Animals known as bilateria, (animals with bilateral symmetry) which includes most vertebrates and invertebrates all have a central and peripheral nervous system, although they vary greatly in the number of cells which make up their nervous system (Franks, 2012). Cells are the fundamental units of the nervous system, responsible for transmitting information throughout the nervous system, from the nervous system to the rest of the body, and from the environment to our brains (Burton, Westen & Kowalski, 2009).
Humans have in the range of 100 billion neurons (nerve cells) in their own nervous system, in comparison to organisms such as simple worms which may have as little as a few hundred cells (Franks, 2012). It is this complex network of neurons which, working in harmony with the many other structures and organs that make up our bodies, can transform otherwise meaningless vibrations into the haunting sounds of Mozarts Requiem Mass in D Minor; wavelengths of light into the emotive brushstrokes of a great artist; and invoke within us a sensation of joy and a smile at the achievements of a child, or tears and despair at the death of a loved one.
This essay first explains the structure and interaction of these neurons, outlining the different types, their functions and the chemical processes that occur between them. It will then cover the structure and functions of both the central and peripheral nervous systems and the interaction between the two, followed by an example illustrating the differences between them, focusing on the processes which would occur during the course of a hypothetical event.
In its entirety this essay will highlight the key differences between the central and peripheral nervous systems, and how they work together harmoniously to provide us with our total experience of the world as we know it. It is incredible to imagine that each unique event that we experience in the course of our day begins with something so tiny as a cell. These microscopic structures reside within us at a population of billons and together transmit important information around the body. These cells primarily fall into two categories: neurons and glial cells (Franks, 2012).
Most neurons are made up of a cell body (or soma), an axon which sends messages away from the cell body, and dendrites which receive messages from other cells and send them towards the cell body. Connected to the axon are terminal buttons, which form synapses with other neurons or their dendrites, and are the vesicles which release chemicals called neurotransmitters. These neurotransmitters, once released and transmitted to the receiving cell will, depending on the type of chemical released, have either an excitatory effect on the receiving cell, causing it to fire, or an inhibitory effect, causing it to stop firing.
In the Central Nervous System (CNS), which consists of the brain and the spinal cord the primary excitatory neurotransmitter is glutamate; when this is secreted by the terminal buttons of one cell and taken up by another it causes the threshold of excitation to lower in the receiving cell, thus rendering it more likely to fire. In contrast the major inhibitory neurotransmitter in the CNS is gamma-aminobutyric acid (GABA), which raises the threshold of excitation on the receiving cell, rendering it less likely to fire.
There are several other neurotransmitters in the CNS other than glutamate and GABA, although these are the most common. In the Peripheral Nervous System (PNS; the network of nerves which lie outside of the CNS and relay information to and from the CNS) however, there are only two neurotransmitters; norepinephrine (also known as noradrenalin), and the major excitatory neurotransmitter; acetylcholine. The other types of cells are glial cells. These are supporting cells which perform important functions which assist neurons in performing at their best.
There are three different types of glial cells in the CNS: oligodendrocytes, astrocytes and microglia. Oligodendrocytes perform the important function of wrapping axons in myelin sheaths, which is a form of insulation that protects the axon from damage and prevents electrical signals from being picked up by nearby axons accidentally. One oligodendrocyte can produce up to fifty segments of myelin (Carlson, 2010). In the PNS, this function is performed by Schwann cells, which differ from oligodentrocytes in that Schwann cells can only produce one segment of myelin, as the entire cell is involved in the creation of the myelin segment.
Schwann cells highlight an important difference between the two nervous systems, as they are able to aid in the reconnection of damaged axons in the PNS, whereas damaged axons in the CNS cannot reconnect (Ashwell, 2010), due largely to the barriers put in place by another type of glial cell; astrocytes (Ashwell, 2010 & Carlson, 2010). Astrocytes are star shaped cells, and their job is to regulate the chemical environment in which neurons reside by providing support and nourishment for neurons.
Astrocytes also serve to fence in and effectively isolate neurons and oligodendrocytes, and constitute around 20 to 50% of the volume of most brain areas (Squire, Berg, Bloom, du Lac, Ghosh & Spitzer, 2008). Microglia are the smallest glial cells, and their job is to clean up debris such as dead cells through a process called phagocytosis (literally; to eat cells). Together, these cells form the bulk of the human brain, and the neurons and their axons create the effect of what we know as grey and white matter effectively what the brain looks like through the human eye.
What we cant see going on at a microscopic level, is that the gray matter is actually great clusters of cell bodies. This is the cerebral cortex, which is the outer skin of the brain itself, and the white matter is the axons leading from the cell bodies to other areas of the brain; their myelin sheathed tendrils making the matter appear white. In vertebrates the cerebral cortex and the rest of the CNS is protected by the meninges; three protective layers consisting of an innermost layer called the pia mater, or pious mother, which is the delicate membrane closest to the brain itself.
Above that is the subarachnoid space which is a cavity that lies between the pia mater and the arachnoid mater, and serves as a channel for the cerebrospinal fluid. The arachnoid mater is called such because of its web-like appearance. The dura mater, or tough mother is the strongest layer of the meninges, this lies between the skull and the arachnoid mater, forming the outer protective layer of the brain. The PNS however, is only protected by two layers of meninges; the pia mater and the dura mater. The arachnoid mater and cerebrospinal fluid do not surround the PNS.
Beneath the skull, inside of these three protective layers is the brain, its delicate outer tissue protected by their embrace, which is lucky for human beings, as the brain has evolved over hundreds of millions of years (Baars and Gage, 2012). The brain itself is divided into three major sections; the hindbrain, the midbrain, and the forebrain. The hindbrain is the earliest developed part of the brain, and includes the cerebellum, which is responsible for coordinated movements, the pons, which plays a part in sleep functions, and the medulla oblongata, which is involved in the control of the respiration and the cardiovascular system.
Rostral to the hindbrain is the midbrain, which consists of the tectum, whose main structures are the superior and inferior colliculi which are part of the visual and auditory systems respectively (Carlson, 2010). These colliculi play a part in visual reflexes, such as when the eye is reflexively drawn towards a moving stimulus in the peripheral vision. The other major structure of the midbrain is the tegmentum which consists largely of part of the reticular formation, which plays a role in movement, sleep, arousal, and vital reflexes. The tegmentum is also involved in species-typical behaviours such as mating and fighting (Carlson, 2010).
The midbrain plays a major and vital role in reactive movements and survival instincts (Squire, et al. , 2008). Together, the hindbrain and the midbrain constitute what is known as the brainstem. The remainder and largest area of the brain is the forebrain. This consists of the cerebral cortex, the basal ganglia, the thalamus, and the hypothalamus and the limbic system. The cerebral cortex is responsible for sensory and motor functions, and everything in between; perception, learning and memory, planning and action (Carlson, 2010); and is divided into four sections; the frontal, parietal, temporal and occipital lobes.
Each region is associated with different cognitive functions including the higher cognitive functions of the prefrontal cortex such as decision-making, speech, strategy, planning, memory, self-control and personality (Baars & Gage, 2012). The basal ganglia are involved in the control of movement. The thalamus is an important structure in the brain, Baars and Gage (2012) call it the second most important structure after the cortex, due to the fact that it is a gateway to the sensory cortex, and is the structure that relays most neural input to the cortex (Carlson, 2010).
The limbic system is a collection of brain regions vital in what is known as the fight or flight reaction which will be described more fully in the example later in this essay. These regions are the amygdala, hippocampus, and parts of the hypothalamus. The amygdala is primarily involved in emotions and expressions including anger, fear, trust and social bonding (Baars & Gage, 2012). The hippocampus is involved in spatial navigation and episodic learning and recall, and the hypothalamus is the control centre for the autonomic nervous system, which is a major part of the Peripheral nervous system.
The spinal cord is the part of the CNS which extends down from the brainstem through the middle of the spinal column and connects much of the PNS to the brain. Its major function is to send motor information to the organs and limbs, and receive sensory information to send to the brain. It does this through the use of spinal or roots, which are bundles of axons which protrude from the spinal cord and out via nerves through the body to various muscles and glands. Ventral roots send information from the brain to the rest of the body via ventral nerves, dorsal roots receive information from the PNS via dorsal nerves and send it to the brain.
The peripheral nervous system is divided by two major functions; the somatic nervous system and the autonomic nervous system. The somatic nervous system regulates activities that are under conscious control, such as the bodys movements and the collection of external sensory stimuli (Franks, 2012). It receives sensory information from the body and sends it to the brain via dorsal nerves; all of this information is carried to the spinal column via unipolar neurons in the dorsal root. The neurons in the ventral root are all multipolar and carry information from the brain to muscles and glands.
The somatic nervous system also consists of twelve pairs of cranial nerves that carry mostly sensory and motor information from the head (Carlson, 2010), and consist of unipolar and bipolar neurons. The autonomic nervous system governs subconscious movement and reactions, and is split into the sympathetic nervous system and the parasympathetic nervous system. The sympathetic nervous system reacts to danger, by shutting down the bodys function which are non-essential to either fight or flight(to prepare to tay and fight, or to flee the danger), and increasing functions which are essential.
These bodily reactions are described in greater detail in the example later in the essay, as are the functions of the parasympathetic nervous system, which is responsible for returning our body to a relaxed and resting state. To use an example to illustrate the processes which the nervous system undergoes during a sequence of events; imagine Sally, who, as she is walking into a room from the hallway is given a sudden fright when her sister, Jodie, jumps out from behind another doorway and shouts Boo! .
The sudden movement into Sallys vision, combined with the sudden explosion of noise into her auditory pathways initiates what is known as the fight or flight response. There are two paths involved in the response: the low road is quick and messy, while the high road takes more time and delivers a more precise interpretation of events. Both processes are happening simultaneously. The idea behind the low road is take no chances. If something large suddenly leaps into Sallys peripheral vision and emits a load roar, it could be her sister trying to scare her. It could also be, as ancient humans learned, a jaguar that wants her as a meal.
It is far less dangerous to Sally for her brain to assume the stimulus is a jaguar and have it turn out to be her sister, than to assume it is her sister and have it turn out to be a jaguar. The low road shoots first and asks questions later. The process looks like this: Jodie jumping out from behind the doorframe and shouting is the stimulus. As soon as Sally hears the sound and sees the motion, her brain sends this sensory data to the thalamus via the cranial nerves. At this point, the thalamus doesnt know if the signals it is receiving are signs of danger or not, but since they might be, it forwards the information to the amygdala.
The amygdala receives the neural impulses and takes action to protect Sally: It tells the hypothalamus to initiate the fight or flight response that could save her life if what she is seeing and hearing turns out to be a predator. To produce the fight or flight response, the hypothalamus activates two systems: the sympathetic nervous system and the adrenal-cortical system (Squire, 2010). The sympathetic nervous system uses nerve pathways to initiate reactions in the body, and the adrenal-cortical system uses the bloodstream.
The combined effects of these two systems are the fight or flight response. When the hypothalamus tells the sympathetic nervous system to kick into gear, the overall effect is that the body speeds up, tenses up and becomes generally very alert. If there is a jaguar coming at Sally, she is going to have to take action and fast. The sympathetic nervous system sends out impulses to glands and smooth muscles and tells the adrenal medulla to release epinephrine (adrenaline) and norepinephrine (noradrenaline) into the bloodstream (Franks, 2012).
These stress hormones (Baars, 2012) cause several changes in the body, including an increase in heart rate and blood pressure. At the same time, the hypothalamus releases corticotropin-releasing hormone (CRH) into the pituitary gland, activating the adrenal-cortical system (Squire, 2010). The pituitary gland (a major endocrine gland) secretes the hormone ACTH (adrenocorticotropic hormone). ACTH moves through the bloodstream and ultimately arrives at the adrenal cortex, where it activates the release of approximately 30 different hormones that get the body prepared to deal with a threat (Squire, 2010; Baars & Gage, 2012).
The sudden flood of epinephrine, norepinephrine and dozens of other hormones cause changes in the body that include: increased heart rate and blood pressure, dilation of pupils to take in as much light as possible, constriction of veins in skin to send more blood to major muscle groups (responsible for the chill sometimes associated with fear, as there is less blood in the skin to keep it warm), increased blood-glucose level, tensing of muscles energized by adrenaline and glucose (responsible for goose bumps when tiny muscles attached to each hair on surface of skin tense up, the hairs are forced upright, pulling skin with them), relaxation of smooth muscle in order to allow more oxygen into the lungs, shutting down of nonessential systems (like digestion and immune system) to allow more energy for emergency functions, and reduced cognitive brain function (brain is directed to focus only on big picture in order to determine where threat is coming from).
All of these physical responses are intended to help Sally survive a dangerous situation by preparing her to either run for her life or fight for her life (thus the term fight or flight). At the same time as the automatic reactions of the fight or flight response are occurring, the thalamus simultaneously sends this information to the sensory cortex, where it is interpreted for meaning. The sensory cortex determines that there is more than one possible interpretation of the data and passes it along to the hippocampus to establish context.