Nervous System

 

The nervous system regulates and coordinates the activity of organisms. The nervous system plays an important role in maintaining body homeostasis. In addition, it is in charge of our perceptions, behaviors, and memories, as well as controlling all voluntary motions.

The nervous system is made up of two main types of cells: Neurons and Neuroglia. The nervous system's basic anatomical and functional elements are neurons, often known as nerve cells. They are designed to respond to physical and chemical stimuli, transmit nerve impulses or action potentials, and release neurotransmitters. Neuroglia, or glial cells, provide support, nourishment, and protection for neurons.

Types of Nerve cells

1. Neurons: Neurons have the ability to respond to stimuli and transform them into action potentials. Most neurons consist of three parts: a Cell Body, Dendrites, and an Axon. The cell body has a nucleus surrounded by cytoplasm, which comprises typical cellular organelles such lysosomes, mitochondria, and a Golgi complex. Dendrites are the receiving or input areas of neurons. They are typically short, tapered, and highly branched. An axon from a neuron transmits nerve impulses to another neuron, muscle fiber, or gland cell.

Classification of neurons on the basis of their structure:

1. Multipolar neurons - These have several processes extending from the cell body, i.e., several dendrites and axons.
2. Bipolar neurons - These have two processes extending from the cell body, i.e., one main dendrite and one axon.
3. Unipolar neurons - Theses have a single processes extending from the cell body which divides into two branches.

Classification of neurons on the basis of their function:

1. Afferent or Sensory neurons - They carry nerve impulses from peripheral sensory receptors to the CNS.
2. Efferent or Motor neurons - They transmit nerve impulses from CNS to effector organs such as muscles and glands.
3. Interneurons or Association neurons - They are located within the CNS in which they form the connecting link between the sensory and motor neurons.

2. Neuroglia: Neuroglia or glial cells account for approximately 90% of all cells in the CNS. It accounts for approximately half of the brain's volume. They do not originate nerve impulses but can divide in the adult nervous system. When neurons are injured, these cells grow to fill the voids left behind. The CNS has four major types of glial cells: astrocytes, oligodendrocytes, microglia, and ependymal cells. PNS contains two minor types: Schwann cells and satellite cells. Myelin sheaths are produced by two types of neuroglia: Schwann cells and oligodendrocytes.

Types of Nervous System

1. Central Nervous System (CNS): The CNS is the central information processing organ of our body, which mainly includes the Brain and the Spinal Cord. CNS is enclosed by hard, bony structures. The cranium, or skull, protects the brain, whereas the vertebral column protects the spinal cord. Three protective connective tissue coverings, known as meninges, are located between the bony covering and the neural tissue within the skull and vertebral column. The brain is protected by the cranial meninges. The dura mater is the outermost meninx, and it is made up of dense, uneven connective tissue. The middle meninx is an avascular covering known as the arachnoid mater. The pia mater is the innermost meninges, a thin translucent connective tissue layer that attaches to the brain's surface. The subarachnoid space is located between the arachnoid mater and the pia mater and is filled with cerebrospinal fluid.

The cerebrospinal fluid (CSF) is a colorless liquid that serves as a shock absorber and protects the brain and spinal cord from physical injuries. It transports oxygen, glucose, and other substances from the blood to neurons and neuroglia (90% of all cells in the CNS). The brain's CSF-filled compartments are known as ventricles. The ventricular system consists of two lateral ventricles, a third ventricle, and a fourth ventricle. Each cerebral hemisphere contains a lateral ventricle. The third ventricle is a thin canal that runs along the midline above the hypothalamus and connects the right and left portions of the thalamus. The fourth ventricle is located between the brain stem and the cerebellum.

2. Peripheral Nervous System: It is made up of nerve fibers that transmit information from the central nervous system to the rest of the body. The CNS communicates with the rest of the body through nerves that exit the brain (cranial nerves) and spinal cord. The peripheral nervous system is made up of 31 pairs of spinal nerves and 12 pairs of cranial nerves that originate in the CNS, as well as aggregations of cell bodies located outside the CNS.

The cranial nerves are named for the fact that they originate in the brain and travel through numerous foramina in the skull bones. There are twelve pairs of cranial nerves. Two of the twelve pairs of cranial nerves originate in the forebrain, whereas the other 10 originate in the midbrain and hindbrain.

1. Olfactory
2. Optic
3. Oculomotor
4. Trochlear
5. Trigeminal, Opthalmic, Maxillary, Mandibular
6. Abducens
7. Facial
8. Auditory
9. Glossopharyngeal
10. Vagus
11. Spinal accessory nerves
12. Hypoglossal

Spinal nerves are 31, grouped into 8 cervical, 12 thoraic, 5 lumbar, 5 sacral and 1 coccygeal according to the region of the vertebral column from which they arise. Each spinal nerve is a mixed nerve composed of both motor and sensory nerves. These fibers are packaged together in the nerve, but they separate near the attachment of the nerve to the spinal cord.

Efferent division of Peripheral Nervous System

The PNS is divided into afferent and efferent divisions. The afferent division contains afferent nerve fibers, which transport information from tissues and organs to the CNS. Instructions from the CNS are communicated to efferent organs, which are muscles or glands that follow out the commands to produce the intended effect, via efferent nerve fibers of the efferent division. The efferent division is further separated into two parts: the somatic nervous system and the autonomic nervous system.

The Autonomic Nervous System, the involuntary branch of the peripheral efferent division, innervates cardiac muscle, smooth muscle, most exocrine glands, a few endocrine glands, and adipose tissue. The Somatic Nervous System innervates skeletal muscle, which is a branch of the efferent nerve that can be controlled voluntarily.

1. Autonomic Nervous System: The autonomic nervous system is made up of preganglionic neurons from the CNS, as well as ganglia and postganglionic neurons. The visceral nervous system gets its name from the fact that autonomic neurons innervate the viscera. This neuron's axon does not directly innervate the effector organ; instead, it synapses with a second neuron within an autonomic ganglion. The first neuron is hence known as a preganglionic neuron. The second neuron in this route, known as a Postganglionic neuron, has an axon that travels from the autonomic ganglia to an effector organ, where it synaptically connects with the target problem. The autonomic nervous system has two subdivisions -

1. Sympathetic Nervous System -> The sympathetic division is often referred to as the fight-or-flight division. Its functions cause enhanced awareness and metabolic activity in order to prepare the body for an emergency. It triggers the body's 'fight-or-flight' response, primarily by releasing norepinephrine from postganglionic fibers and secreting epinephrine, norepinephrine, and dopamine from the adrenal medulla.
2. Parasympathetic Nervous System -> The Parasympathetic division frequently exerts antagonistic effects by releasing acetylcholine from its postganglionic fibers. It is commonly referred to as the rest-and-digest division. It lowers the heart rate while increasing peristalsis and the amount of secretion by the digestive glands.

2. Somatic Nervous System: Motor neurons, whose axons form the somatic nervous system, supply skeletal muscles and cause movement. The Somatic division innervates the skeletal muscle. These neurons' cell bodies are grouped together in the brainstem or spinal cord. A motor neuron's axon is continuous from its start in the CNS to its end in skeletal muscle. It controls our body's voluntary actions (movement).

Major parts of the Brain

Anatomically, the brain consists of three major parts:

1. Forebrain or prosencephalon - It is the largest part of the brain. It includes Cerebrum and Diencephalon. The Diencephalon consists of two main parts, the Thalamus and Hypothalamus.
2. Midbrain or mesencephalon - It includes Tectum and Tegmentum.
3. Hindbrain or rhombencephalon - It includes Pons, Cerebellum and Medulla.

Midbrain, pons and medulla oblongata are together called as the Brain Stem. It is continuous with the Spinal Cord. Posterior of the brain is the Cerebellum.

1. Cerebrum: The cerebrum is the largest portion of the brain. It is divided into two halves: the right and left hemispheres. The hemispheres are linked by a mass of fibers known as the corpus callosum. The left hemisphere governs the right side of the body, while the right hemisphere controls the left. Each cerebral hemisphere's surface contains numerous folds known as Gyri, which are divided by depressions known as Sulci. The deep sulcus is known as Fissure. Each hemisphere is made up of a thin layer of grey matter known as the Cerebral Cortex. It contains the thick central core of white matter. Gray matter is mostly composed of densely packed neuronal cells and dendrites, as well as glial cells. It appears grayish rather than white because these areas contain little or no myelin. The white matter consists of bundles of myelinated axons. The spinal cord's white matter surrounds an inner core of gray matter and is formed like a butterfly. Functionally, each hemisphere's cortex is divided into four lobes or areas -

1. Frontal lobes -> Controls voluntary actions, speaking ability, and elaboration of thought.
2. Parietal lobes -> Controls temperature, pain, taste and touch.
3. Temporal lobes -> Process memory and auditory information and speech and language functions
4. Occipital lobes -> Receive and process visual information.

2. Diencephalon: It consists of three parts - Epithalamus (secretes cerebrospinal fluid), Thalamus (sensory awareness of pain, temperature, touch and pressure, situated between cerebral cortex and midbrain), and Hypothalamus (controls thirst, temperature, food intake, pituitary hormone secretion, emotional behaviours, release of blood and behaviour).

3. Cerebellum: The cerebellum, the second biggest section of the brain, is connected to the back of the upper portion of the brain stem. The vermis is the core restricted area, with cerebellar hemispheres on the lateral wings or lobes. Each hemisphere contains lobes. The cerebellar cortex, or superficial layer, is made mainly of gray matter. The cerebellum controls body posture, muscle tone, voluntary muscular actions, and bodily equilibrium. It may play a role in cognition and language processing.

4. Medulla Oblongata: The Medulla Oblongata is the lowest section of the brainstem and is linked to the cervical spinal cord. In addition to regulating heart rate, blood vessel diameter, and appropriate breathing rhythm, nuclei in the medulla control reflexes for vomiting, swallowing, sneezing, coughing, and hiccupping.

5. Pons: The pons is located directly superior to the medulla and anterior to the cerebellum. It acts as a connection between the midbrain and the medulla oblongata. It also contains nerve nuclei and fibers that govern eye muscles, strengthen facial muscles, and perform other activities.

6. Midbrain: The cerebrum covers only a small fraction of the brain. The four lobes, or optic lobes, are together known as corpora quadrigemina. The superior colliculi are associated with optical activity, while the two smaller posterior colliculi are associated with auditory. These two are known collectively as Tectum.

7. Limbic System: The limbic system refers to the areas in the human brain that regulate emotion, motivation, and emotional association with memory. The Limbic System refers to the area of the cerebral cortex that forms a rim around the corpus callosum on the medial face of the cerebral hemispheres. It works with the hypothalamus to regulate sexual behaviour, mood, long-term memory, and motivation. It is also known as the 'emotional brain' because it is responsible for a wide range of emotions such as pleasure, pain, affection, fear, and rage.

8. Spinal Cord: The spinal cord extends from the base through the foramen magnum, a large aperture in the skull, into the vertebral canal, which is formed by openings in the vertebrae. There are 31 pairs of spinal nerves that arise from the spinal cord. There are 8 pairs of cervical nerves, 12 thoracic nerves, 5 lumbar nerves, 5 sacral nerves, and one coccygeal nerve. Each spinal neuron is a mixture of sensory and motor nerve fibers.

A cross section of the spinal cord reveals the central canal, gray matter, and white matter. The central canal contains cerebrospinal fluid. The gray matter is centrally situated and shaped like a butterfly. The white matter is divided into areas. It contains ascending and descending pathways that transport information to and from the brain. Because the tracts cross soon after entering and exiting the brain, the left side of the brain controls the right side of the body, while the right side controls the left.

The spinal cord facilitates communication between the brain and the peripheral nerves that exit the spinal cord. When someone touches our hands, sensory receptors produce nerve impulses that go through sensory fibers to the spinal cord and ascending tracts to the brain. As a result, if the spinal cord is cut, we lose both sensation and voluntary control.