Nervous System

Nervous System

• The nervous system, composed of millions of nerve cells, stands as the human body's most intricate system
• It acts as the primary control and coordination network, serving as the body's command center
• It regulates and communicates within the body and is central to mental processes like thinking, learning, and memory
• The nervous system maintains homeostasis

Organization/Classification of Nervous System

• The nervous system is divided into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS)
• The CNS includes the brain and spinal cord
• The PNS includes the Autonomic and Somatic Nervous Systems
• The Autonomic Nervous System includes the Sympathetic and Parasympathetic systems

Cells of the Nervous System

• There are two main cell types in the nervous system: neurons, also known as nerve cells, and neuroglia or glial cells

Neurons

• Neurons form the basic structural and functional units of the nervous system
• They generate, carry, and transmit nerve impulses, also known as nerve cells

Structure of Neurons

• A neuron consists of a cell body, dendrites, and an axon
• The cell body, also known as the soma, is the central region containing the nucleus and is the site of metabolic activity
• It ranges from 4-100 micrometers in diameter
• The cytoplasm of a neuron is known as neuroplasm
• Ribosomes of neurons are called Nissle's Granules
• Dendrites and axons are essentially extensions of the cell body
• Axons are thin, long, cylindrical processes that extend from the cell body
• These are crucial for carrying and transmitting nerve impulses from one neuron to another
• Most axons are covered by a fatty substance called the myelin sheath, further wrapped by Schwann cells
• Areas where the myelin sheath is absent are known as Nodes of Ranvier
• The site where the axon extends from the cell body is known as the Axon Hillock and the end part of the axon is known as the Axon Terminal
• Dendrites are extensions of the cell body that receive stimuli or nerve impulses from other neurons

Classification of Neurons/Nerve Fibers

• Neurons can be classified into three categories based on polarity/extension, function, and Myelin Sheath
• Based on the polarity, neurons are of four types: unipolar, bipolar, pseudounipolar, and multipolar
• Unipolar neurons have a single process, bipolar neurons have two processes, pseudounipolar neurons have a single process that divides into two, and multipolar neurons have multiple processes

Classification based on Function

• On the basis of function, neurons are divided into sensory (afferent) neurons, motor (efferent) neurons, and interneurons
• Sensory neurons, also known as afferent neurons, carry nerve impulses from tissues and organs to the central nervous system
• Motor neurons, also known as efferent neurons, carry nerve impulses from the central nervous system to effector organs
• Interneurons, also known as association neurons, are exclusively found in the central nervous system and connect sensory and motor neurons

Classification Based on Myelin Sheath

• On the basis of the presence or absence of myelin sheath, neurons are divided into myelinated and non-myelinated neurons
• Myelinated neurons are covered by a thick layer of myelin sheath
• Impulses are rapidly conducted through myelinated neurons unlike in non-myelinated neurons
• Non-myelinated neurons lack a myelin sheath, and impulse conduction through these neurons is slower compared to myelinated neurons

Special Properties of Neurons

• Neurons contain branches or processes known as Dendrites
• Neurons do not have centrosomes and cannot be further divided

Neuroglia

• Neuroglia are also called non-neuronal or glial cells
• These cells support neurons but not generate or conduct nerve impulses
• Unlike neurons, neuroglia cells have cell division properties

Functions of Neuroglia

• Neuroglia provide nutrition to neurons
• They protect neurons from pathogens
• They provide structural and functional support to neurons
• They form myelin sheath and maintain homeostasis

Types of Neuroglia

• Neuroglia in the Central Nervous System include astrocytes, ependymal cells, oligodendrocytes, and microglia
• Neuroglia in the Peripheral Nervous System include Schwann cells and satellite cells
• Astrocytes are the most abundant neuroglial cells and found in large numbers near blood vessels and are star-shaped
• They form and maintain the Blood-Brain Barrier
• Oligodendrocytes are smaller compared to astrocytes
• They form myelin sheath in the neurons of the central nervous system
• They support the network of neurons
• Ependymal cells form the epithelial lining of the brain and spinal cord, are columnar epithelium, and secrete and absorb Cerebrospinal Fluid
• Microglia are the smallest neuroglial cells and are derived from monocytes
• They are phagocytic in nature and help in neuroglial repair
• Schwann cells are the major glial cells in the Peripheral Nervous System
• They form the myelin sheath in the neurons of the Peripheral Nervous System and are also known as Neurilemma Cells
• Satellite cells form the extracellular space of the nervous system and provide physical support to neurons

Properties of Neurons/Nerve Fibers

• Neurons/nerve fibers possess properties such as conductivity, excitability, refractory period, unfatigability, all or none law, and accommodation
• Excitability is the ability of neurons/nerve fibers to respond to various stimuli and convert them into nerve impulses
• Conductivity is the ability of a nerve fiber to conduct nerve impulses (electrical impulses) along its length from one part to another
• The Refractory Period is when the nerve fiber will not respond to any other stimulus for a selective time period & this time period
• Unfatigability is the ability of nerve fibres to not get fatigued even if they receive continuous stimuli
• The "all or none law" states nerve fibres get excited only at a particular threshold strength
• Nerve fibers will either give a maximum response or no response at all
• Accommodation refers to a decrease in the excitability of nerve fibers if continuous stimuli are applied to the nerve fibers

Electrophysiology

• A neuron is a special type of cell in the nervous system that generates, carries, and transmits nerve impulses
• Electrophysiology is the study of the electrical properties of neurons
• Study includes how a nerve impulse is generated, how a stimulus changes membrane ion concentration, and how a stimulus is converted into a nerve impulse

Action Potential

• An action potential is the potential difference generated across the membrane of a neuron due to the imbalance of positive and negative charges, which ultimately gives rise to the generation of a Nerve Impulse
• Events during the generation of action potential include: polarization, depolarization, repolarization, and hyperpolarization
• Polarization, known as the resting stage, occurs when there is no stimulus and no action potential is generated
• At this resting stage, the inside membrane has a negative charge and high potassium ions, and the outside membrane has a positive charge and high sodium ions which creates the resting membrane Potential of -70 mV
• During depolarization, the stimulus deactivates the sodium-potassium pump which increases the sodium ions inside the cell
• The cell membrane becomes positively charged.
• Membrane rises to +30 mV
• During repolarization, the stimulus passes, the sodium gate closes, and the potassium gate opens
• Potassium is passed out and the membrane is negatively charged.
• ATPase pump activates
• During hyperpolarization, a membrane becomes repolarized but the potassium channels remain open so potassium ions continue to pass outside of the membrane
• Membrane potential becomes -90 mV which closes potassium channels and the membrane rests.

Nerve Impulse

• A nerve impulse is a message in the form of electrical signals generated inside the neuron as a result of action potential developed due to the arrival of a stimulus

Synapse

• A synapse is a junction between two neurons where the transmission of nerve impulses takes place
• Synapses allow communication between two neurons and other cells through the use of neurotransmitters
• This junction is formed between the axon terminal of one neuron and the dendrite of another neuron
• Neurotransmitters transmit signals from a presynaptic neuron to a postsynaptic neuron

Classification of Synapse

• There are two types of Synapses- chemical and electrical

Chemical Synapse

• A chemical synapse is a junction between two neurons through which signals are transmitted by the release of chemical neurotransmitters
• In the chemical synapse, there is no continuity between two neurons because of the presence of a space called the synaptic cleft
• This causes neurotransmitters to release from vessicles
• Most synapses in the human body are chemical synapses

Electrical Synapse

• An electrical synapse is a type of synapse where there is a direct connection between the presynaptic and postsynaptic neuron through a Gap Junction
• Electrical synapse transmission occurs very rapidly
• Electrical synapses are significantly fewer in number compared to chemical synapses

Neurotransmitters

• Neurotransmitters are chemical messengers that transmit signals from one neuron to another across a synapse
• Neurotransmitters are generally packed in synaptic vesicles and are very important for the rapid transfer of nerve impulses between two neurons

Types of Neurotransmitters

• Neurotransmitters can be classified on the basis of chemical nature and function
• Based on chemical property, neurotransmitters can be classified into amino acids, amines, and others
• Example of Amino Acids: GABA, Glutamate, Glycin, Aspartate
• Example of Amines: Adernaline, Nor-adernaline, Histamine, Dopamine
• Example of Others: Acetylcholine, Nitric Oxide

Classification Based on Function

• Neurotransmitters can be classified into three types based on function they perform- excitatory, inhibitory, and both
• Excitatory neurotransmitters stimulate target neurons by increasing the influx of sodium ions inside the cell to cause depolarization such as Acetylcholine, Adernaline, and Serotonin
• Inhibitory neurotransmitters act by inhibiting the target neurons, increasing the outflux of potassium ions outside the cell to cause repolarization such as GABA, Glycine, and Serotonine
• Some neurotransmitters contain both excitatory and inhibitory properties and includes Acetylcholine and Dopamine

Central Nervous System

• The central nervous system (CNS) consists mainly of the brain and spinal cord
• The cranial cavity and vertebral column protect the brain and spinal cord
• The CNS is the major controlling, regulatory, and communicating system in our body
• It controls functions such as feeling and thinking

Parts of CNS

• The CNS is composed of the brain and the spinal cord

Meninges

• The brain and spinal cord are protected by the cranial bones and vertebral column, but given their sensitivity and importance, they are further protected by three layers of covering called meninges
• Meninges provide both protection and nourishment to brain and spinal cord

Layers of Meninges

• The meninges include three layers: Dura Matter (outermost layer), Arachnoid Matter (middle layer), and Pia Matter (innermost layer)
• Dura mater is the toughest and outermost layer and is divided into two sub-layers: Endosteal layer (outer layer towards the skull) and Meningeal layer (inner layer towards arachnoid matter)
• The arachnoid matter is the middle fibrous layer and is separated from the dura matter by the subdural space
• It is separated from pia matter by the subarachnoid space; arachnoid matter and pia matter are collectively known as leptomeninges
• The pia matter is the innermost layer of the meninges and is a highly vascular connective tissue layer, carrying blood vessels into the brain
• It plays a major role in the formation of the choroid plexus
• The subdural space lies between the dura matter and the arachnoid matter, while the subarachnoid space is between arachnoid matter and pia matter

Grey matter and White matter

• The organs of the central nervous system (brain and spinal cord) are composed of two kinds of tissues: grey matter and white matter
• Grey matter is the major component of the central nervous system, mainly consisting of neuronal cell bodies and dendrites
• In the brain, grey matter is present on the outer side, while in the spinal cord, it is present on the inner side
• White matter is the network of fibers mainly consisting of myelinated axons and nerve fibers
• In the brain, it is present on the inner side, while it is present on the outer side in the spinal cord

The Brain

• The brain, one of the largest organs in the body, coordinates the majority of bodily activities and is the body's control center
• The adult human brain weighs about 1.4-1.5 kg on average
• In males, the average weight is about 1370 g, while in females, it is about 1200 grams
• The brain comprises about 100 billion neurons and is one of the most complex living structures in the universe

Ventricles of Brain

• The ventricles are cavities or hollow spaces filled with cerebrospinal fluid (CSF)
• There are four ventricles in the brain that interconnect with each other
• Each ventricle is lined by ependymal cells, which form the choroid plexus that produces CSF
• Four ventricles include the right lateral ventricle, left lateral ventricles, third ventricle, and fourth ventricle
• There are two lateral ventricles found in each hemisphere of the brain and are shaped like a "C" and they are filled with CSF
• Both of these lateral ventricles are separated by a Septum Lucidum
• The third ventricle is located below the lateral ventricles and is a funnel-shaped cavity of the brain and filled with CSF and connects with 4th ventricle by the cerebral aqueduct
• The fourth ventricle, located below the third ventricle, is a diamond-shaped cavity and is connected with the 3rd ventricle by the cerebral aqueduct

Cerebrospinal Fluid (CSF)

• CSF is a clear, colorless, and transparent fluid found in the ventricles, subarachnoid space, and central canal of the spinal cord
• It is mainly formed by the choroid plexus of the lateral ventricles
• Choroid plexus has a network of blood vessels lined by ependymal cells that produce cerebrospinal fluid
• The volume of cerebrospinal fluid is approximately 100-150 ml
• It is secreted at a rate of 0.5 ml/min or 500-700 ml/day, and its pH is slightly alkaline at 7.3
• CSF composition: Water (99%) and Solid Substances (1%)
• Inorganic substances: Sodium, Potassium, Calcium, Magnesium, Bicarbonates, Chlorides
• Organic: Proteins, Sugar, Uric acid, Creatinine
• CSF supports and protects, and acts as a shock absorbent for CNS and provides nutrients to the brain and spinal cord

Parts of the Brain

• The brain is divided into four parts: cerebrum, cerebellum, brain stem, and diencephalon

Cerebrum

• The cerebrum, the largest part of the brain, is divided into two cerebral hemispheres (Right and Left)
• The cerebrum is located in the upper portion of the brain and is responsible for learning and senses
• The superficial layer of the cerebrum is known as the cerebral cortex
• Both hemispheres are connected by the Corpus Callosum
• The right hemisphere controls the left side of the body, while the left hemisphere controls the right side of the body

Lobes of Cerebrum

• The cerebrum of the brain is divided into four major lobes: frontal lobe, parietal lobe, temporal lobe, and occipital lobe
• The frontal lobe, or motor cortex, controls motor functions like personality, movement, planning, decision-making, creativity, concentration, and thoughts
• The parietal lobe, or sensory cortex, controls logical reasoning, direction, distance recognition, and language storage
• The temporal lobe, or auditory cortex, controls hearing, smell, and dreams
• The occipital lobe, or visual cortex, receives input from the eyes and controls vision

Cerebellum

• The cerebellum is the second-largest portion of the brain and sits below the posterior of the cerebrum
• Sometimes called the "little brain," it resembles the cerebrum structurally and is divided into two cerebellar hemispheres
• The cerebellum controls muscle contraction, movement, and balance of the body and posture, and plays a major role in cognition or learning

Brain Stem

• The brain stem is a stem-like part of the brain that connects with the spinal cord
• It is mainly composed of the midbrain, pons, and medulla oblongata
• The midbrain is one of the smallest portions of the brain and controls sensory organs like the eyes and ears
• Pons act like a bridge, the pons connect the cerebellum with the brain
• The pons control the sleep cycle and manage breathing rate and maintain balance between body and mind
• The medulla oblongata is the lowermost part of the brain and directly connects with the spinal cord
• It controls involuntary actions of the body such as respiration, blood circulation, heart rate, yawning, blinking, and vomiting

Diencephalon

• The diencephalon is the posterior part of the brain that connects the midbrain with the cerebrum and is mainly composed of the thalamus, hypothalamus, and epithalamus

Thalamus

• The thalamus is the gateway of the brain as all sensory inputs pass through it
• Thalamus receives information from all the sensory nerves and sends it to the different regions of the brain

Hypothalamus

• The hypothalamus is small but one of the most important portions of the brain
• It controls body temperature and is responsible for nearly all types of feelings and emotions

Epithalamus

• The epithalamus is a small region superior and posterior to the thalamus and consists of the pineal gland, which is an endocrine gland that secretes melatonin

Spinal Cord

• The spinal cord is a thin, tube-like structure that starts from the medulla oblongata of the brain and extends to the 1st and 2nd lumbar vertebra
• It is part of the central nervous system
• In men, the length of the spinal cord is about 45 cm, while in women, it is about 43 cm, and it weighs about 35 g
• It controls and regulates most reflex actions within the body

Covering of Spinal Cord

• The spinal cord is a very sensitive organ of the central nervous system
• Therefore, it is protected and covered by a three-layered tissue called meninges and the bones of the vertebral column

Meninges of the Spinal Cord

• Three layers of membranes also protect and nourish the spinal cord
• Dura matter is the outermost layer
• Arachnoid matter is the middle layer
• Pia matter is the innermost layer

Vertebral Column

• The vertebral column, also known as the spinal column or backbone and the vertebrae protects the spinal cord and is divided into five parts
• Cervical vertebrae consists of 7 parts (C1 to C7)
• Thoracic vertebrae consists of 12 parts (T1 to T12)
• Lumbar vertebrae consists of 5 parts (L1 to L5)
• Sacrum consists of 5 parts (Fused)
• Coccyx consists of 4 parts (Fused)

Spinal Cord Segments

• The spinal cord is divided into 31 segments from which 31 pairs of spinal nerves arise
• Cervical segment/cervical nerves: 8
• Thoracic segment/thoracic nerves: 12
• Lumbar segment/lumbar nerves: 5
• Sacral segment/sacral nerves: 5
• Coccygeal segment/coccygeal nerves: 1

Spinal Cord Enlargements

• There are two major enlargements in the spinal cord: Cervical and Lumbar enlargements

Cervical Enlargement

• Cervical enlargement, the superior enlargement, extends from the 4th cervical vertebra to the 1st thoracic vertebrae [C4 - T1]

Lumbar Enlargement

• Inferior enlargement, extends from the 9th to 12th thoracic vertebrae [T9 - T12]

Internal Anatomy of Spinal Cord

• The spinal cord is composed of an inner core of grey matter surrounded by an outer covering of white matter
• Grey matter consists of the neuronal cell body and white matter consists of the axon part
• In the centre of the grey matter runs a central canal
• The posterior part of the grey matter is known as the dorsal region, which contains two dorsal horns
• The anterior part of the grey matter is known as the ventral region, which contains two ventral horns
• The grey matter in the spinal cord resembles the shape of a butterfly

Reflex Activity

• Reflex activity is an involuntary and immediate response of voluntary muscles to a specific stimulus
• The pathway followed by nerve impulses that produces reflex activity is known as the reflex arc
• A reflex action produced by the brain is known as a cranial reflex
• A reflex action produced by the spinal cord is known as a spinal reflex
• A reflex arc contain 5 major components: a receptor, sensory neurons, control center, motor neurons, and effector