Human A&P1 , Chapter 11

Functions of the nervous system

- Gathers sensory input

- Integrate sensory input

- Causes response or motor output (activates effector organs)

Organization of the Nervous System

Central Nervous System (CNS) and Peripheral Nervous System (PNS) -> sensory + motor division -> somatic + autonomic -> sympathetic + parasympathetic divisions

Central Nervous System (CNS)

Control center for interpreting sensory input and motor output

Brain + Spinal Cord

Peripheral Nervous System (PNS)

Communication lines between CNS and rest of body

Nerves and ganglia OUTSIDE the CNS

Visceral (heart) organ, Skin, Skeletal muscles

Sensory (afferent) division

Division of PNS

Has somatic (skin) and visceral (heart) sensory fibers

Conducts impulses from receptors TO the CNS

Motor (efferent) division

Division of PNS

Has Motor nerve fibers

Conducts impulses FROM the CNS to effectors (muscles + glands)

Somatic nervous system

Division of motor division

Somatic (voluntary) nerve fibers

Conducts impulses FROM the CNS to skeletal muscles

Autonomic nervous system (ANS)

Division of motor division

Visceral (involuntary) nerve fibers

Conducts impulses FROM the CNS to cardiac muscle, smooth muscle and glands

Sympathetic division

Division of ANS

Mobilizes body systems during activity

Parasympathetic division

Division of ANS

Conserves energy, promotes house-keeping functions during rest

2 major cell types

- Supporting cells

- Neurons

Supporting cells

“Nerve glue” surround and wrap neurons, called neuroglia or glial cells

Help the neurons

Astrocytes (Supporting cell of CNS)

- Star-shaped, wrap around neurons, blood/brain barrier

- Anything that goes to the neuron goes through the astrocyte

- Most abundant

- Support neurons; anchor to capillaries

- Mop up leaked K+ and recycle neurotransmitters (ACh, etc.)

Microglia (supporting cells of CNS)

- Transform into a macrophage (which phagocytize neural debris + microorganisms)

- Lying in wait for a breach in the nervous tissue; are protective

Ependymal cells (supporting cells of CNS)

- Line cavities of CNS, brain and spinal cord

- Many have cilia facing fluid-filled cavity 

- Beat their cilia to circulate CSF (cerebrospinal fluid) cavities

Cerebrospinal Fluid Functions

Protect and cushion the brain and spinal cord, has nutrients

Oligodendrocytes (supporting cells of CNS)

- One of two cell types that make a myelin sheath INSIDE THE CNS

- Line up along thicker CNS neurons and wrap them with cytoplasmic extensions

- Produce insulating layers (myelin sheaths, increasing the speed of an action potential)

Satellite cells (Supporting cells of PNS)

- Surround the neuron cell bodies within ganglia

- Function unknown

Schwann cells (supporting cells of PNS)

- Also called neurolemmocytes

- Surround nerve fibers and form myelin sheath in the PNS

- Vital for peripheral nerve fiber regeneration

Neurons (nerve cells)

Excitable nerve cells that transmit signals

- Conduct nerve impulses

Characteristics of nerve cells

- Extreme longevity (lasts long)

- Amitotic (cannot divide, hard to replace)

- High metabolic rate (need LOTS of oxygen and glucose)

Cell body of a neuron (soma)

- Mostly in CNS

- Contains nucleus and cytoplasm

- Is the Biosynthetic/metabolic center + receptive region

Structures in Cell body

Protein and membrane-making machinery (rough ER (chromatophilic substance), ribosomes, golgi apparatus)

Cytoskeletal elements - Maintain cell shape/integrity (microtubules and neurofilaments)

Pigment inclusions - changes color (black melanin, lipofuscin)

Biosynthetic center

In soma, has the rough ER, is most active of any cell, no centrioles, makes protein

Processes of a neuron

Dendrites and axons

Dendrites

Branchlike parts of a neuron that are specialized to receive information, large surface area, convey short-distance signals (graded potentials) TOWARD the cell body

Receptive region

Axon

- Generates nerve impulse AWAY from cell body, starts at axon hillock

- Each neuron has a single axon that relies on cell body

- Ends in terminal branches (have endings called synaptic knobs (boutons)) that release neurotransmitters

- Tracts in the CNS, nerves in the PNS

Boutons

Synaptic knobs in the axon that release neurotransmitters

A tract

Bundle of axons in the CNS

A nerve

Bundle of axons in the PNS

Nuclei

Clusters of cell bodies INSIDE CNS

Ganglia

Cell bodies that lie along nerves in PNS (OUTSIDE CNS)

Axonal transport

Anterograde (away from body), retrograde (toward body)

- Uses ATP dependent motor proteins

Regions of a neuron

Receptive (dendrite and body)

Conducting (axon)

Secretory (axon terminals)

Myelin sheaths

Insulating, whitish/fatty cover that wraps tightly around, protects, and insulates nerve fibers (mostly in CNS)

Formed by Schwann cells in the peripheral nervous system, oligodendrocytes in the CNS, with gaps known as myelin sheath gaps (nodes of Ranvier) between cells

Myelin sheath formation in PNS

- Formed by Schwann cells, wrap around axon in "jelly roll" fashion

-Schwann envelops then rotates, squeezing out cytoplasm

- Adjacent Schwann cells along axon don't touch

Myelin sheath formation in CNS

- Oligodendrocytes, nodes of Ranvier more widely spaced, CNS myelin sheaths lack an external lamina

- Oligodendrocytes can wrap multiple different axons at the same time

White matter (fiber tracts)

CNS regions containing collections of myelinated fibers

Gray matter

CNS regions of nerve cell bodies and unmyelinated fibers

Classification of neurons by processes

Number of processes extending from body

Multipolar, bipolar, unipolar

Multipolar

3 or more processes (1 axon and many dendrites)

99% of neurons

Bipolar

Rare, 2 processes (1 axon and 1 dendrite)

Ex: receptors of the retina and olfactory mucosa

Unipolar (pseudounipolar)

Single, short process divides of cell body into proximal and distal branches

In ganglia of PNS (sensory neurons)

Classification of Neurons

The functional direction the impulse travels

Sensory (afferent), Motor (efferent), Interneurons (association neurons)

Sensory (afferent) neurons

Classification of neuron

From skin/organs to CNS, mostly unipolar

Cell bodies OUTSIDE CNS

Motor (efferent) neurons

Classification of neuron

Carry impulse FROM CNS to the effector organs (muscles + glands), usually multipolar

Cell bodies INSIDE CNS

Interneurons (association neurons)

Classification of neuron

Lie between sensory and motor neurons, shuttle signals through CNS, often multipolar

99% of neurons

Part of Neurophysiology

- When a neuron is stimulated, electrical impulse conducted along length of axon

- Potential energy

Ex: stored potential energy in a battery, complete circuits

Potential energy of neuron

Generated by separated electrical charges of opposite sign

The greater the distance, the greater the voltage

Resting Membrane Potential

The potential difference across a membrane in a resting (polarized) neuron

Measured by a voltameter (-70 mV)

Voltage

-Measure of potential energy difference generated by separated change

- Volts or millivolts

- Measures between two points (inside and outside)

Current

A flow of electric charge from one point to another, flows through fluids when voltage changes

Sodium and Potassium flow

Resistance

- Hinderance to charge flow

- Insulators and conductors

- Plasma membrane provides resistance to current flow

Insulators

Have a high electrical resistance

Conductors

Have a low electrical resistance

Ohm's Law

Current (I) = Voltage (V) / Resistance (R)

Current is directly related to voltage and inversly related to resistance

Nongated channels

Leakage channels, are always open

Types of gated ion channels

Voltage gated, chemically gated, mechanically gated

Chemically (ligand) gated

Open in response to binding of the appropriate neurotransmitter

Voltage gated

Normally wide open, are waiting for an electrical response to changes in membrane potential

Initially activated by local (graded) potentials along the dendritic body/soma membranes

Mechanically gated

Open in response to physical deformation of the receptor

Ex. sensory receptors for touch and pressure

Electrochemical gradient

Concentration (higher to lower concentration) and electrical (toward area of opposite charge) gradient

Ions flowing across membrane create currents and voltage

Maintaining RMP

Concentration of Na+ and K+ inside and outside cell

Selective permeability of the membrane (K+ leaks out more than Na+ leaks in)

Na+/K+ Pump (3 Na+ out, 2 K+ in)

Action potential (nerve impulse)

Long-distance signals of axons with consistent strength; brief switch of RMP

Do not decay

Action Potential generation

Resting state, depolarization

Generated by influx of Na+ (depolarization)

Action Potential propagation 

Axonal hillock regulates the generator potential, and it must rise to the threshold, when it reaches the threshold it fires.

The entire AP must be propagated along the entire axon for a nerve impulse. After depolarization, AP is self-propagating down the axon due to voltage-gated channels opening

Resting state

The state in which there is a negative electrical charge of about -70 millivolts within a neuron

All gated Na+ (two gates) and K+ (one gate) channels are closed.

Depolarization

Decrease in membrane potential where the inside of the membrane becomes less negative/more positive (closer to zero); outside is occupied with more negative ions instead

Na+ channels open during depolarization

Repolarization

Sodium efflux (out), return of the cell to resting state, caused by reentry (influx) of potassium into the cell while sodium exits the cell.

Na+ stops rushing in, K+ rushes out

Hyperpolarization

The inside of the membrane becomes more negative, RMP dips lower than normal but are corrected

Some K+ channels remain open, and Na+ channels rest.

Threshold Point

The membrane potential at which the outward current of Na+ is equal to the inward current of K+, reached when depolarization occurs

Subthreshold stimuli

Brief weak stimuli produce subthreshold depolarizations that do not translate into nerve impulses

Depends on amount of current

Threshold stimuli

Stronger stimuli produce depolarizing currents that push the membrane potential toward and beyond the threshold voltage

Depends on amount of current

Local (graded) potential

Short-lived, localized changes in membrane potential, typically occurring in dendrites or the cell body.

-Excitatory or inhibitory (depolarization or hyperpolarization)

-Magnitude varies directly with stimulus strength

-Triggered by a change in the neuron's environment

Receptor generated potential

Sensory receptor is excited by stimuli such as light, pressure, or chemicals

Postsynaptic potential

Stimulus is a neurotransmitter that is released by another neuron

Neurotransmitter is released into a fluid-filled gap called a synapse and influences the neuron beyond the synapse

Absolute Refractory period

Period from the opening of the Na+ channels until the channels begin to reset to their original state. Ensures each AP is separate and enforces one-way transmission of AP.

Cannot respond to another stimulus, one end of axon to the other

Relative Refractory period

Interval following the absolute refractory period where most Na+ channels have returned to resting state,

Some channels still open and repolarization is occurring. Axons threshold for AP generation is elevated

Generator potential

Local depolarization initiated by some form of energy

Pre-synaptic neuron

houses vesicles filled with neurotransmitter in its synaptic knob

Post-synaptic neuron

contains proteins that function as receptors and ion gates

Neurotransmitter receptor

Specialized protein, often embedded in the cell membrane, that selectively senses and reacts to molecules of a corresponding neurotransmitter or hormone

Synaptic cleft

Separates the pre and post synaptic membranes, filled with fluid.

Conduction Velocity

How fast AP is propagated

Depends based on axon diameter and myelination

Continuous conduction

Occurs in nonmyelinated axons where voltage-gated channels are adjacent. Relatively slow

Saltatory conduction

In myelinated fibers, depolarizing current jumps from one myelin sheath gap to next, 30 times faster

Axodendritic synapse

Occur between the axon endings of one neuron and the dendrites of another neuron.

Axosomatic synapse

Occur between the axon endings of one neuron and the cell body (soma) of another neuron.

Axoaxonal synapse

Occur between the axon of one neuron and the axon endings of another neuron

Electrical synapse

Allow direct passage of ions and signaling molecules through gap junctions, enabling rapid and synchronized communication (Direct ion movement between neurons)

Chemical synapse

Use neurotransmitters to transmit signals across a synaptic cleft, allowing for more complex and modifiable communication (Involves neurotransmitter release, diffusion, and receptor binding, resulting in unidirectional communication.)

EPSP (Excitatory postsynaptic potential)

- Causes a depolarization

- Likely to reach threshold on a postsynaptic neuron, TOWARD threshold

- “Fire!”

IPSP (Inhibitory postsynaptic potential)

- If a neurotransmitter causes the postsynaptic membrane to hyperpolarize (down)

- AWAY from threshold

- “Don’t fire!”

Neuronal integration

Information-processing, decision-making, and memory mechanisms of neurons

No summation, temporal summation, spatial summation, spatial summation of EPSPs and IPSPs

No summation

- Two ESPS's separated in time, don't add together

- Fire... Fire... Fire...

Temporal summation

- Rapidly firing presynaptic potential causes ESPS's that are close in time

- FireFireFire

Spatial summation

- More than one presynaptic neuron fires at the same time, generated at different locations on the neuron

- Three people saying “Fire!” at the same time

Spatial summation of EPSPs and IPSPs

An EPSP can be overridden by an IPSP

- One saying “Fire!” and one saying “Don’t Fire!”

- Do not reach threshold

Nerve fiber regeneration

- Peripheral nerve fibers (in PNS) can sometimes be regenerated if the soma (cell body) isn't damaged and some of the neurilemma is intact

1. Neurilemma forms a regeneration tube, a new axon goes through it, reestablishes its original connection

2. Regeneration occurs due to reestablished connection

- Muscle cell may atrophy if no innervation