Ch 12. Nervous Tissue Notes

Introduction to Nervous Tissue

• The nervous system maintains homeostasis by controlling conditions within healthy limits.

• This chapter will cover the different branches of the nervous system.

• We will identify and describe the various types of cells found in nervous tissue.

Overview of the Nervous System

• The nervous system has sensory, integrative, and motor functions.

• Sensory functions detect changes through sensory receptors.

• Integrative functions analyze sensory information, store aspects, and make decisions.

• Motor functions respond to stimuli via effectors.

Layout of the Nervous System

• The nervous system is divided into two main divisions:

  • Central Nervous System (CNS): Brain and spinal cord.

  • Peripheral Nervous System (PNS): Cranial nerves, spinal nerves, enteric plexuses, and sensory receptors.

Organization of the Nervous System

• The Central Nervous System (CNS) integrates sensory input and initiates motor output.

• The Peripheral Nervous System (PNS) is divided into:

  • Sensory (afferent) division: somatic and special senses.

  • Motor (efferent) division: somatic nervous system, autonomic nervous system, and enteric plexuses.

  • Somatic nervous system: controls skeletal muscle.

  • Autonomic nervous system: controls smooth muscle, cardiac muscle, and glands.

    • Sympathetic division

    • Parasympathetic division

  • Enteric plexuses: control smooth muscle and glands of the digestive canal.

Histology of Nervous Tissue

• Nervous tissue consists of:

  • Neurons: Electrically excitable cells that transmit nerve impulses (action potentials).

  • Neuroglia: Support, protect, and maintain neurons.

Neurons

• Neurons are electrically excitable cells.

• A nerve impulse is called an action potential.

Structural Classification of Neurons

• Neurons are classified by the number of processes extending from the cell body.

Functional Classification of Neurons

• Neurons are classified based on the direction of nerve impulse propagation.

  • Sensory (afferent) neurons: convey information to the CNS.

  • Motor (efferent) neurons: convey action potentials from the CNS to effectors (muscles or glands).

  • Interneurons (association neurons): process sensory information and elicit a motor response; usually multipolar.

Neuroglia

• Neuroglia are not electrically excitable.

• They make up about half the volume of the nervous system.

• They can multiply and divide.

• There are 6 kinds total (4 in CNS, 2 in PNS).

Neuroglia of the CNS

• Astrocytes (protoplasmic and fibrous)

• Oligodendrocytes - produce myelin

• Microglial cells

• Ependymal cells - produces spinal fluid

Neuroglia of the PNS

• Schwann cells - produce myelin

• Satellite cells

Myelination of Neurons

• The myelin sheath is produced by Schwann cells (PNS) and oligodendrocytes (CNS).

• It surrounds the axons of most neurons.

Gray Matter vs. White Matter

• Gray matter contains neuronal cell bodies, dendrites, unmyelinated axons, and neuroglia. - on the outside in the brain and inside with the spine

• White matter contains myelinated axons.

Electrical Signals in Neurons

• Excitable cells communicate via action potentials (AP) or graded potentials (GP).

• Action potentials (AP) allow communication over short and long distances.

• Graded potentials (GP) allow communication over short distances only.

• Production of an AP or GP depends on:

  • Resting membrane potential

  • Existence of certain ion channels

Ion Channels in Neurons

• Leak channels: alternate between open and closed; K+ channels are more numerous than Na^+ channels.

• Ligand-gated channels: respond to chemical stimuli (ligand binds to receptor).

• Mechanically-gated channels: respond to mechanical vibration or pressure stimuli.

• Voltage-gated channels: respond to direct changes in membrane potential.

Resting Membrane Potential

• The membrane of a non-conducting neuron is positive outside and negative inside.

• Determined by:

  1. Unequal distribution of ions across the plasma membrane and the selective permeability of the neuron’s membrane to Na^+ and K^+.

  2. Most anions cannot leave the cell.

  3. Na^+/K^+ pumps.

Factors Contributing to Resting Membrane Potential

• Unequal distribution of ions (e.g., sodium, potassium, chloride, phosphate).

• Selective permeability of the membrane to Na^+ and K^+ through leak channels.

• Most anions cannot leave the cell.

• Sodium-potassium ATPase (pump) maintains the resting membrane potential by pumping 3 Na^+ ions out and 2 K^+ ions into the cell.

Graded Potentials

• Small deviations in resting membrane potential.

• Occur in response to the opening of a mechanically-gated or ligand-gated ion channel.

• The amplitude of a graded potential depends on the stimulus strength.

• Graded potentials can be added together to become larger in amplitude.

Action Potentials

• An action potential is a sequence of rapidly occurring events that decrease and eventually reverse the membrane potential and eventually restore it to the resting state.

• Action Potentials have two phases:

  • Depolarization

  • Repolarization

• Action potentials can only occur if the membrane potential reaches threshold.

Action Potentials: the Status of Na^+ and K^+ Voltage-Gated Channels

• Voltage-gated Na^+ channels open during the depolarizing phase.

• Voltage-gated K^+ channels open during the repolarizing phase.

Comparison of Graded & Action Potentials

Propagation of Action Potentials

• Action potentials must travel from the trigger zone to the axon terminals for communication to occur.

• This traveling is called propagation.

• Action potentials do not die out; they keep their strength as they spread across the membrane of a neuron.

Continuous vs. Saltatory Conduction

• Continuous conduction: Step-by-step depolarization of each adjacent area of the plasma membrane; occurs in unmyelinated axons.

• Saltatory conduction: Nerve impulse jumps from neurofibril node (node of Ranvier) to node; occurs in myelinated axons; faster than continuous conduction.

Factors That Affect Propagation Speed

1. Axon diameter

   • Larger diameter axons propagate APs faster.

2. Amount of myelination

   • Myelin increases speed of AP propagation.

3. Temperature

   • Higher temperature increases speed of AP propagation.

Signal Transmission at Synapses

• A synapse is the junction between neurons or between a neuron and an effector.

  • Electrical Synapse

    • Gap junctions connect cells and allow the transfer of information to synchronize the activity of a group of cells

  • Chemical Synapse

    • One-way transfer of information from a presynaptic neuron to a postsynaptic neuron

Events at the Synapse

1. Nerve impulse arrives at synaptic end bulb of presynaptic neuron.

2. Depolarization opens voltage-gated Ca^{2+} channels, and Ca^{2+} flows inward.

3. Increase in Ca^{2+} inside the presynaptic neuron triggers exocytosis of synaptic vesicles.

4. Neurotransmitter molecules are released into the synaptic cleft.

5. Neurotransmitter binds to receptors on the postsynaptic neuron.

6. Opening of ligand-gated channels results in an excitatory or inhibitory postsynaptic potential (EPSP or IPSP).

7. When summation of EPSPs and IPSPs causes depolarization to threshold, an action potential (nerve impulse) is generated.

Postsynaptic Potentials

• Excitatory postsynaptic potentials (EPSP): A depolarizing postsynaptic potential.

• Inhibitory postsynaptic potentials (IPSP): A hyperpolarizing postsynaptic potential.

• A postsynaptic neuron can receive many signals at once.

Structure of Neurotransmitter Receptors

• Neurotransmitters at chemical synapses cause either an excitatory or inhibitory graded potential

• Neurotransmitter receptors have two structures

  • Ionotropic receptors

    • Contains a neurotransmitter binding site and ion channel

  • Metabotropic receptors

    • Contains a neurotransmitter binding site and is coupled ot a separate ion channel by a G protein

Removal of Neurotransmitter

• Neurotransmitter can be removed from the synaptic cleft by:

  • Diffusion

  • Enzymatic degradation

  • Uptake into cells

Summation

• If several presynaptic end bulbs release their neurotransmitter at about the same time, the combined effect may generate a nerve impulse due to summation.

• Summation may be spatial or temporal.

• Spatial summation: summation of postsynaptic potentials in response to stimuli that occur at different locations in the postsynaptic neuron at the same time.

• Temporal summation: summation of postsynaptic potentials in response to stimuli that occur at the same location in the postsynaptic neuron but at different times.

Summary of Neuronal Structure and Function

Neurotransmitters

• Small molecule neurotransmitters

  • Acetylcholine

  • Amino acids

  • Biogenic amines

  • ATP and other purines

  • Nitric oxide

  • Carbon monoxide

• Neuropeptides

  • Substance P

  • Enkephalins

  • Endorphins

  • Dynorphins

  • Hypothalamic releasing and inhibiting hormones

  • Angiotensin II

  • Cholecystokinin

Neuropeptides

Neural Circuits

• A neural circuit is a functional group of neurons that process specific types of information.

• Types of circuits

  • Simple series

  • Diverging

  • Converging

  • Reverberating

  • Parallel after-discharge

Regeneration & Repair of Nervous Tissue

• Although the nervous system exhibits plasticity, neurons have a limited ability to regenerate themselves

  • Plasticity – the capability to change based on experience

  • Regenerate – the capability to replicate or repair

Neurogenesis in the CNS

• In the CNS, there is little or no repair due to:

  • Inhibitory influences from neuroglia, particularly oligodendrocytes

  • Absence of growth-stimulating cues that were present during fetal development

  • Rapid formation of scar tissue

Damage and Repair in the CNS

• In the PNS repair is possible if the cell body is intact, Schwann cells are functional, and scar tissue formation does not occur too rapidly

• Steps involved in the repair process are:

  • Chromatolysis

  • Wallerian degeneration

  • Formation of a regeneration tube

Neural Disorders – Multiple Sclerosis

• Autoimmune disease that causes progressive destruction of myelin sheath

• Cause is unclear; may be genetic and/or environmental

• Symptoms include muscle weakness, abnormal sensations and double vision

Neural Disorders – Depression

• Several types of depression

  • Major depression

  • Dysthymia

  • Bipolar depression (manic-depressive illness)

  • Seasonal affective disorder (SAD)

• Common feelings are lack of interest in activities, sadness, helpless and possibly suicidal thoughts

• Commonly treated through selective serotonin reuptake inhibitors (SSRIs)

Other Neural Disorders

• Epilepsy

  • Short, recurrent attacks of motor, sensory or psychological function

  • Initiated by abnormal synchronous electrical discharges from the millions of neurons in the brain

• Excitotoxicity

  • Destructions of neurons through prolonged activation of excitatory synaptic transmission

  • Caused by high levels of glutamine in CNS interstitial fluid