are synthesized in the neuronal cell body and transported along axons. Axonal transport occurs throughout the life of a neuron and is essential to its growth and survival. Microtubules (made of tubulin) run along the length of the axon and provide the main cytoskeletal "tracks" for transportation. Kinesin and dynein are motor proteins that move cargoes in the anterograde (forwards from the cell body to the axon tip) and retrograde (backwards to the cell body directions, respectively). Motor proteins bind and transport several different cargoes including mitochondria, cytoskeletal polymers, autophagosomes, and synaptic vesicles containing neurotransmitters. Axonal transport can be fast or slow, and anterograde or retrograde

Synapses

• Synapse: is the site of communication between two neurons

• Presynaptic neuron carries impulse toward synapse.

• Postsynaptic cell may be either a nerve cell (carries impulse away from synapse) or effector ( muscle or gland responds to impulse at synapse).

At Neuromuscular junction, Acetylcholine is a

neurotransmitter.

Neuroglia

• Neuroglia cells constitute about half the volume of the CNS.

• Neuroglia are smaller than neurons.

• Neuroglia cells

– Do not generate or transphere nerve impulses

– Have the ability to multiply and divide in the mature nervous system

– They support the neurons, maintain homeostasis, form myelin, and provide support and protection for neurons in the central and peripheral nervous systems

• Six types of neuroglia:

– Astrocytes, oligodendrocytes, microglia, and ependymal cells all these cells are found only in the central nervous system (CNS)

- Schwann cells (neurolemmocytes) and satellite cells are present in the Peripheral nervous system (PNS).

Function of Glial cells of CNS 1- Astrocytes: The most abundant type of macroglial cell in the CNS, astrocytes have numerous projections that 1- link neurons to their blood supply while forming the blood-brain barrier. 2- They regulate the external chemical environment of neurons by removing excess potassium ions, and recycling neurotransmitters released during synaptic transmission. 3- Astrocytes may regulate vasoconstriction and vasodilation by producing substances such as arachidonic acid, whose metabolites are vasoactive 2- Oligodendrocytes are cells that coat axons in the central nervous system (CNS) with their cell membrane, forming myelin sheath.

3- Ependymal cells : line the spinal cord and the ventricular system of the brain. These cells are involved in the creation and secretion of cerebrospinal fluid (CSF) and beat their cilia to help circulate the CSF and make up the blood-CSF barrier. They are also thought to act as neural stem cells 4- Microglia: are specialized macrophages capable of phagocytosis that protect neurons of the central nervous system Function of Glial cells of PNS 1- Schwann cells: provide myelination to axons in the peripheral nervous system (PNS). They also have phagocytotic activity and clear cellular debris that allows for regrowth of PNS neurons 2- Satellite cells are small cells that surround neurons in sensory, sympathetic, and parasympathetic ganglia. These cells help regulate the external chemical environment

Neuroglia of the Central Nervous System (CNS)

Myelination

• Axons that are surrounded by a multilayered lipid and protein covering, called the myelin sheath, are myelinated.

• Axons without such a covering are unmyelinated.

• Two types of neuroglia produce myelin sheaths: – Schwann cells (in the PNS)

– Oligodendrocytes (in the CNS)

Myelinated and Unmyelinated Axons

Myelinated and Unmyelinated Axons

Copyright © 2014 John Wiley &

Sons, Inc. All rights reserved.

The blood–brain barrier (BBB) Is a highly selective semipermeable membrane barrier that separates the circulating blood from the brain and extracellular fluid in the central nervous system. The blood–brain barrier is formed by brain microvascular endothelial cells . BBB inhibits the free para cellular diffusion of water-soluble molecules by an efficient network of complex tight junctions that interconnects the endothelial cells. combined with the absence of fenestrae and an extremely low pinocytotic activity (which inhibit transcellular passage of molecules across the barrier). it allows the passage of water, some gases, and lipid-soluble molecules by passive diffusion. It also allows the selective transport of molecules such as glucose and amino acids that are crucial to neural function.

It prevents the entry of lipophilic potential neurotoxins (neurotoxins dissolved in lipids) . Astrocytes have been claimed to be necessary to create the blood–brain barrier. Blood-CSF barrier This system is achieved through apical tight junctions between the choroid plexus epithelial cells. It inhibits paracellular diffusion of water-soluble molecules across this barrier. Besides its barrier function, choroid plexus epithelial cells have a secretory function and produce the CSF. The barrier and secretory function of the choroid plexus epithelial cells are maintained by the expression of numerous transport systems allowing the directed transport of ions and nutrients into the CSF and the removal of toxic agents out of the CSF.

Meninges of the brain and spinal cord

Pia mater

Lateral ventricle

Third ventricle

Blood brain barrier

Gray and White Matter

• The white matter is aggregations of myelinated and unmyelinated axons of many neurons.

• The gray matter of the nervous system contains neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia.

The peripheral nervous system (PNS) I The PNS consists of the nerves and ganglia outside the brain and spinal cord. The main function of the PNS is to connect the CNS to the limbs and organs, essentially serving as a relay between the brain and spinal cord and the rest of the body. Unlike the CNS, the PNS is not protected by the vertebral column and skull, or by the blood–brain barrier, which leaves it exposed to toxins and mechanical injuries. The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system The somatic nervous system is under voluntary control. It includes motor nerve fibres that transmits signals from the brain to end organs such as muscles. It also include sensory nerve fibres that transmits signals from senses such as taste and touch to the spinal cord and brain.

The autonomic nervous system is a 'self-regulating' system which influences the function of organs outside voluntary control, such as the heart rate, or the functions of the digestive system.

Peripheral nervous system includes the spinal and cranial nerves. Spinal nerve is a mixed nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body. In the human body there are 31 pairs. Cranial nerves could be mixed or pure sensory or motor nerves Cranial nerves I, II, and VIII are pure sensory nerves. Cranial nerves III, IV, VI, XI, and XII are pure motor nerves. Cranial nerves V, VII, IX, and X are mixed sensory and motor nerves.

Ganglia are primarily made up of neuron cell bodies and dendritic structures . Ganglia often interconnect with other ganglia to form a complex system of ganglia known as a plexus. Ganglia provide relay points and intermediary connections between different neurological structures in the body, such as the peripheral and central nervous systems. There are three major groups of ganglia: Dorsal root ganglia (also known as the spinal ganglia) contain the cell bodies of sensory (afferent) neurons. Cranial nerve ganglia contain the cell bodies of cranial nerve neurons. Autonomic ganglia: contain the cell bodies of autonomic nerves.

In the autonomic nervous system, fibres from the central nervous system to the ganglia are known as preganglionic fibres, while those from the ganglia to the effector organ are called postganglionic fibres

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