Neurophysiology: Comprehensive Study Notes on the Nervous System, Neurons, and Receptors

Introduction to the Nervous System

The nervous system is a complex network divided into two primary sections: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The brain and the spinal cord constitute the Central Nervous System, which serves as the primary analyzer and integrator of information gathered from both the internal and external environments. By processing this information, the CNS generates essential commands that regulate muscles, glands, and blood vessels. It acts as the initiator and coordinator of all bodily actions and is fundamentally involved in higher cognitive processes, including learning and memory. The Peripheral Nervous System consists of cranial nerves and spinal nerves which emerge from the brain and spinal cord, respectively. The PNS is further divided into afferent and efferent pathways. The afferent pathway carries information from somatic and visceral sense organs toward the CNS. The efferent pathway is divided into the somatic nervous system, which controls skeletal muscles via motor nerves, and the autonomic nervous system, which regulates smooth muscle, cardiac muscle, and glands. The autonomic nervous system is further sub-divided into the parasympathetic and sympathetic branches.

The Neuron: Structure and Classification

The neuron, or nerve cell, is the fundamental structural and functional unit of the nervous system. While it shares many characteristics with other body cells, such as containing a nucleus, mitochondria, and a golgi apparatus, it possesses two unique distinctions: it has specialized branches known as axons and dendrites, and it is unable to undergo cell division. The cell body, also known as the soma, is irregular in shape and contains the nucleus alongside various organelles. Mitochondria serve as the powerhouse for ATP production, while the golgi apparatus is responsible for processing and packaging proteins into granules. Protein synthesis occurs within the Nissl bodies, and the cell is supported by neurofibrils consisting of microfilaments and microtubules.

Neurons are classified based on several criteria. Based on the number of poles, neurons are categorized as unipolar, possessing a single pole from which both the axon and dendrites arise; bipolar, featuring two poles where the axon and dendrite emerge from opposite sides; and multipolar, which have multiple poles where one gives rise to an axon and the others to dendrites. Functionally, neurons are divided into motor (efferent) neurons, which carry impulses away from the CNS to peripheral organs like muscles and glands, and sensory (afferent) neurons, which carry impulses from the periphery to the CNS. Based on axon length, neurons are classified as Golgi type I, which have long axons found throughout the CNS, and Golgi type II, which have short axons typically located in the cerebral cortex and spinal cord.

Anatomy of the Nerve and the Myelin Sheath

Individual nerve fibers are organized into a tiered structure of connective tissue. Each individual axon is covered by a layer called the endoneurium. Groups of these nerve fibers are bundled together into a fasciculus, which is encased in the perineurium. The entire nerve, comprising multiple fasciculi, is ultimately wrapped in a tubular sheath known as the epineurium. Within this structure, many axons are insulated by a thick lipoprotein sheath called the myelin sheath. This insulation is not continuous but is interrupted at regular intervals by gaps known as nodes of Ranvier.

The myelin sheath serves two critical functions. First, it significantly increases the speed of impulse conduction. In myelinated fibers, the impulse jumps from one node to another in a process called saltatory conduction, which is approximately 50 times faster than conduction in non-myelinated fibers. Second, the insulating property of the myelin sheath restricts nerve impulses to a single fiber, preventing the accidental stimulation of neighboring fibers.

Neurotrophins and Their Functions

Neurotrophins, or neurotrophic factors, are specific protein substances vital for the growth, development, and functioning of nervous tissue. These proteins are secreted by various tissues, most notably muscles and astrocyte neuroglia cells. Key types of neurotrophins include the Nerve Growth Factor (NGF), found in many peripheral tissues, and the Brain Derived Neurotrophic Growth Factor (BDGF), which is present in the human brain and human sperm. Other factors include the Ciliary Neurotrophic Factor (CNTF) secreted by peripheral nerves and cardiac muscles, the Glia-cell line-derived Neurotrophic Factor (GNDF), the Fibroblast Growth Factor (FGF), and Neurotrophin-3 (NT-3). Collectively, these factors facilitate the initial growth of nerve cells in the CNS and PNS, promote cellular survival and repair, and maintain neural transmission integrity.

Comprehensive Classification of Nerve Fibers

Nerve fibers are classified according to five major systems. Structurally, they are either myelinated or non-myelinated. Functionally, they are either sensory (afferent) or motor (efferent). Based on origin, they are classified as cranial nerves, originating from the brain, or spinal nerves, originating from the spinal cord. There are 12 pairs of cranial nerves: Olfactory (I), Optic (II), Oculomotor (III), Trochlear (IV), Trigeminal (V), Abducens (VI), Facial (VII), Acoustic or Vestibulocochlear (VIII), Glossopharyngeal (IX), Vagus (X), Spinal Accessory (XI), and Hypoglossal (XII). There are 31 pairs of spinal nerves: 8 cervical (C1-C8), 12 thoracic (T1-T12), 5 lumbar (L1-L5), 5 sacral (S1-S5), and 1 coccygeal pair. Distribution-wise, fibers are somatic (supplying skeletal muscle) or autonomic (supplying internal organs). Based on neurotransmitter secretion, they are adrenergic (secreting noradrenaline) or cholinergic (secreting acetylcholine).

General classification categories fibers into types A, B, and C based on diameter and conduction velocity. Type A fibers are the thickest and are myelinated, further subdivided into $\alpha$, $\beta$, $\gamma$, and $\delta$. Type C (or type IV) fibers are the thinnest and are unmyelinated. Conduction velocity is directly proportional to fiber thickness. Sensory physiologists use a specific grouping system: Group 1a consists of fibers from annulospiral endings ($17\,\mu m$ diameter, Type A$\alpha$); Group 1b consists of fibers from Golgi tendon organs ($16\,\mu m$ diameter, Type A$\alpha$); Group II consists of fibers from cutaneous tactile receptors and flower-spray endings ($8\,\mu m$ diameter, Type A$\beta$ and A$\gamma$); Group III carries temperature, crude touch, and pricking pain ($3\,\mu m$ diameter, Type A$\delta$); and Group IV consists of unmyelinated fibers for pain, itch, and temperature ($0.5\,to\,2\,\mu m$ diameter, Type C).

Properties of Nerve Fibers

Nerve fibers exhibit several distinct physiological properties. Excitability refers to the physicochemical change that occurs when a nerve is stimulated. Adequate stimulus strength produces a propagating action potential, while subliminal strength produces a non-propagating electrotonic potential. Conductivity is the ability to transmit impulses; in the body, this typically occurs in one direction through a sequence of depolarization and repolarization. The Refractory Period is a duration where the nerve does not respond to stimuli, divided into absolute and relative phases. Summation occurs when multiple subliminal stimuli applied over a short period combine to produce a response. Adaptation is the gradual decrease in response to a continuous constant stimulus. Infatigability describes the nerve's ability to remain functional without fatigue even after long-term stimulation. Finally, the All-or-none Law states that a nerve fiber will either produce a maximum response or no response at all when stimulated.

Degeneration and Regeneration of Nerve Fibers

Degeneration occurs due to injury, such as blood flow obstruction, transection, or toxic injection. Wallerian (orthograde) degeneration involves pathological changes in the distal end of the axon, starting within 24 hours and involving myelin sheath disintegration and axis cylinder breakage. Retrograde degeneration affects the cell body and proximal axon, beginning within 48 hours. Changes include the disintegration of Nissl granules and the golgi apparatus, cellular swelling, and potential nucleus extrusion, which may lead to cell death. Transneuronal degeneration occurs when the cutting of an afferent fiber causes degeneration in the neuron with which it synapses.

Injury is classified by degree. First-degree injury (Seddon neuropraxia) involves temporary loss of function due to hypoxia or pressure, usually resolving within weeks. Second-degree injury (axonotmesis) involves prolonged compression where repair can take over 18 months. Third-degree injury involves endoneurium degeneration with intact perineurium and epineurium, leading to slow or incomplete repair. Fourth-degree injury involves disorganized fasciculi and affected epineurium/perineurium. Fifth-degree injury is a complete transection requiring surgical approximation. Third, fourth, and fifth-degree injuries are collectively termed neurotmesis.

Regeneration is the regrowth of the nerve fiber, beginning on the 4th day after injury and peaking after 30 days. For successful regeneration, four criteria must be met: the gap between cut ends must not exceed $3\,mm$, the nucleus must remain intact in the cell body, the neurilemma must be present, and the two cut ends must remain aligned.

Neuroglia Cells

Neuroglia, or glia cells, are the non-excitable supporting and nutritive cells of the nervous system, outnumbering neurons. Central neuroglia include Oligodendrocytes, which produce CNS myelin and form supportive connective tissue; Astrocytes, which maintain the neuronal environment and form the Blood-Brain Barrier (BBB); Ependymal cells, which line the brain ventricles and produce Cerebrospinal Fluid (CSF); and Microglia, the small immune cells that phagocytose foreign substances. Peripheral neuroglia include Schwann cells, which produce PNS myelination and assist in regeneration, and Satellite cells, which myelinate PNS cell bodies and provide physical support.

Receptors: Classification and Properties

Receptors are sensory nerve endings that respond to stimuli by producing impulses transmitted through afferent nerves. They are classified into five basic types: Mechanoreceptors (detecting mechanical deformation like pressure, touch, and vibration), Thermoreceptors (detecting temperature changes), Nociceptors (detecting tissue damage or pain), Photoreceptors (detecting light via rods and cones), and Chemoreceptors (detecting chemical concentrations). Specific mechanoreceptors include Pacinian corpuscles for pressure, Meissner corpuscles and Merkel’s Discs for touch/vibration, and proprioceptors like muscle spindles for length and Golgi tendon organs for tension. Thermoreceptors include Ruffini endings for warmth and Krause endings for cold. Chemoreceptors are found in carotid/aortic bodies ($PCO_2$, $PO_2$, $pH$), taste buds, and the hypothalamus (osmolarity).

Receptors possess four primary properties. Transduction is the conversion of natural stimuli into action potentials. During this process, a stimulus like mechanical deformation opens $Na^+$ channels, causing an influx of ions that creates a receptor (generator) potential; once it reaches a certain threshold, it triggers a propagated action potential. Adaptation is the decline in action potential frequency during a maintained constant stimulus. Phasic receptors adapt quickly, while tonic receptors adapt slowly. Fatigue is the inability of a receptor to respond despite increased stimulus strength, distinct from adaptation because the receptor is no longer capable of the response. The Law of Specific Nerve Energies states that stimulation of a specific sensory nerve always produces only one sensation (e.g., light for the optic nerve), regardless of the stimulus type, due to the specific synaptic pathways in the brain.