Anatomy and Physiology HAP101 Chapter Notes - Chapter 12: Central Nervous System, Upper Motor Neuron, Resting Potential

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HAP101 - Chapter 12: Nervous Tissue
LO 12.1: Describe the organization of the nervous system: CNS, PNS, Somatic/Autonomic/Enteric NS
The Central Nervous System (CNS)
o Consists of the brain (located in the skill; contains billions of neurons), and the spinal cord (connected
to brain through the foramen magnum of the occipital bone; is encircled by vertebral column bones;
contains millions of neurons)
o Processes many kinds of incoming sensory information and is the source emotional
thinking/memories.
o Creates signals that stimulate muscles to contract and glands to secrete
The Peripheral Nervous System (PNS)
o Consists of all nervous tissue outside the CNS.
o It is composed of the following:
Nerve: a bundle of many axons, associated connective tissue and blood vessels that lies outside
the brain and spinal cord. There are 12 pairs of cranial nerves and 31 pairs of spinal cord nerves.
Each nerve follows a defined path and serves a specific region of the body
Ganglia: small masses of nervous tissue, consisting primarily of neuron cell bodies, that are
located outside of the brain and spinal cord. They are closely associated with the cranial and
spinal nerves
Enteric Plexuses: extensive networks of neurons located in the walls of organs of the GI tract.
Such neurons help regulate the digestive system
Sensory Receptor: a structure of the NS that monitors changes in the external or internal
environment
o The PNS is divided into a somatic nervous system (SNS), an autonomic nervous system (ANS),
and an enteric nervous system (ENS)
SNS: consists of (1) sensory neurons that convey information to the CNS from somatic receptors
and sensory receptors and (2) motor neurons that conduct impulses from the CNS to the skeletal
muscles only; these actions are consciously controlled i.e. voluntary
ANS: consists of (1) sensory neurons that convey information to the CNs form AS receptors
located primarily in visceral organs, and (2) motor neurons that conduct nerve pulses from the
CNS to smooth/cardiac muscles and glands; these actions are not under conscious control i.e.
involuntary. The motor part of the ANS consists of the sympathetic divisions (helps support
exercise or emergency actions, the “flight – fight” responses) and the parasympathetic division
(takes care of “rest – and digest” activities)
ENS: knowns as the “brain of the gut,” and is involuntary. Consists of millions of neurons in
enteric plexuses that extend the length of the GI tract. Many such neurons function independently
of the ANS and CNS to some extent (communicate with the CNS via symp. and parasymp.
neurons. ENS sensory neurons monitor chemical changes in the GI/stretching of its walls. They
also govern contractions of GI tract smooth muscle to propel food through the GI tract, secretions
of such organs, and activities of the endocrine cells in the tract.
LO 12.2: Identify the functions of the NS
There are three basic functions of the NS
o Sensory Function (input): these receptors detect internal and external stimuli. The sensory
information is then carried into the brain and spinal cord through cranial and spinal nerves
o Integrative Function (process): this system processes sensory information by analyzing it and
making decisions for appropriate responses i.e. integration
o Motor Function (output): once sensory information in integrated, the NS may elicit (provoke) an
appropriate motor response by effectors (muscles and glands) through cranial and spinal nerves.
Muscles contract and glands secrete when effectors are stimulated
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LO 12.3: Identify and describe the functions of a neuron with reference to its structure
Neurons
o Form complex processing networks within the brain and spinal cord
o They are highly specialized cells that are capable of reaching great lengths and creating extremely
intricate connections with other cells
o Provide functions for the NS, such as thinking, sensing, remembering, controlling muscle activity,
regulating glandular secretions
o Most have lost the ability to undergo cell division
Neuroglia
o Smaller cells that are outnumbered by neurons
o They support, nourish, and protect neurons; maintain the interstitial fluid that surrounds neurons; and
are able to divide
NEURONS IN DEPTH
o These nerve cells possess electrical excitability the ability to respond to a stimulus (any change in
the environment that is strong enough to initiate an action potential) and convert it into an action
potential (an electrical signal that travels along the surface of the membrane of the neuron).
o An action potential is initiated due to the movement of sodium and potassium ions between the
interstitial fluid and the inside of a neuron through specific ion channels in its plasma membrane.
o Most neurons have three parts
Cell Body (perikaryon or soma): this contains a nucleus surrounded by cytoplasm consisting of
typical cellular organelles. Such cell bodies also contain Nissl bodies, a cluster of rough
endoplasmic reticulum and free ribosomes (produce protein synthesis)
Dendrite: processes (extensions) that are the receiving or input portion of a neuron. Its plasma
membrane contains numerous receptor sites for binding chemical messengers from other cells.
They are usually short, tapering and highly branched
Axon: a neuron propagates nerve impulses toward another neuron, a muscle fiber or a gland cell.
It is a long, thin, cylindrical projection and contains mitochondria, microtubules, and neurofibrils.
Protein synthesis does not occur here because it does not have rough ER
o Synapse is the site of communication between two neurons or between a neuron and an effector cell.
o Neurotransmitter is a molecule released form a synaptic vesicle that excites or inhibits another
neuron, muscle fiber or gland cell.
o There are two types of transport systems that carry materials from the cell body to the axon terminal
and back
Slow Axonal Transport: are capable of transporting materials ~1-5 mm per day. It conveys
axoplasm in one direction (from cell body toward axon terminal). It supplies new axoplasm to
developing or regenerating axons and replenishes axoplasm in growing and mature axons
Fast Axonal Transport: are capable of transporting materials a distance of 200-400 mm per day.
It uses proteins that functions as “motors” to move materials along the surface of microtubules of
the neuron’s cytoskeleton. It moves materials in both directions (away from and towards the cell
body)
o Structural Classification: neurons are classified according to the number of processes extending
from the cell body
Multipolar Neurons: have several dendrites and one axon. Neurons in the brain and spinal cord,
as well as all motor neurons are of this type
Bipolar Neurons: have one main dendrite and one axon. These are found in the retina of the eye,
the inner ear, and the olfactory area of the small brain
Unipolar Neurons: these have dendrites and one axon that are fused together to form a
continuous process that emerge from the cell body. Such neurons are called pseudo-unipolar
neurons, as they begin in the embryo as bipolar neuron
LO 12.4: Contrast the three types of neurons
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Functional Classification: neurons are classified according to the direction in which the nerve impulse is
conveyed with respect to the CNS
o Sensory (afferent): either contain sensory receptors at dendrites or are located just after sensory
receptors that are separate cells. Once a stimulus activates a sensory receptor, an action potential is
formed in its axon and the action potential is conveyed into the CNS through cranial or spinal nerves.
Most are unipolar in structure.
o Motor (efferent): convey action potentials away from the CNS to effectors in the PNS through
cranial or spinal nerves. Most are multipolar in structure
o Interneurons (association): mainly located with the CNS between sensory and motor neurons. Such
neurons integrate (process) incoming sensory information from sensory neurons and then provoke a
motor response by activating the appropriate motor neurons. Most are multipolar in structure
LO 12.6: Describe the general properties and importance of neuroglia in the NS
NEUROGLIA IN DEPTH
o There are six types of neuroglia: astrocytes, oligodendrocytes, microglia, and ependymal cells are
found in the CNS; Schwann cells and satellite cells are found in the PNS
Astrocytes: there are two types protoplasmic astrocytes have short branching processes and are
found in gray matter (unmyelinated axons); fibrous astrocytes have long unbranched processes
and are located mainly in white matter (myelinated axons). The extensions of astrocytes make
contact with blood capillaries, neurons and the pia matter (thin membrane around the brain and
spinal cord)
They contain microfilaments that give strength, to support neurons
Extensions of astrocytes wrapped around capillaries isolate CNS neurons from harmful
substances in blood
In embryos, these create chemicals that regulate the growth, migration and interconnection
among brain neurons
They help maintain chemical environments to generate nerve impulses
They also play a role in learning and memory by influencing the formation of neural
synapse
Oligodendrocytes: these resemble astrocytes but are smaller and contain fewer extensions.
Processes of these are responsible for forming and maintaining the myelin sheath around CNS
axons
Microglial Cells or Microglia: these are small cells with slender processes that give off
numerous spine-like projections. They function as phagocytes and remove cellular debris formed
during normal development of the NS and phagocytize microbes and damaged nervous tissue
Ependymal Cells: are cuboidal to columnar cells arranged in a single layer that possess
microvilli and cilia. They line ventricles of the brain and central canal of the spinal cord. These
cells produce, possibly monitor and assist in the circulation of cerebrospinal fluid.
Schwann Cells: these cells encircle PNS axons and form the myelin sheath. However, each
Schwann cell myelinate a single axon. Such a cell can enclose as many as 20 or more
unmyelinated axons and they also participate in axon regeneration, which is more easily
accomplished in the PNS than in the CNS
Satellite Cells: these are flat cells that surround cell bodies of neurons of PNS ganglia. They
provide structural support and regulate the exchanges of materials between neuronal cell bodies
and interstitial fluid
LO 12.7: Identify the function and purpose of myelination
Axons are surrounded by a multilayered lipid and protein covering, called the myelin sheath
This sheath electrically insulates the axon of a neuron and increases the speed of nerve impulse
conduction.
Schwann cells (PNS) and oligodendrocytes (CNS) produce myelin sheaths
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