Overview of the Nervous SystemOverview of the Nervous System

Organization of the Nervous SystemOrganization of the Nervous System

Peripheral nervous system - PNS

Paired Spinal and Cranial nerves

Carries messages to and from the spinal cord and brain – links parts of the body to the CNS

Central nervous system - CNS

Brain and Spinal Cord (in dorsal body cavity)

Integration and command center – interprets sensory input and responds to input

• Central Nervous System• brain• spinal cord

• Peripheral Nervous System• peripheral nerves

• cranial nerves• spinal nerves

Divisions of the Nervous SystemDivisions of the Nervous System

Nervous SystemNervous System

Sensory Input – monitoring stimuli occurring inside and outside the body

Integration – interpretation of sensory input

Motor Output – response to stimuli by activating effector organs

Functions:

Divisions Nervous SystemDivisions Nervous System

Levels of Organization in the Levels of Organization in the Nervous SystemNervous System

Sensory Division• picks up sensory information and delivers it to the CNS

Motor Division• carries information to muscles and glands

Divisions of the Motor Division• Somatic – carries information to skeletal muscle• Autonomic – carries information to smooth muscle, cardiac muscle, and glands

Divisions of Peripheral Nervous SystemDivisions of Peripheral Nervous System

Sensory Function• sensory receptors gather information• information is carried to the CNS

Integrative Function• sensory information used to create

• sensations• memory• thoughts• decisions

Motor Function• decisions are acted upon • impulses are carried to effectors

Functions of Nervous SystemFunctions of Nervous System

PNS - Two Functional DivisionsPNS - Two Functional DivisionsSensory (afferent) Division

Somatic afferent nerves – carry impulses from skin, skeletal muscles, and joints to the CNS

Visceral afferent nerves – transmit impulses from visceral organs to the CNS

Motor (efferent) Division

Transmits impulses from the CNS to effector organs, muscles and glands, to effect (bring about) a motor response

Sensory Neurons• afferent• carry impulse to CNS• most are unipolar• some are bipolarInterneurons• link neurons• multipolar• in CNSMotor Neurons• multipolar• carry impulses away from CNS• carry impulses to effectors

Classification of NeuronsClassification of Neurons

Motor Division: two subdivisionsMotor Division: two subdivisionsSomatic Nervous System (voluntary)

Somatic motor nerve fibers (axons) that conduct impulses from CNS to Skeletal muscles – allows conscious control of skeletal muscles

Autonomic Nervous System (ANS) (involuntary)

Visceral motor nerve fibers that regulate smooth muscle, cardiac muscle, and glands

Two functional divisions – sympathetic and parasympathetic

Levels of Organization in the Nervous SystemLevels of Organization in the Nervous System

Histology of Nerve TissueHistology of Nerve TissueTwo principal cell types in the nervous system:Neurons – excitable nerve cells that transmit electrical signals

Supporting cells – cells adjacent to neurons or cells that surround and wrap around neurons

Cell Types of Neural Tissue• neurons• neuroglial cells

Neurons (Nerve Cells)Neurons (Nerve Cells)Highly specialized, structural units of the nervous system – conduct messages (nerve impulses) from one part of the body to another

Structure is variable, but all have a neuron cell body and one or more cell projections called processes.

Long life, mostly amitotic, with a high metabolic rate (cannot survive more than a few minutes without O2)

Generalized NeuronGeneralized Neuron

Neuron StructureNeuron Structure

Nerve Cell Body (Perikaryon or Soma)Nerve Cell Body (Perikaryon or Soma)

Contains the nucleus and a nucleolus

The major biosynthetic center

Has no centrioles

Has well-developed Nissl bodies (rough ER)

Axon hillock – cone-shaped area where axons arise

Clusters of cell bodies are called Nuclei in the CNS and Ganglia in the PNS

ProcessesProcessesExtensions from the nerve cell body. The CNS contains both neuron cell bodies and their processes. The PNS consists mainly of neuron processes.

Two types: Axons and Dendrites

Bundles of neuron processes are called Tracts in the CNS and Nerves in the PNS

Dendrites Dendrites Short, tapering, diffusely branched processes

The main receptive, or input regions of the neuron (provide a large surface area for receiving signals from other neurons)

Dendrites convey incoming messages toward the cell body

These electrical signals are not nerve impulses (not action potentials), but are short distance signals called graded potentials

AxonsAxonsSlender processes with a uniform diameter arising from the axon hillock, only one axon per neuron

A long axon is called a nerve fiber, any branches are called axon collaterals

Terminal branches – distal ends are called the axon terminus (also synaptic knob or bouton)

Axons: FunctionAxons: FunctionGenerate and transmit action potentials (nerve impulses), typically away from the cell body

As impulse reaches the axon terminals, it causes neurotransmitters to be released from the axon terminals

Movement of substances along axons:

Anterograde - toward axonal terminal (mitochondria, cytoskeletal, or membrane components)

Retrograde - away from axonal terminal (organelles for recycling)

Anterograde →

←Retrograde

Myelin SheathMyelin Sheath

Whitish, fatty (protein-lipoid), segmented sheath around most long axons – dendrites are unmyelinated

•Protects the axon•Electrically insulates fibers from one another•Increases the speed of nerve impulse transmission

Myelin Sheath Myelin Sheath Formed by Schwann cells in the PNSA Schwann cell envelopes and encloses the axon with its plasma membrane.

The concentric layers of membrane wrapped around the axon are the myelin sheath

Neurilemma – cytoplasm and exposed membrane of a Schwann cell

Nodes of Ranvier (Neurofibral Nodes)Nodes of Ranvier (Neurofibral Nodes)Gaps in the myelin sheath between adjacent Schwann cells

They are the sites where axon collaterals can emerge

White Matter• contains myelinated axons

Gray Matter• contains unmyelinated structures• cell bodies, dendrites

Myelination of AxonsMyelination of Axons

Axons of the CNSAxons of the CNSBoth myelinated and unmyelinated fibers are present

Myelin sheaths are formed by oligodendrocytes

Nodes of Ranvier are more widely spaced

There is no neurilemma (cell extensions are coiled around axons)

White matter – dense collections of myelinated fibers

Gray matter – mostly soma and unmyelinated fibers

Bipolar• two processes• eyes, ears, nose

Unipolar• one process• ganglia

Multipolar• many processes• most neurons of CNS

Classification of NeuronsClassification of Neurons

Classification of NeuronsClassification of Neurons

Multipolar — three or more processes

Bipolar — two processes (axon and dendrite)

Unipolar — single, short process

Structural

Neuron ClassificationNeuron ClassificationFunctional

Sensory (afferent) – transmit impulses toward the CNS

Motor (efferent) – carry impulses away from the CNSInterneurons (association neurons) – lie between sensory and motor pathways and shuttle signals through CNS pathways

Supporting Cells: NeurogliaSupporting Cells: NeurogliaSix types of Supporting Cells - neuroglia or glial cells – 4 in CNS and 2 in the PNS

Each has a specific function, but generally they:

Provide a supportive scaffold for neurons

Segregate and insulate neurons

Produce chemicals that guide young neurons to the proper connections

Promote health and growth

Schwann Cells• peripheral nervous system• myelinating cell

Oligodendrocytes• CNS• myelinating cell

Astrocytes• CNS• scar tissue• mop up excess ions, etc• induce synapse formation• connect neurons to blood vessels

Microglia• CNS• phagocytic cell

Ependyma• CNS• ciliated• line central canal of spinal cord• line ventricles of brain

Types of Neuroglial CellsTypes of Neuroglial Cells

Supporting Cells: NeurogliaSupporting Cells: NeurogliaNeuroglia in the CNS

Astrocytes

Microglia

Ependymal Cells

Oligodendrocytes

Neuroglia in the PNS

Satellite Cells

Schwann Cells

Outnumber neurons in the CNS by 10 to 1, about ½ the brain’s mass.

Types of Neuroglial CellsTypes of Neuroglial Cells

AstrocytesAstrocytesMost abundant, versatile, highly branched glial cells

Cling to neurons, synaptic endings, and cover nearby capillaries

Support and brace neurons

Anchor neurons to nutrient supplies

Guide migration of young neurons

Aid in synapse formation

Control the chemical environment (recapture K+ ions and neurotransmitters)

MicrogliaMicrogliaMicroglia – small, ovoid cells with long spiny processes that contact nearby neurons

When microorganisms or dead neurons are present, they can transform into phagocytic cells

Ependymal CellsEpendymal CellsEpendymal cells – range in shape from squamous to columnar, many are ciliated

Line the central cavities of the brain and spinal column

OligodendrocytesOligodendrocytesOligodendrocytes – branched cells that line the thicker CNS nerve fibers and wrap around them, producing an insulating covering – the Myelin sheath

Schwann Cells and Satellite CellsSchwann Cells and Satellite Cells

Schwann cells - surround fibers of the PNS and form insulating myelin sheaths

Satellite cells - surround neuron cell bodies within ganglia

Regeneration of A Nerve AxonRegeneration of A Nerve Axon

NeurophysiologyNeurophysiology

Neurons are highly irritable (responsive to stimuli)

Action potentials, or nerve impulses, are:

Electrical impulses conducted along the length of axons

Always the same regardless of stimulus

The underlying functional feature of the nervous system

Levels of PolarizationLevels of Polarization•Depolarization – inside of the membrane becomes less negative (or even reverses) – a reduction in potential•Repolarization – the membrane returns to its resting membrane potential•Hyperpolarization – inside of the membrane becomes more negative than the resting potential –an increase in potential

Depolarization increases the probability of producing nerve impulses. Hyperpolarization reduces the probability of producing nerve impulses.

Changes in Membrane PotentialChanges in Membrane Potential

Graded PotentialsGraded Potentials

Short-lived, local changes in membrane potential (either depolarizations or hyperpolarizations)

Cause currents that decreases in magnitude with distance

Their magnitude varies directly with the strength of the stimulus – the stronger the stimulus the more the voltage changes and the farther the current goes

Sufficiently strong graded potentials can initiate action potentials

Graded PotentialsGraded Potentials

Voltage changes in graded potentials are decremental, the charge is quickly lost through the permeable plasma membrane

short- distance signal

Action Potentials (APs)Action Potentials (APs)An action potential in the axon of a neuron is called a nerve impulse and is the way neurons communicate.

The AP is a brief reversal of membrane potential with a total amplitude of 100 mV (from -70mV to +30mV)

APs do not decrease in strength with distance

The depolarization phase is followed by a repolarization phase and often a short period of hyperpolarization

Events of AP generation and transmission are the same for skeletal muscle cells and neurons

NaNa++ and K and K++ channels are closed channels are closedEach NaEach Na++ channel has two voltage-regulated channel has two voltage-regulated

gates gates Activation gates – Activation gates –

closed in the resting closed in the resting state state

Inactivation gates – Inactivation gates – open in the resting open in the resting statestate

Action Potential: Resting StateAction Potential: Resting State

Depolarization opens the activation gate (rapid) and closes the inactivation gate (slower) The gate for the K+ is slowly opened with depolarization.

Depolarization PhaseDepolarization PhaseNa+ activation gates open quickly and Na+ enters causing local depolarization which opens more activation gates and cell interior becomes progressively less negative. Rapid depolarization and polarity reversal.

Threshold – a critical level of depolarization (-55 to -50 mV) where depolarization becomes self-generating

Positive Feedback?

Repolarization PhaseRepolarization PhasePositive intracellular charge opposes further Na+ entry. Sodium inactivation gates of Na+ channels close.

As sodium gates close, the slow voltage-sensitive K+ gates open and K+ leaves the cell following its electrochemical gradient and the internal negativity of the neuron is restored

HyperpolarizationHyperpolarizationThe slow K+ gates remain open longer than is needed to restore the resting state. This excessive efflux causes hyperpolarization of the membrane

The neuron is insensitive to stimulus and depolarization during this time

Role of the Sodium-Role of the Sodium-Potassium PumpPotassium Pump

Repolarization restores the resting electrical conditions of the neuron, but does not restore the resting ionic conditions

Ionic redistribution is accomplished by the sodium-potassium pump following repolarization