Download Nervous System A basic overview What does it do? Allows us to

Nervous System
A basic overview
What does it do?
 Allows us to
◦ Think, feel, remember, move, being aware of the world around us
 How?
◦ Sensory receptors bring messages to the Central Nervous System(CNS) from both inside and outside
sources.
◦ CNS determines course of action and sends a message out to where it need to go so that the action
can happen.
So... it is the master control?
 Yep, it sure is and it is also our means of communication, too.
 How does it work? Well....
◦ Sensory input – monitoring stimuli
▪ This is all the data the receptors collect... you know... pain, pressure, temperature... that sort of
stuff
◦ Integration – interpretation of sensory input
▪ The data is brought together to create memories, sensations, produce thoughts.. stuff like that
and then we make decisions about it, either consciously or subconsciously
◦ Motor output – response to stimuli
▪ This is how we act on what we have sensed... like running to the restroom, jerking our hand
back from a hot stove...
Okay... so what are the parts?
 The major players are the Nerve cells (technically called the neurons) that use electrochemical impulses
(nerve impulses) to pass messages from one to another and to the effectors (the things that respond to the
message)
◦ They make up the brain, spinal cord, motor nerves, sensory nerves... anything that sends and
receives messages in us has a nerve.
1st major classification
 Are they of the CNS (central nervous system)
◦ Brain and spinal cord
◦ Integration and command center
 Or are they the PNS (peripheral nervous system)
◦ Paired spinal and cranial nerves (peripheral nerves)
◦ Carries messages to and from the spinal cord and brain
The PNS is further divided
 Sensory (afferent) division
◦ Sensory afferent fibers – carry impulses from skin, skeletal muscles, and joints to the brain
◦ Visceral afferent fibers – transmit impulses from visceral organs to the brain
 Motor (efferent) division
◦ Transmits impulses from the CNS to effector organs
The motor division gets to do it again....
 Somatic nervous system
◦ Conscious control of skeletal muscles (Voluntary)
 Autonomic nervous system (ANS)
◦ Regulates smooth muscle, cardiac muscle, and glands (Involuntary)
◦ Divisions – (once again...^-^)
▪
▪
Sympathetic- prepares the body for energy-expending, stressful, or emergency actions
Parasympathetic- used in ordinary restful conditions, and returning to that after the sympathetic
division has had its way.
Okay Okay Okay... we got that, but what does the nerve look like?
 Well... it has three main parts
◦ Dendrites
◦ Cell body
◦ Axon
 And some ancillary parts too
◦ Nodes of Ranvier
◦ Schwann cells & the Neurilemma
◦ Myelin sheaths
Let's start with the main parts
 Dendrites
◦ They are the short, tapering, and diffusely branched processes (extensions)
◦ receive the messages from where ever (sensory receptors, other nerves...) and carries them to the
Cell body
 Axon
◦ It is the slender processes of uniform diameter arising from the hillock (If long called a nerve fiber)
◦ Typically only one and it is usually unbranched
◦ Takes messages from the Cell body to the next stop (dendrites or the effector)
So, what about the Cell body?
 Contains the nucleus and a nucleolus, mitchondria, gogli apparatus... those sorts of things
 Is the focal point for the outgrowth of neuronal processes (Those things we just finished talking about)
 Has no centrioles (hence its amitotic nature – that means it can't divide to make new cells)
 Has well-developed Nissl bodies (kind of like rough ER)
 Contains an axon hillock – cone-shaped area from which axons arise
The Ancillaries? First up... the Myelin sheaths
 Whitish, fatty (protein-lipoid), segmented sheath around most long axons
 It functions to:
◦ Protect the axon
◦ Electrically insulate fibers from one another
◦ Increase the speed of nerve impulse transmission
 It is created by Schwann cells in PNS
◦ Can repair the axon if damages
 It is created by oligodendrocytes in the CNS
◦ Axons usually cannot repair
Schwann cells?
 A Schwann cell:
◦ Envelopes an axon in a trough
◦ Encloses the axon with its plasma membrane
◦ Has concentric layers of membrane that make up the myelin sheath
 Neurilemma – remaining nucleus and cytoplasm of a Schwann cell
So, what are these node things?
 The Nodes of Ranvier (AKS Neurofibral nodes)
 Gaps in the myelin sheath between adjacent Schwann cells
So, what if the axon is short?
 It is called unmyelinated.
◦ A Schwann cell surrounds nerve fibers but coiling does not take place
 In the brain they are called gray matter (the myelinated are called white matter)