Three Functions of Nervous Tissue

1. Sensory Function

2. Motor Function

3. Integrative Function

(The nervous system and the endocrine system are the body's major control centers.)

Sensory Function

Senses stimuli with sensory receptiors in the internal and external environments.

Motor Function

Reacts to those changes with effectors:

(Effectors carry out a function, i.e., muscular contractions which are voluntary or glandular secretions which are involuntary.)

Integration

Interpreting and remembering those changes.

(Analyzes the sensory information, stores some of it and makes decisions regarding apprpriate behaviors.)

Nervous System Division

Peripheral Nervous System (PNS)

Consists of cranial and spinal nerves that contain both sensory and motor fibers.

Connects CNS to muscles, glands and all sensory receptors.

Afferent Neurons

PNS

Sensory neurons that transmit information from sensory receptors to the CNS.

Afferent Neurons: Muscles/Glands-->CNS

Efferent Neurons

PNS

Motor neurons which transmit nerve impulses from the CNS to muscles and glands.

Efferent Neurons: CNS-->Muscles/Glands

Somatic Nervous System

One of Two Major Divisions of the PNS

VOLUNTARY

- Sensory neurons transmit information from cutaneous, and special sense receptiors: sight, hearing, smell, taste, and touch (pressure, temperature, and pain).

- Motor neurons that transmit informaiton from the CNS to skeletal muscles (voluntary part).

PNS Afferent (sensory)-->CNS-->message-->CNS-->PNS Efferent (motor)

VOLUNTARY

Autonomic Nervous System

One of Two Major Divisions of the PNS

INVOLUNTARY

-Sensory neurons that transmit information from visceral receptors to the CNS.

-Motor neurons that transmit information from the CNS to smooth muscle, cardiac muscle, and the glands. (Involuntary Part)

PNS Afferent (sensory)-->CNS-->message-->CNS-->PNS Efferent (motor)

Has two further divisons.

Autonomic Nervous System

Two Branches

1. Sympathetic Division

2. Parasympathetic Division

Sympathetic Divison

Part of the Autonomic Nervous System

Promotes visceral processes that involve expenditure of energy

Parasympathetic Division

Part of the Autonomic Nervous System

Promotes visceral processes that restore and conserve body energy.

Nervous Tissue Consists Of....

Neurons

Neuroglia (nerve glue)

Neuroglia

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• Supporting cells which support, nurture, and protect the neurons.
• Occupy abou 1/2 the volume of the CNS
• Smaller but about 50x more numerous
• Can divide (brain tumors are commonly masses of glial cells called gliomas- rapid mitosis)

4 types of Neuroglia (Glial Cells) in the CNS

1. ASTROCYTES
2. OLIGODENDROCYTES
3. MICROGLIA
4. EPENDYMAL

2 types of Neuroglia (Glial Cells) in the PNS

1. Schwann Cells

2. Satellite Cells

• Star-shaped cells
• MAIN FUNCTION: a barrier to certain chemicals or molecules in the blood stream.
• Form the blood-brain barrier by covering blood capillaries.
• Metabolize NTs
• Regulate the K+ Balance
• Provide structural support

Oligodendrocytes

• Most common glial cell type
• FUNCTION: Each forms myelin sheathe around more than one axon in the CNS. 
• Analogous to Schwann cells of PNS

Microglia

• Small cell found near blood vessels
• FUNCTION: Phagocytic
• Derived from cells that also gave rise to macrophages and monocytes.

Ependymal Cells

• Line the ventricles
• Form epithelial membrane lining cerebral cavities and central canal
• Have cilia
• FUNCTION: Important for CSF production

Satellite Cells

• Flat cells
• FUNCTION: Surround and protect cell bodies in the peripheral ganglia.
• Support Neurons in the PNS ganglia 

Bundle of Axons

• "NERVES" in the PNS
• "TRACT" in the CNS

Schwann Cells

• AKA "Neurolemmocytes"
• Each cell produces part of the myelin that surrounds an axon in the PNS during fetal development
• The cytoplasm and nucleus form the outermost part with the inner portion being the myelin sheath.
• The tube guides growing axons that are repairing themselves.

Neuronal Cell Body

• AKA Soma, or Perikaryon
• Single nucleus with prominent nucleolus
• Contains NISSL Bodies 
• Neurofilaments/fibrils give cell shape and support
• Microtubules move material inside the cell

Nissl Bodies

The chromatophilic substance, or Nissl bodies, is an orderly arrangement of rough endoplasmic reticulum. The proteins from these Nissl bodies are used to restore peripheral nerve fibers.

RER and free ribosomes for protein synthesis.

Dendrites

• Conducts impulses towards the cell body
• Usually short
• Always unmyelinated
• Contain neurofibrils & Nissl bodies
• The recieving part of the neuron

Axons

• Conducts impulses away from the cell body (conductive zone)
• Does NOT contain Nissl bodies or ribosomes
• Excluding sensory neurons, nerve impulses are intiated at the "trigger zone" which is the junction of the axon hillock and initial segment. The initial segment is the first portion of the axon. 
• Collaterals, or side branches end in axon terminals.

Myelin Sheaths

• Protects the axon 
• Electrical insulatpr
• Appear white
• Made of lipoprotein
• Unmyelinated fibers are surrounded by glial cell process but have no myelin sheath for protection. 
• Nodes of Ranvier: gaps located between neighboring neurolemmocytes 
• Neurolemmocytes AKA Schwann cells in PNS
• Oligodendrocytes myelinate axons in CNS (more than 1)

Nerve Fiber

• Usually refers to an axon and its sheaths.
• A NERVE is a bundle of Nerve Fibers that travel along the same path in the PNS
• Most nerves contain both sensory and motor fibers that are surrounded by connective tissue coats.

Nerve Sheaths

• Protective coverings of connective tissue on PNS axons
• Endoneurium: thin collagenous CT covering surrounding individual axons and their myelin sheaths.
• Perineurium: thicker CT sheath around a nerve fascicle (bundle of axons)
• Epineurium: thick dense CT sheath surrounding a whole nerve; often contains adipocytes

Transmisson of Nerve Impulse

• Axon terminals form synapses on dendrites and cell bodies
• Impulse generation: summation takes place at inital segement on axon hillock. 
• Impulse conduction: at threshold Na open and impulse is conducted down the axon.
• Myelinated axon: Na channels open at Nodes of Ranvier -->saltatory conduction
• Unmyelinated axon: Na channels open at adjacent path of membrane and is slower.

Structural Classification of Neurons

• Multipolar: several dendrites and one axon: most common
• Bipolar: one main dendrite and one axon: found in the retina, inner ear, and olfactory
• Unipolar: one process only and develops from a bipolar: are always sensory neurons.

Functional Classification

• Sensory (Afferent) Neurons - transports sensory info from the skin, muscles, joints, sense organs and viscera to CNS
• Motor (Efferent) Neurons - sends motor nerve impulses to muscles and glands
• Interneurons (Association) - connects neurons together which are 90% of neurons in body.

Endoneurium

• thin collagenous CT covering surrounding individual axons and their myelin sheaths.
• CT containing collagen fibres, fibroblasts and mast cells.

Perineurium

• Fascicles are axon bundles that are surronded by perineurium
• helps to establish a peripheral blood-nerve diffusion barrier.

Epineurium

• Surrounds the complete collection of fascicles
• Made up of dense irregular connective tissue
• Associated with adipose tissue.

Nervous System Changes (Plasticity) and Repair

• Plasticity maintained throughout:

 - sprouting of new dendrites, synthesis of new proteins, changes in synaptic contacts with other neurons.

• REPAIR:

- PNA: can repair dendrites or axons

- CNS: no repairs possible.

Repair within the PNS

• Axons and dendrites may be repaired if

- neuron cell body remains intact

- Schwann cells remain active and form a tube

Scar tissue does not form too rapidly

Chromatolysis

• 24-48 hours after injury
• Nissle bodies in the cell body break up into fine granular masses

Wallerian Degeneration

• 3-5 days
• breakdown of axon and myelin sheath distal to injury in anterograde manner (away from the somata). 
• A CT tube (epineurium) will remain relatively intact to form a way for regenerating myelin and axon fibers to follow towards the orginal target organ. 

• Followed by retrograde degeneration which occurs back one node towards the somata but will re-grow outwards along the nerve sheath and re-connect with target sites if regeneration occurs.