Lecture # 17: Nervous Tissue(Chapter 12)

Objectives:1- Explain the general role of the nervous system in maintaining homeostasis. 2- Name the subdivisions of the nervous system and list the components of each. 3- Identify the parts of a neuron and give their functions. 4- Classify neurons functionally and structurally. 5- Contrast characteristics and functions of neuroglia and neurons. 6- Identify and give the specific functions for the types of neuroglial cells.

A Purkinje cell, a neuron from the cerebellum

There are two organ systems dedicated to maintaining the internal conditions (homeostasis):

Overview of the Nervous System

Endocrine System

Nervous System

It communicates by means of chemical messengers (hormones) secreted into to the blood

It employs electrical and chemical means to send messages from cell to cell

Endocrine gland

Target cell (skeletal muscle cell)

Hormone

Neurotransmitter (Ex: Acetylcholine)

It is a bundle of nerve fibers (axons) wrapped in fibrous connective tissue in the PNS

Central Nervous System (CNS)

Peripheral Nervous System (CNS)

It consists of the brain and spinal cord enclosed by cranium and vertebral column.

Brain

Spinal cord

It is all the nervous system except the brain and spinal cord.

It is a knot-like swelling in a nerve where neuron cell bodies are concentrated in the PNS

It consists of nerves and ganglia.

Nerve:

Ganglion:

GanglionNerve

It is responsible for integrating, processing and coordinating sensory data and motor commands.

It deliveries sensory information to the CNS and carries motor commands to peripheral tissues and system.

It is a bundle of nerve fibers (axons) in the CNS (white matter).

It is a concentration of neuron cell bodies in the CNS (gray mater)

Tract:

Nucleus:

Anatomical Subdivision of the Nervous System

Central Nervous System

Sensory or Afferent Division

Motor or Efferent Division

Receptors

Effectors

Overview of the Nervous SystemIt carries sensory signals from various receptors to the CNS

Sensory input

Motor output

The brain and spinal cord processes the information, relates it to past experiences, and determine what response is appropriate to the circumstances

It carries signals from the CNS to gland and muscle cells that carry out the body’s response

Peripheral Nervous System

Sensory or Afferent Division

Motor or Efferent Division

Brain

Spinal cord

Visceral sensory fiber

Parasympathetic

Sympathetic

Somatic motor fiber

Somatic sensory fiber

Nervous Tissue

2- Neuroglia or glial cells1- Neurons

-In the CNS-In the PNS

The glial cells are supporting cells, which are associated to the neurons and provide a supportive scaffolding for neurons

NeuronNeurosoma, cell body, or perikaryon

Dendrites

Glial cell (astrocyte)

Neurons

Axon

Properties of Neurons

When electrical signal reaches end of nerve fiber, a chemical neuro-transmitter is secreted that crosses the gap and stimulates the next cell

Excitability (irritability)

Conductivity

Secretion

They respond to environmental changes called stimuli by producing electrical signals (action potentials)

The action potentials are quickly conducted to other cells at distant locations

Stimulus

Action potential

Universal Properties

Central Nervous System

Receptors

Effectors

Sensory input

Motor output

They conduct signals from receptors to the CNS. Some receptors, such as those for pain and smell, are themselves neurons

Sensory or afferent neurons

InterneuronsThey occur only in the CNS. They receive signals from many other neurons; and process, store, and retrieve information and make decisions that determine how the body respond to stimuli

Motor or efferent neurons

They conduct signals from the CNS to the effectors (gland and muscle cells )

Functional Classes of Neurons

Functional Classes of Neurons

Structure of a Neuron

SomaDendrites

NucleolusNucleus

They carry on the synthesis of protein

Neurofibrils

Nissl bodies

Axon hillock

Axon

Axon collateral

Schwann cell

Node of Ranvier

It is the trigger zone for the nerve impulse

It carries the nerve impulse away from the soma. From 1 to 20 mm in diameter and from a few mm to more than one meter long

They receive nerve impulses from other neurons

They produce the myelin sheath

They release the neurotransmitter

Terminalarborization

Synaptic knobs

Structure of a Neuron

SomaDendrites

Axon

Axon collateral

Schwann cell

Node of Ranvier

It carries the nerve impulse away from the soma. From 1 to 20 mm in diameter and from a few mm to more than one meter long

They receive nerve impulses from other neurons

They produce the myelin sheath

They release the neurotransmitter

Terminalarborization

Synaptic knobs

AxoplasmCytoplasm of axon

Plasma membrane of axon

Axolemma Thick outermost coil of myelin sheath

NeurilemmaSchwann cell nucleus

Myelin sheathIt consists of the plasma membrane of glial cells (20% protein and 80 % lipid). It electrically insulates the axon

Synaptic vesiclesThey contain the neurotransmitter

Structural Classification of Neurons

They have one axon and multiple dendrites. They are the most common neurons in the brain and spinal cord.

They have one axon and one dendrite. Ex: Olfactory cells, retina, inner ear.

They have single process leading away from the soma. A short distance away from the soma, the process branches like a “T”. They carry sensory information from skin and organs to spinal cord.

They have many dendrites but no axon. They are found in the brain, adrenal medulla, and retina where they help in visual processes

Supportive Cells (Neuroglia)

1- Oligodendrocytes: They form myelin sheaths in CNS. Each arm-like process wraps around a nerve fiber forming an insulating layer that speeds up signal conduction

2- Ependymal cells: They form the ciliated cuboidal epithelium that lines internal cavities of the brain. They secrete and circulate cerebrospinal fluid (CSF

3- Microglia: They are small, macrophages formed from white blood cell called monocytes. They wander brain tissues ingesting microorganisms and dead tissue.

4- Astrocytes: They are the most abundant glial cell in CNS covering the entire brain surface. They have extensions (perivascular feet) that contact blood capillaries that stimulate them to form a tight seal called the blood-brain barrier.

The blood-brain barrier isolates the blood from the brain tissue and limits what substances are able to get the brain cells, thus protecting the neurons.

Astrocytes regulate the chemical composition of tissue fluid by absorbing excess neurotransmitters and ions.

Astrocytosis or Sclerosis: When the neurons are damaged, the astrocytes form hardened scar tissue and fill space formerly occupied by the neuron.

Supportive Cells

(Neuroglia)

OligodendrocytesEpendymal cells Microglia Astrocytes

Schwann cellsSatellite cells

Central Nervous System

Peripheral Nervous System

Neuroglia in the CNS:

Neuroglia in the PNS:

1- Schwann cells: They wind repeatedly around a nerve fiber and produce a myelin sheath similar to the ones produced by oligodendrocytes in CNS. Schwann cells assist in the regeneration of damaged fibers.

2- Satellite cells: They surround the neurosomas in ganglia of the PNS and provide electrical insulation around the soma. Satellite cells regulate the chemical environment of the neurons.

Tumors: They are masses of rapidly dividing cells. Mature neurons have little or no capacity for mitosis and seldom form tumors.Brain tumors arise from:- Meninges (protective membranes of CNS).- By metastasis from non-neuronal tumors in other organs.- Most come from glial cells (gliomas) that are mitotically active throughout life.

Gliomas grow rapidly and are highly malignant. The blood-brain barrier decreases effectiveness of chemotherapy. The treatment consists of radiation or surgery.

Clinical Application: