Nervous System

Master controlling and communication system of the body. Most dynamic system in controlling homeostasis in our body. Fast. (Endocrine system also controls homeostasis in our body with release of hormones, but much slower)

Functions of Nervous System

Cells Communicate via electrical and chemical signals. Rapid & Specific, usually cause almost immediate responses.

Nervous system has three overlapping functions

1. Sensory input
   • Information gathered by sensory receptors (touch, pain, etc.) about internal and external changes.
2. Integration
   • Processing and interpretation of sensory input.
3. Motor output
   • Activation of effector organs (muscles and glands) produces a response. (Has an effect on the stimulus)

Nervous system is divided into two principal parts

1. Central nervous system (CNS)
   • Brain and spinal cord of dorsal body cavity
   • Integration and control center
     • Interprets sensory input and dictates motor output
2. Peripheral nervous system (PNS) (this is everything but the brain and spinal cord)
   • The portion of nervous system outside CNS
   • Consists mainly of nerves that extend from brain and spinal cord
     • Spinal nerves to and from spinal cord
     • Cranial nerves to and from brain

Peripheral nervous system (PNS) has two functional divisions:

• Sensory (afferent) division
  • Somatic sensory fibers: convey impulses from skin, skeletal muscles, and joints to CNS
  • Visceral sensory fibers: convey impulses from visceral organs to CNS
• Motor (efferent) division
  • Transmits impulses from CNS to effector organs
    • Muscles and glands
  • Two divisions
    • Somatic nervous system
    • Autonomic nervous system

(To help remember, think "SAME:" Sensory - Afferent, Motor - Efferent)

Somatic nervous system

• Somatic motor nerve fibers conduct impulses from CNS to skeletal muscle
• Voluntary nervous system (we control it)
  • Conscious control of skeletal muscles

Autonomic nervous system

• Consists of visceral motor nerve fibers
• Regulates smooth muscle, cardiac muscle, and glands
• Involuntary nervous system (we can't control it, automatic)
• 2 functional subdivisions:
  • Sympathetic
  • Parasympathetic
  • (Work in opposition to each other)

Cells of Nervous System

• Nervous tissue histology
• Nervous tissue consists of two principal cell types:

- Neuroglia (glial cells): small cells that surround and wrap delicate neurons. (Protect neurons. 8-9x more than neurons. Glial=glue)

- Neurons (nerve cells): excitable cells that transmit electrical signals.
(Creating Electrical Currents, Nerve Impulses...Action Potentials)

Four main neuroglia support CNS neurons

1. Astrocytes
2. Microglial cells
3. Ependymal cells
4. Oligodendrocytes

Astrocytes

• Most abundant, versatile, and highly branched of glial cells
• Cling to neurons, synaptic endings, and capillaries
• Functions include:
  • Support and brace neurons
  • Play role in exchanges between capillaries and neurons (create our blood brain barrier)
  • Control chemical environment around neurons
  • Respond to nerve impulses and neurotransmitters
  • Participate in information processing in brain

Microglial cells

• Phagocytic Cells
• Small, ovoid cells with thorny processes that touch and monitor neurons
• Migrate toward injured neurons
• Can transform to phagocytize microorganisms and neuronal debris

Ependymal cells

• Range in shape from squamous to columnar
• May be ciliated
  • Cilia beat to circulate CSF
• Line the central cavities of the brain and spinal column
• Form permeable barrier between cerebrospinal fluid (CSF) in cavities and tissue fluid bathing CNS cells

Oligodendrocytes

• create myelin - white phospholipid insulating material
• Branched cells
• Processes wrap CNS nerve fibers, forming insulating myelin sheaths in thicker nerve fibers

Two major neuroglia seen in PNS

• Satellite cells
  • Surround neuron cell bodies in PNS
  • Function similar to astrocytes of CNS
• Schwann cells (neurolemmocytes)
  • Surround all peripheral nerve fibers and form myelin sheaths (in PNS) in thicker nerve fibers
    • Similar function as oligodendrocytes
  • Vital to regeneration of damaged peripheral nerve fibers

Neurons

• Neurons (nerve cells) are structural units of nervous system
• Large, highly specialized cells that conduct impulses (Action Potentials - A.P.)
• Special characteristics
  • Extreme longevity (lasts a person’s lifetime)
  • Amitotic, with few exceptions
  • High metabolic rate: requires continuous supply of oxygen and glucose (very active cells)
• All have cell body and one or more processes

Neuron Cell Body

Also called the perikaryon or soma (center of neuron)

Biosynthetic center of neuron
- Synthesizes proteins, membranes, chemicals
- Rough ER (chromatophilic substance, or Nissl bodies)

Contains spherical nucleus with nucleolus (RNA)

Some contain pigments

In most, plasma membrane is part of receptive region that receives input info from other neurons

• Most neuron cell bodies are located in CNS
  • Nuclei: clusters of neuron cell bodies in CNS
  • Ganglia: clusters of neuron cell bodies in PNS

Neuron Processes

Anything coming through dendrites, go through Neuron Processes:

• Armlike processes that extend from cell body
  • CNS contains both neuron cell bodies and their processes
  • PNS contains chiefly neuron processes
• Tracts (Fiber Tract)
  • Bundles of neuron processes in CNS
• Nerves
  • Bundles of neuron processes in PNS
• Two types of processes:
  • Dendrites
  • Axon

(Don't have nerves in CNS, we have tracts. Don't have tracts in PNS, have nerves)

Dendrites

• Motor neurons can contain 100s of these short, tapering, diffusely branched processes
  • Contain same organelles as in cell body
• Receptive (input) region of neuron
• Convey incoming messages toward cell body as graded potentials (short distance signals)
• In many brain areas, finer dendrites are highly specialized to collect information
  • Contain dendritic spines, appendages with bulbous or spiky ends

The axon: structure

The axon: structure:

• Each neuron has only one axon that starts at cone-shaped area called axon hillock
• In some neurons, axons are short or absent; in others, axon comprises almost entire length of cell
  • Some axons can be over 1 meter long
• Axons have occasional branches called axon collaterals
• Axons branch profusely at their end (terminus)
  • Can number as many as 10,000 terminal branches
• Distal endings are called axon terminals

The axon: functional characteristics

The axon: functional characteristics

• Axon is the conducting region of neuron
• Generates nerve impulses and transmits them along axolemma (neuron cell membrane) to axon terminal
  • Terminal: region that secretes neurotransmitters, which are released into extracellular space
• Quickly decay if cut or damaged

Myelin sheath

(Only found around axon only - protects axon to increase speed of electrical current)

• Composed of myelin, a whitish, protein-lipid substance
• Function of myelin:
  • Protect and electrically insulate axon
  • Increase speed of nerve impulse transmission
• Myelinated fibers (axon): segmented sheath surrounds most long or large-diameter axons
• Nonmyelinated fibers (axon): do not contain sheath
  • Conduct impulses more slowly

Myelination in the PNS

• Formed by Schwann cells
  • Wraps around axon in jelly roll fashion
  • One cell forms one segment of myelin sheath
• Outer collar of perinuclear cytoplasm (formerly called neurilemma): peripheral bulge containing nucleus and most of cytoplasm
• Plasma membranes have less protein
  • No channels or carriers, so good electrical insulators
  • Interlocking proteins bind adjacent myelin membranes

(Cell Membrane: plasmalemma)

Myelin Sheath Gaps

• Gaps between adjacent Schwann cells
• Sites where axon collaterals can emerge
• Formerly called nodes of Ranvier

Nonmyelinated fibers

Thin fibers not wrapped in myelin; surrounded by Schwann cells but no coiling; one cell may surround 15 different fibers

Myelin sheaths in the CNS

• Formed by processes of oligodendrocytes, not whole cells
• Each cell can wrap up to 60 axons at once
• Myelin sheath gap is present
• No outer collar of perinuclear cytoplasm

Myelin Sheaths, White Matter

White matter: regions of brain and spinal cord with dense collections of myelinated fibers

Usually fiber tracts

Myelin Sheaths, Gray Matter

Gray matter: mostly neuron cell bodies and nonmyelinated fibers

Classification of Neurons - Structural Classification

Structural classification

3 types grouped by number of processes

1. Multipolar: 3 or more processes (1 axon, others dendrites)
   • Most common and major neuron type in CNS2.
2. Bipolar: two processes (one axon, 1 dendrite)–Rare (ex: retina and olfactory mucosa)
3. Unipolar: one T-like process (2 axons)
   • Also called pseudounipolar
   • Peripheral (distal) process: associated with sensory receptor.
   • Proximal (central) process: enters CNS.

Functional classification of neurons

3 types of neurons grouped by direction in which nerve impulse travels relative to CNS1:

1. Sensory: Transmit impulses from sensory receptors toward CNS. Almost all are unipolar. Cell bodies are located in ganglia in PNS
2. Motor: Carry impulses from CNS to effectors, Multipolar, Most cell bodies are located in CNS (except some autonomic neurons)
3. Interneurons: Also called association neurons; Lie between motor and sensory neurons; Shuttle signals through CNS pathways; Most are entirely within CNS; 99% of body’s neurons are interneurons