nervous system powerpoint AS111

Course Goals

  • To understand the organization of the nervous system and its functions.

  • To recognize neurons and their supporting cells.

  • To grasp the processes of depolarization and repolarization.

  • To understand the autonomic nervous system, including its structures and functions.

Nervous System Purpose and Function

  • The nervous system serves as a communication network that responds to stimuli and coordinates both internal and external body functions.

Main Functions:

  1. Sensory:

    • Sensing changes in the environment or within the body and conveying this information to the brain and spinal cord.

  2. Integration:

    • Information is received, analyzed, stored, and integrated to produce an appropriate response.

  3. Response:

    • Stimulation of muscles or glands to generate movement or produce substances in different parts of the body.

Organization of Nervous System

  • Peripheral Nervous System (PNS):

    • Includes cranial nerves, receptors, and effectors.

    • Afferent Division:

    • Special Sensory Receptors: Smell, taste, vision.

    • Somatic Sensory Receptors: Skin, joints, position.

    • Visceral Sensory Receptors: Heart, lungs, gastrointestinal (GI) tract, urinary system.

    • Efferent Division:

    • Somatic Nervous System:

      • Innervates skeletal muscle.

    • Autonomic Nervous System:

      • Parasympathetic: "Rest & Digest"

      • Sympathetic: "Fight or Flight"

      • Innervates smooth muscle, cardiac muscle, and glands.

  • Central Nervous System (CNS):

    • Composed of the brain and spinal cord, responsible for higher-order functions such as memory, learning, and intelligence.

Cell Types in the Nervous System

1. Neurons

  • The structural and functional units of the nervous system.

  • Transmit information via electrical impulses.

  • Highly metabolic activity and well protected by the body.

2. Glial Cells (Neuroglia)

  • Form the connective tissue network that supports and protects the nervous system.

Neuron Structure

  • Once a nerve cell is destroyed, it loses function.

  • The process of nervous system healing is slow and may take an extended period.

  • After repair, there may be a loss of full functionality.

Types of Neurons:

  1. Sensory Neuron:

    • Afferent neurons that deliver information from sensory receptors to the CNS.

    • Found in sensory organs (such as skin, eyes, ears) and proprioceptors in muscles and joints.

  2. Interneuron:

    • Association neurons that outnumber other neuron types combined.

    • Located primarily within the CNS, they transmit impulses between sensory and motor neurons.

    • Involved in higher functions like memory, learning, and planning.

  3. Motor Neuron:

    • Efferent neurons that deliver information from the CNS to peripheral tissues and organ systems.

    • Can be subdivided into:

      • Somatic motor neurons: Innervate skeletal muscles.

      • Autonomic (visceral) motor neurons: Innervate smooth muscle, cardiac muscle, and glands.

Neuron Structure Components:

  • Cell Body (Perikaryon):

    • Contains cellular organelles; provides energy with a large number of mitochondria.

    • Site of neurotransmitter synthesis.

  • Dendrites:

    • Projections that receive information from other neurons.

  • Axon:

    • Fibers that carry impulses away from the cell body to other neurons or cell types.

    • Can be either myelinated or unmyelinated.

  • Myelin Sheath:

    • Composed of multiple layers of Schwann cells that insulate the axon.

    • Increases conduction velocity of action potentials.

    • Features Nodes of Ranvier, which are gaps in the myelin sheath.

  • Axon Terminal:

    • Contains synaptic vesicles storing neurotransmitters that relay information to other neurons.

Neuron Function

Action Potential:

  • Nerve impulses are carried through the process of depolarization and repolarization.

  • This requires the active transport of Na+- and K+- ions via the sodium-potassium pump.

Properties of Myelinated Axons:

  • Myelinated axons conduct action potentials more rapidly due to saltatory conduction, where the action potential jumps from one node of Ranvier to another.

Synapse:

  • Junction between two neurons or a neuron and a target cell.

  • Includes:

    • Synaptic Cleft: Space between the two neurons.

    • Presynaptic Neuron: Neuron that brings the depolarization wave to the synapse and releases neurotransmitters.

    • Neurotransmitter: Chemical released by the presynaptic neuron.

    • Postsynaptic Neuron: Neuron that contains receptors for the neurotransmitter.

Neurotransmitters:

  • Chemicals allowing signal transmission from one neuron to another, each with specific functions:

    • Acetylcholine: The first discovered and most understood neurotransmitter, primarily involved in the parasympathetic nervous system.

    • Norepinephrine: Released as a stress hormone during stressful events.

    • Dopamine: Involved in pleasure response.

    • Gamma-aminobutyric acid (GABA): Primary inhibitory neurotransmitter, reduces neuronal excitability.

    • Serotonin: Modulates various physiological processes including mood and cognition.

    • Endorphins: Associated with pain relief and pleasure.

    • Epinephrine: Involved in the sympathetic nervous system's fight-or-flight response.

Types of Neurotransmitters:

  • Excitatory Neurotransmitters: Cause Na+- influx, leading to depolarization of the postsynaptic cell.

  • Inhibitory Neurotransmitters: Hyperpolarize the cell and inhibit action potential generation.

Synaptic Transmission Process:

  1. As an action potential reaches the axon terminal, calcium channels open.

  2. Calcium influx causes neurotransmitter release into the synapse.

  3. Neurotransmitters bind to receptors on the postsynaptic neuron, causing depolarization, contraction of muscles, or hormone release.

  4. After exerting their effects, neurotransmitters are broken down and recycled (e.g., acetylcholine is broken down by acetylcholinesterase).

Potential Issues with Neurotransmission:

  • Inhibition of acetylcholinesterase (e.g., by organophosphate insecticides) can lead to overstimulation of effectors, resulting in symptoms such as salivation, gastrointestinal cramping, and seizures.

Neuroglia (Support Cells of the CNS)

Types of Neuroglia:

  1. Ependymal Cells:

    • Line fluid-filled structures in the brain and spinal cord; some produce cerebrospinal fluid (CSF).

  2. Astrocytes:

    • Largest and most numerous in CNS; maintain the blood-brain barrier (BBB), regulate interstitial environment, and repair damaged neural tissue.

  3. Oligodendrocytes:

    • Form the myelin sheath around CNS neurons; contribute to ‘white matter’ appearance in dissections.

  4. Microglia:

    • Smallest and most numerous; engulf cellular debris, waste, and pathogens.

Central Nervous System Components

  • Introduction: Brain and spinal cord, encased in protective structures:

    • Cerebrospinal Fluid (CSF): Provides mechanical support, protects against trauma, and nourishes the neural tissue.

Grey Matter vs. White Matter:

  • Grey Matter: Composed of neuronal cell bodies; located on the brain's exterior and spinal cord's interior.

  • White Matter: Composed of myelinated axon tracts; located on the brain's interior and spinal cord's exterior.

Brain Regions:

  1. Cerebral Cortex:

    • Largest brain portion, divided into two hemispheres connected by the corpus callosum.

    • Responsible for higher-order functions: behavior, conscious thought, voluntary movement, and memory.

  2. Cerebellum:

    • Responsible for coordination of movement, balance, equilibrium, and posture.

  3. Diencephalon:

    • Contains important structures:

      • Thalamus: Relay station for sensory impulses.

      • Hypothalamus: Interface between the endocrine and nervous systems, regulates hunger, thirst, body temperature, and circadian rhythms.

      • Pituitary Gland: The master gland of the endocrine system, regulating hormone production.

  4. Brain Stem:

    • Connects the brain and spinal cord; regulates autonomic functions, cardiovascular, and respiratory functions; origin of most cranial nerves.

    • Key divisions: Midbrain, Pons, Medulla Oblongata.

Spinal Cord:

  • Divided into four regions: cervical, thoracic, lumbar, and sacral; serves as a two-way conduction system for signals between the brain and peripheral nerves.

  • No regenerative powers, information moves sensory signals up to the brain and motor signals down from the brain.

Protective Layers of the CNS

Meninges:

  • Connective tissue layers surrounding the brain and spinal cord:

    • Dura Mater: Tough outer layer.

    • Arachnoid Mater: Layer tightly fused to the dura.

    • Pia Mater: Adheres tightly to the brain surface, extends into folds, and forms part of the BBB.

Cerebrospinal Fluid (CSF):

  • Provides mechanical support, cushioning within the cranial cavity, and exchanges metabolites for chemical stability.

Blood-Brain Barrier (BBB):

  • Formed by tightly joined endothelial cells of CNS capillaries, critical for protecting the brain from potentially harmful substances.

Peripheral Nervous System (PNS)

  • Encodes cranial nerves originating from the brain; classified as sensory, motor, or both; labeled I - XII.

Cranial Nerves:

  1. Olfactory (I): Sensory; sense of smell.

  2. Optic (II): Sensory; vision.

  3. Oculomotor (III): Motor; controls eye muscles.

  4. Trochlear (IV): Motor; controls superior oblique muscle of the eye.

  5. Trigeminal (V): Both; sensory for the face and muscles of mastication.

  6. Abducens (VI): Motor; external rectus muscle of the eye.

  7. Facial (VII): Both; facial expression and taste from anterior tongue.

  8. Vestibulocochlear (VIII): Sensory; hearing and balance.

  9. Glossopharyngeal (IX): Both; taste from posterior tongue and pharynx control.

  10. Vagus (X): Both; innervates organs including the heart, lungs, and gastrointestinal tract.

  11. Accessory (XI): Motor; neck muscles.

  12. Hypoglossal (XII): Motor; tongue muscles.

Autonomic Nervous System (ANS):

  • Controls involuntary functions of smooth muscle, cardiac muscle, and glands to maintain homeostasis.

  • Divisions:

    • Sympathetic Nervous System: Executes "fight or flight" response. Increases heart rate, blood pressure, and blood flow to muscles, while reducing GI activity.

    • Parasympathetic Nervous System: Executes "rest and digest" activities. Decreases heart rate and increases GI secretions.

Somatic Nervous System:

  • Involves all neurons that transmit motor impulses to skeletal muscles and relays sensory impulses to the CNS.

Nerve Plexuses:

  • Networks of interwoven nerve fibers from different spinal nerves:

    • Brachial Plexus: Innervates forelimbs. Damage can result in diminished sensation/function.

    • Lumbosacral Plexus: Innervates hind limbs, with critical nerves for motor functions in the legs.

Reflexes:

  • Role: Rapid, automatic responses to stimuli for protection and homeostasis maintenance.

    • Types of reflexes:

    • Somatic Reflexes: Involve skeletal muscles.

    • Autonomic Reflexes: Involve smooth muscle, heart, and glands.

Examples of Reflex Arcs:

  1. Patellar Reflex:

    • A two-neuron reflex arc triggered by tapping the patellar tendon, causing muscle contraction.

  2. Withdrawal Reflex:

    • A three-neuron reflex arc triggered by painful stimuli, leading to muscle contraction or relaxation to protect from injury.

  3. Crossed Extensor Reflex:

    • Engages different sides of the body to maintain balance and adjust position when a painful stimulus is applied.

  4. Palpebral Reflex:

    • Eyelash blinking in response to touch, involving trigeminal and facial nerves.

  5. Pupillary Light Reflex:

    • Constriction of pupils in response to light stimulus, involving optic and oculomotor nerves.

Additional Reflexes:

  • Panniculus Reflex: Reaction to skin pinch.

  • Anal Reflex: Contraction of the anal sphincter upon stimulation of the perineal area.