lec9 anatomy

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Anatomy Dr.Israa H. Mohsen
Lecture 9
The nervous system—consisting of the brain, spinal cord, and nerves—constantly receives signals about changes within the body as well the external environment.
The nervous system contains two main divisions:
1.The central nervous system consists of the brain and spinal cord.
2.The peripheral nervous system consists of the vast network of nerves throughout the body.
Two types of cells make up the nervous system: neurons and neuroglia.
Neurons are the excitable, impulse-conducting cells that perform the work of the nervous system, while neuroglia protect ,support and bind the neurons together.
The nervous system contains five major types of glia .
Neuroglia of CNS are include Oligodendrocytes, Ependymal cells, Microglia and Astrocytes.
Neuroglia of PNS is Schwann cells
Neurons
Nerve cells called neurons handle the nervous system’s role of communication. There are three classes of neurons:
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1. Sensory (afferent) neurons: detect stimuli—such as touch, pressure, heat, cold,
or chemicals— and then transmit information about the stimuli to the CNS.
2. Interneurons: which are found only in the CNS, connect the incoming sensory
pathways with the outgoing motor pathways. Besides receiving, processing,
and storing information, the connections made by these neurons make each of
us unique in how we think, feel, and act.
3. motor (efferent) neurons: relay messages from the brain (which the brain emits
in response to stimuli) to the muscle or gland cells.
Types of Neurons
Neurons vary greatly in both size and shape. They also vary according to the type,
number, and length of projections.
Multipolar neurons
Multipolar neurons have one axon and multiple dendrites
Bipolar neurons
Bipolar neurons have two processes: an axon and a dendrite with the cell body in
between the two processes.
Unipolar neurons
Unipolar neurons have one process—an axon—that extends from the cell body
before branching in a T shape
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Neuron structure
Myelin
Not all nerve fibers are myelinated. However, because myelin helps speed impulse
conduction, unmyelinated fibers conduct nerve impulses more slowly. Typically,
unmyelinated nerve fibers perform functions in which speed isn’t essential, such as
stimulating the secretion of stomach acid. In contrast, nerve fibers stimulating skeletal
muscles, where speed is more important, are myelinated.
In the peripheral nervous system, the myelin sheath is formed when Schwann cells
wrap themselves around the axon, laying down multiple layers of cell membrane. It’s
these inside layers that form the myelin sheath. The nucleus and most of the
cytoplasm of the Schwann cell are located in the outermost layer. This outer layer,
called the neurilemma, is essential for an injured nerve to regenerate.
In the CNS, the myelin sheath is formed by oligodendrocytes. one oligodendrocyte
forms the myelin sheath for several axons. As a result, there is no neurilemma, which
prevents injured CNS neurons from regenerating.
cell body
Dendrites
axon
myelin sheath
synaptic knob
nodes of Ranvier
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Structure of the Spinal Cord
The spinal cord sits inside a protective, bony tunnel created by the stacked vertebrae.
A cross section clearly shows the two types of nervous tissue (white matter and gray
matter) that make up the spinal cord.
Gray matter—which appears gray because of its lack of myelin— contains mostly
the cells bodies of motor neurons and interneurons. This H-shaped mass is divided
into two sets of horns: the posterior (dorsal) horns and the ventral (anterior) horns.
White matter appears white because of its abundance of myelin. It contains bundles
of axons (called tracts) that carry impulses from one part of the nervous system to
another.
A small space—called the epidural space—lies between the outer covering of the
spinal cord and the vertebrae; it contains a cushioning layer of fat as well as blood
vessels and connective tissue.
Meninges of the Spinal Cord
The spinal cord is further protected by three layers of fibrous connective tissue, called
the meninges. (The meninges also covers the brain.) The three layers of the meninges,
from the inside out, are the pia mater, the arachnoid mater, and the dura mater.
1. The pia mater is the innermost layer. This transparent membrane clings to the
outer surface of the brain and spinal cord. It also contains blood vessels.The
subarachnoid space lies between the arachnoid mater and the pia mater. It is
filled with cerebrospinal fluid.
2. The arachnoid mater—a delicate layer resembling a cobweb—lies between
the dura mater and the pia mater.
3. The dura mater is the tough outer layer.
NOTE:
Cell bodies of the dorsal neurons are clustered in a knot-like structure called a
ganglion.
epidural
space
A minute opening called
the central canal carries
cerebrospinal fluid
through the spinal cord.
White matter
Gray matter
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Categories of Spinal Nerves
Thirty-one pairs of spinal nerves connect to the spinal cord. They include:
? 8 cervical nerves (C1-C8)
? 12 thoracic nerves (T1-T12)
? 5 lumbar nerves (L1-L5)
? 5 sacral nerves (S1-S5)
? 1 coccygeal nerve (Co)
The first cervical nerve exits the spinal cord between the skull and the axis. Once outside the
spinal column, each spinal nerve forms several largebranches. Some of these branches
subdivide further to form nerve networks called plexuses. The four major plexuses are the
cervical plexus, the brachial plexus, the lumbar plexus, and the sacral plexus.
Cerebrospinal fluid (C SF)
Cerebrospinal fl uid is produced by the choroid plexus in the ventricles of the brain
There is approximately 150 ml of CSF circulating around the brain, in the ventricles
and around the spinal cord. The CSF is replaced every eight hours . It is a
thin fluid similar to plasma and has several important functions:
• It acts as a cushion supporting the weight of the brain and protecting it from damage
• It helps to maintain a uniform pressure around the brain and spinal cord
• There is a limited exchange of nutrients and waste products between neurons and
CSF.
pia mater
subarachnoid
space
arachnoid
mater
dura mater
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The brain
The brain is the site for thought, learning, reasoning, memory, and creativity. Indeed,
the brain performs numerous amazing functions, many of which remain beyond our
grasp. The brain is divided into four major regions: the cerebrum, the diencephalon,
the cerebellum, and the brainstem.
1.The cerebrum is the largest portion of the brain. Its surface is marked by thick
ridges called gyri (singular: gyrus). Shallow grooves called sulci (singular: sulcus)
The cervical plexus contains
nerves that supply the muscles
and skin of the neck, tops of
the shoulders, and part of the
head.
The brachial plexus
innervates the lower part
of the shoulder and the
arm
The lumbar plexus —
supplies the thigh and
leg. A key nerve in this
region is the large
femoral nerve.
The sacral plexus is formed
from fibers from nerves L4,
L5, and S1 through S4.
The sciatic nerve, the
largest nerve in the body,
arises here and runs down
the back of the thigh.
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divide the gyri. Deep sulci are called fissures. Your ability to think, remember, feel,
use judgment, and move can be credited to the cerebrum.
2.The diencephalon sits between the cerebrum and the midbrain.It contain thalamus
and the hypothalamus. The thalamus acts as a gateway for nearly every sensory
impulse (including smell, sight, taste, pain, pressure, heat, cold, and touch) travelling
to the cerebral cortex while hypothalamus Controls the autonomic nervous system
(which is responsible for such vital functions as heart rate and blood pressure);
Contains centers responsible for hunger, thirst, and temperature regulation;Controls
the pituitary gland—often called the “master gland” because of its influence on most
endocrine glands (such as the thyroid, testes, ovaries, and adrenal glands) and Is
involved in multiple emotional responses, including fear, anger, pleasure, and
aggression
3.The cerebellum is the second largest region of the brain. Although smaller than the
cerebrum, it contains more neurons than the rest of the brain combined. In brief, the
cerebellum:
• Joins forces with the cerebral cortex to monitor body movements and send messages
crucial for balance, coordination, and posture
• Stores the information necessary for muscle groups to work together to perform
smooth, efficient, and coordinated movements
• Evaluates sensory input, such as touch, spatial perception, and sound
4.The brainstem makes up the rest of the brain. It consists of three structures:
• Midbrain: contains tracts that relay sensory and motor impulses. It also contains centers for
auditory and visual reflexes as well as clusters of neurons integral to muscle control.
• Pons: contains tracts that convey signals to and from different parts of the brain.
• Medulla oblongata :attaches the brain to the spinal cord. Besides relaying sensory and motor
signals between the brain and spinal cord, the medulla contains nuclei that perform functions
vital to human life. These include: • The cardiac center, which regulates heart rate • The
vasomotor center, which controls blood vessel diameter, which, in turn, affects blood
pressure • Two respiratory centers, which regulate breathing The medulla also houses reflex
centers for coughing, sneezing, swallowing, and vomiting.:
cerebrum
diencephalon
cerebellum
The brainstem It consists
of three structures:
• Midbrain
• Pons
• Medulla oblongata
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AUTONOMIC NERVOUS SYSTEM
The autonomic nervous system (ANS) is a subdivision of the nervous system responsible for regulating the activities that maintain homeostasis. These activities include such things as the secretion of digestive enzymes, the constriction and dilation of blood vessels for the maintenance of blood pressure, and the secretion of hormones. Most of these activities occur without your awareness or control; in other words, they happen independently, or autonomously, which is how the ANS received its name. The ANS sends motor impulses to cardiac muscle, glands, and smooth muscle (as opposed to skeletal muscle, which is innervated by the peripheral nervous system). Because the ANS targets organs, it’s sometimes called the visceral motor system.
The ANS consists of two divisions: the sympathetic division and the parasympathetic division. These two divisions have separate neural pathways and perform different functions; however, they often both innervate the same organ.
Sympathetic Division
• Increases alertness
• Increases heart rate
• Dilates bronchial tubes to increase air flow in the lungs
• Dilates blood vessels of skeletal muscles to increase blood flow
• Inhibits intestinal motility
• Stimulates secretion of thick salivary mucus
• Stimulates sweat glands
• Stimulates adrenal medulla to secrete epinephrine
• Has no effect on the urinary bladder or internal sphincter
• Causes “fight or flight” response
Parasympathetic Division
• Has a calming effect
• Decreases heart rate
• Constricts bronchial tubes to decrease air flow in lungs
• Has no effect on blood vessels of skeletal muscles
• Stimulates intestinal motility and secretion to promote digestion
• Stimulates secretion of thin salivary mucus
• Has no effect on sweat glands
• Has no effect on adrenal medulla
• Stimulates the bladder wall to contract and the internal sphincter to relax to cause urination
• Causes the “resting and digesting” state