Fundamentals of the Nervous System

Fundamentals of the Nervous System and Nervous Tissue

• Chapter 12 is about the nervous system and nervous tissue.
• The nervous system is described as the master controlling and communicating system of the body.
• Cells communicate through electrical and chemical signals.
• This communication is rapid and specific.

Functions of the Nervous System

• Sensory input involves gathering information, and sensory receptors monitor for both internal and external changes.
• Integration involves processing and interpreting what the sensory receptors have found.
• Motor output involves activating effector organs, like muscles and glands, to produce a response.

Divisions of the Nervous System

• The central nervous system(CNS) consists of the brain and the spinal cord; it functions as the integration and control center, interpreting sensory input and dictating motor output.

• The peripheral nervous system(PNS) is the portion of the nervous system outside the CNS.

• The PNS Consists mainly of nerves that extend from the brain and spinal cord It includes:

  • Spinal nerves that go to and from the spinal cord
  • Cranial nerves that go to and from the brain

• The peripheral nervous system(PNS) has two functional divisions: sensory (afferent) and motor (efferent).

• The sensory division conveys impulses from the skin, skeletal muscles, and joints to the CNS. Visceral sensory fibers convey impulses from visceral organs to the CNS.

• The motor division transmits impulses from the CNS to effector organs such as muscles and glands.

• The motor division has two divisions of its own: somatic nervous system and autonomic nervous system

• The somatic nervous system allows for conscious control of skeletal muscles.

• The autonomic nervous system regulates smooth muscle, cardiac muscle, and glands, controlling such things as the middle of ribs, lungs, and ovaries.

Nervous Tissue

• Nervous tissue is highly cellular with little extracellular space.
• It is made up of two principal cell types:
  • Neuroglia (glial cells or nerve glue), which are small cells that surround and wrap delicate neurons
  • Neurons (nerve cells), which are basic functional units of the nervous system that are excitable and transmit electrical signals.

Neurons

• Neurons are the structural units of the nervous system.
• Neurons are specialized cells that conduct impulses.
• Neurons have a very high metabolic rate, requiring a continuous supply of oxygen and glucose; they cannot survive more than a few minutes without oxygen(O2).
• All neurons have a cell body (soma) and one or more processes.
• Processes are armlike extensions from the soma; they are called tracts in the central nervous system(CNS) and nerves in the peripheral nervous system(PNS) and are either axons or dendrites.

Nerve Cell Body (Perikaryon or Soma)

• The nerve cell body contains the nucleus and a nucleolus.
• It’s the major biosynthetic center.
• Cytoplasm contains cytoskeleton elements(neurofibrils). -Nissl bodies (chromatophilic substances; rough ER) are involved in metabolic activities.
• It contains an axon hillock, which is a cone-shaped area from which axons arise and can be damaged by drugs and medication.
• Most neuron cell bodies are located in the central nervous system(CNS), protected by bones, skull, and vertebral column (nuclei)
• Cell bodies located in the peripheral nervous system(PNS) are known as ganglia.

Dendrites

• Motor neurons have hundreds of short, tapering, diffusely branched processes.
• Dendrites are the receptive region of the neuron.

The Axon

• Each cell has one axon arising from the axon hillock.
• Long axons are called nerve fibers.
• There can be occasional branches (axon collaterals).
• Axons branch profusely at the end (terminus).
• Distal endings are called axon terminals or terminal boutons.

Function of the Axon

• Generates and transmits nerve impulses along the axolemma (neuron cell membrane) to the axon terminal.
• It secretes neurotransmitters from the axonal terminals.

Structural Classification of Neurons

• Neurons are classified by the number of processes issuing from the cell body.
  • Multipolar: three or more processes. One axon with the others being dendrites. They are the most common major neuron in the CNS.
  • Bipolar: There are two processes: one axon and one dendrite. These are rare, but can be found in the retina and olfactory mucosa.
  • Unipolar: It has one short process that divides T-like into both branches that considered axons. The distal (peripheral) process is associated with a sensory receptor and the proximal (central) process enters the central nervous system.

Functional Classification of Neurons

• Sensory neurons transmit impulses from sensory receptors toward the CNS.
• Motor neurons carry impulses from the CNS to effectors.
• Interneurons (association neurons) shuttle signals through CNS pathways and are mostly confined to the CNS.
  • 99% of the body's neurons are interneurons.

Neuroglia

• Astrocytes (CNS) are associated with nerve and capillary (blood).
• Microglial cells (CNS) are most closely associated with the nerve.
• Ependymal cells (CNS) are associated spinal chord(CSF).
• Oligodendrocytes (CNS) wrap the nerve in CNS.
• Satellite cells (PNS) recognize.
• Schwann cells (PNS) are mostly associated with arms and legs.

Astrocytes

• Provide support, play a role in exchanges between capillaries and neurons
• Control the chemical environment around neurons and respond to nerve impulses and neurotransmitters.

Microglial Cells

• Small ovoid, cells with thorny processes that increase with headaches and respond to situations.
  • They can transform to phagocytize debris (dead cells, bacteria, etc)

Ependymal Cells

• They range in shape from squamous to columnar.
• Cilia beat to circulate cerebrospinal fluid(CSF), acting as a cushion to protect the brain and maintains the homeostasis in the brain.
• Ependymal cells line the central cavities, ventricles of the brain, and spinal column.
• They form a permeable barrier between CSF in cavities and tissue fluid bathing CNS cells.

Oligodendrocytes

• They have branched cells, The processes wrap CNS nerve fibers and forming insulating myelin sheaths around the fibers.

Satellite Cells and Schwann Cells (PNS)

• Satellite cells surround neuron cell bodies(ganglia). Their function is similar to astrocytes of (CNS).

• Schwann cells (neurolemmocytes) surround all peripheral nerve fibers and form myelin sheaths in thicker nerve fibers and has similar function to as oligodendrocytes

• It is vital to regeneration of damaged peripheral nerve fibers.

Myelin Sheath

• It is a whitish, fatty (protein-lipid) segmented sheath around long axons.
• Its functions:
  • Protects the axon from the outside
  • Electrically insulates fibers from one another
  • Increases the Speed of nerve impulse transmissions

Myelin Sheath and Neurilemma Formation

• Formed by Schwann cells and Oligodendrocytes.
• A Schwann cell:
  • Envelopes an axon in a trough
  • Encloses the axon with its plasma membrane
  • Has concentric layers
• Consists of Nodes of Ranvier: Gaps in the myelin sheath between adjacent Schwann cells

Gray and White Matter in the CNS

• Gray matter, which is gray-colored surrounds hollow central cavities of the CNS.

  • It forms H-shaped region in the spinal cord: dorsal half contains cell bodies of interneurons, ventral half contains cell bodies of motor neurons.
  • Composed primarily of neuronal cell bodies, dendrites, nonmyelinated axons.

• It surrounds white matter of CNS in cerebral cortex and cerebellum.

• White matter lies external to the gray matter of the CNS.

  • It is composed of myelinated axons, consists of axons passing between specific regions of the CNS, Tracts are bundles of axons traveling to similar locations.

Neuronal Integration

• The central nervous system(CNS) and peripheral nervous system(PNS) are functionally interrelated.
• Nerves of the PNS:
  • Afferent PNS fibers respond to sensory stimuli.
  • Efferent PNS fibers transmit motor stimuli from CNS to muscles and glands
• Nerves of the CNS:
  • Receive sensory information
  • Direct and transport information to specific CNS regions
  • Initiate appropriate motor responses

Reflex Arcs

• Reflex arcs are simple chains of neurons that explain reflex behaviors.
  • Responsible for reflexes and rapid automatic motor responses that can be visceral or somatic.
• Components of a reflex arc:
  • Receptor is sited where stimulus effects.
  • Sensory neuron transmits afferent impulses to the CNS
  • Integration which consists of one or more synapses.

Integration center

• Monosynaptic reflex*
• Only one synapse
• Fastest of all reflexes e.g Knee-jerk reflex
• Polysynaptic reflex*
• Most common type of reflex One or more interneurons signal the motor neuron to contract muscle involved

Neuronal Circuits

• Diverging circuit: one presynaptic neuron synapses with multiple other neurons.
• Converging circuit: many neurons synapse on a single postsynaptic neuron.
• Reverberating circuit : This circuit receives feedback via a collateral axon from a neuron in the circuit

Integration Between PNS and CNS

• Neuronal circuits form networks of interneurons.
  • The pain is then felt after reflexive withdrawal.

Disorders of the Nervous System

• Multiple sclerosis: An autoimmune disease where immune system attacks the myelin around axons damages myelin sheath.
  • Is more prominent in Women than men are affected
  • When men are affected, disease becomes devastating
• Regeneration of an axon in a peripheral nerve. If axons alone are destroyed cell bodies can survive & axons may regenerate. PNS-microphages invade and destroy axon distal to the injury . Axon filaments grow peripherally from injured site.
• In CNS Neuroglia form bands to guide regrowing axons.
• No effective regeneration after injury to spinal cord or brain