CH 11- Functional Organization of Nervous Tissue Pt 1.

Nervous Tissue- Cells of nervous tissue

• Neurons

• Glial Cells

Neurons

Electrically excitable cells of the nervous system

Glial Cells

Supportive cells

Functions of Nervous System

• Maintaining homeostasis

• Receiving sensory input

• Integrating information

• Controlling muscles and glands

• Establishing and maintain mental activity

Divisions of the Nervous System

• Central nervous system (CNS)

• Peripheral nervous system (PNS)

Central Nervous System (CNS)

• receives information from and sends information to the body

• decision maker

• consists of the brain and spinal cord 

Peripheral Nervous System (PNS)

• Detects stimuli in and around the body

• Carries information to the CNS and from the CNS to the body

• Consists of nerves, ganglia, and sensory receptors

Structure of the CNS

• Brain

  • housed in the skull

• Spinal Cord

  • housed in the vertebral columns

Structure of the PNS

Nerves

• collection of axons outside the brain and spinal cord

• can carry electrical signals away from or towards the CNS

• Cranial nerves- 12 pairs or nerves originating from the brain

• Spinal nerves- 31 pairs of nerves originating from the spinal cord

• Plexus- bundle of nerves outside the brain and spinal cord

Ganglia

• group of neuron cell bodies outside the brain and spinal cord

Sensory receptors

• cells that respond to a specific stimuli

• can be neurons or specialized cells

• distributed throughout the body

Divisions of the PNS

• Sensory (afferent) division

• Motor (efferent) division

Sensory (afferent) division

transmits electrical signals from receptors tot he CNS

Motor (efferent) division

• Transmits electrical signals from the CNS to effector organs

• Effector organs include muscle (skeletal, cardiac, smooth) and glands

Divisions of Motor Nervous System

• Somatic Nervous System

• Autonomic Nervous Sytem

Somatic Nervous System 

• voluntary division

• controls movement of skeletal muscles

Autonomic Nervous System

• involuntary division

• regulates contract of cardiac and smooth muscles and secretions of glands

Division of the Autonomic Nervous System 

• Sympathetic nervous system 

• Parasympathetic nervous system 

• Enteric nervous system 

Sympathetic nervous system

• prepares the body for physical activity

• “fight or flight”

Parasympathetic nervous system 

• regulates resting function (digesting food)

• “rest and digest”

Enteric nervous system

• neuronal networks in the wall of the digestive tract

Flow of the nervous system

Cells of Nervous Tissue

• Neurons

• Glial Cells

Neurons

• electrically excitable cells of the nervous system

• ~100 billion neurons in the body

Glial cells

• supportive cells

• 50% of the brain’s weight

• 10-50 times more glial cells than neurons in various brain regions 

Neuron Struture

• Cell body (soma)

  • nissl bodies

• Dendrites

  • dendritic spines

• Axons

  • axoplasm

  • axolemma

  • axon hillock

  • initial segment

  • trigger zone

  • presynaptic terminal

Cell body (Soma)

• Single, centrally located nucleus with nucleolus

• Nissl bodies- extensive rough endoplasmic reticulum 

• Abundant intermediate filaments (neurofilaments) and microtubules forming bundles int he cytoplasm

Dendrites

• Processes off the cell body

• Short, often highly branched

• Tapered from base to tip 

• Receive input from other neurons and other sensory receptors

• Dendritic spines

  • small extension on the surface where synapses are formed

Axons

• Single process off the cell body

• Constant diameter, varied length 

• Axoplasm

• Axolemma

• Axon hillock

• Initial segment

• Trigger zone

• Presynaptic terminal

Axoplasm

cytoplasm of the axon

Axolemma

plasma membrane of the axon

Axon hillock

cone shaped area coming off cell body

Initial segment

formed by the narrowing of the axon hillock

Trigger zone

axon hillock and initial segment, where action potentials are generated

Presynaptic terminal

region at the end of the axon that house synaptic vesicles storing neurotransmitters

• synapse

Synapse

point of contact between the axon ending and its effector

Fucntional classes of neurons are based on:

the direction of action potential conduction

Types of functional classes of neurons

• Sensory (afferent) neurons

• Motor (efferent) neurons

• Interneurons

Sensory (afferent) neurons

conduct action potential toward CNS

Motor (efferent) neurons

conduct action potentials away from the CNS toward muscles or glands

Interneurons

conduct action potentials within the CNS

Structural classes of neurons

based on the number of dendrites

Types of structural classes of neurons:

• Multipolar neurons

• Bipolar neurons

• Pseudo-unipolar neurons

• Anaxonic neurons

Multipolar neurons

• many dendrites and a single axon

• dendrite number vary with varying branching 

• motor neurons of the PNS and most neurons within the CNS

Bipolar neurons

• one dendrite and one axon

• dendrites are often specialized to receive stimulus 

• axons conduct action potentials

• located in sensory organs (retina of the eye, nasal cavity)

Pseudo-unipolar neurons

• single process exits teh cell body and divides into two branches that work as a single axon

• peripheral process

  • extends to the periphery and has dendrites that act as sensory receptors or communicate with sensory receptors

• central process 

  • extend to the CNS

• most sensory neurons

Anaxonic neurons

• do not have axons, only dendrites

• found within the brain and retina

• communicate using only graded potentials

Glial Cells of the CNS

• Astrocytes

• Ependymal cells

• Microglia

• Oligodendrocytes

Glial Cells of the CNS: Astrocytes

• Cytoplasmic processes extending from cell body

• Foot processes cover blood vessels, neurons, and pia mater

• Regulate the composition of extracellular brain fluid

  • produce chemicals that promote formation of tight junction between endothelial cells of capillaries to form the blood-brain barrier

• Blood-brain barrier

• Play a role in response to tissue damage in CNS

  • Reactive astrocytosis

• Promote development of synapses and help regulate synaptic activity by synthesizing, absorbing, and recycling neurotransmitters

Blood brain barrier

• controls substances that pass from blood into the brain and spinal cord

• protects neurons from toxins 

• lets nutrients and waste products to be exchanged

• prevents fluctuations in blood composition

Reactive astrocytosis

• caused by injuries in the CNS

• astrocytes wall off injury site

• limit spread of inflammation

• limit regeneration of axons on injured neurons

Glial Cells of the CNS: Ependymal Cells

• Line ventricles of the brain and central canal of the spinal cord 

• Choroid plexuses

  • specialized ependymal cells and blood vessels located in regions of the ventricles

  • secretes cerebrospinal fluid

• May of cilia to move CSF

• Extension of the basal surface extedn deep into brain and spinal cord

Glial Cells of the CNS: Microglia

• CNS specific immune cells

• Become mobile and phagocytic in response to inflammation

  • phagocytize necrotic tissue, microorganisms, and other foreign substances

Glial Cells of the CNS: Oligodendrocytes

• Form the myelin sheath 

• Cytoplasmic extension wrap around multiple axons

• Insulate axons

Glial Cells of the PNS

• Schwann cells

• Satellite cells

Schwann cells

• Form the myelin sheath 

• Schwann cell wraps around only one axon

• Neurilemma

  • outermost layer of each Schwann cell

  • contains majority of Schwann cell cytoplasm, nucleus, and organelles

Satellite cells

• Surround neuron cell bodies in sensory and autonomic ganglia

• Give support and nutrition

• Protects neurons from heavy metal poisons

Myelinated Axons 

• Schwann cells (PNS) or oligodendrocytes (CNS) wrap around axons

• Forms layers of phospholipids with small amounts of cytoplasm 

• Give myelinated axons a white appearance 

• Nodes of Ranvier- gaps in the myelin sheath 

  • Schwann cells or oligodendrocytes extend across and connect

• Protect and electrically insulate the axons

Unmyelinated Axons

• Not devoid of myelin

• Axons rest in invaginations of Schwann cells or oligodendrocytes

• Protects axons

Development of Myelin Sheath

• begins in late fetal development

• continues rapidly until end of first year after birth 

• slows and continues after

Multiple sclerosis

• chronic disease of the CNS 

• gradual loss of myelin sheath

• slows action potential transmission

• impairs control of skeletal and smooth muscle