C - THE MUSCULAR TISSES.
These tissues form the muscles. They are made up of muscle cells, which are typically elongated and arranged in parallel array, allowing them to work together effectively and they are called muscles fibers . Three kinds of muscles are known in the animal body: -
These tissues form the muscles. They are made up of muscle cells, which are typically elongated and arranged in parallel array, allowing them to work together effectively and they are called muscles fibers . Three kinds of muscles are known in the animal body: -
1-Striated or Skeletal Muscles.
In which the cells exhibit cross-striations at the light microscope level . Note the striated muscle fibers which appear as large elongated cylindrical cell, each showing a large number of fine alternating dark and light cross striations commonly known as the dark bands (myosin filaments) and light bands (actin filaments), they called (A,I) bands and contains a large number of peripherally situated nuclei. The striated muscles are usually found connected to the skeleton and their movement is controlled by the will of the animal. Hence they are called skeletal or voluntary muscles.
2- Unstriated or Smooth Muscles.
In which the cells do not exhibit cross striations. Note the bundles of unstriated muscle fibers. Each fiber is a long, spindle shaped cell with pointed ends and a thickened central part housing the nucleus . The unstriated muscles are usually found in the walls of the viscera and work without the interference of the will of the animal. Hence they are also called visceral or in voluntary muscles . (i.e. Alimentary canal, blood vessels).
3- Cardiac Muscles.
Note that the cardiac muscle fibers are also cylindrical, but not much elongated. Each fiber has one ovoid central nucleus, the cardiac fiber branch and unite with each other . Every two connected fibers have a darkly stained transverse band in between them known as the intercalated discs , and contains alternating dark and light bands . The cardiac muscles are found only in the wall of the heart and contract rhythmically.
D-Nervous Tissue (system).
Is divided into the central nervous system (CNS), consisting of the brain and the spinal cord , and the peripheral nervous system (PNS) , composed of nerve fibers and nerve cells which called nerve ganglia or neurons .
Neurons .
Are anatomic and functional units consist of three parts :-
1- Dendrites , multiple elongated processes specialized in receiving stimuli from the environment
.
2- Cell body, perikaryon the trophic center.
3- The axon , single process specialized in generating or conducting nerve impulses to other cells .
Neurons are classified on the basis of the number of processes extending from the cell body to :-
• Multipolar, neurons have an axon and two or more dendrites
• bipolar , neurons have one axon and one dendrites
• Unipolar, (actually pseudounipolar) have one process the axon , that divides close to cell body to two long processes.
Synapses .
Are specialized junctions between neurons that facilitate transmission of impulses from one neuron to another. Synapses between neurons may be classified morphologically as :
• Axodendritic, occuring between axons and dendrites.
• Axosomatic, occuring between axons and cell body
• Axoaxonic , occuring between axons and axons.
• Dendrodendritic, occuring between dendrites and dendrites.
Myelinated and Unmyelinated Nerves.
Nerves are composed of bundles of nerve fibers held together by perineurium and connective tissue . Each nerve fiber consist of an axon that is surrounded by a cellular investment called the neurilemma or the sheath of Schwann . In addition , the fiber may be myelinated or unmyelinated which do not have myelin , if present, ediately around the axon then called myelinated nerves.The myelin sheath is segmented and this site is called the node of Ranvier .
Autonomic nervous system
Is that portion of PNS that conduct impulses to smooth muscle, cardiac muscle, and glandular epithelium.
Spinal cord
In cross section, the spinal cord is organized into two discrete parts exhibits a butterfly - shaped grayish-tan inner substance, the gray matter surrounding the central canal , and a whitish peripheral substance , the white matter which contains only myelinated and unmyelinated axons traveling to and from other parts of the spinal cord and to and from the brain . The gray matter contains neuronal cell bodies and their dendrites, along with axons and neuralgia .
The brain (Cerebrum & Cerebellum )
Is contains gray matter and white matter but in a different arrangement , the gray matter forms an outer covering or cortex , which consists of a six layers :
I -Molecular layer.
II -Outer glranular layer.
III-Outer pyramidal cell layer.
IV- Inner granular layer.
V -Ganglionic layer (large pyramidal cell).
VI -Fusiform (polymorphic) cell layer.
Nerve ending
• Pacinian corpuscles,are large ovoid structures found in the deeper dermis and hypodermis also it found in the pancreas
.
• Meissner ,s corpuscles , are touch receptors that found in the papillary layer of hairless skin
• Ruffini endings , are the simplest mechanoreceptors ,they have an elongated fusiform shape .
• Motor end plates , are the physiologic contact between nerve and muscle.