Structure, Function, Communication & Disorders

The Nervous System Structure, Function, Communication & Disorders First Year Biology — Neuroscience Module Academic Year 2025/2026 Figure 1: Hierarchical Organization of the Human Nervous System 1. Introduction The nervous system is one of the most complex and fascinating organ systems in the human body. It serves as the primary communication network of the body, receiving, processing, and transmitting information that coordinates virtually every bodily function — from the beating of the heart and regulation of breathing, to conscious thought, emotion, memory, and voluntary movement. Without the nervous system, no organ, tissue, or cell could function in a coordinated manner. It is the body's master control and communication system, working in concert with the endocrine system to maintain homeostasis and allow the organism to interact meaningfully with its environment. 1.1 Why Study the Nervous System? Understanding the nervous system is essential for anyone entering the health sciences, biology, or related fields. Neurological disorders are among the leading causes of disability worldwide. Knowledge of how the nervous system is organized, how it functions at the cellular level, and how it can fail provides the foundation for clinical diagnosis, pharmacological treatment, and emerging therapies. ? Learning Objectives By the end of this chapter, students should be able to: (1) Describe the overall organization of the nervous system and its two main divisions. (2) Identify the structure and function of neurons and glial cells. (3) Explain how neurons communicate via electrical signals and chemical synapses. (4) Outline the anatomy and function of the central nervous system. (5) Describe the subdivisions of the peripheral nervous system. (6) Identify common nervous system disorders and their neurological basis. 1.2 Historical Overview Ancient scholars believed the heart was the seat of intelligence and sensation. It was not until the 17th and 18th centuries that scientists such as René Descartes, Luigi Galvani, and later Santiago Ramón y Cajal began to unravel the true role of the nervous system. Cajal's neuron doctrine — the principle that the nervous system is made up of discrete, individual cells — revolutionized neuroscience and earned him the Nobel Prize in 1906. Modern neuroscience continues to build upon these foundations using tools such as MRI, electrophysiology, optogenetics, and computational modeling. 2. The Central Nervous System (CNS) The Central Nervous System (CNS) consists of the brain and the spinal cord. Encased in protective bone — the cranium and vertebral column respectively — and surrounded by three membranous layers called the meninges, the CNS serves as the processing and integration centre for all sensory input and motor output in the body. Figure 2: Major Regions of the Brain and Their Functions 2.1 The Brain The brain is the most complex organ in the known universe. In an adult human, it weighs approximately 1.3–1.4 kg and contains roughly 86 billion neurons and an equal number of glial cells. The brain is divided into three major regions: the cerebrum (forebrain), the cerebellum, and the brainstem. A. The Cerebrum The cerebrum is the largest part of the brain and is responsible for higher cognitive functions such as thought, voluntary movement, language, reasoning, perception, and emotion. It is divided into two cerebral hemispheres — left and right — connected by a thick band of nerve fibres called the corpus callosum. Each hemisphere is further divided into four lobes: Frontal Lobe — Located at the front of the brain. Responsible for voluntary movement (via the motor cortex), executive function, decision-making, planning, working memory, personality, and speech production (Broca's area). Parietal Lobe — Located behind the frontal lobe. Processes sensory information such as touch, temperature, pain, and spatial awareness (via the somatosensory cortex). Temporal Lobe — Located on the sides. Involved in auditory processing, language comprehension (Wernicke's area), and memory formation (hippocampus). Occipital Lobe — Located at the back. Dedicated to visual processing and interpretation of what the eyes see. B. The Cerebellum The cerebellum is located at the rear and lower portion of the brain, beneath the cerebrum. It coordinates voluntary motor movements, maintains posture and balance, and fine-tunes motor activity. The cerebellum receives input from the sensory systems, spinal cord, and other parts of the brain, and integrates this information to produce smooth, coordinated movement. Damage to the cerebellum results in ataxia — a loss of coordination and balance. C. The Brainstem The brainstem is the lower part of the brain that connects the cerebrum and cerebellum to the spinal cord. It consists of three parts: Midbrain (Mesencephalon) — Involved in visual and auditory reflexes and the control of eye movement. Pons — Acts as a relay station between the cerebrum and cerebellum. Also involved in regulating breathing, sleep, and swallowing. Medulla Oblongata — Controls vital autonomic functions including heart rate, blood pressure, and respiratory rhythm. Damage to the medulla can be immediately life-threatening. ? Clinical Note: The Blood-Brain Barrier The brain is protected from potentially harmful substances in the bloodstream by a highly selective semipermeable membrane known as the Blood-Brain Barrier (BBB). Formed by tight junctions between endothelial cells lining brain capillaries, the BBB permits only certain molecules (oxygen, glucose, lipid-soluble drugs) to enter the brain. This has important implications for drug delivery in neurological disorders. 2.2 The Spinal Cord The spinal cord is a long, cylindrical bundle of nervous tissue extending from the medulla oblongata down to the lumbar region of the vertebral column. It is approximately 45 cm in length and contains both ascending (sensory) and descending (motor) tracts of nerve fibres. The spinal cord performs two major functions: Relay of Information — It transmits sensory signals from the body to the brain (afferent pathways) and motor commands from the brain to the body (efferent pathways). Reflex Integration — It mediates spinal reflexes, which are rapid, involuntary responses to stimuli that do not require brain involvement (e.g., the knee-jerk reflex). In cross-section, the spinal cord shows a butterfly-shaped region of grey matter (containing neuron cell bodies) surrounded by white matter (containing myelinated axons). The dorsal horn of grey matter receives sensory input; the ventral horn contains motor neurons that innervate skeletal muscles. 3. Neurons and Glial Cells The nervous system is composed of two principal cell types: neurons, which are the electrically excitable signalling cells, and glial cells (neuroglia), which provide structural and functional support to neurons. Together, these cells form the cellular basis of all neural activity. 3.1 Neurons A neuron (nerve cell) is the fundamental structural and functional unit of the nervous system. Neurons are specialised for receiving stimuli, integrating information, and transmitting electrical impulses over long distances. The human nervous system contains approximately 86 billion neurons, each forming thousands of connections with other neurons. Figure 3: Detailed Anatomy of a Motor Neuron A. Structure of a Neuron All neurons share a common structural plan, though they vary greatly in size and shape. The major components are: Cell Body (Soma) — Contains the nucleus and most cellular organelles. This is the metabolic centre of the neuron where protein synthesis and energy production occur. Dendrites — Short, branching extensions that receive incoming signals from other neurons or sensory receptors. They increase the surface area available for receiving synaptic input. Axon — A single, elongated process that conducts electrical impulses (action potentials) away from the cell body. Axons can range from less than 1 mm to over 1 metre in length (e.g., the sciatic nerve axon from the spinal cord to the toe). Axon Hillock — The junction between the soma and axon. This is where action potentials are initiated when the membrane potential reaches threshold. Myelin Sheath — A fatty insulating layer formed by Schwann cells (in the PNS) or oligodendrocytes (in the CNS). It greatly increases the speed of signal conduction through saltatory conduction. Nodes of Ranvier — Gaps in the myelin sheath where the axon membrane is exposed. Action potentials "jump" between nodes during saltatory conduction. Axon Terminals (Synaptic Knobs) — The distal ends of axons where neurotransmitters are stored in vesicles and released into the synaptic cleft. B. Types of Neurons Based on their function, neurons are classified into three main types: Sensory (Afferent) Neurons — Carry signals from sensory receptors (e.g., in the skin, eyes, ears) toward the CNS. They convey information about the external and internal environment. Motor (Efferent) Neurons — Carry signals from the CNS to effectors (muscles and glands), initiating movement or glandular secretion. They include somatic motor neurons (controlling skeletal muscle) and autonomic motor neurons. Interneurons (Association Neurons) — Located entirely within the CNS. They connect sensory and motor neurons and are responsible for integration, analysis, and coordination of neural signals. Interneurons constitute the vast majority of neurons in the brain. Neurons can also be classified by structure: unipolar (one process), bipolar (two processes), or multipolar (multiple processes — the most common type in the CNS). 3.2 Glial Cells (Neuroglia) Glial cells outnumber neurons in the brain and are essential for normal neural function. Far from being mere scaffolding, glial cells actively modulate synaptic activity, regulate the chemical environment of neurons, and protect the CNS from damage. There are six main types of glial cells. ? Key Fact: Glial Cells Glial cells make up approximately 50% of the brain's volume and play active roles in information processing, not just passive support. Recent research has shown that astrocytes, for example, can regulate synaptic strength — a discovery that has transformed our understanding of learning and memory. Glial Cell Type Location Primary Function Astrocytes CNS Regulate synaptic environment; maintain BBB; provide nutrients to neurons; repair CNS after injury Oligodendrocytes CNS Form myelin sheaths around axons in the CNS; allow saltatory conduction Microglia CNS Immune cells of the CNS; phagocytose pathogens, dead cells, and debris (brain's macrophages) Ependymal Cells CNS Line ventricles and central canal; produce and circulate cerebrospinal fluid (CSF) Schwann Cells PNS Form myelin sheaths around peripheral axons; guide axon regeneration after injury Satellite Cells PNS Surround and protect neuron cell bodies in peripheral ganglia Table 1: Types of Glial Cells and Their Functions 4. How Neurons Communicate Neurons communicate by generating and transmitting electrical signals called action potentials, and by releasing chemical messengers called neurotransmitters at specialised junctions known as synapses. This process — known as synaptic transmission — is the foundation of all neural activity, from a simple reflex to complex conscious thought. 4.1 The Resting Membrane Potential In its resting state, a neuron maintains an electrical potential difference across its plasma membrane known as the resting membrane potential, which is approximately -70 millivolts (mV) in most neurons. This means the inside of the cell is negatively charged relative to the outside. This potential is maintained by: The Sodium-Potassium (Na+/K+) ATPase Pump — Actively transports 3 Na+ ions out and 2 K+ ions in per cycle, consuming ATP. Selectively Permeable Membrane — At rest, the membrane is more permeable to K+ than Na+, allowing K+ to diffuse out, leaving behind negatively charged proteins. Ion concentration gradients — Na+ is more concentrated outside; K+ is more concentrated inside. 4.2 The Action Potential An action potential is a brief, self-propagating reversal of membrane polarity that travels along an axon. It follows the all-or-none principle: once threshold is reached (~-55 mV), the action potential fires completely, with constant amplitude regardless of stimulus strength. The phases of an action potential: Resting State — Membrane at -70 mV. Na+ channels closed; K+ channels open. Depolarisation — Stimulus causes Na+ channels to open; Na+ rushes in; membrane potential rises rapidly to approximately +30 to +40 mV. Repolarisation — Na+ channels inactivate; K+ channels open; K+ flows out; membrane returns toward resting potential. Hyperpolarisation (After-Hyperpolarisation) — Membrane overshoots to approximately -80 mV as K+ channels close slowly. Recovery — Na+/K+ pump restores original ion concentrations; neuron returns to resting state. ? Key Concept: All-or-None Law Neurons communicate stimulus intensity not through the size of individual action potentials (which is always the same), but through the frequency of action potentials — a process called rate coding. A stronger stimulus produces more action potentials per second, not larger ones. 4.3 Synaptic Transmission Communication between neurons occurs at synapses — the specialised junctions between neurons. There are two types of synapses: Electrical Synapses — Direct electrical coupling through gap junctions. Signals pass almost instantaneously. Found in cardiac muscle and some brain regions. Chemical Synapses — The predominant type in the nervous system. Involve the release of chemical messengers (neurotransmitters) into the synaptic cleft. Figure 4: The Process of Chemical Synaptic Transmission Steps in Chemical Synaptic Transmission Action Potential Arrives — An action potential travels along the presynaptic axon to the axon terminal (synaptic knob). Calcium Influx — The action potential opens voltage-gated Ca2+ channels at the terminal; Ca2+ flows into the cell. Vesicle Fusion — Calcium triggers synaptic vesicles (containing neurotransmitters) to fuse with the presynaptic membrane via SNARE proteins. Neurotransmitter Release — Neurotransmitters are released into the synaptic cleft by exocytosis. Receptor Binding — Neurotransmitters diffuse across the cleft and bind to specific receptors on the postsynaptic membrane. Postsynaptic Response — Receptor activation either opens ion channels (ionotropic receptors) or activates second messenger systems (metabotropic receptors), causing excitation or inhibition. Termination — The signal is ended by: (a) reuptake of neurotransmitter into the presynaptic terminal, (b) enzymatic degradation, or (c) diffusion away from the cleft. 4.4 Excitatory and Inhibitory Neurotransmitters Neurotransmitters can be broadly classified as excitatory (increasing likelihood of an action potential) or inhibitory (decreasing likelihood). Common neurotransmitters include: Neurotransmitter Type Location / Role Acetylcholine (ACh) Excitatory (NMJ); Inhibitory (heart) Neuromuscular junction, ANS, brain Glutamate Excitatory Most common excitatory NT in the CNS; learning and memory GABA Inhibitory Primary inhibitory NT in the brain; reduces neural excitability Dopamine Modulatory (Excitatory/Inhibitory) Reward, motivation, motor control; reduced in Parkinson's disease Serotonin Modulatory (Inhibitory) Mood, sleep, appetite; implicated in depression Norepinephrine Excitatory Arousal, attention, stress response (fight-or-flight) Endorphins Inhibitory Pain suppression; natural opioids released during exercise or injury Table 2: Major Neurotransmitters and Their Functions 4.5 Summation and Integration A postsynaptic neuron receives hundreds or thousands of signals simultaneously from many presynaptic neurons. The neuron integrates these signals through a process called summation before deciding whether to fire an action potential. Temporal Summation — Multiple action potentials arrive at the same synapse in rapid succession, and their effects are added together over time. Spatial Summation — Simultaneous inputs from multiple synapses are added together across the postsynaptic membrane. If the combined excitatory postsynaptic potentials (EPSPs) outweigh inhibitory postsynaptic potentials (IPSPs) and threshold is reached at the axon hillock, an action potential is generated. 5. The Peripheral Nervous System (PNS) The Peripheral Nervous System consists of all neural structures outside the brain and spinal cord — that is, all cranial and spinal nerves and their associated ganglia. The PNS serves as the communication link between the CNS and the rest of the body, carrying sensory information to the CNS and motor commands from the CNS to effectors. Figure 5: Organisation of the Peripheral Nervous System 5.1 Divisions of the PNS The PNS is divided into two major functional subdivisions: the Somatic Nervous System and the Autonomic Nervous System. A. Somatic Nervous System (SNS) The Somatic Nervous System is responsible for voluntary, conscious control of skeletal muscles and for transmitting sensory information from the body's surface and special senses (vision, hearing, taste, smell, balance) to the CNS. Afferent (Sensory) Division — Carries impulses from skin, joints, and special sense organs to the CNS. Conveys information about touch, pain, temperature, proprioception, and special senses. Efferent (Motor) Division — Carries motor commands from the CNS to skeletal muscles. All somatic motor neurons release acetylcholine (ACh) at neuromuscular junctions and produce excitatory responses. B. Autonomic Nervous System (ANS) The Autonomic Nervous System controls involuntary, visceral functions — those that occur without conscious effort — including heart rate, blood pressure, digestion, respiratory rate, pupil dilation, urination, and sexual arousal. The ANS operates largely below the level of consciousness. The ANS is subdivided into two antagonistic divisions: Sympathetic Division ("Fight-or-Flight") — Prepares the body for stressful or emergency situations. Effects include: increased heart rate and blood pressure, dilation of bronchi, pupil dilation, decreased digestive activity, increased blood glucose, redistribution of blood to skeletal muscles. The primary neurotransmitter at effectors is norepinephrine. Parasympathetic Division ("Rest-and-Digest") — Promotes restorative, conserving functions under non-stressful conditions. Effects include: decreased heart rate, increased digestive activity, pupil constriction, and contraction of the bladder. The primary neurotransmitter is acetylcholine (ACh). ?? Remember: Sympathetic vs. Parasympathetic In most organs, the sympathetic and parasympathetic systems have opposing effects. A useful memory aid: Sympathetic = "4 E's" — Exercise, Excitement, Emergency, Embarrassment. Parasympathetic = "4 D's" — Digestion, Defecation, Diuresis, Dilation (of blood vessels to digestive organs). 5.2 Cranial and Spinal Nerves The PNS includes 12 pairs of cranial nerves and 31 pairs of spinal nerves. Cranial nerves emerge directly from the brain and brainstem, and are numbered I–XII (e.g., Olfactory I, Optic II, Vagus X). Spinal nerves emerge from segments of the spinal cord and are named by region: 8 cervical (C1–C8), 12 thoracic (T1–T12), 5 lumbar (L1–L5), 5 sacral (S1–S5), and 1 coccygeal. 6. Nervous System Disorders Disorders of the nervous system (neurological disorders) are among the most prevalent and debilitating conditions worldwide. The World Health Organization estimates that neurological disorders affect up to one billion people globally. Understanding the neurological basis of these conditions is essential for appreciating how dysfunction at the cellular or systems level translates to clinical disease. 6.1 Overview of Common Disorders Disorder Pathological Basis Key Signs & Symptoms Alzheimer's Disease Progressive neurodegeneration of the cortex and hippocampus; accumulation of amyloid plaques and tau tangles Memory loss, cognitive decline, personality changes; no cure currently Parkinson's Disease Loss of dopaminergic neurons in the substantia nigra, leading to dopamine deficit in the basal ganglia Resting tremor, rigidity, bradykinesia, postural instability Multiple Sclerosis Autoimmune demyelination of CNS axons, leading to disrupted signal conduction Visual disturbances, limb weakness, sensory loss, fatigue Epilepsy Recurrent, unprovoked seizures due to excessive, synchronised neuronal firing Convulsions, altered consciousness, sensory disturbances, automatisms Stroke (CVA) Interruption of blood supply to brain (ischaemic) or vessel rupture (haemorrhagic), causing neuron death Sudden weakness, speech difficulties, vision loss, headache, death if severe Meningitis Inflammation of the meninges, usually due to bacterial or viral infection Severe headache, neck stiffness, photophobia, fever Motor Neuron Disease Progressive degeneration of upper and lower motor neurons (e.g., ALS) Muscle wasting, weakness, paralysis; respiratory failure in advanced stages Table 3: Common Nervous System Disorders 6.2 Principles of Neurodegeneration Many neurological disorders involve neurodegeneration — the progressive loss of neurons and their function. Key mechanisms include: excitotoxicity (excessive glutamate leading to calcium overload and cell death), oxidative stress, mitochondrial dysfunction, protein misfolding and aggregation, and neuroinflammation mediated by microglial activation. ? Emerging Research: Neuroplasticity and Neurogenesis The human brain was once thought to be fixed and unable to change after childhood. We now know the brain retains neuroplasticity throughout life — the ability to reorganise its connections in response to learning, experience, and injury. Adult neurogenesis (production of new neurons) has been demonstrated in the hippocampus, offering hope for novel therapeutic approaches to neurodegenerative diseases. 7. Key Terms Glossary The following terms are essential vocabulary for understanding the nervous system. Students are expected to define and use all terms correctly in assessments. Term Definition Action Potential A rapid, self-propagating electrical signal along a neuron's membrane resulting from changes in membrane permeability to Na+ and K+ ions. Afferent Carrying signals toward the CNS (sensory pathways). All-or-None Principle The rule that an action potential either occurs at full strength or does not occur at all, regardless of stimulus intensity. Autonomic Nervous System The division of the PNS that controls involuntary visceral functions including heart rate, digestion, and glandular secretion. Axon The single long process of a neuron that conducts impulses away from the cell body. Depolarisation A decrease in membrane potential (making it less negative) due to influx of Na+ ions. Efferent Carrying signals away from the CNS (motor pathways). Glial Cells Non-neuronal cells that support, protect, and nourish neurons; also called neuroglia. Interneuron A neuron located entirely within the CNS that connects sensory and motor neurons. Membrane Potential The electrical voltage difference across the neuronal plasma membrane. Meninges Three protective membranes (dura mater, arachnoid mater, pia mater) surrounding the brain and spinal cord. Myelin Sheath An insulating fatty layer around axons that speeds up signal conduction via saltatory conduction. Neuron A specialised excitable cell that is the basic structural and functional unit of the nervous system. Neurotransmitter A chemical messenger released from a presynaptic terminal into the synaptic cleft to influence the postsynaptic neuron. Nodes of Ranvier Gaps in the myelin sheath between Schwann cells where action potentials are regenerated during saltatory conduction. Resting Membrane Potential The stable electrical potential across the neuronal membrane when not generating a signal; approximately -70 mV. Saltatory Conduction The jumping of action potentials from node to node along a myelinated axon, which greatly increases conduction velocity. Soma The cell body of a neuron, containing the nucleus and most organelles. Synapse The specialised junction between two neurons (or a neuron and an effector) where signals are transmitted. Synaptic Cleft The microscopic gap between the presynaptic and postsynaptic membranes at a chemical synapse. Threshold Potential The critical membrane potential (~-55 mV) at which an action potential is triggered. Table 4: Key Terms Glossary — The Nervous System 8. Chapter Summary This chapter has provided a comprehensive introduction to the human nervous system. Below is a structured summary of the major concepts covered. 8.1 Core Concepts Review Organisation The nervous system is divided into the Central Nervous System (brain + spinal cord) and the Peripheral Nervous System (all nerves outside the CNS). The PNS is further divided into the Somatic NS (voluntary) and Autonomic NS (involuntary), with the ANS subdivided into sympathetic and parasympathetic divisions. The CNS The brain consists of the cerebrum (4 lobes), cerebellum, and brainstem (midbrain, pons, medulla oblongata). The spinal cord relays signals between the brain and body and mediates spinal reflexes. The CNS is protected by bone, meninges, cerebrospinal fluid, and the blood-brain barrier. Neurons and Glial Cells Neurons are specialised for electrical signalling. Key structures: dendrites, soma, axon hillock, axon, myelin sheath, and axon terminals. Three functional types: sensory (afferent), motor (efferent), and interneurons. Glial cells (astrocytes, oligodendrocytes, microglia, ependymal cells, Schwann cells, satellite cells) provide essential support functions and outnumber neurons. Neural Communication Resting membrane potential = approximately -70 mV; maintained by the Na+/K+ pump and selective permeability. Action potentials follow the all-or-none law; stimulus intensity is coded by firing frequency. Chemical synaptic transmission involves: action potential ? Ca2+ influx ? vesicle fusion ? neurotransmitter release ? receptor binding ? postsynaptic response. Integration occurs via temporal and spatial summation at the axon hillock. Nervous System Disorders Common disorders include Alzheimer's disease, Parkinson's disease, multiple sclerosis, epilepsy, and stroke. Many involve neurodegeneration through excitotoxicity, oxidative stress, protein misfolding, or demyelination. 8.2 Self-Assessment Questions Attempt the following questions without referring to your notes first: Draw and label the major structural components of a multipolar neuron. Include the myelin sheath, nodes of Ranvier, and synaptic terminals. Describe the sequence of events during an action potential, naming the ion channels involved at each stage. Distinguish between the sympathetic and parasympathetic nervous systems in terms of function, neurotransmitters, and physiological effects. Explain what is meant by saltatory conduction. Why is it more efficient than continuous conduction? A patient presents with loss of coordination and balance but intact cognition and sensation. Which brain structure is most likely affected? Justify your answer. Compare and contrast the roles of astrocytes and microglia in the CNS. Using your knowledge of dopamine pathways, explain the neurological basis of Parkinson's disease and the rationale for L-DOPA therapy. 8.3 Recommended Further Reading Kandel, E.R. et al. (2021). Principles of Neural Science, 6th Edition. McGraw-Hill. Bear, M.F., Connors, B.W., & Paradiso, M.A. (2020). Neuroscience: Exploring the Brain, 4th Edition. Wolters Kluwer. Purves, D. et al. (2018). Neuroscience, 6th Edition. Sinauer Associates. Blumenfeld, H. (2021). Neuroanatomy Through Clinical Cases, 3rd Edition. Sinauer Associates. ? Examination Note This chapter is examinable in full. Students should be able to draw clearly labelled diagrams of the neuron structure, the action potential graph (voltage vs. time), and a synaptic transmission diagram. Definitions of all key terms in the glossary may be directly assessed. Case-based questions will require application of concepts to clinical scenarios. End of Chapter — The Nervous System First Year Biology | Neuroscience Module | Academic Year 2025/2026