Nervous Tissue

Nervous System

  • The master controlling and communicating system of the body

    • The most powerful system in the body and can override anything

  • Functions

    • Sensory input—monitoring stimuli occurring inside and outside the body

      • Sensory = afferent; toward CNS

      • CNS: brain & spinal cord; our computer

    • Integration—interpretation of sensory input; computing

    • Motor output—response to stimuli by activating effector organs

      • Motor = efferent; away from CNS

      • Effector organs: muscles & glands

        • Carries out orders of brain and spinal cord





  • PNS consist of nerves and branches that come off of brain & spinal cord

Organization of the Nervous System

  • Central nervous system (CNS)

    • Brain & spinal cord

    • Integration (interprets sensory input) & command center

  • Peripheral nervous system (PNS)

    • Paired spinal and cranial nerves

    • Ganglia—clusters of cell bodies of neurons

      • Neurons—individual nerve cells

    • Carries messages to (sensory/afferent) and from (motor/efferent) the spinal cord and brain


Peripheral Nervous System (PNS): Two Functional Divisions

  • All of the branches that are coming off of the brain and spinal cord; does not actually include brain and spinal cord

  • Sensory (afferent) division

    • Sensory afferent fibers—carry impulses from skin, skeletal muscles, joints, and special senses organs to the brain

      • Gonna be carrying info from the outside of the body (skeletal muscles, joints, and special sense organs are considered external)

    • Visceral afferent fibers—transmit impulses from visceral organs and blood vessels to the brain

      • Asks what’s going on internally (are you hungry/is your stomach empty, is your appendix hurting, are you about to have a baby, etc.)

  • Motor (efferent) division

    • Transmits impulses from CNS to effector organs

Motor Division: Two Main Parts

  • Somatic nervous system

    • Conscious control of skeletal muscles (somatic & skeletal both begin w/ an S)

  • Autonomic nervous system (ANS)

    • Regulates smooth muscle, cardiac muscle, and glands

    • Divisions: sympathetic & parasympathetic


Receptor Class by Location

  • Sensory receptors are going to begin afferent pathway, which sends info toward CNS

  • Exteroceptors

    • Respond to stimuli arising outside the body

    • Sensitive to touch, pressure, pain, and temperature

    • Include the special sense organs (vision, hearing, taste, smelling)

  • Interoceptors

    • Respond to stimuli arising within the body

    • Found in internal viscera and blood vessels

    • Sensitive to chemical changes, stretch, and temp changes

  • Proprioceptors

    • Gets us to where we need to be and helps maintain balance

    • Detects body’s position by detecting stretch in all of these things down below (skeletal muscles, tendons, etc.)

    • Respond to stretch in skeletal muscles, tendons, joints, ligaments, and CT coverings of bones and muscles

    • Constantly “advise” the brain of one’s movements

Histology of Nerve Tissue

  • The two principal cell types of the nervous system are

    • Neurons—excitable cells that initiate and transmit electrical signals

    • Supporting cells—cells that surround and wrap neurons

      • Helps neurons grow, live, prop them up, and sometimes even feed them and take away waste

Neurons (Nerve Cells)

  • Structural units of the nervous system

    • Composed of a body, axon, and dendrites

    • Long-lived, amitotic, and have a high metabolic rate

  • Their plasma membrane functions in

    • Electrical signaling

    • Cell-to-cell signaling during development


Neuron Cell Body (Perikaryon or Soma)

  • Contains the nucleus and nucleolus (inside the nucleus that produces parts of ribosomes, which lead to the production of proteins)

  • Is the major biosynthetic center

  • Is the focal point for the outgrowth of neuronal processes

  • Has no centrioles (hence its amitotic nature)

    • They can not get cancer bc the only way to grow a tumor is to have actively dividing cells

  • Has well-developed Nissl bodies (rough ER) that cause gray color in gray matter

    • Makes stuff for ribosomes, which produce proteins; proteins are big building blocks of the cell body

  • Contains an axon hillock (aka trigger center)—cone-shaped area from which axons and action potentials (sparks) arise


Neuron Processes

  • Armlike extensions from the cell body

  • Bundles of these are called tracts in the CNS and nerves in the PNS

    • There are no nerves in the brain and spinal cord

  • Two types

    • Dendrites—convey electrical signals (varying in amount) toward the cell body

    • Axons—longer and also called nerve fibers

      • Convey electrical signals away from the cell body toward another neuron, muscle, or gland

      • Release NT (Neurotransmitter)


    • Also an example of a multipolar neuron

Neuron Structural Classification

  • Neurons vary in the number of cell body processes

    • Unipolar—one process; sensory neurons

    • Bipolar—two processes; typically only found in special sense organs

    • Multipolar—at least 3 processes; motor neurons, interneurons, etc.


Interneurons (aka Association Neurons)


  • A) Sensory (afferent)—transmit impulses toward CNS

  • B) Motor (efferent)—carry impulses away from the CNS

  • C) Interneurons—multipolar neurons entirely within the CNS that shuttle signals through CNS pathways

    • Communicate w/ sensory and motor neurons

Supporting Cells: Neuroglia

  • The supporting cells (neuroglia or glial cells)

    • Not going to send electrical signals

    • Provide a supportive scaffolding for neurons in CNS and PNS

    • Segregate and insulate neurons

      • Makes sure that some cells or parts of cells are not inappropriately affected by electrical signals

    • Guide young neurons to the proper connections

    • Promote health and growth

    • Very numerous, can divide, do not transmit impulses/action potential/electrical signals/sparks

      • Can form tumors due to dividing nature

Glial Cells: Astrocytes

  • Most abundant, versatile, and highly branched glial cells

  • Clings to neurons and their synaptic endings, and covers capillaries (forms the blood-brain barrier)

  • Functionally, they:

    • Support and brace neurons

    • Anchor neurons to their nutrient supplies

    • Guide migration of young neurons

    • Control the chem environ


Microglia and Ependymal Cells

  • Microglia—small, ovoid (oval-shaped) cells w/ spiny processes

    • Phagocytes that destroy wastes and pathogens

    • Kinda like spiders waiting on a web for bad things to come along

  • Ependymal cells—range in shape from squamous to columnar

    • Line central cavities of the brain and spinal column to circulate cerebrospinal fluid (CSF)

  • Satellite cells—squamous cells surrounding ganglia

    • Ganglia (singular: ganglion)—Clusters of cell bodies in PNS

    • Clusters of cell bodies are like mini brains scattered around the body

    • Regulate chem of nutrients, gases, and NT (neurotransmitter) for ganglia



Oligodendrocytes and Schwann Cells Perform Myelination

  • Myelin sheath—whitish, fatty (protein-lipoid), segmented sheath that protects the axon, electrically insulates fibers from one another, and incs the speed of nerve impulse transmission

    • Forms a protective outer structure

    • Fats insulate against electricity

  • Schwann cells (neurolemmocytes)—myelinate nerve fibers (axon) of PNS



  • Oligodendrocytes—branched cells that myelinate CNS nerve fibers (axons)


    • Oligodendrocytes can perform myelination on multiple axons

  • These guys basically do the same functions, but in different parts of the body (CNS v. PNS)

Saltatory Conduction

  • Current passes through a myelinated axon only at the nodes of Ranvier (neurofibral nodes)

    • Jumps over myelin sheath and lands on neurofibral nodes


  • Action potentials are triggered only at the nodes and jump from one node to the next

  • Much faster (and uses less ATP) than conduction along entire length of unmyelinated axons

Conduction Velocities of Axons

  • Varies widely among neurons

  • Rate of impulse propagation is determined by

    • Axon diameter—the larger the diameter (thicker), the faster the impulse

    • Presence of a myelin sheath—myelination dramatically incs impulse speed

Axon Regeneration

  • CNS axons generally can’t regenerate

  • Damaged PNS axons may regenerate if some of the neurolemma is still present

    • Depends on distance btwn severed ends

    • Depends on secretion of nerve growth factors

Nerve/Tract CT

  • Axons are bundled by 3 CT

    • Epineurium—surrounds entire nerve or tract; dense irregular CT

    • Perineurium—surrounds fascicles of axons; dense irregular CT

    • Endoneurium—surrounds the individual axon; loose (areolar) CT


Synpases

  • Axons end w/ axonal terminals at synapses

  • Presynaptic neurons—release NT into synapse for comms

  • Postsynaptic neurons—receive NT from presynaptic neuron

  • Axons can have synaptic contacts at any unmyelinated surface on another neuron


  • Synpase--little spaces where neurotransmitter are released and received

Neurotransmitters

  • Nerve impulses reach the axonal terminal of the presynaptic neuron and open Ca2+ channels

  • Neurotransmitter is released into the synaptic cleft via exocytosis (when vesicle bubble merges w/ cell membrane to release NT)

  • Neurotransmitter crosses the synaptic cleft and binds to receptors on the postsynaptic neuron

  • Postsynaptic membrane experiences Na+ influx, causing an excitatory or inhibitory effect

  • Synaptic delay is the time elapsed

  • Nerve impulse travels down presynaptic neuron’s axon → opens the gateways for Ca2+ → Ca2+ triggers synaptic vesicles which carry NT (ACh) → vesicles move to end and merge w/ cell membrane → exocytosis (release NT) → now we’re in the synapse! → once NT hits postsynaptic receptors, allows Na+ to come in (can be either excitatory or inhibitory)


Neuronal Pools/Circuits

  • Interneurons form patterns (pools) in the CNS, based on function

  • A pool may be localized, or may be distributed regionally throughout the CNS