NERVOUS SYSTEM-
• The
nervous system is an intricate, highly organized network of billions of neurons
and even more neuroglia. The structures that make up the nervous system include
the brain, cranial nerves and their branches, the spinal cord, spinal nerves
and their branches, ganglia, enteric plexuses, and sensory receptors
• The
nervous system is a major communicating and control system within the body. It
works with the endocrine system to control many body functions. It is a vast
topic and we will cover in multiple lectures. Today we will discuss the
introduction, overview and organization of nervous system with structure of
neuron and nerves.
CLASSIFICATION OF NERVOUS SYSTEM
• The
nervous system is classified into two main types
• Central
Nervous System
• Peripheral
Nervous system
CENTRAL NERVOUS SYSTEM-
• The
central nervous system consists of the brain and spinal cord and is the control
and integration center for many body functions.it is central as it lies in the
central line of the body system.
PERIPHERAL NERVOUS SYSTEM-
• The
peripheral nervous system carries sensory information to the central nervous
system and motor information out of the central nervous system. It mainly
consists of nerves. The peripheral nervous system is further divided into
somatic nervous system and autonomic nervous system
SOMATIC NERVOUS SYSTEM-
• The
somatic nervous system is under voluntary control, and the effector (tissue or
organ responding to instruction from the central nervous system) is skeletal
(voluntary) muscle. The central nervous system’s response to sensory
information may be to activate the somatic nervous system, eliciting a
voluntary response involving skeletal muscle movement.
AUTONOMIC NERVOUS SYSTEM-
• The
autonomic nervous system is responsible for involuntary motor responses. The
effector organ may be smooth or cardiac muscle (both involuntary muscles) or a
gland. The autonomic nervous system is further divided into the sympathetic
(fight or flight) andthe parasympathetic (rest and digest) divisions.
NERVOUS TISSUE-
• The
nervous system is composed of –
• Neurons
• Neuroglia
and
• Nerves
NEURONS-
• The
structural and functional unit of the nervous system is the neuron or nerve
cell. Neurons consist of an axon, dendrites and a cell body. Their function is
to transmit nerve impulses. Nerve impulses only travel in one direction: from
the receptive area – the dendrites –to
the cell body, and down the length of the axon
• Cell
bodies- Nerve cells vary
considerably in size and shape but they are all too small to be seen by the
naked eye. Cell bodies form the grey matter of the nervous system and
are found at the periphery of the brain and in the centre of the spinal cord.
• Axons
and dendrites- Axons and dendrites are extensions of cell bodies and form
the white matter of the nervous system. Axons are found deep in the
brain and in groups, called tracts, at the periphery of the spinal cord.
Each neurone has only one axon that conducts information away from the cell
body. Dendrites carry nerve impulse towards the cell body. Axons are longer
than the dendrites, sometimes as long as 100 cm.
• Large
axons are surrounded by a myelin
sheath . This consists of a series of Schwann cells arranged along
the length of the axon. Each one is wrapped around the axon so that it is
covered by a number of concentric layers of Schwann cell plasma membrane.
Between the layers of plasma membrane there is a small amount of fatty
substance called myelin. The outermost layer of the Schwann cell plasma
membrane is the neurilemma.
• There
are tiny areas of exposed axolemma between adjacent Schwann cells, called nodes
of Ranvier, which assist the rapid transmission of nerve impulses in
myelinated neurons.
• Classification
of neurons- Neurons are classified in two
ways – structural classification and functional classification.
• Structural
Classification of neurons-
• 1.
Multipolar neurons usually have several dendrites and one axon. Most
neurons in the brain and spinal cord are of this type.
• 2.
Bipolar neurons have one main dendrite and one axon. They are found in the
retina of the eye, in the inner ear, and in the olfactory area of the brain.
• 3.
Unipolar neurons have dendrites and one axon that are fused together to
form a continuous process that emerges from the cell body
• Functional
Classification of neurons-
• 1.
Sensory Neurons - Sensory neurons (afferent neurons) carry impulses from
receptors to the central nervous system. Sensory neurons from receptors in
skin, skeletal muscles, and joints are called somatic sensory neurons and
those from receptors in internal organs are called visceral sensory neurons
.
• 2.
Motoer Neurons - Motor neurons (efferent neurons) carry impulses from the
central nervous system to effectors. The two types of effectors are
muscles and glands. In response to impulses, muscles contract or relax and
glands secrete.
• 3.
Interneurons - Interneurons or
association neurons are mainly located within the CNS between sensory and motor
neurons. Interneurons integrate (process) incoming sensory information from
sensory neurons and then elicit a motor response by activating the appropriate
motor neurons. Most interneurons are multipolar in structure.
NEUROGLIA-
• Neuroglia
make up about half the volume of the CNS. Their name derives from the idea
of early histologists that they were the “glue” that held nervous tissue together.
Neuroglia are not merely passive bystanders but rather actively participate in
the activities of nervous tissue
• There
are six types of neuroglia. Out of these four neuroglias— astrocytes,
oligodendrocytes, microglia, and ependymal cells are found only in the CNS. The
remaining two types Schwann cells and satellite cells are present in the PNS.
• ASTROCYTES-
These stars haped cells have many
processes and are the largest and most numerous of the neuroglia. Astrocytes
help in forming blood brain barrier and supports neurons. There are two types
of astrocytes.
• Protoplasmic
astrocytes
• Fibrous
astrocytes
• OLIGODENDROCYTES-
These resemble astrocytes, but are smaller and contain fewer processes. Oligodendrocyte
processes are responsible for forming and maintaining the myelin sheath
around CNS axons
• MICROGLIA -These neuroglia are small cells with
slender processes that give off numerous
spine like projections. Microglia
function as phagocytes. and phagocytize microbes and damaged nervous
tissue.
• Ependymal
cells are cuboidal to columnar cells arranged in a single layer that
possess microvilli and cilia. These cells line the ventricles of the brain and
central canal of the spinal cord. ependymal cells produce cerebrospinal fluid.
• SCHWANN
CELLS- These cells encircle PNS axons. Like oligodendrocytes, they form the
myelin sheath around axons. Schwann cells participate in axon regeneration,
which is more easily accomplished in the PNS than in the CNS.
• SATELLITE
CELLS- These flat cells surround the cell bodies of neurons of PNS ganglia.
Besides providing structural support, satellite cells regulate the
exchanges of materials between neuronal cell bodies and interstitial fluid
NERVES-
• A
nerve is a group of axons and/or dendrites of many neurons, with blood
vessels and connective tissue.
• Innermost
layer is endoneurium. Endoneurium is a delicate tissue which surrounds
individual axon.
• Perineurium
is a smooth connective tissue, surrounding each bundle of fibres. A
Bundle contains a group of axons.
• Epineurium
is the fibrous tissue which surrounds and encloses a number of bundles of
nerve fibres. Most large nerves are
covered by epineurium. It is outermost layer of a nerve.
TYPES OF NERVES-
• Nerves
may be sensory, motor or mixed.
• 1.
Sensory Nerves- Sensory nerves are made only of sensory neurons. The
optic nerves for vision and olfactory nerves for smell are examples of nerves
with a purely sensory function These nerves
carry information from the body to the spinal cord or brain.
• 2.
Motor Nerve- Motor nerves are
made only of motor neurons; some crainial nerves are motor nerves.
• 3.
Mixed Nerves- A mixed nerve contains both sensory and motor neurons.
Most of our spinal nerves, such as the sciatic nerves in the legs, are mixed nerves.
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