Nervous System Overview

Overview of the Nervous System

• The nervous system works with the endocrine system to maintain homeostasis
• Behaviors, memories, and movements are all functions of the nervous system
• Neurology is the area of medical science focused on the nervous system

Nervous System Structures

• The nervous system consists of the Central Nervous System (CNS) and the Peripheral Nervous System (PNS)
• The CNS is made up of the brain and spinal cord, which contain neurons and neuroglia
• Nerves, ganglia, and plexuses make up the PNS
• Nerves are bundles of axons, connective tissue (CT), and blood vessels
• There are 12 pairs of cranial nerves and 31 pairs of spinal nerves
• Sensory nerves contain receptors that are neurons or specialized cells monitoring the internal and external environment
• Motor nerves stimulate muscles and glands (effectors)
• Ganglia are small masses of nervous tissue with neuron cell bodies, located outside the brain and spinal cord
• Enteric plexuses comprise neuron networks that facilitate digestive system regulation

Functions of the Nervous System

• Sensory Function: Sensory (afferent) neurons detect internal and external environmental changes via sensory receptors
• Integrative Function: Association or interneurons analyze sensory data, store data and determine appropriate responses
• Motor Function: Motor (efferent) neurons respond to stimuli via initiating action

Nervous System Divisions

• Central Nervous System (CNS): contains the brain and spinal cord
• Peripheral Nervous System (PNS): contains cranial and spinal nerves with sensory and motor fibers
• The PNS connects the CNS to muscles, glands (effectors), and sensory receptors (sensors)

Histology of Nervous Tissue: Neurons

• Neurons are the functional units of the nervous system
• Neurons sustain a resting potential
• Neurons possess the ability to generate action potentials, indicating electrical excitability
• Cell body: contains a single nucleus with a distinctive nucleolus and Nissl bodies (rough ER and free ribosomes for protein synthesis)
• Cell fibers/processes: has multiple dendrites and one axon

Axons and Dendrites

• Dendrites are short and branched, they receive information
• Axons originate from the axon hillock
• Possess one or more terminals, synaptic end bulbs and terminate at the synapse
• Synaptic vesicles contain neurotransmitters
• Often covered in a myelin sheath

Structural Classification of Neurons

• Neurons are classified based on the number of processes extending from the cell body
• A multipolar neuron has many processes
• A bipolar neuron has two processes
• A unipolar neuron has one process

Functional Classification of Neurons

• Neurons are classified based on the direction of nerve impulse propagation
• Sensory/afferent neurons carry sensory information to the CNS
• Motor/efferent neurons carry motor commands from the CNS
• Inter/association neurons connect to other neurons

Functional Categories of Neurons

• Sensory neurons:
• General somatic: detect touch, pain, among others
• Special somatic: deal with vision, balance, hearing
• Special visceral: deal with taste and smell
• General visceral: autonomic
• Motor neurons:
• General somatic: impact skeletal muscles
• General visceral: impact smooth/cardiac muscle & glands
• Special visceral: impact some skeletal muscles, facial expression and parts of head/neck

Neuroglia

• These are not electrically excitable
• They account for approximately half the volume of the nervous system, and are more numerous than neurons (5-20x)
• Neuroglia can multiply and divide
• Six types exist: four in the CNS and two in the PNS
• CNS: astrocytes, oligodendrocytes, microglia, ependymal cells
• PNS: Schwann cells, satellite cells

Astrocytes

• Protoplasmic astrocytes are located in that gray matter together with neuronal cell bodies
• Fibrous astrocytes are located in the white matter along with axons
• Strong cells provide support
• Astrocytes can support the blood-brain barrier, maintain a chemical environment, and regulate brain development in the embryo
• Astrocytes influence the formation of synapses among neurons

Microglia

• Small, phagocytic cells that remove debris, microbes, and damaged tissue

Ependymal Cells

• Cuboidal to columnar shape with microvilli and cilia
• Line the brain ventricles and central canal of the spinal cord
• Produce cerebrospinal fluid and help form the blood-CSF barrier

Oligodendrocytes and Schwann Cells

• Similar to astrocytes in the CNS
• Generate a myelin sheath around axons in the CNS (oligodendrocytes) and PNS (Schwann cells)

Myelinated Axons in CNS

• Oligodendrocytes myelinate many axons each
• Axon regeneration is limited

Myelinated Axons in PNS

• Schwann cells myelinate (wrap around) axons in PNS during fetal development, separated by Nodes of Ranvier
• Schwann cell cytoplasm and nucleus create outermost layer (neurolemma) with the myelin sheath as an inner portion
• Tubes guide growing axons that are repairing

Myelination of Neurons

• Schwann cells (in the PNS) and oligodendrocytes (in the CNS) produce myelin sheaths to surround most neuron axons
• Axons with a myelin sheath from Schwann cells in the PNS or oligodendrocytes in the CNS are myelinated

Satellite Cells

• Flat cells that surround neuron cell bodies in PNS ganglia
• Provide support and regulate the exchange of materials

Gray vs White Matter

• White matter consists mainly of myelinated axons
• Gray matter consists of cell bodies, unmyelinated axons, dendrites, and glia

Electrical Signals in Neurons

• Excitable cells communicate via action potentials or graded potentials
• Action Potentials (AP) facilitate short and long distance communication
• Graded Potentials (GP) facilitate short distance communication

Resting Membrane Potential

• Potential depends on resting membrane potential and the presence of ion channels
• Non-conducting neurons have a membrane that is positive outside and negative inside
• Determined by unequal distribution of ions across the plasma membrane, selective permeability of neuron Na+ and K+, and Na+/K+ pumps

Graded Potentials

• Small deviations in resting membrane potential
• In hyperpolarizing graded potential, internal membrane potential is more negative than resting level
• In depolarizing graded potential, the internal membrane potential is less negative

Action Potentials

• A sequence of rapidly occurring events that decrease and eventually reverse the membrane potential (depolarization)
• Action potentials occur on the membrane potential reaches threshold