Human Physiology | Module 3 Unit 1 | Calicut University First Semester BSc. Zoology Notes PDF |

 Human Physiology: Calicut University Free Study Material 

Hello everyone! In this study material I've put together notes from Module 3: Physiology of excitation, Unit 1 - Nervous system.

 I’ve tried to keep the notes clear, structured, and exam-friendly, so that you can revise easily and score well. If you find these notes helpful, please do share them with your friends and classmates. Also, feel free to leave a comment or suggestion if you’d like me to add extra explanations, diagrams, or simplified versions of tough concepts.

Summary of what’s covered in this study material

🔬 Components of the Nervous System

The nervous system is composed of neurons, neuroglia, and nerve fibres. Each component plays a specific role in impulse generation, support, and transmission.

🧩 Neurons

The neuron is the structural and functional unit of the nervous system. It consists of three main parts: the cell body (cyton or soma), dendrites, and axon.

The cell body contains the nucleus and cytoplasmic organelles such as Nissl granules, mitochondria, and Golgi bodies. It integrates incoming signals. Dendrites are short branching extensions that receive impulses and conduct them toward the soma. The axon is a long projection that carries impulses away from the cell body to other neurons, muscles, or glands. The terminal branches end in synaptic knobs that release neurotransmitters.

Based on structure, neurons may be apolar, unipolar (or pseudounipolar), bipolar, or multipolar. Functionally, they are classified as sensory neurons (afferent), motor neurons (efferent), and interneurons, which link sensory and motor pathways.

🛡 Neuroglia

Neuroglia are non-nervous supporting cells that provide structural, metabolic, and protective functions. Their number increases in higher vertebrates, reflecting increased neural complexity.

Microglia function as phagocytic cells, protecting the CNS from pathogens and cellular debris. Macroglia include astrocytes, oligodendrocytes, ependymal cells, radial glia, Schwann cells, and satellite cells.

Astrocytes regulate ion balance and repair injured nervous tissue. Oligodendrocytes form myelin sheaths in the CNS, while Schwann cells perform myelination in the PNS. Ependymal cells line brain ventricles and help circulate cerebrospinal fluid. Satellite cells support neurons in peripheral ganglia.

⚡ Nerve Fibres

Nerve fibres are axons covered with protective sheaths. They are classified as myelinated or non-myelinated.

Myelinated fibres possess a myelin sheath interrupted by nodes of Ranvier, enabling saltatory conduction which is a rapid mode of impulse transmission.  Myelination significantly enhances conduction speed and energy efficiency.

Non-myelinated fibres lack myelin and conduct impulses more slowly.

⚡ Nerve Impulse

A nerve impulse is an electrochemical change that travels along the neuron’s membrane. It involves three fundamental events: maintenance of membrane potential, generation of action potential, and propagation of action potential.

🔋 Maintenance of Membrane Potential

At rest, a neuron exhibits a polarized membrane with a negative charge inside and a positive charge outside. This resting membrane potential is maintained by three mechanisms: fixed intracellular anions, ion leak channels, and the sodium–potassium pump.

The sodium–potassium pump actively transports three sodium ions out of the cell and two potassium ions into the cell using ATP. This unequal exchange maintains ionic gradients and electrical polarity across the membrane.

⚡ Generation of Action Potential

When a neuron is stimulated beyond threshold level, sodium channels open, allowing rapid influx of Na⁺ ions. This causes depolarization, where the inside of the membrane becomes positive relative to the outside.

Subsequently, potassium channels open, allowing K⁺ to exit the cell, restoring the negative resting potential in a process called repolarization. This rapid and transient reversal of polarity constitutes the action potential.

🔁 Propagation of Action Potential

Once generated, the action potential spreads along the axon. Local currents stimulate adjacent resting segments, causing sequential depolarization. In myelinated fibres, the impulse “jumps” between nodes of Ranvier, increasing conduction velocity.

🔗 Synapse

A synapse is a functional junction between two neurons or between a neuron and an effector cell such as a muscle. The transmitting neuron is called the presynaptic neuron, and the receiving neuron is the postsynaptic neuron.

Synapses may be axodendritic, axosomatic, or axoaxonic depending on contact location.

Functionally, synapses are classified as electrical or chemical. Electrical synapses allow direct ion flow through gap junctions helping in rapid transmission. Chemical synapses use neurotransmitters to transmit signals across a synaptic cleft.

🧪 Chemical Synaptic Transmission

In chemical synapses, arrival of an action potential at the synaptic knob triggers calcium influx. This stimulates synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitters into the synaptic cleft.

The neurotransmitters bind to specific receptors on the postsynaptic membrane, altering ion permeability and generating either excitatory or inhibitory responses.

Excitatory neurotransmitters increase the likelihood of depolarization, while inhibitory neurotransmitters reduce it.

🧬 Neurotransmitters

Neurotransmitters are chemical messengers involved in synaptic transmission.

• Acetylcholine plays a crucial role at neuromuscular junctions and is broken down by acetylcholinesterase after signal transmission.
  
  
• Noradrenaline functions mainly in the sympathetic nervous system and is associated with fight-or-flight responses.
  
  
• Gamma-aminobutyric acid is a major inhibitory neurotransmitter in the CNS and helps regulate anxiety, sleep, and neuronal excitability.
  
  

⏳ Synaptic Delay and Fatigue

Synaptic delay refers to the brief time gap between arrival of an impulse at the presynaptic terminal and the postsynaptic response. It is typically less than a millisecond and is caused by neurotransmitter release and diffusion.

Synaptic fatigue occurs during prolonged stimulation when neurotransmitter stores become depleted. This temporary reduction in transmission acts as a protective mechanism against overstimulation.

🧠 Sensory Phenomena: Numbness, Tingling and Tickling

Numbness is a partial or complete loss of sensation, while tingling is a “pins and needles” sensation caused by abnormal nerve activity.

Tickling is a complex sensory response. It exists in two forms: knismesis, a light-touch sensation likely evolved as a protective mechanism against parasites, and gargalesis, a deeper stimulus that evokes laughter and involuntary movement.

📝 Important Exam Questions

• Types of synapse
• Neuroglia and their functions
• Chemical synaptic transmission
• Explain the mechanism of action potential generation and propagation.
• Describe synaptic transmission in detail.
• Discuss structure and classification of neurons.

A downloadable PDF version of this module is available for structured revision and easy reference before exams.
Feel free to use it for your personal study and exam preparation.

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