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The Nervous System
The nervous system is, perhaps unsurprisingly, the system of nerves in our body. It has several parts, as described below.
The Central and Peripheral Nervous Systems are separated from one another. The Sensory-Somatic and Autonomic Nervous Systems are also separated from one another although they both overlap the CNS and PNS.
The Central Nervous System is made up of the brain and the spinal chord.
The brain is made up of three main areas, sometimes known as the 'lizard brain', the 'leopard brain' and the 'learning brain'. The lower 'lizard brain' contains basic motor functions that all animals have. The central 'leopard brain' is common to all mammals and adds emotions to the brain's functions. The upper cortex 'learning brain' adds complex thinking functions, language and more.
The brain receives about 20% of the body's blood. It floats in a bath of cerebrospinal fluid (CSF). CSF is extracted from the blood as a form of plasma.
The spinal cord is the main 'superhighway' for signals between the body and the brain. It is protected by the spine, which is a clever system of interlocking bones with a hole up the middle.
The cord is delicate, and breaking the spine can result in damage to the cord and hence paralysis. Break your lower back and you lose the use of your legs. Break your neck and your arms can become immobile too.
The Peripheral Nervous System is the 'rest of the nervous system', carrying signals outside the brain and spinal chord. These are the 'wires' that run throughout the rest of the body, carrying signals to muscles that tell them how to move, as well as sending feedback to the brain about how things are working. This includes touch and pain signals from the skin.
This system overlaps the above CNS and PNS and include neurons to provide sense information to the brain and motor neurons to drive muscles and glands. This system thus provides the interface between the outside world and the brain.
Cranial nerves are in the head connect facial systems with the brain. There are 12 pairs of cranial nerves, as follows:
There are 31 pairs of spinal nerves. These are all mixed, containing both sensory and motor neurons.
This system connects the brain and internal organs, such as the heart, lungs and various glands. Whilst we have a significant conscious control over the Sensory-Somatic system, the Autonomic system largely works by itself, regulating temperature, keeping the heart pumping, keeping lungs breathing and so on.
The ANS includes both sensory and motor neurons. Preganglionic neurons start in the CNS and run to a ganglion in the body where they connect with postganglionic neurons that connect with the target organ.
The ANS has a pair of matched subdivisions: The sympathetic and parasympathetic nervous systems. Examples of these are given in the following table:
The sympathetic system has two chains of ganglia than run parallel to and either side of the spinal cord. These are linked to the spinal cord with preganglionic neurons.
The sympathetic neurotransmitter which stimulates the postganglionic neurons is acetylcholine (ACh). Postganglonic neurons release noradrenaline (also called norepinephrine).
The sympathetic nervous system thus prepares the body for fight-or-flight.
Sympathetic activation is quite general not only because there is a one-to-many pattern of pre- to post-ganglionic neurons, but also because adrenaline that is released into the blood ensures all body cells are subject to sympathetic stimulation even if they are not directly affected by postganglionic stimulation.
The parasympathetic system is mostly driven by the vagus nerve, plus a few additions such as the medulla for saliva stimulation and the spinal cord for bladder contraction.
The parasympathetic neurotransmitter is also mostly acetylcholine (ACh), although there is also some nitric oxide (NO) used.
The parasympathetic nervous system reverses the effect of the sympathetic system, calming the body and returning it to a steady state.
The nervous system is protected by a tough tissue called the meninges (sing. meninx). The outer layer is called the dura mater ('hard mother'). The middle layer is the soft and web-like arachnoid membrane. Cushioning the nerves and the blood vessels is the pia mater ('pious mother').
In the CNS, between the pia mater and the arachnoid membrane is the sub-arachnoid space, which is filled with cerebrospinal fluid.
Arachnoid granulations (or arachnoid villi) are small protrusions of the arachnoid membrane through the dura mater into the superior sagittal sinus (the venous area above and behind the brain). These pass cerebrospinal fluid from the brain into the blood stream.
The PNS has only a fused double layer of dura mater and pia mater.