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Nervous System - PPT/PDF

Download notes (PPT/PDF) providing a comprehensive overview of the Nervous System. These notes cover the functions, anatomy, organization, and physiology of the nervous system, including detailed discussions of supporting cells, neurons, the brain (cerebral hemispheres, diencephalon, brain stem, cerebellum), protection of the CNS (meninges, cerebrospinal fluid, blood-brain barrier), the spinal cord, the peripheral nervous system (cranial nerves, spinal nerves), the autonomic nervous system (sympathetic and parasympathetic divisions), nerve impulses, synaptic transmission, and autonomic functioning. Suitable for B.Pharm and related health science programs.

Keywords: Nervous System, Functions, Anatomy, Organization, Physiology, Supporting Cells, Neurons, Brain, Cerebral Hemispheres, Diencephalon, Brain Stem, Cerebellum, Meninges, Cerebrospinal Fluid, Blood-Brain Barrier, Spinal Cord, Peripheral Nervous System, Cranial Nerves, Spinal Nerves, Autonomic Nervous System, Sympathetic, Parasympathetic, Nerve Impulse, Synaptic Transmission, B.Pharm, PPT, PDF, Download.

Nervous System - Detailed Explanation

1. Functions of the Nervous System

The nervous system is the master controlling and communicating system of the body. Its primary functions include:

  • Sensory Input: Gathering information about internal and external changes (stimuli) via sensory receptors.
  • Integration: Processing and interpreting sensory input and deciding what action, if any, should be taken.
  • Motor Output: Activating effector organs (muscles and glands) to cause a response.
  • Higher mental function and emotional response.

2. Anatomy of the Nervous System

Anatomically, the nervous system can be divided as follows:

  • Central nervous system- Brain and spinal cord.
  • Peripheral nervous system- outside the brain and spinal cord

3. Organization of the Nervous System

  • Structural Classification:
    • Central Nervous System (CNS): Brain and spinal cord. The integrating and control center.
    • Peripheral Nervous System (PNS): All nervous tissue outside the CNS (nerves, ganglia, sensory receptors). Connects the CNS to the rest of the body.
  • Functional Classification: (Focuses on the PNS)
    • Sensory (Afferent) Division: Carries sensory information *from* receptors *to* the CNS.
    • Motor (Efferent) Division: Carries motor commands *from* the CNS *to* effector organs (muscles and glands). Further divided into:
      • Somatic Nervous System (SNS): Controls voluntary movements of skeletal muscles.
      • Autonomic Nervous System (ANS): Controls involuntary functions (e.g., heart rate, digestion, smooth muscle contraction). Further divided into:
        • Sympathetic Division: "Fight-or-flight" responses.
        • Parasympathetic Division: "Rest-and-digest" functions.

4. Nervous Tissue: Structure and Function

  • Supporting Cells (Neuroglia or Glial Cells): Non-excitable cells that support, protect, and insulate neurons. Types include:
    • Astrocytes (CNS): Most abundant glial cells. Support neurons, regulate the chemical environment, and contribute to the blood-brain barrier.
    • Microglial Cells (CNS): Defensive cells; act as phagocytes.
    • Ependymal Cells (CNS): Line the cavities of the brain and spinal cord; help circulate cerebrospinal fluid.
    • Oligodendrocytes (CNS): Form the myelin sheath around axons in the CNS.
    • Satellite Cells (PNS): Surround neuron cell bodies in ganglia; provide support and regulate the environment.
    • Schwann Cells (PNS): Form the myelin sheath around axons in the PNS.
  • Neurons: The excitable cells that transmit electrical signals. Key features:
    • Cell Body (Soma): Contains the nucleus and other organelles.
    • Dendrites: Branching extensions that receive signals from other neurons.
    • Axon: A single, long extension that transmits signals to other neurons, muscles, or glands. The axon may be myelinated (covered with a myelin sheath), which increases conduction speed.
    • Axon Terminals: The branches at the end of the axon, where neurotransmitters are released.
    Types of Neurons:
    • Sensory neuron
    • Motor neuron
    • Interneuron

Central nervous system

  • Brain and spinal cord included.

5. Brain

The brain is the control center of the nervous system, responsible for complex functions like thought, memory, emotion, and regulation of bodily processes.

  • Cerebral Hemispheres (Cerebrum):
    • The largest part of the brain, divided into two hemispheres (left and right) connected by the corpus callosum.
    • Responsible for higher-level functions, including:
      • Sensory Perception: Interpreting sensory input from the body.
      • Motor Control: Initiating and controlling voluntary movements.
      • Language: Speech production and comprehension.
      • Thought, Memory, and Learning: Cognitive functions.
    • The surface of the cerebrum is highly folded, forming *gyri* (ridges) and *sulci* (grooves). Deep sulci are called *fissures*.
    • Each hemisphere is divided into four lobes:
      • Frontal Lobe: Motor control, planning, decision-making, personality.
      • Parietal Lobe: Sensory perception (touch, pressure, pain, temperature), spatial awareness.
      • Temporal Lobe: Auditory processing, memory, language comprehension.
      • Occipital Lobe: Visual processing.
    • The cerebral cortex is the outer layer of gray matter, responsible for higher-level functions. The inner white matter consists of myelinated axons that connect different parts of the brain.
  • Diencephalon: Located between the cerebral hemispheres and the brainstem. Includes:
    • Thalamus: The major relay station for sensory information going to the cerebral cortex.
    • Hypothalamus: Controls the autonomic nervous system, regulates body temperature, hunger, thirst, sleep-wake cycles, and controls the endocrine system via the pituitary gland.
    • Epithalamus: Includes the pineal gland, which produces melatonin.
  • Brain Stem: Connects the cerebrum and diencephalon to the spinal cord. Includes:
    • Midbrain: Contains visual and auditory reflex centers, and motor nuclei.
    • Pons: Relays information between the cerebrum and cerebellum, and contains respiratory centers.
    • Medulla Oblongata: Contains vital centers that control heart rate, blood pressure, and breathing. Also contains reflex centers for coughing, sneezing, swallowing, and vomiting.
  • Cerebellum:
    • Located at the back of the brain, inferior to the occipital lobes.
    • Coordinates movement, balance, and posture. Receives input from the motor cortex, proprioceptors (in muscles and joints), and the vestibular system (inner ear).
  • Protection of the Central Nervous System: The brain and spinal cord are protected by:
    • Bones: The skull (cranium) protects the brain, and the vertebral column protects the spinal cord.
    • Meninges:
      • Three layers of connective tissue membranes that surround the brain and spinal cord:
        • Dura Mater: The outermost, tough layer.
        • Arachnoid Mater: The middle layer, with a web-like appearance.
        • Pia Mater: The innermost layer, which adheres closely to the brain and spinal cord.
      • The subarachnoid space, between the arachnoid mater and pia mater, contains cerebrospinal fluid (CSF).
    • Cerebrospinal Fluid (CSF):
      • A clear fluid that circulates around the brain and spinal cord.
      • Provides cushioning and protection.
      • Provides a stable chemical environment for the CNS.
      • Formed by the choroid plexuses in the ventricles of the brain.
    • The Blood-Brain Barrier:
      • A protective barrier that helps maintain a stable environment for the brain.
      • Formed by tight junctions between endothelial cells in brain capillaries, along with astrocytes.
      • Selectively permeable, allowing only certain substances (e.g., glucose, oxygen, essential amino acids) to pass from the blood into the brain tissue. Restricts the passage of many harmful substances (e.g., toxins, pathogens).

6. Spinal Cord

  • The spinal cord extends from the foramen magnum (the opening at the base of the skull) to the level of the first or second lumbar vertebra.
  • It's a two-way conduction pathway, carrying sensory information *up* to the brain and motor commands *down* from the brain.
  • It's also a major reflex center.
  • Gray Matter of the Spinal Cord and Spinal Roots:
    • The inner core of the spinal cord, shaped like a butterfly or the letter "H."
    • Contains neuron cell bodies, dendrites, and unmyelinated axons.
    • Divided into horns:
      • Dorsal (Posterior) Horns: Contain interneurons and the axons of sensory neurons.
      • Ventral (Anterior) Horns: Contain cell bodies of somatic motor neurons.
      • Lateral Horns: (Present in thoracic and upper lumbar regions) Contain cell bodies of autonomic motor neurons.
    • Spinal Roots:
      • Dorsal Root: Contains sensory (afferent) fibers. The dorsal root ganglion contains the cell bodies of sensory neurons.
      • Ventral Root: Contains motor (efferent) fibers.
    • The dorsal and ventral roots join to form a spinal nerve.
  • White Matter of the Spinal Cord:
    • Surrounds the gray matter.
    • Contains myelinated axons, organized into tracts (bundles of axons with similar functions).
    • Ascending Tracts: Carry sensory information *up* the spinal cord to the brain.
    • Descending Tracts: Carry motor commands *down* the spinal cord from the brain.

7. Peripheral Nervous System (PNS)

The PNS consists of all nervous tissue outside the CNS.

  • Structure of a Nerve:
    • A nerve is a bundle of axons in the PNS.
    • Axons are wrapped in layers of connective tissue:
      • Endoneurium: Surrounds individual axons.
      • Perineurium: Surrounds bundles of axons (fascicles).
      • Epineurium: Surrounds the entire nerve.
    • Nerves can be classified as:
      • Sensory (Afferent) Nerves: Carry only sensory fibers.
      • Motor (Efferent) Nerves: Carry only motor fibers.
      • Mixed Nerves: Carry both sensory and motor fibers. (Most nerves are mixed.)
  • Cranial Nerves:
    • 12 pairs of nerves that arise from the brain.
    • Most are mixed nerves, but some are primarily sensory or motor.
    • Serve the head and neck region (except for the vagus nerve (X), which extends to the thorax and abdomen).
    • Examples:
      • Olfactory (I): Sensory - smell.
      • Optic (II): Sensory - vision.
      • Oculomotor (III): Motor - eye movements, pupil constriction.
      • Trochlear (IV): Motor - eye movements.
      • Trigeminal (V): Mixed - sensory from face, motor for chewing.
      • Abducens (VI): Motor - eye movements.
      • Facial (VII): Mixed - sensory for taste, motor for facial expressions, lacrimation, salivation.
      • Vestibulocochlear (VIII): Sensory - hearing and balance.
      • Glossopharyngeal (IX): Mixed - sensory for taste, motor for swallowing and salivation.
      • Vagus (X): Mixed - sensory and motor to thoracic and abdominal viscera (parasympathetic).
      • Accessory (XI): Motor - controls muscles of the neck and shoulder.
      • Hypoglossal (XII): Motor - controls tongue movements.
  • Spinal Nerves and Nerve Plexuses:
    • 31 pairs of spinal nerves arise from the spinal cord.
    • All are mixed nerves.
    • Named according to the region of the vertebral column from which they emerge:
      • Cervical (C1-C8): 8 pairs
      • Thoracic (T1-T12): 12 pairs
      • Lumbar (L1-L5): 5 pairs
      • Sacral (S1-S5): 5 pairs
      • Coccygeal (Co1): 1 pair
    • Except for the thoracic nerves (T2-T12), spinal nerves branch and join to form *nerve plexuses*. A plexus is a network of intersecting nerves.
    • Major plexuses:
      • Cervical Plexus (C1-C5): Innervates the skin and muscles of the neck, shoulder, and diaphragm (phrenic nerve).
      • Brachial Plexus (C5-T1): Innervates the upper limb.
      • Lumbar Plexus (L1-L4): Innervates the anterior and medial thigh.
      • Sacral Plexus (L4-S4): Innervates the posterior thigh, leg, and foot (sciatic nerve).

8. Autonomic Nervous System (ANS)

The ANS controls involuntary functions (e.g., heart rate, digestion, smooth muscle contraction, glandular secretions).

  • Anatomy of the Parasympathetic Division:
    • "Rest-and-digest" system.
    • Preganglionic neurons originate in the brainstem (cranial nerves III, VII, IX, X) and the sacral region of the spinal cord (S2-S4). Craniosacral outflow.
    • Long preganglionic fibers, short postganglionic fibers.
    • Ganglia are located near or within the effector organs (terminal ganglia).
    • Neurotransmitter: Acetylcholine (ACh) at both preganglionic and postganglionic synapses.
  • Anatomy of the Sympathetic Division:
    • "Fight-or-flight" system.
    • Preganglionic neurons originate in the thoracolumbar region of the spinal cord (T1-L2). Thoracolumbar outflow.
    • Short preganglionic fibers, long postganglionic fibers.
    • Ganglia are located near the spinal cord (sympathetic trunk ganglia and prevertebral ganglia).
    • Neurotransmitters: Acetylcholine (ACh) at preganglionic synapses; norepinephrine (NE) at most postganglionic synapses (some release ACh, e.g., sweat glands).

9. Physiology of the Nervous System

  • Nerve Impulse (Action Potential):
    • A rapid, temporary change in the membrane potential of a neuron, allowing it to transmit signals.
    • Resting Membrane Potential: The neuron is at rest, with a negative charge inside compared to the outside (around -70mV). Maintained by the sodium-potassium pump.
    • Depolarization: A stimulus causes voltage-gated sodium channels to open, allowing Na+ to flow into the cell, making the inside more positive.
    • Threshold: If the depolarization reaches a certain threshold, an action potential is triggered (all-or-none principle).
    • Repolarization: Sodium channels close, and voltage-gated potassium channels open, allowing K+ to flow out of the cell, restoring the negative charge inside.
    • Hyperpolarization: A brief period where the membrane potential becomes more negative than the resting potential (due to continued K+ outflow).
    • Return to Resting Potential: The sodium-potassium pump restores the ion balance.
    • Refractory Period: A brief period after an action potential during which another action potential cannot be generated (absolute refractory period) or is more difficult to generate (relative refractory period). Ensures unidirectional propagation of the action potential.
    • Saltatory Conduction: In myelinated axons, the action potential "jumps" from one Node of Ranvier (a gap in the myelin sheath) to the next, greatly increasing conduction speed.
  • The Nerve Impulse Pathway:
    • Stimulus → Receptor → Sensory Neuron → CNS (Integration) → Motor Neuron → Effector
  • Communication of Neurons at Synapses:
    • A synapse is the junction between two neurons, or between a neuron and a target cell (e.g., muscle fiber).
    • Electrical Synapses: Direct connection between cells via gap junctions; allow rapid transmission. Less common.
    • Chemical Synapses: More common. Involve the release of neurotransmitters.
      • An action potential arrives at the axon terminal of the presynaptic neuron.
      • Voltage-gated calcium channels open, allowing Ca2+ to enter the axon terminal.
      • Ca2+ triggers the fusion of vesicles containing neurotransmitters with the presynaptic membrane.
      • Neurotransmitters are released into the synaptic cleft (the space between the neurons).
      • Neurotransmitters bind to receptors on the postsynaptic neuron (or target cell).
      • This binding causes ion channels to open, leading to either excitation (depolarization, EPSP - excitatory postsynaptic potential) or inhibition (hyperpolarization, IPSP - inhibitory postsynaptic potential) of the postsynaptic neuron.
      • Neurotransmitters are removed from the synaptic cleft by reuptake (into the presynaptic neuron), enzymatic degradation, or diffusion.
  • Autonomic Functioning:
    • The ANS controls involuntary functions, maintaining homeostasis.
    • It has two divisions, sympathetic and parasympathetic, which often have opposing effects.

Sympathetic division

  • The sympathetic division prepares the body for "fight-or-flight" situations:
    • Increases heart rate and contractility.
    • Increases blood pressure.
    • Dilates bronchioles (improves airflow).
    • Increases blood glucose levels.
    • Redirects blood flow to skeletal muscles and away from the digestive system.
    • Dilates pupils.

10. Parasympathetic Division

  • The parasympathetic division promotes "rest-and-digest" functions:
    • Decreases heart rate.
    • Increases digestive activity (increased salivation, gastric motility, and secretion).
    • Constricts pupils.
    • Constricts bronchioles.

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