Nervous System Fundamentals Quiz

Nervous System Overview

• The nervous system is the major control, regulatory, and communication system of the body
• It's the center of mental activities like thoughts, learning, and memory.
• It works together with the endocrine system to regulate and maintain homeostasis
• The nervous system is made up of: brain, cranial nerves & their branches, the spinal cord, spinal nerves & their branches, ganglia, enteric plexuses, and sensory receptors.

Organization of the Nervous System

• The nervous system is organized into two main subdivisions: central nervous system (CNS) and peripheral nervous system (PNS)
• CNS consists of the brain and the spinal cord
  • The brain is located in the skull and contains 100 billion neurons
  • The spinal cord is surrounded by the vertebral column and contains 100 million neurons connected to the brain.
  • The brain and spinal cord are continuous at the foramen magnum
• PNS consists of cranial nerves & branches, spinal nerves & branches, ganglia, enteric plexuses in the small intestine, and sensory receptors in the skin

Peripheral Nervous System (PNS)

• Cranial nerves and their branches
• Spinal nerves and their branches
• Ganglia
• Enteric plexuses in the small intestines
• Sensory receptors in the skin

Function of the Nervous System

• Sensory Function: sensory receptors detect internal or external stimuli. Cranial and spinal nerves carry this sensory information to the brain & spinal cord.
• Integrative Function: the nervous system integrates/processes information via analyzing, storing, and making decisions for appropriate responses.
• Motor Function: carry out the processed response to effectors, (muscle & glands). Stimulations to the effector lead to muscle contraction and gland secretions.

Nervous Tissue

• Two main types of tissue: neurons and neuroglia/glial cells
• Neurons are the functional units of the nervous system. They are nerve cells that conduct nerve impulses. They consist of a cell body, one axon, and many dendrites
  • Require oxygen and glucose for survival
  • Do not undergo mitosis
  • Supported by connective tissue—neuroglia
• Neuron cell bodies form the grey matter of the nervous system, found in the periphery of the brain and center of the spinal cord. Groups of cell bodies are called nuclei in the CNS and ganglia in the PNS
• Axons and dendrites form the white matter of the nervous system. Axons and dendrites are extensions of cell bodies, referred to as nerves or nerve fiber outside the brain & spinal cord.

Structure of Neurons (continued)

• Dendrites are shorter and branching extensions. Their function is to receive and carry incoming impulses towards the cell body from another neuron.
• Axons function in carrying impulses.

Structure of Neurons (continued)

• Axons are extensions of cell bodies that carry impulses away from the cell body to another neuron; typically found deep in the brain and nerves.
• Bundles of axons in the CNS are called tracts
• Axons are typically covered in a myelin sheath. Myelin is a white, fatty substance that speeds up the transmission of nerve impulses.
• The unmyelinated region between the myelin is called the nodes of Ranvier.

Synapse and Neurotransmitters

A synapse is the point where the nerve impulse passes from one neuron to another or effector cell.

• Neurotransmitters are chemical substances which release at the synaptic cleft.

Synapse continued

• Synaptic knobs (or terminal buttons): small swellings at the end of axons in nerve fibers; are close to dendrites and cell body of postsynaptic neurons
• Synaptic vesicles: small sacs that contain neurotransmitters in synaptic knobs

Synapse continued

Neurotransmitters are chemical substances which allow transmission of signals from one neuron to the next across synapses; released into the synaptic cleft.

Types of Neurons (Function)

• Sensory neurons: carry messages from the receptors to the Central Nervous System (CNS)
• Interneurons/Association neuron: located in the CNS, receives signals from many neurons, carry out integrative functions (make decisions on responses)
• Motor neurons: carry messages from the CNS to effectors (muscles & glands)

Types of Neurons (Structure)

• Multipolar neuron: have several dendrites & 1 axon; located in the brain and spinal cord
• Bipolar neuron: have 1 main dendrite & 1 axon; found in the eye, ear, & nose
• Unipolar neuron: have 1 axon extension and the cell body is on one side of the axon.

Neuroglia

• Also known as glia, these cells make up about ½ of the volume of the CNS. They do not conduct nerve impulses.
• Their functions include:
• supplying nutrients and oxygen
• surrounding neurons and holding them in place
• destroying pathogens and removing dead neurons
• maintaining homeostasis in the interstitial fluids.

Types of Neuroglia

• Oligodendrocytes: produce and maintain myelin sheath around the axons of CNS neurons.
• Astrocytes: cover the blood vessels in the CNS and form the blood-brain barrier, protecting the brain from potentially harmful substances; Actively involved in the formation & circulation of cerebrospinal fluid (CSF).
• Ependymal cells: actively involved in formation and circulation of cerebrospinal fluid (CSF)
• Microglia: Protect CNS cells from disease by engulfing invading microbes and migrate to areas of injured nerve tissue; clear away debris of dead cells.

Type of Neuroglia (PNS)

• Schwann cells: form the myelin sheath around axons in the PNS and play a role in nerve fiber regeneration
• Satellite cells: support neurons in PNS ganglia and regulate materials exchange between neurons and interstitial fluid

Myelination

• Myelin sheath: white, fatty substance surrounding the axon, covering axons.
• Myelinated axon: axon with myelin
• Unmyelinated axon: axon without myelin
• Nodes of Ranvier: unmyelinated regions between myelin sheath. Function: speed up transmission of nerve impulses.

White and Grey Matter

• White matter: primarily myelinated axons; whitish color due to myelin
• Grey matter: contains neuronal cell bodies, dendrites, unmyelinated axons & axon terminals, neuroglia; appear grey in color

Nerves

• Nerves consist of neurons bundled together. Bundles of nerve fibers in the CNS are referred to as tracts
• Nerve coverings: Endoneurium- covers individual axons or nerve fibers ; Perineurium- encases fascicles (bundles of nerve fibers); Epineurium- encases the entire nerve

Types of Nerves

• Sensory (afferent) nerve: carries information from the body to the spinal cord, then the brain
• Sensory receptors: specialized endings that respond to various stimuli (inside or outside of the body)
• Somatic sense: sensory information from skin; feels pain, touch, heat, and cold
• Proprioceptors sense: information from muscles and joints used for posture and balance.
• Special senses: vision, hearing, balance, smell and taste
• Autonomic afferent nerve: Originates in internal organs, glands, and tissues. E.g. baroreceptors control blood pressure, chemoreceptors control respiration
• Motor (efferent) nerve: originates in brain, spinal cord, or autonomic ganglia. Transmits impulses to effector organs (muscle & glands)
• Two types of motor nerves: somatic nerves involved in voluntary and reflex skeletal muscle contractions, and autonomic nerves (sympathetic and parasympathetic) are involved in cardiac & smooth muscle contractions, and glandular secretions
• Mixed nerves: consist of both sensory and motor fibers

Action Potentials (The Nerve Impulse)

• An impulse is initiated by stimulation of sensory nerve endings (sensory receptors).
• Action potential is due to movement of ions across the nerve cell membrane.
• Two major characteristics of neurons: irritability (ability to initiate nerve impulses in response to stimuli from inside or outside the body) and conductivity (ability to transmit impulses).

Action Potentials Con't

• Ions move along axon membrane, reach synapse, neurotransmitters release, allowing communication between neurons and other cells.
• When a neuron reaches a certain stimulation level, an electrical impulse is generated.
• The principle ions involved are sodium (major positive ion in extracellular fluid) and potassium (major positive ion in intracellular fluid).
• Membrane potential and presence of voltage-gated channels for Na+ and K+ are essential for action potential generation.
• During action potential, voltage-gated Na+ and K+ channels open in sequence.

Generation of Action Potentials

• Resting membrane potential: a neuron not conducting an impulse
• Depolarization: a very rapid process when a nerve cell is stimulated, Na+ channels open, causing inflow of Na+ ions from the extracellular fluid into the neuron that changes the neuronal membrane to be positive.
• Repolarization: occurs at the end of the depolarization phase; K+ gates open; K+ ions leave the neuron; K+ gates close slowly
• Refractory Period: condition where a neuron is recovering; Na channels are closed, and K+ channels are open
• Following action potential, the neuron temporarily cannot conduct another impulse until the Na+ gates have closed.

Action Potentials continued

• An action potential travels down the nerve axon to the terminal region.
• At the terminal, the action potential causes neurotransmitter molecules to be released into the synapse. These molecules stimulate receptors in the next cell and transmit the action potential.
• Local anesthetics and certain neurotoxins prevent opening of voltage-gated Na+ channels preventing nerve impulses from passing.

Transmission of Impulse in Synaptic Cleft

• Once the impulse reaches the presynaptic neuron, synaptic vesicles in the synaptic knobs will release neurotransmitters into the synaptic cleft (exocytosis).
• Neurotransmitter binds to specific receptors on the postsynaptic neuron or effector organ (e.g., muscle).
• Neurotransmitters initiate an electrical response in the postsynaptic neuron, to either excite or inhibit.
• The action is short-lived because postsynaptic receptors are inactivated by enzymes or taken back into the synaptic knob.

Neurotransmitters

• There are about 100 types of neurotransmitters.
• Acetylcholine: most common in CNS & PNS; involved in arousal, dreaming, regulating mood.
• Norepinephrine (noradrenaline): involved in arousal (awakening from deep sleep), dreaming, and regulating mood.
• Dopamine: regulates skeletal muscle tone.
• Serotonin: chemical that allows transmission of signals from one neuron to the next across synapses.
• Histamine: chemical involved in CNS & PNS, other roles.