Nervous System and Tissue Ch 13

• Chapter 13 focuses on the nervous system and nervous tissue.
• Steven Bassett from Southeast Community College is the presenter.

Introduction

• The nervous system controls and adjusts the body's activities.
• Provides swift, but brief responses.

Overview of the Nervous System

• Nervous tissue consists of the:
• Central Nervous System (CNS)
• Peripheral Nervous System (PNS)
• The CNS is associated with the brain and spinal cord.
• The PNS is associated with tissue outside the CNS.
• The CNS is responsible for integrating, processing, and coordinating sensory input.
• Also responsible for integrating, processing, and coordinating motor output.
• The CNS is the seat of intelligence, memory, learning, and emotion.
• The PNS provides sensory information to the CNS and carries motor commands away from the CNS.
• PNS can be divided into afferent and efferent divisions.
• The afferent division brings sensory information to the CNS, starting at the receptors.
• The efferent division carries motor commands to muscles and glands, ending at the effectors.
• The PNS can be further subdivided into afferent and efferent divisions.
• The afferent division can be further subdivided into somatic and visceral sections.
• The efferent division can be further subdivided into the somatic nervous system (SNS) and autonomic nervous system (ANS).
• The autonomic nervous system can be further divided into the parasympathetic and sympathetic divisions.

Functional Overview of Nervous System

• CNS consists of the brain and spinal cord.
• Sensory information passes through the afferent division.
• Information processing happens after sensory input.
• Motor commands pass through the efferent division.
• PNS consists of nervous tissue outside the CNS.
• The efferent division includes the somatic nervous system (SNS) and the autonomic nervous system (ANS).
• The ANS includes the parasympathetic and sympathetic divisions.
• Special sensory receptors monitor smell, taste, vision, balance, and hearing.
• Visceral sensory receptors monitor cardiovascular, respiratory, digestive, urinary, and reproductive systems.
• Somatic sensory receptors monitor skeletal muscles, joints, body position, skin surface and provide position sense, touch, pressure, pain, and temperature sensations.
• Skeletal muscles, smooth muscle, cardiac muscle, glands, and adipose tissue are all effectors.

Cellular Organization in Nervous Tissue

• Nervous tissue consists of two cell types:
• Neurons: responsible for the transfer and processing of information.
• Neuroglia: supporting cells that protect the neuron.
• Neurons consist of a soma (cell body), axon, dendrites, perikaryon, dendritic spines, and axon terminals.

Neuron Structure and Function

• Dendrites are stimulated by environmental changes or activities of other cells.
• The cell body contains the nucleus, mitochondria, ribosomes, other organelles, and inclusions.
• The axon conducts nerve impulses (action potential) toward axon terminals.
• Axon terminals affect other neurons or effector organs (muscle or gland).
• Neurons consist of:
• Perikaryon: neuron cytoplasm
• Neurofilaments/Neurotubules: make up the neuron cytoskeleton
• Neurofibrils: bundles of neurofilaments
• Nissl bodies: clusters of free ribosomes
• Axon hillock: area where the initial segment connects to the cell body
• Axoplasm: cytoplasm of the axon
• Collaterals: branches off the axon
• Telodendria: ends of the axon and collaterals
• Axon terminals: ends of the telodendrias
• Axoplasmic transport: movement of material between the cell body and the axons

Neuroglia

• Neuroglia of the CNS:
• Astrocytes: largest and most numerous, make up the blood-brain barrier (BBB)
• Oligodendrocytes: form myelin sheath, internodes, and myelin sheath gaps.
• Areas with mostly myelinated axons are called white matter.
• Areas devoid of myelinated axons are called gray matter.
• Gray matter is also composed of cell bodies and dendrites.
• Microglia: Phagocytic cells
• Ependymal cells: Involved in cerebrospinal fluid (CSF) production and makes up a cellular lining called the ependyma.
• Neuroglia of the PNS:
• Cell bodies in the PNS are clustered in ganglia.
• Axons are bundled together to form peripheral nerves.
• Satellite cells surround cell bodies and schwann cells surround axons
• Schwann cells are also called neurolemmocytes and consist of the axolemma/neurolemma.
• The central nervous system contains:
  • Astrocytes, oligodendrocytes, microglia, and ependymal cells.
• Astrocytes maintain the blood-brain barrier, provide structural support, regulate ion/nutrient/dissolved gas concentrations, absorb and recycle neurotransmitters, and form scar tissue after injury.
• Oligodendrocytes myelinate CNS axons and provide structural framework.
• Microglia remove cell debris/wastes/pathogens by phagocytosis.
• Ependymal cells line ventricles (brain) & central canal (spinal cord) and assist in producing, circulating, and monitoring cerebrospinal fluid.
• Peripheral nervous system contains satellite cells and schwann cells.
• Satellite cells surround neuron cell bodies in ganglia and regulate O2, CO2, nutrient, and neurotransmitter levels.
• Schwann cells surround all axons in the PNS, myelinate peripheral axons, and participate in the repair process after injury.
• Axolemma is the plasma membrane of the axon

Myelination

• A schwann cell first encloses a segment of the axon within a groove of its cytoplasm when myelinating a peripheral axon.
• The schwann cell then rotates around the axon.
• The inner membranous layers are compressed and the cytoplasm is forced into superficial layers as the schwann cell rotates.
• The myelin sheath consists only of the phospholipid bilayers of the plasma membrane, with the Schwann cell nucleus and cytoplasm at the surface when completed.
• Each Schwann cell myelinates only about 1 mm of an axon; many Schwann cells are needed to myelinate an entire axon.
• The portion of a myelinated nerve axon between 2 successive Schwann cells is an internode, and the small gaps that separate internodes are nodes, or nodes of Ranvier.
• A single Schwann cell shields multiple unmyelinated axons within superficial grooves.
• Chains of Schwann cells are needed to enclose an entire axon.

Neuron Classification

• Can be classified based on structure or function.

Structural Classification

• Based on the number of processes extending from the cell body.
  • Anaxonic: Has many processes but cannot differentiate between axons and dendrites and is found only in the CNS.
• Bipolar: The cell body is between the dendrite and axon. Axons are not myelinated.
• Pseudounipolar: The cell body is off to one side of the axon.
• Multipolar: Typically has a single axon and multiple dendrites and is the most common type in the CNS.

Functional Classification

• Sensory, motor, and interneurons

Functional Classification of Neurons

• Sensory Neurons (afferent division):
  • Axons are afferent fibers, send information from the PNS to the CNS, and include somatic sensory neurons and visceral sensory neurons.
  • Interoceptors monitor internal organ activity.
  • Exteroceptors provide information about the external environment.
  • Proprioceptors monitor position and movement.
• Motor Neurons:
  • Axons are efferent fibers which send information from the CNS to the periphery.
  • Consist of two divisions; -Somatic Nervous System (SNS): consists of somatic motor neurons. -Autonomic Nervous System (ANS): consists of visceral motor neurons and also consists of preganglionic and postganglionic fibers.
• Interneurons:
  • Located entirely in the CNS and situated between the motor and sensory neurons.
  • They analyze sensory input, coordinate motor outputs and can be excitatory or inhibitory.

Regeneration of Nervous Tissue

• Steps involved in Wallerian degeneration and nerve regeneration after fragmentation of the axon and myelin in the distal stump.
• Schwann cells form a cord, grow into the cut, unite stumps as macrophages engulf degenerating axon and myelin.
• The axon sends buds into network of Schwann cells and then starts growing along cord of Schwann cells, continuing to grow into distal stump and is enfolded by schwann cells.

The Nerve Impulse

• Excitability is the ability to conduct the impulse.
• A nerve impulse is known as the action potential of a nerve.
• Membrane potential: the uneven distribution of positive and negative ions across the plasma membrane.
• A stimulus creates a change in membrane permeability.
• Threshold stimulus: the level of stimuli to cause a change in membrane permeability.
• The impulse will propagate the length of the axon.
• The speed of the impulse depends on presence of a myelin sheath (fast impulse) or lack of a myelin sheath (slow impulse).
• Myelinated axons with a large diameter have fast impulses (up to 140 m/sec).
• Unmyelinated axons with a small diameter have slow impulses (less than 1 m/sec).

Synaptic Communication

• A synapse is the junction between a neuron and another cell.
• Two major types of synapses:
  • Chemical (vesicular) and Electrical (nonvesicular).
• Impulses are conveyed in one direction only in chemical synapses.
  • Action potential arrives at the presynaptic membrane.
  • Triggers the release of a neurotransmitter from the axon vesicles.
  • The neurotransmitter diffuses across the synapse.
  • The neurotransmitter binds to the postsynaptic membrane.
  • Binding action causes a change in permeability of the postsynaptic membrane.
  • Change in permeability results in an action potential of the next neuron.
• Impulses can be conveyed in any direction in electrical synapses.
  • The presynaptic membrane of one neuron is tightly bound to the postsynaptic membrane of another neuron.
  • Binding permits the passage of ions from one neuron to the next.

Neuron Organization and Processing

• Neurons can be organized into smaller organized groups called neuronal pools
• The neuronal pools are identified by their neural circuits such as:
  • Divergence: The spread of information from one neuron to several neurons. It permits broad distribution of a specific input & information enters the CNS and then spreads to the brain and spinal cord at the same time.
  • Convergence: Information going from several neurons to a single neuron
• Movements of the diaphragm muscle are involuntary, but at times we can move the diaphragm muscle voluntarily.
• Includes parallel and serial processing

Anatomical Organization of the Nervous System

• Neurons in the PNS:
  • Consist of ganglia in which axons are bundled together in nerves.
• Neurons in the CNS:
  • Collection of cell bodies called a center.
  • If this center has a distinct boundary, it is called a nucleus.
  • White matter consists of axons in bundles called tracts and columns
• Central Nervous System (CNS) Gray Matter Organization: -Neural Cortex: Gray matter on the surface of the brain -Nuclei: Collections of neuron cell bodies in the interior of the CNS and Centers that are collections of neuron cell bodies. -Each center has specific processing functions with higher centers that are the most complex centers in the brain. White Matter Organization: -Bundles of CNS axons sharing a common origin and destination (Tracts) Several tracts forming an anatomically distinct mass (Columns)
• Peripheral Nervous System contains Ganglia (Collections of neuron cell bodies in the PNS) and Bundles of Axons in the PNS (Nerves)
• Tracts: Centers and pathways connecting the brain with other organs and systems in the body; ascending (sensory) and descending (motor).