ANHB1102 Study Guide - Final Guide: Lower Motor Neuron, Postganglionic Nerve Fibers, Preganglionic Nerve Fibers
3 Jun 2018
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TOPIC ONE: THE NERVOUS SYSTEM
Overview of the Nervous System:
There are two main divisions of the nervous system – the central nervous system
(CNS) and peripheral nervous system (PNS). Both the CNS and PNS contain
specialized cells that carry information and commands – neurons.
The Central Nervous System
The central nervous system is comprised of the brain and spinal cord. The brain
and spinal cord are enclosed by the cranium and vertebral column respectively,
and protected by the meninges. The meninges is made up of three layers:
1. The dura mater – the outmost, thickest layer
2. Arachnoid mater – the middle layer
that forms arachnoid granulations
that form projections for
cerebrospinal fluid to be reabsorbed
3. Pia mater – the thinnest, most
delicate layer that adheres to the
surface of the brain and spinal cord,
rich with blood vessels
Also protecting the central nervous system
is the ventricular system, containing cerebrospinal fluid (CSF). There are large
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cavities present in the brain stem and cerebral hemispheres, which serve as a
channel for CSF. The CSF is primarily used for protection, chemical stability and
buoyancy.
The Peripheral Nervous System
The PNS is all of the nervous system except for the brain and spinal cord. It is
composed of two substances
1. Nerves – bundles of nerve fibers (axons) wrapped in a fibrous connective
tissue, and
2. Ganglia – knot-like swellings in a nerve where the cell bodies lie
The parasympathetic nervous system has two main divisions – the sensory
division and motor divisions. The sensory and motor divisions have further
subdivisions, the somatic (voluntary) and visceral (involuntary) subdivisions.
Sensory and Motor Divisions
The sensory division (called the afferent division) carries information from
receptors to the central nervous system. The somatic sensory division carries
signals from receptors in the skin, muscles, bones and joints while the visceral
sensory division carries signals from visceral organs (i.e. heart, lungs, stomach
and urinary bladder.)
The motor division (called the efferent division) carries signals from the CNS to
effectors, which are usually glands or muscles that carry out the bodys response.
The somatic motor division carries signals to skeletal muscles, which produces
muscular contraction and somatic reflexes. The visceral motor division carries
signals to glands, cardiac and smooth muscle, and their involuntary responses
are called visceral reflexes.
The Autonomic Nervous System
The visceral motor division of the peripheral nervous system is referred to as the
autonomic nervous system. It has parts present in both the CNS and PNS, making
it anatomically not a distinct division. Similar to the CNS and PNS, the autonomic
nervous system has an efferent and afferent component. The efferent component
sends signals to muscles and glands, and can cause an excitatory (sympathetic)
or inhibitory (parasympathetic) effect. The afferent component is largely
imperceptible as the sensations are all within normal limits. However, if there is
excessive pressure or stretch on visceral organs, warning signals felt as pain are
sent.
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Neurons:
Neurons are defined as a bundle of nerve fibers wrapped in a fibrous connective
tissue. While the structure of neurons differ depending on their function, all
neurons share the same universal properties:
• Neurons are excitable, meaning they respond to environmental changes
called stimuli
• Neurons are conductive, meaning they respond to stimuli by producing
electrical signals that are conducted to other cells at distant locations
• Neurons are able to release neurotransmitters, which are released when
the electrical signal reaches the end of the cell, influencing the next cell
Structure of a Neuron
A neuron contains the following structures:
• A cell body (soma) that is the control center of the neuron. It contains a
single, centrally located nucleus with a large nucleolus. The cytoplasm of
the soma contains organelles.
• Branching off of the soma are structures called dendrites. Dendrites are
the primary site for receiving signals from other neurons. The more
dendrites a neuron has, the more information it can receive
• Nerve fibers, called axons. Axons are cylindrical structures that take
information away from the soma.
• Some axons are covered by the myelin sheath. The myelin is insulation
around a nerve fiber that creates greater resistance to electrical impulses
and elicits an action potential response.
• The myelin sheath is segmented by the Nodes of Ranvier, which allow
electrical messages to be passed along an axon more quickly
• The terminal part is a little swelling that forms a synapse, or junction with
the adjacent neuron. They contain synaptic vesicles full of
neurotransmitters.
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Document Summary
There are two main divisions of the nervous system the central nervous system specialized cells that carry information and commands neurons. (cns) and peripheral nervous system (pns). The central nervous system is comprised of the brain and spinal cord. The brain and spinal cord are enclosed by the cranium and vertebral column respectively, and protected by the meninges. Also protecting the central nervous system is the ventricular system, containing cerebrospinal fluid (csf). There are large cavities present in the brain stem and cerebral hemispheres, which serve as a channel for csf. The csf is primarily used for protection, chemical stability and buoyancy. The pns is all of the nervous system except for the brain and spinal cord. It is composed of two substances: nerves bundles of nerve fibers (axons) wrapped in a fibrous connective, ganglia knot-like swellings in a nerve where the cell bodies lie. The parasympathetic nervous system has two main divisions the sensory division and motor divisions.
