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Neuroanatomy Bio ISU
Transcript of Neuroanatomy Bio ISU
What is Neuroanatomy? Neurons are the basic
units of nervous tissue,
otherwise known as nerve
cells. They make up the
brain, spinal cord and the
retina in the central nervous
system. The nerve processes are made up of dendrites and axons. Dendrites are hair-like structures surrounding the cell body that conduct incoming signals. Axons, or nerve fibres, can vary in length from one millimeter to a metre. They conduct outgoing signals emitted by other neurons. Thus, the point at which an axon from one neuron and a dendrite from another neuron is called a synapses. Any given neuron is connected to thousands of neurons. Axons are encased in a fat
like sheath called myelin, which acts as an insulator and speeds up impulse transmission. Messages passed to and from the brain between neurons are called electrical impulses or action potentials, which are produced by a chemical reaction that moves along the axon. At the synapse, the message/impulse causes the release of neurotransmitters, and these drive the impulse to the next neuron. The speed at which these impulses travel along neurons is roughly 250 miles/hour. Neurons are anatomically divided into
two sections: the nerve processes and the nerve body. Nerve bodies contain the neuron's nucleus, organelles, and cytoplasm. Neurons are generally located in groups, and in the brain and spinal cord, these groups are called nuclei. Outside of the brain and spinal cord, they are called ganglia. Nerve bodies are called gray matter because of their colour. The axons of the neuron are called white matter because of their white mylein sheath.
The nervous system is divided into two categories, the Central Nervous system (CNS) and the Peripheral Nervous system (PNS). The CNS involves all neurons in the brain and the spinal cord, while the PNS involves neurons pretty much everywhere else. The PNS is divided into two categories: the somatic nervous
system and the autonomic nervous system (ANS). The somatic nervous system is made up of peripheral nerve fibres that send sensory information to the CNS and motor nerve fibres that project to skeletal muscle. The autonomic nervous system is divided into
three subcategories: the sympathetic nervous
system, the parasympathetic nervous system
and the enteric nervous system. Think of the sympathetic nervous system as the accelerator and the parasympathetic nervous system as the brake. The sympathetic functions in actions requiring quick responses, while the parasympathetic functions with actions that do not require immediate reaction.
The enteric nervous system is a meshing network of nerve fibres that supplies nerves to the GI tract, the pancreas and the gall bladder.
By Peter Loudfoot
Why Choose Neuroanatomy?
I wanted to know how the knee jerk reflex worked, but in order to know that, I needed to know about the nervous system and neuroanatomy.
All in all, it's a pretty interesting subject.
Who wouldn't want to know what make their reflexes react? There are 3 types of neurons we'll be looking at. Motor Neurons Efferent neuron: moving towards a central organ or point.
Relays messages from the brain or spinal cord to the muscles and organs. Sensory Neurons Afferent Neuron: moving away from a central organ or point.
Relays messages from receptors to the brain or spinal cord. Interneurons (relay neuron) Relays information from sensory
neurons to motor neurons.
Makes up the brain and spinal cord If the brain is the central computer
of your body, your spinal cord is like
a network that relays messages back
and forth to the brain from different
parts of your body. The spinal cord contains threadlike nerves, branching out to every organ and body part. Neural circuits in the mammalian spinal cord generates motor commands for rhythmic movements such as
motion. The nervous system is the body's information gatherer, storage center
and control system. The overall function of the nervous system is to
collect information about the body's internal/external states and
transfer this info to the brain (afferent system) to analyze information,
and then to send impulses out (efferent system) to initiate the
appropriate motor responses to meet the body's needs. CNS The brain is the body's biggest organ, with over
100 billion neurons and weighing in at roughly 3
pounds. Spinal cord serves 2 functions:
provides the two way conduction route for afferent and efferent systems between the brain and nerves
serves as a reflex centre for all spinal reflexes.
Thirty-one pairs of nerves are connected to the spinal cord, all of which have no special names, but are numbered according to what vertebrate they emerge from. Each nerve has two roots in
the spinal cord: the posterior (dorsal)
root is the sensory root and the
anterior (ventral) root is the motor root.
Some fibres synapse with other
neurons in the dorsal horn,
while still others continue up to
the brain. The cell bodies in the
ventral horn send axons through
the ventral root to muscles to
You all know a reflex as an involuntary movement in response to a stimulus/trigger, but in regards to microanatomy, it is "a relatively stereotyped movement or response caused by a stimulus applied to the periphery, transmitted to the central nervous system and then transmitted back out to the periphery."
Sensory neurons convey info from tissues and organs into the CNS. Motor neurons transmit signals from the central nervous system to the effecter cells.
A reflex action, or reflex is an involuntary movement in response to a stimulus/trigger. Reflex actions in humans are controlled/mediated by the reflex arc.
The reflex arc is a neural pathway that mediates a
reflex. Most sensory neurons do not pass into the
brain, but react/synapse in the spinal cord. This
allows reflexes to occur quickly because of the
activation of spinal motor neurons without the
delay of routing signals via neurons to the brain.
The brain does receive sensory information while
the action occurs.
Reflex Arcs Reflex arcs are divided into two different categories: the autonomic reflex arc (inner organs) and the somatic reflex arc (affecting muscles). There are two classifications of reflex arcs: monosynaptic and polysynaptic. Monosynaptic and Polysynaptic Reflex Arcs A reflex arc consisting of two neurons (1 sensory neuron and 1 motor neuron) is monosynaptic, referring to the presence of one chemical synapse. Monosynaptic reflexes are the simplest of reflexes (ex. knee jerk- when you hit your knee with a hammer and it swings out).
Polysynaptic reflexes are one or more interneurons connecting afferent (sensory) and efferent (motor) signals, and can be processed in the spinal cord and inhibited. This is because polysynaptic reflexes are connected to the brain via longitudinal spinal cords, which, exactly like they sound, run the length of the spine. This means that conscious control of the brain can stop reflexes, ex, touch a hot object but override your reflex and leave it there.
Sensory receptors send info to the spinal cord (and to the brain) through the spinal nerve. These cell bodies for these sensory receptors are located in the dorsal root ganglion and enter the spinal cord through the dorsal/posterior root.
Knee Jerk Reflex Simplest reflex is monosynaptic reflex, or “two-neuron reflex”,
an example of which would be the tendon jerk reflex or
tendon tap reflex, otherwise called myotatic reflex. This reflex
is evoked (brought forth) by tapping the tendon just below
the patella, aka your kneecap. This tap, applied to the tendons
of your quadriceps muscles, stretches the muscles and their
muscle spindles. The muscle spindles trigger the stimulus of
a sensory receptor in the muscle, sending a signal to the
spinal cord (afferent system), where it enters through the
dorsal root, then synapses with a sensory nerve axon in the
ventral horn. From here, the receiving axon carries the impulse
from the synapses through the ventral root back to the muscle,
whose specific tendon was tapped (efferent system). The
Invention Regulates Cells Electronically Researchers at Linkoping University and Karolinska Institute in Sweden have made a breakthrough in making artificial nerve cells that respond with the impulses and messages from other nerve cells. They have made an ion transistor (semiconductor device that amplifies or switches the flow of current between two terminals) of plastic that can transport ions and charged biomolecules, thereby addressing and regulating cells. In other words, they can make neurons synapse. This means that they are beginning to explore and possibly fix neurons that might have previously been damaged. So, it might be able help restore feeling and nerves back into paraplegics. This could also mean that they might be able to create an artificial reflex arc in the future, and thus, fully restore motion to paraplegics. Bibliography Expertanswer. "Intervention regulates nerve cells electronically". ScienceDaily. May 22, 2010. May 31, 2010.
Mann D., Michael. The Nervous System in Action. Oklahoma City, OK. McGraw-Hill Ryerson. 1997.
"Reflex Arc". Wikipedia. May 25, 2010. http://en.wikipedia.org/wiki/Reflex_arc
"Nervous System: CNS and PNS". PATTS. July 2001. May 25, 2010. http://webschoolsolutions.com/patts/systems/nervous.htm
"The Human Nervous System". Biologymad.com. April 17, 2004. May 25, 2010. http://www.biologymad.com/NervousSystem/nervoussystemintro.htm
"The Nervous System". Maricopa.edu. May 18, 2010. May 25, 2010.
http://www.emc.maricopa.edu/faculty/farabee/biobk/biobooknerv.html Trivia time 1. Nerve processes are made up of what? What does afferent and efferent mean? What is the peripheral system divided into? What is an example of a polysynaptic reflex? What is an example of a monosynaptic reflex? (other than the knee jerk reflex) What is another name for a myotatic reflex? What are neurons? What is a reflex arc? What is neuroscience?