What is it, how do I remember it all and how on earth do you pronounce it? We can help …
What is it?
The nervous system is a single system, but it is divided into two sections; the central nervous system or CNS, which is the brain and spinal cord, and the peripheral nervous system or PNS that includes everything outside the brain and spinal cord.
The brain and spinal cord combine the information that they receive with other kinds of information, compare it to information from past experiences, and make decisions about how to respond. Once that decision is made, the ‘output’ side of the nervous system is activated, which carries instructions from the brain and spinal cord. This ‘output’ side is the motor or efferent system that carries information to all three types of muscle (smooth, skeletal and cardiac) and also to the body’s glands, which elicits a response. The motor system is divided into two branches, the somatic, which controls skeletal muscle and voluntary movements, and the autonomic, which controls smooth and cardiac muscle. The autonomic branch is involuntary and not under conscious control, and is further divided into the sympathetic and parasympathetic branches. The sympathetic branch is known as the ‘fight or flight’ response system whereas the parasympathetic branch is known as the ‘rest and digest’ response system.
Specialised cells in the nervous system called neurons make the nervous system function. Neurons are excitable cells that change their electrical charges and transmit information or effect muscular contractions. Neurons are rather bizarre looking cells, with many branches and what looks like a tail.
Each part of a neuron has a specific function. The dendrites receive information from the environment or other cells. The axon generates and sends signals to other cells. The signal travels down the axon until it reaches the axon terminal, which then connects to a receiving cell, called a synapse. Neurons are classified by how their structure and function.
The nervous system works by generating electrical current. Each time a charged particle flows across the cell membrane in enough amounts, an electrical current is produced. These electrical currents are generated by substances called electrolytes. Positively charged electrolytes are called cations and negatively charged electrolytes are called anions. Examples of cations include potassium (K+) and sodium (Na+) and an example of an anion is chloride (Cl-). The speed of impulse conduction is determined by two characteristics: the presence of myelin and the diameter of the axon. Myelin is an insulator and makes the nerve impulse go faster. When an impulse arrives at the end of an axon terminal, neurotransmitters carry the impulse to the next axon.
The spinal cord runs from the foramen magna to the second lumbar vertebrae and is divided into 31 segments, each with a pair of spinal nerves. The spinal cord is divided in half by a ventral median fissure and a dorsal median sulcus. The interior of the spinal cord is divided into a series of sections of white matter columns and grey matter horns. The dorsal horn is involved in sensory functions, the ventral horn is involved in motor functions, and the lateral horn is involved in autonomic functions.
Five quick facts
- The nervous system is organised into two sections, the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The CNS is made up of the brain and spinal cord and everything else forms the PNS. The CNS controls all basic bodily functions and responds to external changes. The PNS provides a complete network of sensory and motor fibres that connect the CNS to the rest of the body.
- Some nerve impulses can travel at 100 metres per second (200 miles an hour)
- Nervous tissue is made up of two different cell types, neuroglia (or glial cells) and neurons. There are four types of neuroglia in the central nervous system, astrocytes; microglia; ependymal cells; and oligodendrocytes, and two types in the peripheral nervous system, Schwann cells and Satellite cells. Neurons are classified according to how they look, their structure, and their function, and can be unipolar, bipolar or multipolar
- Neurons are excitable cells; they carry electrical current caused by changes in cell permeability to certain ions.
- Tiny electrical currents can be all-or-none (action potentials), can change depending on the size of the stimulus (local potentials), can travel down axons (impulse conduction), or can be used to transmit information from one cell to another (synapses).
Hint to learning the system
Review the organisation of the nervous system and try to draw out a diagram that shows the differences between the CNS and PNS. You can see that the PNS has sub-divisions (the somatic and autonomic) and further divides into the parasympathetic and sympathetic divisions of the autonomic nervous system.
In the spinal cord, directions terms – anterior, posterior, dorsal and ventral – are used because function is linked to location. Spinal roots are always dorsal and ventral.
- astrocytes(AST row SIGHT)
- efferent(EFF er ent)
- electrolytes (eh LEK trow light)
- ependymal (ep PEN dye mal)
- glialcells (GLY all sells)
- microglia (my crow GLY ah)
- myelin (My lin)
- neuroglia (new ROH gly ah)
- neurotransmitters (new row TRANS mit ter)
- oligodendrocytes (OLLY go DEN drow sites)
- parasympathetic (para sim pah THET ik)
- Schwann (SHWON)
- Somatic (soh MAT ick)
- sulcus (SULL cuss)
- synapses (SIH naps)
Martin Steggall is the co-author of Anatomy and Physiology for Nursing and Healthcare Professionals 2nd Ed.