Human Communication Systems
The human body has two communication systems that allow us to respond to any changes in our environment. These are;
- The nervous system uses nerve impulses (electrical impulses) to react quickly to a stimulus.
- The hormonal system uses hormones to react slowly to a stimulus.
|Nervous system||Hormonal system|
|Nature of response||Electrical impulses||Hormones/chemicals that travel in blood|
Our bodies cannot operate without the nervous system – the complex network that coordinates our actions, reflexes, and sensations. Broadly speaking, the nervous system is organized into two main parts, the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).
The Central Nervous System
The brain and spinal cord form the Central Nervous System (CNS). The CNS controls and coordinates responses between receptors and effectors.
The CNS is the processing centre of the body and consists of the brain and the spinal cord. Both of these are protected by three layers of membranes known as meninges. For further protection, the brain is encased within the hard bones of the skull, while the spinal cord is protected with the bony vertebrae of our backbones. A third form of protection is cerebrospinal fluid, which provides a buffer that limits impact between the brain and skull or between spinal cord and vertebrae.
Grey and White Matter
In terms of tissue, the CNS is divided into grey matter and white matter. Grey matter
comprises neuron cell bodies and their dendrites, glial cells, and capillaries. Because of the abundant blood supply of this tissue, it is actually more pink-coloured than grey.
In the brain, grey matter is mainly found in the outer layers, while in the spinal cord it forms the core ‘butterfly’ shape.
White matter refers to the areas of the CNS which host the majority of axons, the long cords that extend from neurons. Most axons are coated in myelin – a white, fatty insulating cover that helps nerve signals travel quickly and reliably. In the brain, white matter is buried under the grey surface, carrying signals across different parts of the brain. In the spinal cord, white matter is the external layer surrounding the grey core.
Image: QBI/Levent Efe
If the CNS is the processing centre of the human body, the brain is its headquarters. It is broadly organised into three main regions –
The forebrain is the largest region of the brain. It contains the large outermost layer of the brain, the wrinkly cerebral cortex, and smaller structures towards its centre, such as the thalamus, hypothalamus, and the pineal gland.
The midbrain, serves as the vital connection point between the forebrain and the hindbrain. It is the top part of the brainstem, which connects the brain to the spinal cord.
The hindbrain is the lowest back portion of the brain, containing the rest of the brainstem made up of medulla oblongata and the pons, and also the cerebellum – a small ball of dense brain tissue nestled right against the back of the brainstem.
Parts of the Brain
The brain’s cerebral cortex is the outermost layer that gives the brain its characteristic wrinkly appearance. The cerebral cortex is divided lengthways into two cerebral hemispheres connected by the corpus callosum, each of which traditionally has been divided into four lobes: frontal, parietal, temporal and occipital.
Although it is now know that most brain functions rely on many different regions across the entire brain working in conjunction, it is still true that each lobe carries out the bulk of certain functions.
Different Regions of the Brain and Their Functions
Bumps and Grooves of the Brain
In humans, the lobes of the brain are divided by a number of bumps and grooves. These are known as gyri (bumps) and sulci (groves or fissures). The folding of the brain, and the resulting gyri and sulci, increases its surface area and enables more cerebral cortex matter to fit inside the skull.
The frontal lobe is separated from the parietal lobe by a space called the central sulcus, and from the temporal lobe by the lateral sulcus.
The frontal lobe is generally where higher executive functions including emotional regulation, planning, reasoning and problem solving occur. The frontal lobe also contains the primary motor cortex, the major region responsible for voluntary movement.
The parietal lobe is behind the frontal lobe, separated by the central sulcus. Areas in the parietal lobe are responsible for integrating sensory information, including touch, temperature, pressure and pain.
Because of the processing that occurs in the parietal lobe, we are able to, for example, discern from touch alone that two objects touching the skin at nearby points are distinct, rather than one object. This process is called two-point discrimination. Different areas of the body have more sensory receptors, and so are more sensitive than others in discerning distinct points.
Separated from the frontal lobe by the lateral fissure, the temporal lobe also contains regions
dedicated to processing sensory information, particularly important for hearing, recognizing language, and forming memories.
The temporal lobe contains the primary auditory cortex, which receives auditory information from the ears and secondary areas, and processes the information so we understand what we’re hearing (e.g. words, laughing, a baby crying).
Certain areas in the temporal lobe make sense of complex visual information including faces and scenes.
The medial (closer to the middle of the brain) temporal lobe contains the hippocampus, a region of the brain important for memory, learning and emotions.
The occipital lobe is the major visual processing centre in the brain. The primary visual cortex, also known as V1, receives visual information from the eyes. This information is relayed to several secondary visual processing areas, which interpret depth, distance, location and the identity of seen objects.
Structure and Functions of the Spinal Cord
The spinal cord is part of the central nervous system and consists of a tightly packed column of nerve tissue that extends downwards from the brainstem through the central column of the spine.
Even though it is a relatively small bundle of tissue, weighing a mere 35g and just about 1cm in diameter, the spinal cord plays a crucial role in facilitating our daily activities. It carries nerve signals from the brain to other parts of the body, commanding the muscles we use to move. It also receives sensory input from the body, partially processes it, and transmits that information to the brain.
As well as carrying motor and sensory signals between the brain and periphery, the spinal cord provides separate neural circuits for many of our reflexes – automatic, involuntary responses to sensory inputs. Some reflexes, such as the knee-jerk and the withdrawal reflex (for example, when touching something hot), are built into the nervous system and bypass the brain, while others can be learned over time.
Structure of the Spinal Cord
Externally, the spinal cord is protected by 26 bones called vertebrae, which are sandwiched between cartilage disks to cushion the cord from any jarring caused by bodily movement. Just like the brain itself, the spinal cord is also protected by three layers of meninges (membranes).
Inside, the spinal cord consists of grey matter and white matter. When looked at, the grey matter of a cross-section, takes on the shape of a butterfly, with four ‘wings’ called horns.
The horns in the front contain motor neurons, which carry information from the brain and spinal cord to the body’s muscles, stimulating their movement.
The horns in the back contain sensory neurons which carry sensory information – about, for example, touch, pressure or pain – from the body back to the spinal cord and the brain.
The spinal cord’s grey matter is enveloped in a column of white matter, which contains axons that allow different parts of the spinal cord to communicate smoothly. These axons travel in both directions – some carry signals from the body to the brain, while others deliver signals from the brain to neurons located elsewhere in the body.
Two rows of spinal nerves – bundles of axons – emerge on either side of the cord through the bony ridges of the vertebrae.
There are 31 pairs of these nerves, each representing a segment of the spinal cord which is further divided into five regions.
From top to bottom, these are:
8 cervical (neck)
12 thoracic (chest)
5 lumbar (abdominal)
5 sacral (pelvic)
1 coccygeal (tailbone) segment
Comparison of Reflex and Voluntary Actions
Reflex action or reflex is an involuntary action in response to a stimulus. This is a spontaneous action without thinking. For example, we adjust our eyes when exposed to bright light. The peripheral nervous system (PNS) is a system of nerves which connect the central nervous system (CNS) (includes the brain and spinal cord) with other parts of the body. Reflex action is the result of the coordination of the spinal cord and peripheral nervous system. This action does not involve the brain. The pathway in which impulses travel during the reflex action is called a reflex arc.
A reflex action happens quickly because the reflex pathway is kept short and involves the smallest number of neurones and synapses possible. One example of this is rapidly removing your hand from a hot cup of tea before it gets burned.
There are 3 neurones and 2 synapses involved in a reflex arc.
- The sensory neurone – carries information from the receptor to the spinal cord (coordinator).
- The association neurone – joins the sensory neurone and motor neurone.
- The motor neurone – carries information from the spinal cord to an effector that can cause a response.
The association and motor neurones both begin with the cell body.
When an action is produced with the involvement of thoughts, they are called voluntary action. It involves actions like walking, eating, jumping and running. These actions are produced consciously. Both spinal cord and brain are involved and these coordinate with PNS to generate necessary movements.
Actions which take place without consciousness or willingness of an individual are called the involuntary action. Digestion, heart beating, sneezing, etc are few examples of involuntary actions.
Both involuntary and voluntary actions are controlled by the same parts of the brain. Hindbrain and midbrain control involuntary actions like salivation, vomiting, etc. All the body’s voluntary actions are controlled by the motor cortex in the frontal lobe of the cerebrum
Following is the table which is a quick summary of difference between involuntary action and reflex action:
|Involuntary actions||Reflex actions|
|Involuntary actions takes place without the conscious choice of an organism.||Reflex actions are those actions takes place along with stimuli.|
|These actions are controlled by the medulla oblongata or the mid brain.||These actions are controlled by spinal cord.|
|The speed is relatively slower.||The speed is very quick.|
|Example is beating of heart.||Example is blinking of eyes.|
Peripheral Nervous System
The peripheral nervous system refers to the parts of the nervous system that are outside the central nervous system, that is, those outside the brain and spinal cord.
Thus, the peripheral nervous system includes
- The nerves that connect the head, face, eyes, nose, muscles, and ears to the brain (cranial nerves)
- The nerves that connect the spinal cord to the rest of the body, including the 31 pairs of spinal nerves
- More than 100 billion nerve cells that run throughout the body
Peripheral Nervous System
Unlike the brain and the spinal cord of the central nervous system that are protected by the vertebrae and the skull, the nerves and cells of the peripheral nervous system are not enclosed by bones, and therefore are more susceptible to trauma.
If the entire nervous system is considered as an electric grid, the central nervous system would represent the powerhouse, whereas the peripheral nervous system would represent long cables that connect the powerhouse to the outlying cities (limbs, glands and organs) to bring them electricity and send information back about their status.
Basically, signals from the brain and spinal cord are relayed to the periphery by motor nerves, to tell the body to move or to conduct resting functions (like breathing, salivating and digesting), for example. The peripheral nervous system sends back the status report to the brain by relaying information via sensory nerves.
As with the central nervous system, the basic cell units of the peripheral central nervous system are neurons. Each neuron has a long process, known as the Axon, which transmits the electrochemical signals through which neurons communicate.
Axons of the peripheral nervous system run together in bundles called fibres, and multiple fibres form the nerve, the cable of the electric circuit. The nerves, which also contain connective tissue and blood vessels, reach out to the muscles, glands and organs in the entire body
Nerves of the peripheral nervous system are classified based on the types of neurons they contain – sensory, motor or mixed nerves (if they contain both sensory and motor neurons), as well as the direction of information flow – towards or away from the brain.
The afferent nerves, contain neurons that bring information to the central nervous system. In this case, the afferent are sensory neurons, which have the role of receiving a sensory input – hearing, vision, smell, taste and touch – and pass the signal to the CNS to encode the appropriate sensation.
The afferent neurons have also another important subconscious function. In this case, the peripheral nervous system brings information to the central nervous system about the inner state of the organs (homeostasis), providing feedback on their conditions, without the need for us to be consciously aware. For example, afferent nerves communicate to the brain the level of energy intake of various organs.
The efferent nerves, contain neurons that transmit the signals originating in the central nervous system to the organs and muscles, and put into action the orders from the brain. For example, motor neurons (efferent neurons) contact the skeletal muscles to execute the voluntary movement of raising your arm and wiggling your hand about.
Peripheral nervous system nerves often extend a great length from the central nervous system to reach the periphery of the body. The longest nerve in the human body, the sciatic nerve, originates around the lumbar region of the spine and its branches reach until the tip of the toes, measuring a meter or more in an average adult.
The peripheral nervous system can be divided into somatic, autonomic and enteric nervous systems, determined by the function of the parts of the body they connect to.
Some differences between the peripheral nervous system (PNS) and the central nervous system (CNS):
- In the CNS, collections of neurons are called nuclei. In the PNS, collections of neurons are called ganglia.
- In the CNS, collections of axons are called tracts. In the PNS, collections of axons are called nerves.
In the peripheral nervous system, neurons can be functionally divided in three ways:
- Sensory (afferent) – carry information INTO the central nervous system from sense organs or motor (efferent) – carry information away from the central nervous system (for muscle control).
- Cranial – connects the brain with the periphery or spinal – connects the spinal cord with the periphery.
- Somatic – connects the skin or muscle with the central nervous system or visceral – connects the internal organs with the central nervous system.
Autonomic Nervous System (ANS)
The Autonomic Nervous System (ANS) receives input from parts of the central nervous system (CNS) that process and integrate stimuli from the body and external environment. These parts include the hypothalamus, nucleus of the solitary tract, reticular formation, amygdala, hippocampus, and olfactory cortex.
It controls the involuntary functions and influences the activity of internal organs. The autonomic nervous system is regulated by the hypothalamus and is required for cardiac function, respiration, and other reflexes, including vomiting, coughing, and sneezing.
The autonomic nervous system controls blood pressure (BP), heart rate, body temperature, weight, digestion, metabolism, fluid and electrolyte balance, sweating, urination, defecation, sexual response, and other processes.
Although most of the autonomic nervous system responses are involuntary, they can integrate with the somatic nervous system, which is responsible for the voluntary movements. For example, in the case of defecation, there is an interplay between voluntary and involuntary movements.
The peripheral nervous system consists of the Somatic Nervous System (SNS) and the Autonomic Nervous System (ANS).
The SNS consists of motor neurons that stimulate skeletal muscles. In contrast, the ANS consists of motor neurons that control smooth muscles, cardiac muscles, and glands. In addition, the ANS monitors visceral organs and blood vessels with sensory neurons, which provide input information for the CNS.
The ANS is further divided into the Sympathetic Nervous System and the Parasympathetic Nervous System. Both of these systems can stimulate and inhibit effectors. However, the two systems work in opposition—where one system stimulates an organ, the other inhibits. Working in this fashion, each system prepares the body for a different kind of situation, as follows:
- The sympathetic nervous system prepares the body for situations requiring alertness or strength, or situations that arouse fear, anger, excitement, or embarrassment (“fight‐or‐flight” situations). In these kinds of situations, the sympathetic nervous system stimulates cardiac muscles to increase the heart rate, causes dilation of the bronchioles of the lungs (increasing oxygen intake), and causes dilation of blood vessels that supply the heart and skeletal muscles (increasing blood supply). The adrenal medulla is stimulated to release epinephrine (adrenalin) and norepinephrine (noradrenalin), which in turn increases the metabolic rate of cells and stimulates the liver to release glucose into the blood. Sweat glands are stimulated to produce sweat. In addition, the sympathetic nervous system reduces the activity of various “tranquil” body functions, such as digestion and kidney functioning.
- The parasympathetic nervous system is active during periods of digestion and rest. It stimulates the production of digestive enzymes and stimulates the processes of digestion, urination, and defecation. It reduces blood pressure and heart and respiratory rates and conserves energy through relaxation and rest.
The sympathetic and parasympathetic systems each consist of 2 sets of nerve bodies:
- Preganglionic: This set is located in the CNS, with connections to another set in ganglia outside the CNS.
- Postganglionic: This set has efferent fibers that go from the ganglia to effector organs (see figure The Autonomic Nervous System below).
Simply, the sympathetic and parasympathetic nervous systems have opposite actions. Many organs are controlled primarily by either the sympathetic or parasympathetic system, although they may receive input from both; occasionally, functions are reciprocal (eg, sympathetic input increases heart rate; parasympathetic decreases it).
The Sympathetic Nervous System is catabolic; it activates fight-or-flight responses. Sympathetic fibres, located in spinal nerves are responsible for the “fight or flight” response, which is an acute response that takes place in case of an imminent harmful event or intense mental distress. To activate this response, the sympathetic fibres use the neurotransmitter noradrenaline to activate the blood flow in skeletal muscles and lungs, dilating lungs and blood vessels and raise the heart rate.
In other words, they initiate the physiological events that prepare the body for self-defence through a fight or an escape and therefore the type of synapses are of the excitatory type.
On the contrary, the Parasympathetic Nervous System is anabolic; it conserves and restores. The parasympathetic fibres regulate resting responses such as heart rate, salivation, lacrimation (secreting tears), digestion, with the only exception being sexual arousal. Parasympathetic motor fibres are found in four of the 12 pairs of cranial nerves. So, synapses established by the parasympathetic fibres are typically inhibitory, with Acetylcholine as main neurotransmitter.
Two major neurotransmitters in the autonomic nervous system are
- Acetylcholine: Fibers that secrete acetylcholine (cholinergic fibers) include all preganglionic fibers, all postganglionic parasympathetic fibers, and some postganglionic sympathetic fibers (those that innervate piloerectors, sweat glands, and blood vessels).
- Norepinephrine: Fibers that secrete norepinephrine (adrenergic fibers) include most postganglionic sympathetic fibers. Sweat glands on the palms and soles also respond to adrenergic stimulation to some degree.
A comparison of the sympathetic and parasympathetic pathways.