nervous system notes

Chapter 5: Nervous System of Man

5.1 Receptors

Receptors are biological transducers that convert energy from both external and internal environments into electrical impulses. They may be massed together to form a sense organ (e.g., the eye or ear) or scattered throughout tissues like the skin and viscera.

5.2 Path of a Message Transmitted to the Central Nervous System

Sensory information travels from the body to the spinal cord before reaching the brain. This pathway involves several types of neurons:

5.2.1 First Order Neuron (Sensory Neuron)

  • Description: The first-order neuron is the initial neuron in a sensory pathway that transmits sensory information from the periphery (e.g., skin, muscles, organs) to the Central Nervous System (CNS).

  • Function: Its primary role is to detect stimuli such as touch, pain, and temperature and relay this information to the second-order neuron, typically located in the spinal cord or brainstem.

5.2.2 Second Order Neuron (Interneuron)

  • Location: Usually located in the spinal cord or brainstem, depending on the specific sensory pathway.

  • Function: It receives input from the first-order neuron, processes the information (possibly integrating it with other sensory inputs), and transmits it to higher brain centers, primarily the thalamus. For pain pathways, it may be located in the dorsal horn of the spinal cord.

5.2.3 Third Order Neuron

  • Function: The third-order neuron relays the impulses received from the thalamus to the somatosensory cortex of the cerebrum. Somatosensory sensations include pressure, pain, temperature, and body sensations.

Fig. 5.1: Pathway of message transmitted to the CNS

5.3 Neuron

Nervous coordination comprises highly specialized cells called neurons, which are the basic structural and functional unit of the nervous system.

5.3.1 Structure of Neuron

Although neurons vary in size and shape, they all possess three basic components:

  1. Cell Body: Contains granular cytoplasm, cell membrane, a centrally placed nucleus with prominent nucleolus, Golgi apparatus, mitochondria, and Nissl's granules (ribosomes and rough ER for protein synthesis).

  2. Dendrites: Short, thin, often highly branched cytoplasmic extensions that taper from the base to the tips, functioning to receive stimuli and conduct impulses to the cell body. Synapses with axons from other neurons occur at dendrites.

  3. Axon: A long, thick nerve fiber with a constant diameter, varying from a few mm to over a meter in length. It may be branched, with presynaptic terminals terminating at its end. Axons conduct action potentials from the neuron's cell body away to the presynaptic terminals.

Fig. 5.2: Structure of neuron
5.3.2 Types of Neurons

Neurons vary in size, shape, and characteristics depending on function:

  1. Sensory Neurons (Afferent Neurons): Convert signals from the environment into stimuli. They are pseudo-unipolar in structure and carry sensory information to the brain and spinal cord.

  2. Motor Neurons (Efferent Neurons): Multipolar neurons located in the CNS that transmit information from the brain to body effectors (muscles or glands).

  3. Interneurons: Multipolar in structure and connect sensory and motor neurons (also known as relay neurons or local circuit neurons).

Fig. 5.3: Types of neuron based on function


Fig. 5.4: Types of neuron based on structure
5.3.3 Effectors

  • Definition: Effectors are muscles and glands that respond to signals sent by the nervous system. Muscles contract, while glands produce secretions in response to these signals.

5.3.4 Detailed Structure of Neurons
a. Sensory Neurons (Afferent Neurons)

  • Location: The cell body is in the dorsal root ganglion outside the CNS. They have long dendrites that receive signals from sensory receptors and convey them to the CNS.

  • Structure: Generally unipolar, with one long process branching into two. One connects to sensory receptors, while the other connects to the CNS.

b. Motor Neurons (Efferent Neurons)

  • Location: The cell body is located either in the spinal cord or the brain's cortex (upper motor neurons).

  • Structure: Multipolar, with many dendrites and a long axon. They transmit signals from the CNS to muscles and glands for movement and reaction.

c. Interneurons

  • Location: Entirely within the brain and spinal cord. Their highly branched dendrites receive signals from various neurons.

  • Structure: Generally multipolar with short, unmyelinated axons for local signal transmission.

Fig. 5.5: Detailed structures of each type of neuron
5.3.5 Relate Structure to Function

  • Motor Neurons: Long axons enable communication over great distances; myelination enhances speed.

  • Sensory Neurons: Long axons and dendrites allow effective signal reception and transmission to CNS.

  • Interneurons: Shorter axons and numerous branches foster local processing and connectivity.

5.3.6 Differences Between Myelinated and Non-Myelinated Neurons

  • Myelinated Neurons:

    • Have a myelin sheath produced by Schwann cells.

    • Conduct nerve impulses rapidly, appearing white.

    • Contain nodes of Ranvier for action potentials to jump between nodes, increasing speed.

  • Non-Myelinated Neurons:

    • Lack a myelin sheath and appear grey.

    • Conduct nerve impulses slowly, lacking nodes of Ranvier.

Table 5.2: Differences between myelinated and non-myelinated neurons

5.4 Reflex Arc

  • Definition: A reflex action is an immediate, automatic, and involuntary response to environmental changes. The pathway of nerve impulses is termed a reflex arc.

Example of Reflex Arc

  • Components:

    • Receptors: Respond to stimuli.

    • Sensory Neurons: Transmit impulses to the spinal cord.

    • Interneuron: Processes the information locally in the spinal cord.

    • Motor Neurons: Transmit signals to muscles.

    • Effectors: Cause muscle contractions to withdraw from harmful stimuli.

Fig. 5.10: A reflex arc

5.5 Nerve Impulse

  • Definition: A nerve impulse is a signal transmitted from receptors to the CNS and to effectors.

  • Technical Definition: A wave of electrochemical changes traveling along the neuron.

5.5.1 Generation and Transmission of Nerve Impulse

  • Electrochemical: Refers to the electrical potential (membrane potential) across the neuron's membrane.

  • Forms: Resting Membrane Potential (RMP) and Active Membrane Potential (AMP).

5.5.2 Resting Membrane Potential

  • Characteristics: More positive outer surface compared to the inner surface, typically around -70 mV.

  • Factors Influencing RMP:

    • Distribution of Ions: More potassium (K+) inside; more sodium (Na+) outside due to the sodium-potassium pump.

    • Presence of Negative Organic Ions: Adjust the internal negativity.

    • K+ Leakage: K+ ions leak through K+ channels, enhancing the RMP.

5.5.3 Active Membrane Potential (Action Potential)

  • Characteristics: More positive inside compared to outside (depolarized state).

  • Establishment:

    • Threshold Stimulus: Initiates action potential if strong enough.

    • Influx of Na+ Ions: Opening of sodium channels allows Na+ to enter, causing depolarization.

    • Repolarization: K+ channels open, allowing K+ to exit, restoring the membrane potential.

    • Hyperpolarization: Occurs due to delayed closing of K+ channels, making the neuron temporarily more negative than resting potential.

Fig. 5.12: Conduction of nerve impulse
5.5.4 Velocities of Nerve Impulse

  • Myelinated Fibers: Conduct impulses at high speeds (up to 120 m/s) due to saltatory conduction, which occurs as impulses jump between nodes of Ranvier.

  • Non-Myelinated Fibers: Conduct impulses slower (1-3 m/s).

  • Diameter Influence: Thicker fibers conduct impulses faster due to decreased resistance.

  • Synaptic Delay: The short time taken for impulses to cross the synapse.

5.5.5 Role of Local Circuits

  • Local circuits of current induce depolarization in the next section of the axon, critical in saltatory conduction. Sodium ions diffuse along the axon, triggering further depolarization and action potential progression at each node.

Fig. 5.14: Synaptic transmission

5.6 Synapse

  • Definition: A synapse is a junction where one neuron's axon terminal transmits information to another neuron's dendrite across a synaptic cleft.

5.6.1 Structure of Synapse

  • Presynaptic Neuron: Transmits impulse towards the synapse.

  • Post-synaptic Neuron: Receives impulse across the synaptic cleft.

5.6.2 Mechanism of Synaptic Transmission

  • Process:

    1. Action potential arrives at the synaptic knob.

    2. Calcium channels in presynaptic membrane open, allowing calcium ions to enter.

    3. Synaptic vesicles fuse with the plasma membrane, releasing neurotransmitters into the cleft.

    4. Neurotransmitters diffuse and bind to receptors on the postsynaptic membrane, opening Na+ channels and causing depolarization (EPSP).

    5. Neurotransmitters are metabolized by enzymes (e.g., acetylcholinesterase for acetylcholine).

Fig. 5.15: Synaptic transmission
5.6.3 Classifications of Neurotransmitters

  • Excitatory Neurotransmitters: Increase membrane permeability to sodium (e.g., acetylcholine, epinephrine).

  • Inhibitory Neurotransmitters: Decrease membrane permeability to sodium (e.g., GABA, glycine).

5.7 Main Components of Nervous System

5.7.1 Architecture of Human Brain and Spinal Cord

  • The CNS consists of brain and spinal cord, protected by meninges (dura mater, arachnoid mater, pia mater). Cerebrospinal fluid (CSF) cushions the brain.

Fig. 5.18: Structure of human brain
5.7.2 Cranial and Spinal Nerves in Man
Cranial Nerves

  • Description: Twelve cranial nerves originate from the brain and assist with functions like smell, sight, taste, and movement.

  • Examples: Olfactory, Optic, Oculomotor, etc.

Spinal Nerves

  • Description: Mixed nerves formed from dorsal and ventral roots, facilitating communication between CNS and body.

  • Composition: 31 pairs of spinal nerves (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal).

5.7.3 Peripheral Nervous System
Somatic Nervous System

  • Controls voluntary actions and connects the CNS to skin and skeletal muscles.

Autonomic Nervous System

  • Regulates involuntary functions for visceral organs. Divided into sympathetic (fight or flight) and parasympathetic (rest and digest) systems.

Diagrammatic Overview of the Nervous System

Fig. 5.16: Organization of human nervous system

5.8 Effects of Drugs on Nervous Coordination

Classifications of Neuroactive Drugs

  1. Opioids/Narcotics: Mimic the effects of endorphins, binding to pain receptors in the brain.

  2. Local Anesthetics: Block sodium channels to prevent pain signal transmission.

  3. Antidepressants and Anticonvulsants: Modulate neurotransmitter activity involved in pain perception.

  4. Corticosteroids: Reduce inflammation-related pain.

  5. Other medications: Various pain-relieving methods via localized numbing or muscle relaxation.

5.9 Disorders of the Nervous System

5.9.1 Vascular Disorders

  • Example: Stroke, characterized by a sudden loss of function due to blood circulation issues, including symptoms like paralysis and aphasia.

5.9.2 Infectious Disorders

  • Example: Meningitis, inflammation of the meninges caused by infection, with symptoms such as headache and stiffness.

5.9.3 Structural Disorders

  • Example: Tumors, resulting from abnormal cell division, leading to diverse neurological symptoms.

5.9.4 Functional Disorders

  • Example: Headaches, categorized as primary (migraine) or secondary (structural problems).

5.9.5 Degenerative Disorders

  • Example: Alzheimer's disease, affecting memory and cognitive function without an identifiable external cause.

Parts of the Brain and Their Functions

  1. Cerebrum

    • Function: Responsible for higher brain functions, including thought, action, and emotion. It is divided into two hemispheres (left and right), each controlling opposite sides of the body. Contains four lobes:

      • Frontal Lobe: Involved in reasoning, motor control, emotion, and language.

      • Parietal Lobe: Processes sensory information such as touch, temperature, and pain.

      • Temporal Lobe: Responsible for hearing, memory, and language comprehension.

      • Occipital Lobe: Governs visual processing, interpreting information from the eyes.

  2. Cerebellum

    • Function: Coordinates muscle movements, maintains posture, and balances. It ensures precision in timing and control of movements.

  3. Brainstem

    • Function: Controls basic life functions such as breathing, heart rate, and blood pressure. It includes:

      • Midbrain: Involved in vision, hearing, motor control, and alertness.

      • Pons: Connects different parts of the brain and plays a role in breathing regulation, sleep, and arousal.

      • Medulla Oblongata: Regulates vital functions, including heart rate and respiratory rhythms.

  4. Thalamus

    • Function: Acts as a relay station for sensory and motor signals to the cerebral cortex. It also regulates sleep and alertness.

  5. Hypothalamus

    • Function: Regulates bodily functions such as temperature, hunger, thirst, and circadian rhythms. It also links the nervous system to the endocrine system via the pituitary gland.

  6. Limbic System

    • Function: Plays a vital role in emotion, behavior, and memory. Components include the hippocampus (memory formation) and amygdala (emotion regulation).

Spinal Cord

  • Function: The spinal cord is a long, thin bundle of nervous tissue that extends from the brainstem down the vertebral canal. It serves as a conduit for signals between the brain and the rest of the body.

  • Sections: Divided into segments that correspond to the regions of the vertebral column:

    • Cervical (C1-C8): Innervates the arms, neck, and some head functions.

    • Thoracic (T1-T12): Controls the torso and some functions of the abdominal organs.

    • Lumbar (L1-L5): Supplies nerves to the lower limbs and pelvic organs.

    • Sacral (S1-S5): Innervates the pelvic region and lower limbs.

    • Coccygeal (Co1): Supplies sensation and mobility to the coccyx area.

  • Reflex Actions: The spinal cord is also involved in executing reflex actions, allowing for quick responses to stimulus.