Neural Tissue

Chapter 12Part A

Homeostasis

Homeostasis refers to maintaining internal environment.

How does body maintain homeostasis??

1. Each cell, tissue or organ maintain their own internal environment (autoregulation).

2. Two systems help the entire body in maintaining homeostasis (extrinsic regulation):Nervous systemEndocrine system

Homeostasis – Extrinsic Regulation

How do nervous system and endocrine system help maintain homeostasis??

Nervous system Endocrine system

Sensory system Chemical system

Generates and transmits impulses through nerves

Releases hormones into blood

Impulses follow a definite route and go directly to the target cells

Hormones travel to all body parts but act only on specific target cells that have its receptors

Rapid system Slower system

Example:You step on a nail impulses are generated impulses travel from the foot to the spinal cord impulses then travel to thigh muscle thigh muscle contracts foot islifted

Example:You eat a candy bar blood sugar increases pancreas is stimulated hormone insulin is secreted into blood insulin travels to all body parts liver has insulin receptors and receives insulin insulin helps liver to store blood sugar as glycogen blood sugar goes down

Nervous tissue

In this chapter, you will concentrate on nervous tissue.

Nervous system

Sensory system

Generates and transmits impulses through nerves

Impulses follow a definite route and go directly to the target cells

Rapid system

Example:You step on a nail impulses are generated impulses travel from the foot to the spinal cord impulses then travel to thigh muscle thigh muscle contracts foot islifted

Neurobiology: branch of biology that deals with nervous system.Neurology: branch of medicine that deals with structure, function and disorders of

nervous system.

Organization of Nervous SystemNervous system can be divided into:1) Central nervous system (CNS)- brain and spinal cord…the integrative center.2) Peripheral nervous system (PNS)- cranial and spinal nerves.

PNS can be divided into:

2A) Sensory/afferent nervous system- bring impulses from the sensory receptors to the CNS. 2B) Motor/efferent nervous system-Take impulses away from CNS to the peripheral organs and tissues that respond (effectors).

* AUTONOMIC(not automatic)

Autonomic nervous system is divided into:2Bii a) Sympathetic nervous system- cause increase/stimulation of the

effector…Fight or Flight response…to prepare the body for increasedphysical activity (increased cardiovascular & respiratory activity, increased sweating; decreased digestive activity).

2Bii b) Parasympathetic nervous system- cause decrease/inhibition of the effector…Rest and Digest…to bring the body back to resting state (decreased cardiovascular & respiratory activity; stimulation of digestive activity).

2Bi) Somatic nervous system- take impulses from CNS to voluntary effectors(skeletal muscles).

2Bii) Autonomic nervous system- take impulses from CNS to involuntary effectors(cardiac & smooth muscles; glands).

Motor/Efferent nervous system is divided into:-Organization of Nervous System

* AUTONOMIC(not automatic)

Examples:1. If you step on a nail, which nerves bring impulses from the foot to the spinal

cord…….afferent or efferent?? Which nerves take impulses from spinal cord to the thigh muscles…. afferent or efferent??

Thigh muscles contract to lift the foot…..is this somatic or autonomic??

2. If it gets very hot outside and your skin temperature starts to rise, which nerves bring impulses from the skin to the brain…….afferent or efferent??

Which nerves take impulses from brain to sudoriferous glands…. afferent or efferent?? Sudoriferous glands secrete sweat…..is this somatic or autonomic??Is this sympathetic or parasympathetic??

Afferent.

Efferent.Somatic.

Afferent.

Efferent.Somatic.

Sympathetic.

AFFERENT or EFFERENT or

* AUTONOMIC(not automatic)

Functions of Nervous SystemNervous system is a sensory system that detects changes in the

environment and helps to respond to the changes in order to maintain homeostasis.

Functions of nervous system:1. Sensory input: senses changes in external and internal

environment.A change is referred to as a stimulus.

2. Conduction: generates impulses and sends them across the body through the nerves.

3. Integration: composed of brain and spinal cord that act as integrative centers to analyze information and make a decision on the action.

4. Motor output: sends impulses through nerves to the effectors (muscles and glands) that respond/act.

5. Maintain homeostasis: help maintain pH, water, gases, temperature…….

6. Center for mental activities: controls thinking, memory, emotions.

Histology of Nervous Tissue

Nervous tissue is composed of two types of cells:NeuronNeuroglial cells

Neuroglial cell

Neuron

Neurons are nerve cells that could be 1 mm or 1 meter long.They generate/conduct impulses that travel at a speed of about 1-100 meters/secNumber of neurons increases till age 4 years.Neurons are composed of:1) Cell body/soma: the main part of the neuron- contains large round nucleus.

Perikaryon- cytoplasm surrounding the nucleus.Nissl bodies- clusters of rough endoplasmic reticulum….appear as dark stained patches(gray matter- regions containing neuron cell bodies).

2) Nerve processes: extensions of the cell body.

DendriteNissl bodies

AxonTelodendria

Nucleus

Direction of action potential

Axon terminals

Neurons - Structure

Nerve processes: two types.Dendrites- usually short, one-many per cell.

Receive stimulus and conduct impulses towards the cell body.

Axon- usually long, one per cell.Conduct impulses from the cell body to the next cell….could be another neuron or a muscle fiber or a gland.Arise from a cone-shaped extension (axon hillock) where sensory impulses from the dendrite are summated before being transmitted to the axon.Branch at the end smaller branches are called telodendria.Each telodendria may end in an axon terminal or a swelling…synaptic bulb/bouton.

DendriteNissl bodies

AxonTelodendria

Nucleus

Direction of action potential

Axon terminals

Axon hillock

Neurons - Structure

Neurons – Structural Classification

Multipolar neuron: have one axon and many dendrites…most common in brain and spinal cord.

Bipolar neuron: have one axon and one dendrite…found in sensory organs- eye, ear, nose.

Unipolar/Pseudounipolar neuron:-single elongated process…dendrites and axon are continuous (fused).. cell body lies off to one side...most sensory neurons of PNS are unipolar.

Anaxonic neuron: small and have numerous dendrites, but no axon…do not produce an impulse…function unknown…found in brain and special sense organs (eye).

Organization of Nervous SystemNervous system can be divide into:1) Central nervous system (CNS)- brain and spinal cord…the integrative center.2) Peripheral nervous system (PNS)- cranial and spinal nerves.

PNS can be divided into:2A) Sensory/afferent nervous system- bring impulses from the sensory receptors to the CNS. 2B) Motor/efferent nervous system-Take impulses away from CNS to the peripheral organs and tissues that respond (effectors).

Motor/Efferent nervous system is divided into:-

2Bi) Somatic nervous system- take impulses from CNS to voluntary effectors(skeletal muscles).

2Bii) Autonomic nervous system- take impulses from CNS to involuntary effectors(cardiac & smooth muscles; glands).

Autonomic nervous system is divided into:2Bii a) Sympathetic nervous system- cause increase/stimulation of the

effector…Fight or Flight response…to prepare the body for increasedphysical activity (increased cardiovascular & respiratory activity, increased sweating; decreased digestive activity).

2Bii b) Parasympathetic nervous system- cause decrease/inhibition of the effector…Rest and Digest…to bring the body back to resting state (decreased cardiovascular & respiratory activity; stimulation of digestive activity).

Neurons – Functional Classification

A. Sensory/Afferent neuron: bring impulses to CNS (brain or spinal cord)-Usually unipolar neurons.a) Somatic sensory neurons: bring impulses from skin and skeletal muscles to monitor external environment.b) Visceral sensory neurons: bring impulses from internal organs to monitor internal environment.

B. Motor/Efferent neuron: take impulses from CNS to the effectors that respond-usually multipolar neurons.a) Somatic motor neurons: take impulses to skeletal muscles.b) Visceral motor neurons: take impulses to internal organs.

C. Interneuron/Association neuron: connect sensory to motor neurons.Usually located in CNS….brain and spinal cord.

Sensory/AfferentNeuron

Motor/EfferentNeuron

Interneuron/Association

Neuron

Histology of Nervous Tissue

Nervous tissue is composed of two types of cells:NeuronNeuroglial cells

Neuroglial cell

Neuron

Neuroglial Cells

Neuroglial cells: also called glial cells.Smaller and 5-50 X more in number as compared to the neurons.As opposed to neurons that stop dividing, glial cells retain their capacity to multiply.Sometimes form brain tumors….gliomas.Present in CNS (brain and spinal cord)….4 types.Present in PNS (associated with nerves)….2 types.

Neuroglial cell

Neuron

1. Ependymal cells: - Lines the fluid (cerebrospinal fluid [CSF]) filled central canal of the spinal cord and

ventricles (CSF-filled chambers or cavities) of the brain.

- Forms a single layer of epithelial lining called ependyma.

- Cells are simple cuboidal to columnar shape, with cilia on surface.

- Helps in producing, monitoring and circulating CSF.

Neuroglial Cells – 4 Types in CNS-Ependymal cells

Neuroglial Cells – 4 Types in CNS-Astrocytes

2. Astrocytes: Largest and most numerous glia in CNS.- Helps to maintain blood-brain barrier (BBB; endothelial cells in blood vessels of

brain tightly fit together-act as a barrier between circulating blood in body and blood vessels in brain.

- Repair damaged neural tissue…if brain tissue is injured…..fill in to form scar tissue in damaged areas.

- Provides nutrients to neurons, helps neuronal survival.

- Absorb and recycle neurotransmitters.

Neuroglial Cells – 4 Types in CNS-Oligodendrocytes

3. Oligodendrocytes: cells with fewer processes and smaller cell bodies compared to astrocytes.

- Wrap around axons of neurons in brain and spinal cord form myelin sheathsprotect, insulate and speed conduction of nerve impulses.

Neuroglial Cells – 4 Types in CNS-Microglial cells

4. Microglial cells: - Least numerous and smallest neuroglia in CNS.- Migrate throughout the neural tissue. - Phagocytic cells…engulf pathogens….act as the first and main form of active immune

defense in the central nervous system.- Clear debris from tissue damage due to infections, stroke or injuries.Which type of neuroglia would increase in number in the brain tissue of a person with CNS infection?

CNS vs. PNS

Ganglion

Neuroglial Cells – 2 Types in PNS

1. Satellite cells: - function is similar to astrocytes in CNS.- surround, protect and provide nutrition for neuron cell bodies in ganglia.

Neuroglial Cells – 2 Types in PNS

2. Schwann cells: cells that wrap around axons that are part of a nerve (axons in PNS) form myelin sheaths protect, insulate and speed conduction.

Schwann cell

Myelin sheath

Axon

Axon

Schwann cells

Oligodendrocytes vs. Schwann cells

Similarities:- Both produce myelin sheath, insulate axons and speed upconduction of an action potential.

Differences:- Oligodendrocytes are seen in CNS.- Schwann cells are in PNS.

- A single oligodendrocyte can myelinate several adjacent axons.- A Schwann cell surrounds a small segment (about 1 mm) of a single axon

and many Schwann cells are needed to myelinate an axon depending on thelength.

Axon

Schwann cells

Myelination

Why do we need myelin sheaths??In an electrical cable, wires are plastic wrapped to insulate and

prevent current leakage.Nerve processes exist as bundles (tracts in CNS and nerves in PNS) they need to be insulated to prevent current leakage from axon and helps in electrical conduction along the length of the axon.

Which cells form myelin sheaths??Glial cells: Oligodendrocytes in CNS and Schwann cells in PNS.

How do they form myelin sheaths??They wrap around nerve processes form layers and layers of

plasma membrane wrapped around called myelin sheaths….gives a silvery white sheen to the tissue or nerve!

Dendrite

Myelinatedinternode

Axon

Myelin covering

internode

Nodes of

Ranvier

Schwann cells or

Oligodendrocyte

What is the function of myelin sheaths??Insulate and protect neurons and speed up conduction (Saltatory

conduction).

Myelinated vs. Unmyelinated Neurons

Myelinated neurons: have myelin sheaths wrapped around their axons or dendrites.Insulated and conduct faster.A series of Schwann cells (in PNS) or oligodendrocytes (in CNS) wrap around the axons to form thick myelin sheaths.Myelination is not continuous…have unmyelinated interruptions called nodes of Ranvier with myelinated internodes.

Neurons can be:MyelinatedUnmyelinated Dendrite

Myelinatedinternode

Axon

Myelin covering internode

Schwann cells or Oligodendrocyte

Nodes of Ranvier

Myelinated vs. Unmyelinated Neurons

Unmyelinated neurons: do not have myelin sheaths.Conduct slower.In CNS….oligodendrocytes do not completely cover unmyelinated axons.In PNS….a single Schwann cell enclose a group of unmyelinated axons.

Multiple sclerosis: Autoimmune disorder affecting CNSLoss of myelination in optic nerve, brain and spinal cord loss of vision, loss of muscle coordination, slurred speech, problems with digestive and urinary system movements.

Diphtheria: Caused by toxins from a bacteria destroys Schwann cells peripheral nerves are demyelinated sensory and motor problems…paralysis….can be fatal.Vaccine to prevent.

Guillain-Barre syndrome: Autoimmune disorder destruction of myelination in peripheral nerves weakening and tingling of extremities paralysis…can be fatal.

Myelination and Muscle Coordination

Demyelination: Progressive destruction of myelin sheath both in CNS and PNS…results in loss of sensation and motor control…affected regions become numb & paralyzed.

Remember: myelin sheaths wrap around the nerve fibers myelin is made up of 80% lipids and 20% proteins…lipids give the tissue a silvery white coloration.

Nerves are silvery white….made of a bundle of myelinated nerve fibers!

In brain and spinal cord, the tissue can be divided into:Gray matter: refers to the tissue with neuron cell bodies and unmyelinated nerve fibers-appears dusky

gray in color.Present towards the center of the spinal cord…where cell bodies and unmyelinated fibers are located.Present along the periphery of the brain….where nuclei….control centers with groups of cell bodies are located.

White matter: refers to the tissue with myelinated nerve fibers-appears glossy white.Present along the periphery in the spinal cord.Present towards the center in the brain.

Gray Matter vs. White Matter

- Neurons stop dividing at age 4 but glial cells retain the capacity to divide.

- Primary CNS tumors in adults- division of abnormal neuroglia rather than from the division of abnormal neurons.

- Primary CNS tumors involving abnormal neurons occur in young children.

- Secondary tumors arise from metastasis (spread of cancer cells that originate in other parts of the body)

In CNS:- very limited axon regeneration occurs after injury.- astrocytes form scar tissue that prevents axon growth across damaged area.- astrocytes release chemicals that blocks regrowth of axons.- slower cellular debris clearance impede axonal regrowth.

Injury Repair & Regeneration of Nervous TissueIf nervous tissue is damaged regeneration depends on the extent of injury and its

location.Most injuries in nervous tissue are permanent.

CNS Tumors

In PNS:If the cell body of the neuron is damaged no regeneration.

If an axon is damaged Wallerian degeneration process is triggered.The axon past the injury breaks down macrophages clean up Schwann cells multiply to form a pseudo-tunnel axon grows through the tunnel full recovery including synapse.

Injury Repair & Regeneration of Nervous Tissue