Human anatomy and physiology Nervous system 1

Nervous system functions

Master regulatory system, sends and receives info, sensory input, integration and processing, motor output, maintains homeostasis, acts as center for thought, learn, memory

Sensory input Nervous system function

detects changes

Integration and processing nervous system function

making decisions

motor output nervous system functions

stimulates muscles and glands to respond

Main cell types of nervous system

Neurons (nerve cells) and Neuroglia

Neurons (nerve cells_

Respond quickly to changes/stimuli. Conduct electrical impulses via neurotransmitters

Neuroglia

Protect, support, insulate, and nourish neurons. Do not conduct electrical impulses like neuron

CNS

Brain and spinal cord

PNS

Connects CNS to other body parts. Consists of cranial and spinal nerves. subdivided into afferent (sensory) and efferent (motor)

Sensory (afferent) system

Sensory receptors perform sensory function (detect changes). Receptors convert information into impulses.Impulses conducted along peripheral nerves to CNS for integration

motor (efferent) division

Neurons that transmit impulses from CNS to effectors

perform motor function. Effectors are muscles or glands outside nervous system. 2 subdivisions somatic and automatic

Somatic motor division

transmits voluntary commands to SKELETAL MUSCLES

autonomic

Transmits INvoluntary commands to VISCERA

Neurons

Vary in size and shape. May differ in length, number, and size of axons and dendrites. Share certain structural features like cell body, dendrites, and axon

Cell body of neuron

Contains nucleus, cytoplasm, organelles, neurofilaments, chromatophilic substance

Dendrites of neuron

Branched receptive surfaces; a neuron may have many

Axon of neuron

Transmits impulses and releases neurotransmitters to another neuron or effector (another neuron, a muscle cell, or a gland cell); a neuron may have only 1 axon

Structural feature of axons

Axon hillock: Cone-shaped area of cell body from which axon arises

Collaterals: Branches from axon

Axon terminal: Specialized endings of extensions from axon

Synaptic knob: Rounded ending of a synaptic terminal

Schwann cells

Neuroglia of the PNS that wrap around some axons in layers

Schwann cells MYELIN

Mixture of fats and proteins that fill layers made by Schwann

cell membranes

Schwann cells MYELIN SHEATH

A wrapped coating around some PNS axons, composed of layers of Schwann cell membranes and myelin; acts as electrical insulator

Schwann cells NODES OF RANVIER

Gaps in myelin sheath between Schwann cells

Myelinated and unmyelinated axons

NOT all axons are myelinated

Myelinated axons

Are coated by a myelin sheath. Produced by a series of Schwann cells lined up along axon in PNS. Produced by Oligodendrocytes in CNS. Groups of myelinated axons in CNS comprise White Matter. Increase conduction speed for electrical impulses

Unmyelinated axons

Encased by Schwann cell cytoplasm in PNS, but there is no wrapped coating of myelin surrounding the axons. Groups of unmyelinated axons in CNS comprise Gray Matter

Neurons classified by structure

multipolar, bipolar, unipolar

Multipolar neurons

Many processes extend from cell body (many dendrites, 1 axon). 99% of neurons. Most neurons of CNS, some in autonomic NS

Bipolar neurons

Two processes extend from cell body (1 dendrite, 1 axon). Not that common. Eyes, ears, nose

Unipolar neurons

One process extends from cell body. Two branches that function as 1 axon (peripheral and central processes). Cell bodies are mainly found in ganglia of PNS

Neurons classified by function

Sensory (afferent), Interneurons, Motor (efferent)

Sensory (afferent) neurons

Carry impulses from periphery to CNS (brain or spinal cord). At distal ends, contain sensory receptors to detect changes. Most are unipolar, some are bipolar

Interneurons

Link neurons in the CNS. Relay information from one part of CNS to another. Multipolar. Some cell bodies cluster to form nuclei in CNS

Motor (efferent) neurons

Carry impulses from CNS to effectors (muscles or glands). Multipolar. In somatic NS, control voluntary skeletal muscles. In autonomic NS, control involuntary smooth and cardiac muscle, glands

Neuroglia FUNCTIONS

Provide structural support for neurons. In embryo, guide neurons into position, may stimulate specialization. Produce growth factors to nourish neurons and remove excess ions and neurotransmitters. Aid in formation of synapses

Neuroglia of the CNS

Astrocytes, oligodendrocytes, microglia, ependyma

Astrocytes

Connect neurons to blood vessels, exchanging nutrients and growth factors. Form scar tissue. Aid metabolism of certain substances. Regulate ion concentrations, such as K+. Part of Blood Brain Barrier

Oligodendrocytes

Myelinate CNS axons; also provide structural support

Microglia

Phagocytic cells; also provide structural support

Ependyma/ependymal cells

Line central canal of spinal cord & ventricles of brain, cover choroid plexus. Help regulate composition of cerebrospinal fluid. Ciliated cuboidal or columnar cells

Neuroglia of PNS

Schwann cells and satellite cells

Schwann cells PNS

Produce myelin sheath found on some peripheral axons. Speed up speed of nerve impulse transmission

Satellite cells PNS

Support clusters of neuron cell bodies (ganglia). Nourish and balance ionic concentrations

Neuroglia and axonal regeneration

Mature neurons do not divide. If cell body is injured, the neuron usually dies

Neuron regeneration in PNS

If a peripheral axon is injured, it may regenerate. Axon separated from cell body and its myelin sheath will degenerate. Schwann cells and neurilemma remain. Remaining Schwann cells provide guiding sheath for growing axon. If growing axon establishes former connection, function will return; if not, function may be lost

Neuron regeneration in CNS

CNS axons lack neurilemma to act as guiding sheath. Oligodendrocytes do not proliferate after injury. Regeneration is unlikely

Cell membrane potential

Inside of membrane is negatively charged with respect to

the outside. Results from unequal distribution of ions on the inside and

outside of membrane. Important in conduction of impulses in neurons and muscle

fibers

Polarized

When cell membrane is electrically charged

Polarity

Charge difference in local area

Excitable cells

Rapidly change internal charge

Membrane potential

Charge inside a cell. “Potential” to transport charges across membrane

Resting membrane potential

Charge inside cell when it is inactive. About −70 mV

Potassium (K+) ions

higher concentration inside than Outside cell. more permeable to resting membranes

Sodium (Na+) ions

in higher concentration outside than inside cell

Gated channels

allow for movement of these ions at certain times. many chemical and electrical factors affect opening and closing of channels

Na+/K+ pump

maintains balance in ion movement across membrane: When resting potential is disturbed, it pumps 3 Na+ ions out of cell and 2 K+ ions into cell. Uses energy of ATP to actively transport these ions in opposite directions across membrane

Action potential

Sequence of electrical events in an excitable cell, involving changes in membrane potential, first positive and then negative, to return to resting potential; action potentials are used for communication between cells

Stimulus

anything that can change resting potential of −70 mV in

either direction. Neurons detect stimuli, and respond by changing their resting potential. Excitatory stimulus opens chemically gated Na+ channels, and Na+ ions enter cell

local potential change

Change in membrane potential that occurs only in the area of stimulation. Local potential changes are graded

Graded

greater stimulus intensity, greater potential change

Chemically gated Na+ channels

opened by excitatory stimulus. Makes inside of neuron less negative

Threshold stimulus

Graded stimuli can add together to produce this

Threshold stimulus definition

Excitatory stimulus that causes enough Na+ ions to flow into cell that it reaches Threshold Potential of −55 mV

Action potential

many voltage-gated Na+ channels open, at Trigger Zone, and charge changes to about +30 mV;

depolarization

Change from negative to positive charge inside cell, making both sides of membrane positive

All-or-none law

Reaching an action potential; either achieved or not. If action potential is reached, it sends signal all the way down the axon

repolarization

Return to resting potential after action potential; occurs as K+ channels open and K+ ions rush out of cell; polarity returns

hyperpolarization

Slight overshoot at end of repolarization, in which potential drops below −70 mV for a moment before returning to −70 mV

What does Na/k+ pump do now after polarizations

returns ions to original locations and concentrations

Refactory period

During an impulse, the portion of the axon actively conducting the action potential is not able to respond to another threshold stimulus

2 parts of refractory period

Absolute refractory period, relative refractory period

Absolute refractory period

Time when threshold stimulus cannot generate another action potential. Voltage-gated Na+ channels are briefly unresponsive

Relative refractory period

Time when only high-intensity stimulus can generate another action potential. Repolarization is not complete, and membrane is re-establishing. resting potential

Refractory period limits number of action potentials generated per second

Impulse conduction

speed of this varies with myelination. Unmyelinated axons conduct impulses over entire length

Myelin impulse conduction

Myelin is rich in lipids, and prevents water and water-soluble substances

(such as ions) from crossing membrane; acts as electrical insulator

nodes of ranvier IMpulse conduction

Ions can cross membrane only through gaps in myelin sheath,

Myelinated axons IMPULSE conduction

transmit impulses through saltatory conduction, in which action potentials “jump” from node to node down the axon

Saltatory conduction is much faster than impulse conduction in unmyelinated axons.

Axon diameter impulse conduction

also affects conduction speed; thick axons transmit faster than thin axons:

Thick, myelinated axons: 120 m/sec.

Thin, unmyelinated axons: 0.5 m/sec.