Thursday, September 23, 2010

9.4 Central Nervous System Overview

Physician's Notebooks 9 - - See Homepage 

4. The Nervous System: Overview - Update 12 April 2018
Note to Reader: this chapter is packed with info; so it should be read in small segments slowly in a relaxed state, using references like internet Wikipedia, and re read in particular parts as needed. It is a good intro to many later chapters and will familiarize readers with the terminology, structures and functions. The chapter, in order, includes the following headings which may be used for search & find.

Today, the brain is likened to a personal computer
Your nervous system is
The paired cerebral hemispheres
the White Matter
The basal ganglia
The cerebellum
The amygdala
The Mid Brain - Pons & Medulla
The spinal cord
The neurons
The Wiring of your Nervous System
The key to your survival is right behavior
Feedback and Feed Forward
The nervous system's sensory input
The output is in the motor system
The Glia, or the Glue that Knits Up the CNS
Peripheral Nervous System
The Autonomic Nervous System 
 The independent nerve network of the GI tract

Today, the brain is likened to a personal computer, and this can be useful in understanding how the brain functions, such as the working memory being equivalent to a computer's RAM memory (Both are the immediate memory that is lost within minutes unless registered in long term memory) and memory storage being equivalent to a computer's magnetic tape long term memory. But the brain does more: it pays attention, expresses feelings, solves problems, invents ideas, creates consciousness. 

Your nervous system is composed of the Central (CNS; the brain and spinal cord) and the Peripheral (PNS, every nerve element outside the CNS). The division is a bit artificial because outgoing and incoming signals that pass through the PNS originate or terminate from CNS, but still it is useful in directing attention to the PNS nerves that run outside the CNS under skin and deep in all parts of our bodies, passing signals that cause muscle movement (motor nerves) and receiving signals that tell the center what the periphery is feeling and experiencing (sensory nerves). The PNS also has the nerves of the autonomic nervous system (ANS) which controls involuntary actions that keep us alive during sleep and also monitors what our internal organs are feeling and controls our glands. And PNS is not only the nerve fibers it includes the distal neuron ganglia.
   The brain is made up, from top to bottom (rostral to caudal), of the left and right cerebral cortex and, just underneath, its subcortical parts; the diencephalon (mid line it unites right and left cerebral cortex) which includes the thalamus and hypothalamus; then, caudally, comes the brainstem (the midbrain, pons and medulla). The cerebellum is like a small brain perched on the rear of the pons and 4th ventricle.The medulla connects the brain to the spinal cord. The brainstem and spinal cord are sources of the peripheral nerves, ie, they contain the nuclei of the proximal neurons that provide the nerve fibers in the cranial and spinal PNS nerves and through which signals are passed to the body and periphery from the CNS, and also signals are passed back to the brain from the periphery as sensory data for the CNS.
External view of brain in skull (on your left) with attached spinal cord hanging down in vertebral bony canal. Note on your right the surface of Brain is wrinkled. This is the cerebral cortex. The front of Brain overlies the eye orbits, and also note the median (mid line) aspect of right half of the Brain. In both views note the cerebellum on the rear of  the brainstem and between the occipital lobe and the rear of the pons below. Then note the pons on the front part of the brainstem, and below it the medulla which is the most caudal part of the brain, connecting with spinal cord.
The lower brain is, from rostral to caudally, the mid brain (not obvious in the above figures, it connects forebrain with pons), the pons and cerebellum, and the medulla oblongata (often just called medulla) that connects to spinal cord.
The paired cerebral hemispheres are bridged by a deep inner part (diencephalon) which on both sides has important subcortical pair-groups of neurons - the thalamus and basal ganglia - and in mid line, the left-right, right-left crossing fiber connections - the main crossed-fiber connection is called corpus callosum -  that put each side of your brain in touch with the other side and may be surgically cut to separate in cases of severe epilepsy. The brain's wrinkled surface is the cerebral cortex, several millimeters thick and packed with cortical neurons in 6 layers that, with input from all other nervous system elements, produces in its output the effect we call Mind  and Consciousness. 
Beneath the neuron-cellular gray matter cortex is the white matter of billions of micro size nerve fibers inputting sensory data from the outside world and the body, and outputting impulses between brain cortex upper neurons and lower neurons and the periphery. If you could peel away the cerebral cortex, you would see the subcortical inner brain neuron structures – the right & left Basal Ganglia, the right and left Thalamus and the single but right-left divided Hypothalamus; and the right and left Amygdala, as well as the major nerve fiber tracts that connect everything in the brain with everything else plus the periphery. 
The right and left thalamus  (pl. thalami)  at the brain's center is a central switchboard that is receiving information from the periphery via the spinal cord and also getting a back and forth signaling from the basal ganglia and cerebellum and upward sensory tracts and constantly feeding back and feeding forward; the thalamus keeps all parts of the brain in touch with each other. Central switchboard is an apt name for its function. 
The basal ganglia act like an inhibitory throttle-down for motor action; they smooth out our too sudden, too strong, awkward movements and, by selective release of the throttle, make for smoothly executed motor action. They are involved in rewarding actions that accomplishes good things and become habit. They are important for sports leaders in skillful learning. When the BG are damaged or under-function, we have the slow motion and at-rest tremor Parkinson disease, and when the BG because of hereditary disease are unbalanced, we have the jerky motion, psychosis-ridden Huntington disease. 
The amygdala are left and right brain, subcortical neuron group nuclei in the the temporal lobes.
        The left and right amygdala locations in medial temporal lobes are indicated by the red.
Amyg.png The amygdalae (plural) are involved in strong emotions like, fear and rage. When they get damaged or removed on dominant or both sides, placidity results.
The cerebellum coordinates fine movements. It is a coincidence timer and also measures space. A great child-prodigy violinist, like the late Yehudi Menuhin, was born with a superbly functioning cerebellum. When you reach for something, the success of your accurately fingering it depends on cerebellum. A sports champion has a wonderful cerebellum. You may test your cerebellum easily, right now, by closing your eyes and trying to touch the tip of your nose with right and then left index fingertip. If you keep missing one side or another, you have a malfunctioning cerebellum on that side and should get an MRI to check it. The cerebellum also is important for memory of physical things like your fingers' recalling the unlocking of a combination lock without your actually naming the numbers in your mind. 
The basal ganglia, and the cerebellum are closely connected with all parts of the cerebral cortex via the thalamus and function as modulators of conscious actions by feed-back and feed-forward connective mechanisms. They are the source of important movement and emotional disorders (the BG of Parkinson's & Huntington's disease) (the cerebellum is involved in schizophrenia).
The Mid Brain, Pons & Medulla:  The midbrain with pons connects the upper brain (cerebrum) and the lower pons/cerebellum/medulla, and, with the pons and medulla, composes the brainstem and has nerve centers that affect respiration, heartbeat, eye movement, reflexes and that monitor other vital body functions, during sleep and coma that cannot depend on voluntary control. The brainstem has most of the cranial nerve nuclei. Through the midbrain, pons and medulla run every nerve fiber that passes signals to and from every brain neuron to the spinal cord and then to the periphery of the body, and also the connections to and from the cerebellum. Damage to the midbrain, pons and/or the medulla - as seen after some brain strokes - is catastrophic. 
The spinal cord is a super highway for signals to and from the Brain and also has its own auto pilot controls if it gets disconnected from the upper brain as it may in motor vehicle, diving and horse thrown accidents. (the 1970s and 80s Superman, Christopher Reeve thrown from his horse)
Neurons are terminally differentiated cells, meaning we have the full set from childhood and cannot reproduce new ones. Each neuron grows out 2 sets of nerve fibers: a single large, long fiber that may branch, the axon, and that only sends out signals; also a number of thinner, shorter fibers, the dendrites, that receive signals into their neuron. The axons and dendrites may connect with each other or with each one's neuron cell body through a connection switchboard called the synapse. This results in signals from a neuron heading in one direction, either away from it or to it (neuron to axon to dendrite to the next neuron). Long- and short-term changes in transmission at the synapse are the mechanism of memory and learning.
The neurons are the main cells in the nervous system. Neurons are of various types and may function as groups (called "ganglia" or "nuclei"); an important one is the suprachiasmatic nuclei above the mouth's upper (hard) palate; it acts like a clock that has a near 24-hour cycle and it resets according to the seasonal daily light/dark signal coming into the brain through the eye retina. Our whole body's cycles are set to this body clock.
All neurons are capable of producing electric current so they function as nano-generators and their fibers are like electric wiring. The connections between neuron fibers - the synapses - are capable of change (also known as plasticity) which is the basis of memory and learning.
The Wiring of Your Nervous System: The central purpose of the brain is to cause a body's DNA to be passed on, and to donate its unique combinations to the future. To do that, your brain must keep the hunk of flesh that is you safe and alive long enough to reproduce and to raise offspring, It must deal with the threats of the outside environment and maximize survival value. 
The key to your survival is right behavior, (to ensure survival), and behavior is effected by actions of the motor part of the CNS. In any wiring system of a brain, the motor part is its output. And the motor includes, in addition to all the voluntary muscles of movement, the involuntary muscles like heart, and the glands and also the glandular production. So visualize a wiring system where the types of input feed back and feed forward ("Feedback" means signals that bounce back, like when a doctor hits your knee tendon with his reflex hammer and your knee jerks; "feed forward" means nerve signals pass forward in the nervous system to affect anticipated behavior).
Feedback and Feed Forward  Here is an example of feedback vs. feed forward: I make an appointment to meet you in the lobby of my office at 3pm. In feedback, I instruct you to message me on mobile phone as soon as you arrive in the lobby so I can know you just arrived and decide when I ought to come down to meet you. In feed forward I anticipate from previous experience or just from my instructions to you that you should arrive at 3pm so I come down without you feeding back the info to me. Feedback has the advantage of certainty (I know exactly you arrived) but the disadvantage of being a bit slow (I need to await your messaging). Feed forward has the advantage of speed (If I anticipated correctly we have pinpoint timing for our meeting) but the disadvantage of inexact knowledge (I may over-anticipate and come down to the lobby before you arrive and have to wait uselessly). This is a rather gross analogy from daily life but as we shall see the central nervous system makes great use of these 2 concepts on a neuronal level.
Now let us deal with the 3 main inputs in the human nerve wiring system: sensory, internal body state, and cognitive.
The nervous system's sensory input from the outside world is from sensors in eyes, ears, skin and other sensory receptors. All input is transmitted into the central nervous system along sensory nerve fibers (“afferent”, or incoming nerves) that are extensions of the sensory neurons. The sensory nerves transmit from the sensory receptors in the periphery and their fibers enter the spinal cord or the brainstem as ascending sensory fibers, and after 2 or 3 neuron-relays, to reach centers in the thalamus. In the thalamus, the input is sorted and distributed to the parts of the cerebral hemispheres (the cortex) to be used as data for decision-making and consciousness. The data are also flashed to the cerebellum so that it may estimate accurate timing and location. And also the information is flashed to the basal ganglia so that if the cerebellum determines there is danger in the behavior the basal ganglia can suddenly act to stop it. This sensory data (hear, see, feel) is correlated in the thalamus and integrated into conscious decision-making in parts of the cerebral cortex, and then fed back to the motor nervous system (or sometimes just stored in memory and later fed forward to affect actions).
   A second type of input is coming from your internal body -- the degree of alert/sleep state, the timing clock that determines your cycles of activity. This input is coming from special sensory neurons of the autonomic nervous system (ANS) located in the vital organs and inputted into key brain nuclei mostly in the hypothalamus. This input (from the basal ganglia) may also  act as a throttle or gain mechanism on motor activity and behavior. For example if your alertness is low or near sleep, even strong voluntary motor activity will be reduced and weakened due to the imput; if your gland hormone is low because of old age, even a situation that would normally make you highly sexually aggressive will then barely stimulate you. 
   A third type of input is what is called cognitive; it comes from the cerebral cortex and is your consciousness and cognition. As already mentioned, this cognitive input to the motor nervous system is the result of both sensory data and internal body data plus your memories of past experiences especially those that have rewarded your body or have harmed your body. Here is the real computer function of the human brain. The inputs from the internal part and from the periphery, integrated with your remembrance of your past is made into moment-by-moment decisions that are passed to the motor nervous system in a constantly modulated mode and produce your ongoing behavior. In this function, the plasticity of the synapses are of importance. Also to be mentioned here are the neurotransmitters - like serotonin, dopamine and norepinephrine - the chemical molecules that are liberated at the synapses and pass the signal onward.
The output is in the motor system, which includes the glands. As implied from the above, the output is constantly throttled down or gained up - by feedback or feed forward input; that is what is meant by modulated. But also the motor output has certain spontaneous features - such as vital organs like the heart or the GI tract being able to function autonomously, even after being cut off from the CNS, and like the central motor patterning that is present at birth as part of DNA inheritance and that explains why running range-animals like the horse are able to give birth to newborns that can run fast immediately they drop from their mother's womb even though they obviously never had a lesson in their lives.
The Neuroglia, or the Glue that Knits Up the CNS Neurons are not the only cells of the nervous system; they are the ones that receive and send signals. Structural cells in the CNS are called the (Neuro) Glia. The glia cells hold the brain together, provide scaffolding to guide neuron fiber direction and targeting, and reproduce and thus are responsible for nervous system tumors. Glia cells also assist the function of neurons producing energy for use by the neurons and assisting with neurotransmitters at the synapses. Glia cells make the myelin cover of nerve fibers. This cover is like the rubber insulation of electric wire and also greatly speeds nerve transmissions, and when the process is interfered with we get the demyelinating diseases, foremost of which is Multiple Sclerosis (MS). 
Peripheral Nervous System: Twelve pairs of cranial nerves exit and enter the lower brain through tunnels at base of skull, and, similarly, 31 pairs of spinal nerves attach to the CNS spinal cord segments. These nerve extensions and their branches are part of the peripheral nervous system (PNS) whose function is to transmit sensory data from the outer world and from the internal body milieu into the CNS; and, oppositely, to pass command signals down from the brain to muscles and other effector organs (glands, heart) through efferent (outgoing), motor nerves.
The Autonomic Nervous System (ANS) is a semi-independent set of mostly peripheral nerves whose first-neurons are located in base of brain and in the spinal cord. The first-neurons send signals via each one’s fibers at the levels of spinal cord from T1 to T12 (T= thoracic); from lower spinal cord; and also from the cranial nerves. The fibers synapse (pass the signal by special connection) with the second-neurons of the system in autonomic ganglia (ganglia, pl. of ganglion, a group of same type, same place neurons), and the second-neurons relay the signal to key body organs (heart, lungs, liver, bladder), smooth muscle tissue (eye pupil muscle, accessory eyelid muscle) and glands of the body (adrenals, thyroid). The ANS consists of 2 parts: excitatory sympathetic nerves that prepare animals for fight or flight, and inhibitory, conserving parasympathetic nerves that prepare animals to rest and digest. The autonomic system affects vital functions - like breathing, heartbeat, blood pressure and small blood vessel flow control, vision, and glandular secretions - that cannot depend only on voluntary higher brain functions that must be constantly monitored and controlled during sleep and other states of low consciousness. Important to note that the ANS is actually under higher CNS feedback and feed forward control through the hypothalamus; it also transmits signals from the vital organs to the brain that allow the brain to keep its body alive during sleep and other types of unconsciousness and that produce important visceral reflexes like blood pressure and pulse responses to body conditions without which we could not stay alive.
  The independent nerve network of the GI tract rounds out the peripheral nervous system. The basic nerve system of the GI tract is almost completely autonomous. A disease called Hirschsprung's occurs because a person is born with a segment of intestine that has no intrinsic nerves and will die of intestinal obstruction unless detected early.
END OF CHAPTER. To read next click 9. Brain Membranes - The Water On and In It/Brai...

No comments: