CHAPTER 14 Nervous Tissue

CHAPTER 14CHAPTER 14

Nervous TissueNervous Tissue

Common course objectives:

1.1. Functions of the nervous systemFunctions of the nervous system

2.2. Organization of the nervous systemOrganization of the nervous system

3.3. Nerve tissue and nerve cell typesNerve tissue and nerve cell types

4.4. Structure of a typical neuronStructure of a typical neuron

5.5. Structure of a chemical synapseStructure of a chemical synapse

Nervous Tissue Histology

Composed of: Neurons are true conducting cells in nervous

tissue

Neuroglial (supporting) cells-Astrocytes -Schwann cells-Oligodendrocytes -Satellite

cells-Microglia-Ependymal

The Nervous The Nervous SystemSystem Defined: Defined: like the CPU of a computer, the like the CPU of a computer, the

nervous system is the nervous system is the master controlling master controlling systemsystem of the body. It is designed to of the body. It is designed to constantly and rapidly adjust and respond constantly and rapidly adjust and respond to stimuli the body receives. It includes the to stimuli the body receives. It includes the brain, cranial nerves, spinal cord, and brain, cranial nerves, spinal cord, and associated peripheral nerves. associated peripheral nerves.

Divisions of the nervous sytem:Divisions of the nervous sytem: CNS = Brain + spinal cordCNS = Brain + spinal cord PNS = Cranial nerves (12) + Spinal nerves (31 PNS = Cranial nerves (12) + Spinal nerves (31

pairs)pairs) PNS = ANS, SS and SMSPNS = ANS, SS and SMS

Properties of NeuronsProperties of Neurons

1.1. ExcitabilityExcitability (irritability): ability to respond to (irritability): ability to respond to environmental changes or stimuli.environmental changes or stimuli.

2.2. ConductivityConductivity: respond to stimuli by initiating : respond to stimuli by initiating electrical signals that travel quickly to other electrical signals that travel quickly to other cells at distant locations.cells at distant locations.

3.3. SecretionSecretion: Upon arrival of the impulse at a : Upon arrival of the impulse at a distant location the neuron usually secretes a distant location the neuron usually secretes a chemical neurotransmitter at a synapse that chemical neurotransmitter at a synapse that crosses the synaptic gap and stimulates the next crosses the synaptic gap and stimulates the next cell.cell.

NeuronsNeurons

Nerve cell Nerve cell proper:proper:

Cell body Cell body (soma)(soma)

Dendrites - Dendrites - TOTO

Axons -FROMAxons -FROM

Functional Classes of Neurons

Sensory (afferent) neuronsSensory (afferent) neurons – afferent neurons – afferent neurons are specialized to detect stimuli and transmit the are specialized to detect stimuli and transmit the information to CNS. They begin in any organ in information to CNS. They begin in any organ in the body, but end in the brain or spinal cord.the body, but end in the brain or spinal cord.

Interneurons (association neurons):Interneurons (association neurons): lie lie entirely in the CNS. They receive signals from entirely in the CNS. They receive signals from many different neurons and perform an many different neurons and perform an integrative function “decision making” to respond integrative function “decision making” to respond to the different stimuli. to the different stimuli.

Motor (efferent) neuronsMotor (efferent) neurons – – efferent neurons efferent neurons transmit the appropriate response from the transmit the appropriate response from the interneuron to an end organ (muscle and gland interneuron to an end organ (muscle and gland cells) to carry out the body’s response to the cells) to carry out the body’s response to the stimuli.stimuli.

Functional Classification of Functional Classification of NeuronsNeurons

Based on the direction of conductionBased on the direction of conduction Sensory or afferent conduct toward the CNS Sensory or afferent conduct toward the CNS

~ 100 million~ 100 million Motor or efferent conduct away from the Motor or efferent conduct away from the

CNS ~ 500,000CNS ~ 500,000 Interneuron interposed between sensory and Interneuron interposed between sensory and

motor ~ 500 billionmotor ~ 500 billion

Organization of the Nervous System

Two main divisions: The Central Nervous System (CNS)

- Consists of the brain and spinal cord with tracts and nucleiNucleus = a collection of nerve cell bodies in the CNS.Tract = bundle of nerve fibers within the CNS

The Peripheral Nervous System (PNS) -Consists of ganglia, cranial nerves, spinal nerves and peripheral receptorsGanglia = a collection of nerve cell bodies in the PNSNerve = bundle of nerve fibers in the PNS

Organization of the Nervous System

Functional divisions of nervous system

CNSCNS = brain = brain and spinal and spinal cordcord

Peripheral Nervous System (PNS)

Composed of cranial nerves and spinal Composed of cranial nerves and spinal nerves and their branches, ganglia and nerves and their branches, ganglia and sensory receptors. sensory receptors.

PNS PNS is subdivided into sensory and is subdivided into sensory and motor divisions:motor divisions: somatic nervous system (SNS)somatic nervous system (SNS) autonomic nervous system (ANS) and autonomic nervous system (ANS) and

thethe enteric nervous system (ENS)enteric nervous system (ENS)

Sensory or Afferent Division

Somatic sensory = = senses touch, senses touch, pressure, pain, temperature, vibration pressure, pain, temperature, vibration and proprioception in skin, body wall and and proprioception in skin, body wall and limbs.limbs.

Visceral sensory = Autonomic = Autonomic sensory division-sensory division- sensessenses stretch, pain, stretch, pain, temperature, chemical changes and temperature, chemical changes and irritation in viscera; nausea and hungerirritation in viscera; nausea and hunger. .

Motor or Efferent Division

Somatic motor--motor control to all skeletal muscles except motor control to all skeletal muscles except pharyngeal muscles.pharyngeal muscles.

Visceral Motor = Autonomic Nervous = Autonomic Nervous SystemSystem-Sensory receptors convey information from -Sensory receptors convey information from visceral organs (e.g. heart, lungs, intestines, visceral organs (e.g. heart, lungs, intestines, etc.) to the CNS for integration and etc.) to the CNS for integration and interpretation. interpretation. -A motor response is initiated that conducts -A motor response is initiated that conducts impulses from CNS to smooth muscle, cardiac impulses from CNS to smooth muscle, cardiac muscle and/or glands for appropriate responsemuscle and/or glands for appropriate response

Autonomic Nervous System

Two divisions of ANSTwo divisions of ANS Sympathetic division – Fight or Sympathetic division – Fight or

FlightFlight

Parasympathetic division – Food or Parasympathetic division – Food or SexSex

CNS/PNS summary

Structural Classification of Neurons

Neurons may be: Multipolar, Bipolar or Unipolar

Determined by the number of processes attached to the cell body

Neurons

Most (99%) neurons in the body are multipolar.

Bipolar neurons are rare and occur in special sense organs of ear, nose and eye.

Unipolar neurons begin as bipolar but processes fuse into one. They are primarily sensory neurons.

ex. dorsal root ganglion

Neuroglia cells

Found in CNS and PNS Perform a supporting function for

neurons CNS PNS

Oligodendrogliocytes Schwann cells Astrocytes Satellite cells Ependymal cells Microglia

Neuroglia cells

Oligodendrogliocytes -CNS

Form myelin sheath in CNS Fewer branches than astrocytes

Myelin

Insulating layer around a nerve Formed by oligodendrocytes in CNS and

Schwann cells in PNS Composed of a lipoprotein with

phospholipids, glycolipids and cholesterol. Myelination is the process of myelin

formation Myelin allows nerve conduction to be 150

x faster than nonmyelinated nerves. This occcurs by “Saltatory conduction” and the impulse jumps from Node to Node.

Microglia - CNS

Thorny bushes in appearance and the smallest glia

Phagocytic function in CNS Originate from monocytes

Astrocytes - CNS

Star shapedStar shaped Most Most numerous numerous

Blood brain barrierBlood brain barrier

Ependymal cells - CNS Epithelial cells that line ventricles and central Epithelial cells that line ventricles and central

cavities of brain and spinal cord-secrete CSFcavities of brain and spinal cord-secrete CSF Ciliated to help circulate CSFCiliated to help circulate CSF

Schwann cells- PNS Form myelin sheath around peripheral Form myelin sheath around peripheral

axonsaxons Look like jelly roll with neurolemma Look like jelly roll with neurolemma

covercover Node of Ranvier separates each Schwann Node of Ranvier separates each Schwann

cellcell

Myelin and Unmyelinated fibers

Nerve conduction Nerve conduction velocityvelocity Velocity is dependent on size and Velocity is dependent on size and

myelination.myelination.

Satellite cells -PNS Surround neuron cell bodies within Surround neuron cell bodies within

gangliaganglia Provide nutrients, remove metabolites Provide nutrients, remove metabolites

etc.etc.

Nerve structureNerve structure

Nerves are only in the peripheryNerves are only in the periphery Cable-like organs in PNS = cranial and spinal Cable-like organs in PNS = cranial and spinal

nervesnerves Consists of 100’s to 100,000’s of myelinated and Consists of 100’s to 100,000’s of myelinated and

unmyelinated axons (nerve fibers).unmyelinated axons (nerve fibers). EndoneuriumEndoneurium surrounds each axon (nerve fiber). surrounds each axon (nerve fiber). Axons are grouped into bundles of Axons are grouped into bundles of fasciclesfascicles PerineuriumPerineurium surrounds each fascicle surrounds each fascicle Epineurium Epineurium surrounds each nerve bundlesurrounds each nerve bundle Conduction is saltatory (i.e. jumps node to node) Conduction is saltatory (i.e. jumps node to node)

in myelinated nerves and continuous in in myelinated nerves and continuous in nonmyelinated.nonmyelinated.

Nerve anatomyNerve anatomy

Synapse

The connection between 2 or more The connection between 2 or more nerves and they are separated by a nerves and they are separated by a space or cleft. space or cleft.

Synaptic terminology

SynapseSynapse – site where two nerves – site where two nerves communicate with each other.communicate with each other.

Presynaptic neuronPresynaptic neuron – neuron that is – neuron that is conducting information toward the next conducting information toward the next neuronneuron

Postsynaptic neuronPostsynaptic neuron – transmits information – transmits information away from synapseaway from synapse

Most synaptic communication is via Most synaptic communication is via chemical messengerschemical messengers (e.g. acetylcholine, (e.g. acetylcholine, serotonin, norepinephrine, dopamine, serotonin, norepinephrine, dopamine, endorphins, GABA, glycine, glutamic acid, endorphins, GABA, glycine, glutamic acid, etc.)etc.)

Neurotransmission

Chemical (99%) Electrical (1%)

Types of synapses

Axodendritic = axon to dendriteAxodendritic = axon to dendrite

Axosomatic = axon to cell bodyAxosomatic = axon to cell body

Axoaxonic = axon to axonAxoaxonic = axon to axon

Types of synapses

Types of synapses

Types of Neuronal Integration

Neurons form many different types of Neurons form many different types of connections and in so doing can result in connections and in so doing can result in finite control over the neuronal circuits. finite control over the neuronal circuits.

Such pathways may create converging, Such pathways may create converging, diverging or reverberating circuits as is diverging or reverberating circuits as is shown in the next slide.shown in the next slide.

Such circuits may produce EPSP’s or Such circuits may produce EPSP’s or IPSP’s and help modulate the neuronal IPSP’s and help modulate the neuronal signals.signals.

Types of Neuronal Circuits

Axonal regeneration Nerve tracts in the CNS are incapable of Nerve tracts in the CNS are incapable of

regeneration on their own and there may be regeneration on their own and there may be hope for stem cells carrying out this process.hope for stem cells carrying out this process.

In the PNS, nerves can regenerate but vey slowly In the PNS, nerves can regenerate but vey slowly and under only ideal conditions. Regeneration is and under only ideal conditions. Regeneration is dependent on 3 things: (a). Amount of damage, dependent on 3 things: (a). Amount of damage, (b). Neurolemocyte secretion of nerve growth (b). Neurolemocyte secretion of nerve growth factor and (c). The distance from the site of the factor and (c). The distance from the site of the damage to the end organ being reinnervated.damage to the end organ being reinnervated.

Regeneration occurs at a rate of ~ 1 to 5 mm/day.Regeneration occurs at a rate of ~ 1 to 5 mm/day.

Neuronal regeneration in the PNS

Axonal regeneration

Unfortunately, even Unfortunately, even “Superman” “Superman”

(Christopher Reeve) succumbed (Christopher Reeve) succumbed

to a severe spinal cord injury. to a severe spinal cord injury.

But maybe in the not too distant But maybe in the not too distant

future stem cells will lead to a future stem cells will lead to a

partial cure for these partial cure for these individuals. individuals.