Human Nervous System

Nervous Systems

Importance of the Nervous System

  • The nervous system is essential for sensing and responding to the environment.

Scenarios without a Nervous System

  • Lack of sensory perception:

    • No sight, hearing, feeling, taste, or smell.

    • Inability to sense or respond to dangers, e.g., an approaching bus.

    • No awareness of environmental conditions such as warmth or cold.

Consequences of Dysfunction

  • Without the nervous system to coordinate organ systems, the following would occur:

    • Inability to regulate body temperature (Tb), urine volume, hormone release, or blood distribution.

    • Absence of thoughts, emotions, and memory.

    • Muscles would not function in a coordinated manner.

Role of the Nervous System

  • The nervous system provides the body with information regarding both external and internal environments, generating appropriate actions or responses.

  • It directs complex processes to maintain homeostasis.

Nervous Tissue

  • Nervous tissue consists of specialized cells known as neurons.

  • Neurons are supported by specialized support cells called glial cells that protect and assist them.

Types of Glial Cells

  • Astrocytes: The most abundant cell type, providing structural support and regulating the chemical environment around neurons.

  • Ependymal Cells: Line the cavities of the brain and spinal cord, producing cerebrospinal fluid (CSF).

  • Microglia: Act as immune cells in the CNS, cleaning up debris and pathogens.

  • Oligodendrocytes: Produce myelin in the CNS.

Neurons

  • Neurons are specialized for receiving and transmitting information, serving as the principal structural and functional units of nervous tissue.

Structure of Neurons

  • Neurons are variable in size and shape, with common categories:

    • Unipolar Neurons: Found in invertebrates, not in humans.

    • Bipolar Neurons: Transmit information from sense organs (sight, smell, hearing).

    • Multipolar Neurons: Constitute the majority of neurons found in the CNS, including motor neurons and interneurons.

    • Pseudo-unipolar Neurons: Lack dendrites; their branched axon serves dual roles and are found in sensory functions of the PNS.

    • Purkinje Neurons: Major neuron type in the cerebellum, integrating vast amounts of sensory and motor information.

    • Pyramidal Neurons: Major neuron type in the cortex, amygdala, and hippocampus, associated with higher cognitive functions.

Parts of the Neuron

  • Cell Body (Soma): Contains the cytoskeleton, nucleus, mitochondria, and organelles typical of eukaryotic cells.

  • Dendrites: Highly branched extensions receiving information from other neurons and transmitting it to the soma; surfaces covered with numerous dendritic spines.

  • Axon: Conducts messages away from the cell body to other neurons or muscles; can be long, called nerve fibers. End of axons divide into terminals with synaptic knobs, forming the pre-synaptic half of a synapse.

Myelin Sheath

  • Myelin: A white, lipid-rich substance that wraps around axons of many neurons in the PNS to insulate them and enhance nerve conduction speed.

  • Myelin is produced by Schwann cells in the PNS and oligodendrocytes in the CNS.

  • Nodes of Ranvier: Gaps between successive Schwann cells where the axon is unmyelinated; crucial for action potential transmission.

Types of Neurons

  • Interneurons: Located exclusively in the CNS, connecting neurons.

Main Neuron Functions

  • Sensory Neurons: Typically possess long dendrites; convey information from sensory receptors to the CNS.

  • Motor Neurons: Characterized by long axons; carry responses from the CNS to muscles or glands.

Neuronal Properties

  • The brain consists of approximately 86 billion neurons and requires a large amount of the body’s oxygen and glucose; hence, neurons are sensitive to oxygen variations.

  • Irreversible damage can occur if there is prolonged lack of blood supply to the brain.

Neuronal Regeneration

  • Mature neurons generally do not regenerate, except in the dentate gyrus of the hippocampus.

  • Diseases like Parkinson's, Huntington's, and Alzheimer's are associated with neuron death.

Neural Function and Transmission

Key Activities

  • Neural functions include:

    • Reception and detection of environmental information.

    • Transmission of this information to the CNS (brain).

    • Generation and transmission of responses via motor neurons.

    • Processing and integration of information.

Electrical Properties

Membrane Potential

  • Nerve cells have an uneven distribution of ions across their membrane, leading to a resting potential of approximately -70 mV, signifying electrical polarization.

Ions’ Distribution Example

  • Intracellular Fluid:

    • Sodium (Na+): 10 g/l

    • Potassium (K+): 16 g/l

    • Protein: 300 g/l

  • Extracellular Fluid:

    • Sodium (Na+): 1 g/l

    • Potassium (K+): 0.8 g/l

    • Chloride (Cl-): 20 g/l

Ion Movement and Channels

  • Ions move across cell membranes through channels which can be voltage-gated (opened/closed by electrical signals) or chemically-gated (opened/closed by neurotransmitters/hormones).

Action Potential

Generation of Action Potential

  • Neurons become easily excited by stimuli. A sufficient stimulus alters the resting potential, making the membrane more permeable to Na+ ions, which lead to:

    • If the neuron reaches the threshold level (~ -55 mV), action potential generation occurs.

    • At threshold, voltage-activated sodium channels open, allowing Na+ influx.

Phases of Action Potential

  1. Depolarization: Na+ rushes in, leading to potential overshoot (~ +35 mV).

  2. Repolarization: Voltage-gated K+ channels allow K+ to flow out; Na+ channels close.

  3. Restoration: Na+/K+ pump restores resting potential (3 Na+ out, 2 K+ in).

Process Timeframe

  • The process of depolarization and repolarization takes less than 1 millisecond.

Propagation of the Action Potential

  • The action potential causes a collapse in resting potential in adjacent membrane areas, creating a wave-like transmission down the neuron.

  • In myelinated neurons, this transmission is accelerated via saltatory conduction, occurring at nodes of Ranvier, approximately 50 times faster than unmyelinated nerves.

Neural Transmission and Communication

Synapses

  • Synapses are specialized junctions where one neuron's axon connects to another's dendrite.

  • Communication occurs chemically through neurotransmitters, termed chemical synapses.

  • A neuromuscular junction is a specialized case where a motor neuron connects to a muscle cell.

Synaptic Mechanism

  1. Action potential arrives at the presynaptic knob.

  2. Neurotransmitter is released into the synaptic cleft.

  3. Neurotransmitter binds to post-synaptic receptors, opening ion channels.

  4. If enough neurotransmitter is present, depolarization and an action potential may occur in the post-synaptic neuron.

  5. Neurotransmitters are later inactivated by enzymes or reabsorbed by the pre-synaptic neuron.

Neurotransmitter Roles

  • Types of neurotransmitters:

  1. Excitatory Neurotransmitters: Generate action potentials in the postsynaptic neuron.

  2. Inhibitory Neurotransmitters: Block the generation of action potentials.

  • Balance between excitation and inhibition is vital for effective brain communication.

Predominant Neurotransmitters

  • Epinephrine: Autonomic nervous system involvement.

  • Norepinephrine: Autonomic 'fight or flight' response.

  • Dopamine: Functions mainly in the brain.

  • Acetylcholine: Acts in both autonomic and peripheral nervous systems.

  • Serotonin: Inhibitory, acting in brain and body.

  • Gamma-Aminobutyric Acid (GABA): Inhibitory neurotransmitter in the brain.

  • Glutamate: Important for excitatory functions in the brain.

Neurotransmitters and Toxins

  • Certain toxins can block neurotransmitter actions, leading to critical health implications.

  • Tetrodotoxin: Found in pufferfish, blocks voltage-gated sodium channels, crucial for muscle function.

  • Various snake venoms can act on neural transmission to inhibit motor functions.

Divisions of the Nervous System

Evolution and Structure of the Nervous System

  • CNS and PNS Structure

    • A collection of axons is termed a tract in the CNS, while groups of cell bodies are called nuclei.

    • A nerve consists of several bundles of axons (fascicles) held by connective tissue with sensory neurons grouped into masses called ganglia.

  • Complex nervous systems, such as the human system, contain billions of neurons and supporting glial cells, showing an evolutionary trend towards the centralization of neural tissue in the anterior body region, known as cephalization.

  • In vertebrates, the nervous system is divided into:

    • Central Nervous System (CNS): Comprising the brain and spinal cord.

    • Peripheral Nervous System (PNS): Comprising peripheral nerves.

Peripheral Nervous System (PNS)

  • Comprises sensory receptors and nerves communicating information from the body to the CNS.

  • Divided into two main subdivisions:

    • Somatic Nervous System: Controls skeletal muscles and sensory receptors.

    • Autonomic Nervous System: Controls internal organs; includes the sympathetic and parasympathetic systems that function antagonistically to maintain homeostasis.

Afferent and Efferent Neurons

  • Afferent Neurons: Carry information from sensory receptors to the CNS (Arrives).

  • Efferent Neurons: Relay motor commands from the CNS to muscles and glands (Exits).

Central Nervous System (CNS)

Brain and Spinal Cord Protection

  • The brain is shielded by the skull, which reinforces facial structures and protects against injuries.

  • Skull bone formation is incomplete at birth, solidifying by around 20 years of age.

Spinal Cord Protection

  • Enclosed within the protective vertebrae, providing structural support, flexibility, mobility, and attachment for muscles, tendons, and ligaments.

Damage Consequences

  • Damage to the spinal cord can lead to conditions such as paraplegia or quadriplegia, which might be permanent due to inadequate nerve fiber regeneration.

  • Composed of the brain (including the cerebrum) and spinal cord.

  • The cerebrum, as the largest part, has a complex structure divided into two hemispheres, each containing four lobes:

    • Frontal Lobe: Functions related to reasoning, planning, movement, emotions, and problem-solving.

    • Parietal Lobe: Linked with movement, orientation, recognition, and sensory perception.

    • Temporal Lobe: Auditory perception, memory, and speech.

    • Occipital Lobe: Visual processing.

Additional Brain Structures and Functions

  • Hypothalamus: Regulates homeostasis, thirst, hunger, body temperature, and blood pressure, linking the nervous and endocrine systems.

  • Cerebellum: Involved in sensory perception, muscle coordination, posture, and balance.

  • Medulla Oblongata: Connects brain with spinal cord; regulates heart rate, breathing, and various reflexes.

Spinal Cord Structure

  • Organized into white matter (outer fibers) and gray matter (internal neuronal nuclei + cell bodies).

  • Motor and sensory fibers travel through spinal nerves, grouped by function.

Spinal Nerves Overview

  • Includes:

    • Cervical Nerves (8): More than cervical vertebrae.

    • Thoracic Nerves (12).

    • Lumbar Nerves (5).

    • Sacral and Coccygeal Nerves (5).

Functions of Spinal Roots

  • Motor neurons exit via ventral roots; sensory neurons enter through dorsal roots accompanied by dorsal root ganglia, housing sensory neuron cell bodies.

Summary

  • Neural transmission relies on ion redistribution across nerve cell membranes, generating action potentials.

  • The neuron, as the functional unit of the nervous system, consists of a soma, dendrites, and axons. Nerves comprise bundles of these neurons.

  • The peripheral nervous system is split into voluntary (somatic) and involuntary (autonomic) systems.

  • The nervous system consists of the CNS and PNS, where the brain is divided into the cerebrum (four lobes), cerebellum (coordination), and medulla oblongata (regulatory functions).