Download Nervous Tissue

Nervous Tissue
Overview of the Nervous System:
The nervous system is part of the body’s 11 systems and though small, it’s
extremely complex. The nervous system consists of 2 types of cells, neurons and
neuroglia that work together to form an extremely intricate network. It is made up
of the body’s most important structures the brain, spinal cord, cranial, spinal, and
peripheral nerves, and their motor and sensory endings.
Organization of the Nervous System
To better understand the nervous system, it is often divided anatomically
into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).
The CNS is made up of the brain and the spinal cord, which together processes the
body’s sensory information. The PNS encompasses all of the nervous system outside
of the CNS. It includes the cranial nerves, spinal nerves, and sensory receptors.
The peripheral nervous system can be further divided into 3 subdivisions:
the somatic nervous system, the autonomic nervous system, and the enteric nervous
system. The somatic nervous system is made up of somatic sensory neurons that
provide information from sensory receptors to the CNS. It plays a fundamental role
in the input information that the CNS processes. It also includes the somatic motor
neurons that provide information from the CNS to the skeletal muscles. It’s part of
the output of information that the CNS is involved in. The autonomic nervous
system of the PNS also has sensory and motor neurons. However, these neurons are
autonomic in that it is done involuntarily. The motor neurons of the ANS are again
divided into sympathetic and parasympathetic divisions. The sympathetic division is
involved in the body’s “fight-or-flight” responses and the parasympathetic in the
“rest and digest” responses of the body. A third division of the PNS is the enteric
nervous system (ENS), which refers to the neurons in the gastrointestinal tract. It
has sensory and motor neurons that monitor the gastrointestinal tract.
Neurons vs Neuroglia
Nervous tissue consists of neurons and neuroglia, which are intermixed in
the body in complex networks. Neurons are highly specialized cells that provide the
backbone for body’s unique functions of thinking, remembering, and regulating.
Because of this, they are unable to undergo mitotic division. Neuroglia on the other
hand, are support structures to the neurons, there to nourish and protect the
neurons. Unlike they neurons, they continually divide through a lifetime.
Neurons’ Structures
A neuron consists of a cell body where the nucleus, mitochondria, and other
cell structures can be found. At one end of the neuron are the dendrites, multiples
tree-like structures that acts as the receiving portion of the neuron. The other end is
the axon, where the nerve impulse travels through to the next neuron. It is typically
an long and thin projection. Between neurons is the synapse, the site of
communication between two axons. Here, a neurotransmitter is usually released
where it produces an action potential.
Neuroglia
Neuroglia are all the other cells other than the neurons and they play a
supportive role in the nervous system. There are 6 types of neuroglia, 4 that are
found in the CNS and the other 2 found in the PNS. The four found in the CNS are the
astrocytes, oligodendrocytes, microglia, and ependymal cells. They all perform a
vital role in the nervous system and are essential to its functioning. Neuroglia in the
nervous system far outnumbers neurons. With a 10 to 1 ratio, neurons form the
minority of structures making up the nervous system. They are easily
distinguishable from the neurons as they lack axons and contain only one type of
processes. They don’t form synapses; instead they are supportive structures to the
neurons. And unlike neurons, they are able to divide through their lifetime.
Astrocytes are star-shaped cells with many processes. They are the largest
and most numerous of the neuroglia. Their primary functions are to support the
neurons, create a blood-brain barrier, and secret chemicals that regulates the
neurons. The blood-brain barrier is extremely important in the nervous system in
that it prevents toxins and pathogens from reaching the neurons. It’s a buffer that
restricts the movement of harmful organisms. Oligodendrocytes are similar to
astrocytes in shape, but small and with fewer processes. Their main purpose is to
create a myelin sheath around the axons of the neurons in the CNS. This sheath
increases the speed of the nerve impulse of conduction. Microglia cells are
fundamentally phagocytes, responsible for removing waste. It is essentially an
immune cell, there to protect the neurons. The ependymal cells lines the ventricles
of the brain and it produces and helps in the movement of cerebrospinal fluid.
The Neuroglia found in the PNS are the Shwann cells and the satellite cells.
Schwann cells are almost identical to the oligodendrocytes of the CNS. It creates a
myelin sheath that surrounds the axons of the neurons. However, unlike the
oligodendrocytes, it can regenerate. Satellite cells are flat cells that monitor the
exchange of materials from the interstitial fluid and the neurons.
Myelination
Myelin, an insulating sheath, is found on the axon of most neurons. It is
created by the Schwann cells in the Peripheral Nervous System and
oligodendrocytes in the Central Nervous System. These cells are wrapped around
the axons of a neuron, 15 to 20 times, creating a phospholipid bilayer. They are
interrupted along the axons by gaps called the nodes of Ranvier. The purpose of
myelination and which cells are myelinated and which cells aren’t is not yet fully
understood. However, it is believed that myelin sheaths conduct nerve impulses
faster along the axons with impulses jumping from node to node.