The Nervous System

The nervous system is the system comprising of a complex collection of nerves and neurons responsible for transmitting electrical signals between different parts of the body. The nervous system is basically a pathway to the brain which sends and receives information to any changes that occur inside and outside the body.

The nervous system is the core of all mental activities that we do such as thought, memory, and learning. It is also in charge of regulating and maintaining body homeostasis. Further, the nervous system also controls our body’s internal functions such as blood vessel dilation and muscle movement.

Further, the nervous system manages all of the things we feel, react to, and all the other things we do internally and externally. It manages all of this through sending lightning fast chemical and electrical action between cells.

Similar to the other systems in our body, the nervous comprises of organs such as the spinal cord, brain, nerves, and a group of nerve cells called ganglia. Following this, these organs comprises of various tissues, namelyconnective tissue, nerves, and blood. Further, these organs and connective tissues are responsible for executing the complex actions of the nervous system.

Three General Activities of the Nervous System

The various complex actions of the nervous system can be organized into three common, coinciding functions. These are Sensory, Integrative, and Motor.

Sensory

There are millions of sensory neurons, also known as afferent neurons that detect changes known as stimuli happening inside and outside of your body. These stimuli can be detected because the sensory neuron carry nerve impulses that sends messages to the central nervous system, specifically the brain or nervous system.

The sensory receptors can be divided into four classifications, three of which monitor changes from the external environment. These three receptors are thermoreceptors, mechanoreceptors, and photoreceptors. Thermoreceptors monitor the temperature, photoreceptors monitor light, and mechanoreceptors monitor touch, and sound.

The fourth classification of sensory receptors is chemoreceptors. The chemoreceptors monitor the changes in carbon dioxide concentration, pH level, levels of various electrolytes, pressure, and presence of chemicals. All of the gathered information perceived by our senses from the external and internal environments can be called the sensory input.

Integrative

Integration occurs when the sensory input converts into electrical signals known as nerve impulses that transmits to the brain. The nerve impulses create sensations to add to memory, or to produce thoughts, when brought together. Integration occurs instantaneously since decisions will be made right away, for every sensory input transmitted to the brain.

Motor

The motor function or motor output refers to the effect caused in response to the command from the nervous system. This effect takes place based on the sensory input and integration. Following this, the nervous system will respond to the sensory input and integration by transmitting signals to the muscles.

Further, the muscles will then contract, or the glands will produce secretions such as sweat, saliva, or tears in accordance to the directions from the central nervous systems. The effectors, specifically the muscles or glands, will only act accordingly with the efferent impulse.

Nerve Tissue

The nerve tissue is the basic unit of the nervous system and consists of two main types of cells, despite the nervous system being very complex. The original cell is the neuron also known as the conducting cell. The conducting cell is the structural unit of the nervous system, responsible for transmitting impulses. The second main type of cell is neuroglia also known as the nerve glue.

Neuroglia or glial cell is the nerve glue simply because it’s a special type of connective tissue serving as a support system for neurons.

Neurons

Neurons carry out the functions of the nervous system through detecting and reacting to stimuli. They generate and conduct nerve impulses at a lightning-quick pace from one body part to another. Since neurons are highly specialized nerve cells, it cannot be replaced once destroyed because they are amitotic.

The typical structure of a neuron has a cell body and long arms, the structure can be divided into three basic parts. These are the cell body, dendrites, and axon.

• Cell body. Similar to other types of cells, the cell body consists of a nucleus with one or more nucleolus and cytoplasmic organelles. Since neurons in the nervous system are amitotic, they lack centrioles, which is responsible for cell division.
• Dendrites. Dendrites are small cytoplasmic extensions branching off from the nerve cells. It is the receiver of stimulus which carry the impulses towards the cell body. A neuron can have one or more dendrites. The number of dendrites vary for each neuron, the fewer the dendrite the more surface area they have for receiving nerve impulses from other neurons.
• Axon. An axon is larger cytoplasmic extensions, sometimes called nerve fibers. It normally develops side branches referred to as axon collaterals. Similar to the roots of a tree, axon collaterals divide into small, thin clusters called terminate branches, or telodendria. Further, telodendria have rounded tips called synaptic knobs that carry neurotransmitter molecules.

Neurons can be divided into three functions, these functions are afferent, efferent, and interneurons. The three functions of the neurons vary on the direction of neuronal communication that receives sensory input, and generates muscle response.

• Afferent neuron. Carries and communicates signals of information that causes reaction from the stimulus towards the brain and spinal cord. The structure of this neuron has long dendrites and short axons.
• Efferent neuron. Carries neural impulses and communicates the signals of information from the brain and spinal cord towards the appropriate portion of muscle of the body. The structure of this neuron has short dendrites and long axons.
• Interneurons. Responsible for creating neural circuits to enable the communication between afferent and efferent neurons in the central nervous system. The structure of this neuron has short dendrites and either a long or short axon.

Neuroglia

Neuroglia or glial cells serve as the nerve glue because glia means glue in Greek. Unlike neurons, neuroglia doesn’t transmit nerve impulses, since they serve as support cells for neurons. Neuroglia is responsible for the maintenance of homeostasis, nutrient support, and produce myelin sheath to protect the nerve cells. Neuroglia can be classified into two classifications, these are macroglia and microglia.

Macroglia has seven subclassifications, and each subclass have special roles.

1. Astrocytes.Astrocytes are cells that provide structural integrity to neurons by filling in the spaces between neurons, and can be found in the central nervous system.
2. Ependymal Cells. Ependymal cells have three types, these are ependymocytes, tanycytes, and choroidal epithelial cells. Ependymocytes enable the molecules to move freely between the neurons and cerebrospinal fluid. Tanycytes transfer chemical signals from the cerebrospinal fluid towards the central nervous system. Choroidal epithelial cells produces and controls the chemical composition of the cerebrospinal fluid.
3. Oligodendrocytes. Oligodendrocytes produce the myelin sheaths found in the central nervous system.
4. Schwann cells. Schwann cells produce the myelin sheaths found in the peripheral nervous system.
5. Satellite cells. Satellite cells maintain a stable chemical balance of the environment by surrounding the neurons of the autonomic system and the sensory system.
6. Radial glia.  Radial glial cells act as scaffolds so that new neurons can travel from their place of origin towards the brain, and can be found in the central nervous system.
7. Enteric glia. Enteric glial cells aid the maintenance of homeostasis and digestion in the gastrointestinal tract. The enteric glia can be found in the peripheral nervous system.

Microglia are phagocytic cells that act as the main form of active immune defense for the nervous system. The microglia can be found throughout the brain and spinal cord, they engulf particulate material by altering their shape.

Organization of the Nervous System

There is only one nervous system in our body, but is divided into two major parts which is the central nervous system or CNS and the peripheral nervous system or PNS. Each part of the highly integrated nervous system have their own functional and structural characteristics that make it different from each other.

Specifically, the central nervous system consists of the brain and spinal cord, while the peripheral nervous system consists of the nerves and ganglia. Further, the motor function mentioned above is further divided into two major subdivisions known as the somatic system and the autonomic system.

Central Nervous System

The central nervous system consists of the brain, located in the cranial cavity, and the spinal cord, located in the vertebral cavity. These two organs are vitally important because they are the main control center for the body’s entire activities. Thus, the brain is encased in the skill, while the spinal cord is encased in the spine, for protection.

The Parts of the Brain

The brain acts as the central role for controlling most of our bodily functions. Further, the brain is made up of three main sections that constantly deals with hundreds of messages and information from your body, and the world around you. These three main parts are the forebrain, midbrain, and hindbrain.

The Forebrain

The largest and most complex part of the brain is the forebrain. The cerebrum can be found in the brain, specifically in the upper part of the cranial cavity. The cerebrum holds the instructions for all of the things we do in our everyday life. Essentially, the cerebrum is what makes us who we are.

Functionally, the cerebrum obtains information from the external environment and from our body, and interprets the obtained knowledge, it then decides what action must be done. The cerebrum contains our intelligence, personality, emotion, speech, memory, ability to feel, and ability to move.

Further, the cerebrum have four regions in charge of processing different types of knowledge, called lobes. The four lobes are the frontal, parietal, temporal, and occipital lobes.

• Frontal lobe. The frontal lobe is responsible for personality traits, solving complex tasks, form complete sentences, and voluntary movement of body parts.
• Parietal lobe. The parietal lobe is responsible for general sensation and feeling.
• Temporal lobe. There are two temporal lobes, one in the right hemisphere, and one in the left hemisphere. The temporal lobes are responsible for auditory processing and may also be involved in learning, emotion, and learning a new language.
• Occipital lobe. The occipital lobe is responsible for processing visual information.

The cerebrum has two halves called the right hemisphere and the left hemisphere. Further, the right hemisphere controls the left side of the body, while the left hemisphere controls the right side of the body. The two hemispheres are connected by a bundle of neural fibers called the corpus callosum. The corpus callosum is allows the two hemispheres to communicate with one another by passing messages between the two halves.

The outer layer of the cerebrum is the cortex and is often referred to as gray matter. The cortex is referred to as the gray matter because it’s made up of tightly packed neurons but has no myelin.

The inner part of the forebrain consists of the thalamus, hypothalamus, and pituitary gland.

• The thalamus is responsible for carrying messages from the eyes, ears, nose, and fingers towards the cortex.
• The hypothalamus is responsible for controlling appetite, thirst, pulse, sleep patterns, and other process in our body that automatically happens.
• The pituitary gland is controlled by the hypothalamus. Further, it develops hormones that control metabolism, mineral balance, response to stress, growth, and sexual maturity.

The Midbrain

The midbrain can be found underneath the middle of the forebrain. The midbrain is responsible for coordinating all the messages that goes in and out of the brain towards the spinal cord.

The Hindbrain

The hindbrain can be found underneath the back part of the cerebrum. Within the hindbrain, there is the cerebellum, medulla, and pons. The cerebellum is also known as the little brain, for its appearance resembles a smaller version of the cerebrum. The cerebrum is responsible for coordination, movement, and balance.

While the pons and medulla is known as the brainstem because it’s along the midbrain. The brainstem is responsible for controlling the body’s automatic functions such as heart rate, blood pressure, digestion, swallowing, blinking, and breathing.

Peripheral Nervous System

The peripheral nervous system consists of organs such as the nerves and ganglia. Similar to muscles, nerves are bundles of nerve fibers. The two main nerves are the cranial nerves and spinal nerves. These two nerves extend from the central nervous system towards the peripheral organs, like muscles and glands.

The peripheral nervous system is subdivided into the somatic nervous system and autonomic nervous system.

Somatic Nervous System

The somatic nervous system consists of peripheral nerve fibers responsible for picking up sensations or sensory information from organs far away from the brain such as limbs. The peripheral nerves then carry the sensory information towards the central nervous system.

The whole process of transmitting information takes less than a second. The somatic nervous system is sometimes called the voluntary nervous system because the nerves in the somatic nervous system allow conscious control of the skeletal muscles.

Autonomic Nervous System

The autonomic nervous system is responsible for controlling the nerves of the inner organs of the body. The autonomic nervous system is subdivided into sympathetic, parasympathetic and enteric nervous system.

The autonomic nervous system is sometimes called the involuntary nervous system because we don’t have conscious control of them such as digestion, heartbeat, and breathing.

The sympathetic nervous system is responsible for preparing our body to react to stresses such as injury or threat. Further, it causes our heart rate to increase and our muscles to contract.

The parasympathetic nervous system is responsible for controlling the functions of our body when we’re relaxing or resting. Further, it causes our heart rate to decrease and our muscles to relax. Also, it’s responsible for maintaining homeostasis in the body.

The enteric nervous system consists of a complex network of nerve fibers that detects changes in the physiological condition of our gastrointestinal tract. Further, the detected information is integrated and controls the movement of our gut. Essentially, The enteric nervous system is responsible for controlling the digestive system and all of the body’s physiological demands.

Takeaway

The nervous system is a highly organized but the most complex system of our body. All the things we know, do, and feel goes through the nervous system to be processed by the brain. For our nervous system to work well, we have to keep ourselves busy and happy to be active, while maintaining a healthy diet.

Since the nervous system communicates between the brain and all the parts of our body, it’s vital that we take care of it. If the nervous system doesn’t function properly, our perceptions, function, and ability to move will be altered.