Functional Organization of Nervous System

Introduction

• The nervous system is a highly organized communication network that enables interaction between the body and the external environment.
• It integrates and coordinates the activities of various organ systems to maintain normal body function.
• Neural pathways provide rapid anatomical and physiological communication between different body regions.
• Through these connections, the nervous system regulates gastrointestinal activity, endocrine secretion, cardiovascular function, respiration, renal function, and musculoskeletal activity, thereby maintaining homeostasis and coordinated body responses.

Divisions of Nervous System

• The nervous system is divided into the central nervous system, peripheral nervous system, and autonomic nervous system.

Central Nervous System

• The central nervous system consists of the brain and spinal cord.
• The brain is enclosed within the cranial cavity, whereas the spinal cord extends from the brainstem through the vertebral canal.

Brain

The brain is broadly divided into the forebrain, midbrain, and hindbrain.

Prosencephalon

• The forebrain consists of the telencephalon and diencephalon.
• The telencephalon includes the two cerebral hemispheres.
• It is responsible for sensory perception, cognition, learning, memory, voluntary motor activity, and planning of responses.
• The diencephalon comprises structures such as the thalamus, hypothalamus, epithalamus, and subthalamus.
• The thalamus serves as a major relay center for sensory information, whereas the hypothalamus regulates autonomic and endocrine functions.

Mesencephalon

• The midbrain contains the cerebral peduncles, tectum, tegmentum, substantia nigra, and several cranial nerve nuclei.
• It participates in motor control, visual and auditory reflexes, posture, and locomotion.
• The midbrain also contains a portion of the reticular formation.

Rhombencephalon

• The hindbrain consists of the pons, cerebellum, and medulla oblongata.
• he midbrain, pons, and medulla together form the brainstem, which regulates cardiovascular, respiratory, visceral, and sleep-wake functions.

Spinal Cord

• The spinal cord is a continuation of the medulla oblongata and extends from the foramen magnum to approximately the level of the first or second lumbar vertebra in adults.
• Because the spinal cord is shorter than the vertebral column, it does not occupy the entire vertebral canal.
• The subarachnoid space below the termination of the spinal cord is commonly used for lumbar puncture to obtain cerebrospinal fluid.
• The spinal cord consists of 31 segments, each giving rise to a pair of sensory and motor nerve roots.
• These roots unite to form 31 pairs of spinal nerves.
• The spinal cord contains ascending sensory tracts and descending motor tracts.
• Its major functions include transmission of sensory information to the brain, conduction of motor commands to peripheral tissues, and integration of spinal reflexes.

Peripheral Nervous System

• The peripheral nervous system consists of nerves and ganglia located outside the brain and spinal cord.
• It includes spinal nerves and cranial nerves that connect the central nervous system with peripheral tissues.
• Sensory fibers of the peripheral nervous system transmit information from receptors in the skin, muscles, joints, and other tissues to the central nervous system.
• Motor fibers convey impulses from the central nervous system to skeletal muscles and other effector organs.

Autonomic Nervous System

• The autonomic nervous system regulates the activity of visceral organs, smooth muscle, cardiac muscle, and glands.
• Autonomic fibers arise from the brainstem and spinal cord and reach target organs through peripheral nerves.
• The autonomic nervous system is divided into sympathetic and parasympathetic divisions.
• These two divisions usually exert complementary or opposing effects to maintain internal homeostasis.

Functions of The Nervous System

• The nervous system provides communication between different parts of the body and integrates the functions of various organ systems.
• It regulates body activities and coordinates responses to internal and external stimuli.
• The nervous system enables appropriate interaction of the body with the external environment.
• Neural control is achieved through specialized cells called neurons, which transmit information rapidly.
• Sensory receptors detect stimuli and transmit information to the central nervous system as nerve impulses.
• The central nervous system processes incoming information and generates conscious sensations when appropriate.
• After integration, the central nervous system formulates suitable motor or autonomic responses.
• Command signals are transmitted to effector organs, producing specific and coordinated responses.

Stimulus Recognition and Transmission of Information to CNS

• Stimulus recognition enables the body to detect and respond to changes in the internal and external environment.
• A sensation begins when an adequate stimulus acts on a specialized sensory receptor.
• Sensation represents the conscious awareness of a specific type of stimulus.
• Sensory receptors convert different forms of environmental energy into electrical signals, a process known as sensory transduction.
• Receptors are specialized to detect specific modalities of stimuli.
• Common forms of stimuli include mechanical, chemical, thermal, light, sound, and electrical stimuli.
• Activation of receptors generates action potentials in sensory neurons.
• Sensory neurons transmit these impulses to the central nervous system through specific ascending pathways.

Information Processing and Generation of Command Signal

• Sensory information reaching the central nervous system undergoes extensive processing and integration.
• Many sensory pathways relay information through the thalamus before it reaches the cerebral cortex.
• The sensory cortex analyzes and interprets incoming signals, leading to conscious perception.
• Processed information is integrated with previously stored experiences and ongoing neural activity.
• The central nervous system then formulates appropriate responses through complex planning and programming mechanisms.
• Motor, autonomic, and behavioral command signals are generated according to the nature of the stimulus and the physiological state of the individual.
• Learning occurs through repeated sensory experiences, and the acquired information is stored in memory for future use.

Responses

• Responses produced by the nervous system may be somatic, autonomic, or behavioral.
• Motor responses involve contraction of skeletal muscles and movement of body parts.
• Autonomic responses modify the activity of visceral organs, blood vessels, and glands.
• Behavioral responses include alterations in cognition, emotion, learning, and social interaction.
• Responses may therefore produce either observable external actions or internal physiological and psychological changes.

Cellular Components of Cns

• The central nervous system consists primarily of two major cell types: neurons and glial cells.
• Glial cells, collectively known as neuroglia, provide structural, metabolic, and protective support to neurons.
• Neuroglia also help maintain the extracellular environment and contribute to neural homeostasis.
• Neurons are specialized cells that generate and transmit nerve impulses in the form of action potentials.

Glial Cells

• Glial cells are considerably more numerous than neurons in the central nervous system.
• Unlike neurons, most glial cells do not generate action potentials or form typical synaptic connections.
• Glial cells can respond to nearby neuronal activity and actively participate in maintaining normal nervous system function.
• Unlike mature neurons, many glial cells retain the capacity to proliferate throughout life.

Functions of Glial Cells

• Glial cells provide structural support and form the framework in which neurons are embedded.
• They supply metabolic and nutritional support essential for neuronal survival.
• Glial cells participate in regulating cerebral blood flow and maintaining the extracellular ionic environment.
• They electrically insulate axons and synapses through myelination.
• Certain glial cells remove damaged cells and neural debris by phagocytosis, thereby contributing to tissue repair and maintenance.

Types of Glial Cells

• The central nervous system contains four major types of glial cells: astrocytes, oligodendrocytes, microglia, and ependymal cells.
• Schwann cells perform similar supportive functions in the peripheral nervous system.

Astrocytes

• Astrocytes are star-shaped glial cells distributed throughout the brain and spinal cord.
• Their processes surround neurons, synapses, and blood vessels.
• Astrocytes provide structural support and form the framework within which neurons are embedded.
• They supply metabolic and nutritional support to neurons.
• Astrocytic processes surround synapses, thereby electrically isolating neighboring synapses and regulating the extracellular environment.
• Astrocytes also contribute to maintenance of the blood-brain barrier and regulation of extracellular ion concentrations.

Oligodendrocytes

• Oligodendrocytes are located adjacent to axons in the central nervous system.
• These cells form the myelin sheath by wrapping their membrane around axons.
• Myelin electrically insulates axons and reduces current leakage across the axonal membrane.
• Myelination enables saltatory conduction, which markedly increases the speed of impulse transmission.
• A single oligodendrocyte can myelinate segments of multiple axons.
• Oligodendrocytes perform a function analogous to that of Schwann cells in peripheral nerves.

Microglia

• Microglia are the resident immune cells of the central nervous system and are the smallest glial cells.
• They continuously monitor the neural environment for injury or infection.
• Following tissue damage, microglia become activated and function as phagocytic cells.
• They remove cellular debris, damaged tissue, and invading microorganisms.

Ependymal Cells

• Ependymal cells line the cerebral ventricles and the central canal of the spinal cord.
• They form an epithelial-like lining that separates cerebrospinal fluid from neural tissue.
• Specialized ependymal cells in the choroid plexus participate in the production and circulation of cerebrospinal fluid.

Neuron

A neuron is the structural and functional unit of the nervous system.

Structure of a Neuron

Each neuron consists of a cell body (soma), dendrites that receive signals, and an axon that conducts impulses away from the cell body.

Soma

• The soma or cell body contains the nucleus and surrounding cytoplasm and serves as the metabolic center of the neuron.
• Neuronal cytoplasm contains organelles such as rough endoplasmic reticulum, Golgi apparatus, mitochondria, microtubules, and neurofilaments.
• Nissl bodies are aggregates of rough endoplasmic reticulum and ribosomes responsible for protein synthesis within neurons.
• Nissl bodies are present throughout the cell body and proximal dendrites but are absent in the axon hillock.
• The neuronal nucleus is usually large, centrally located, and contains a prominent nucleolus, reflecting high metabolic activity.

Dendrites

• Dendrites are branched cytoplasmic processes that arise from the cell body.
• They contribute more than 90% of the neuronal surface area available for synaptic contact.
• Proximal dendrites contain Nissl bodies and Golgi apparatus, whereas distal dendrites lack Nissl bodies.
• Dendrites contain abundant microtubules and neurofilaments that provide structural support.
• The cell body and dendrites constitute the major receptive region of the neuron.
• Dendrites receive synaptic inputs from other neurons and conduct electrical signals toward the cell body.

Axon

• The axon is the longest neuronal process and conducts impulses away from the cell body toward target cells.
• It originates from a specialized region of the soma known as the axon hillock.
• The axon hillock lacks Nissl bodies, rough endoplasmic reticulum, and Golgi apparatus, unlike proximal dendrites.
• The axon hillock continues into the initial segment, which is usually unmyelinated.
• The initial segment serves as the trigger zone where action potentials are most commonly generated.
• Action potentials arise at this site because of summation of excitatory and inhibitory signals received by the neuron.
• The interior of the axon, called the axoplasm, contains mitochondria, neurofilaments, microtubules, and transport vesicles.
• Nissl bodies are absent within the axon.
• Most neurotransmitters and axonal proteins are synthesized in the cell body and transported along the axon by axonal transport mechanisms.
• Axons surrounded by a myelin sheath are termed myelinated fibers, whereas those lacking myelin are called unmyelinated fibers.
• Myelination greatly increases the speed and efficiency of nerve impulse conduction.

Types of Neuron

• Neurons are classified on the basis of the number of processes, axonal length, function, and dendritic pattern.

A. According to the Arrangement of Axon or Processes

• Unipolar neurons possess a single process arising from the cell body.
• True unipolar neurons are uncommon in vertebrates and are mainly observed in the autonomic nervous system and invertebrates.
• Pseudounipolar neurons have a single process that divides into peripheral and central branches.
• These neurons transmit sensory information from peripheral receptors to the central nervous system.
• Dorsal root ganglion cells are typical examples of pseudounipolar neurons.
• Bipolar neurons possess two processes, one dendrite and one axon, arising from opposite poles of the cell body.
• They are found in specialized sensory pathways, such as the retina, olfactory epithelium, and vestibulocochlear system.
• Multipolar neurons possess one axon and multiple dendrites.
• They are the most abundant neurons in the nervous system.
• Spinal motor neurons are typical examples.

B. According to the Length of Axon

• Neurons with long axons are referred to as Golgi type I neurons.
• These neurons transmit impulses over long distances.
• Corticospinal motor neurons are examples of Golgi type I neurons.
• Golgi type II neurons have short axons that terminate near the cell body.
• They usually function as local interneurons within neural circuits.
• Cortical inhibitory interneurons are examples of Golgi type II neurons.

C. According to Function

• Sensory neurons or afferent neurons conduct impulses from receptors to the central nervous system.
• Motor neurons or efferent neurons transmit impulses from the central nervous system to muscles, glands, and other effector organs.
• Interneurons integrate and relay information between sensory and motor pathways within the central nervous system.

D. According to Dendritic Pattern

• Pyramidal neurons possess pyramid-shaped cell bodies with characteristic apical and basal dendrites.
• They are prominently present in the cerebral cortex and hippocampus.
• Stellate neurons exhibit a star-shaped arrangement of dendrites.
• They are commonly found within the cerebral cortex and serve important local processing functions.