9.1 introduction neurons: masses of nerve cells

9.1 INTRODUCTION• Neurons: masses of nerve cells.• Structural and functional units of the nervous

system.• Specialized to react to physical and chemical

changes in their surroundings.• Nerve impulses: electrochemical changes

neurons transmit.

NEURONS• Cell body: rounded area• Extensions: dendrites and axons• Dendrites: receive electrochemical messages.• Axons: extensions that send information in the

form of nerve impulses.• Neurons usually have only one axon.

NERVES• Bundles of axons.• Neuroglial cells: provide physical support, insulation,

and nutrients for neurons.ORGANS OF NERVOUS SYSTEM• Central nervous system: brain, spinal cord.• Peripheral nervous system: nerves that connect the

central nervous system to other body parts.3 FUNCTIONS1. Sensory2. Integrative3. motor

9.2 GENERAL FUNCTIONS OF THE NERVOUS SYSTEM

• Sensory receptors: ends of peripheral neurons.

• Gather information by detecting changes inside and outside the body.

• Monitor external environmental factors, such as light and sound intensities, and conditions of the body’s internal environment, such as temp. and oxygen level.

• Convert environmental information into nerve impulses.

• Transmitted over peripheral nerves to the CNS.• Signals are integrated; brought together,

creating sensations, adding to memory, or helping to produce thoughts that translate sensations into perceptions.

• Integrative function: we make conscious or subconscious decisions.

• Motor functions: to act on them.

• Effectors: carry impulses from the CNS to responsive structures.

• Outside the nervous system, include muscles that contract and glands that secrete when stimulated by nerve impulses.

MOTOR FUNCTIONS OF THE PERIPHERAL NERVOUS SYSTEM

2 categories• Somatic NS: consciously controlled, controls skeletal

muscle.• Autonomic NS: controls effectors that are involuntary,

hear, smooth muscle in blood vessels, and various glands.

9.3 NEUROGLIAL CELLS• Fill spaces between neurons, provide

structural frameworks, produce the fatty lipoprotein myelin, and carry on phagocytosis.

• Neuroglial cells greatly outnumber neurons.

NEUROGLIAL CELLS1. Microglial cells: scattered throughout the CNS. They

support neurons and phagocytize bacterial cells and cellular debris

2. Oligodendrocytes: align along nerve fibers. They provide insulating layers of myelin, called a myelin sheath around axons within the brain and spinal cord.

3. Astrocytes: commonly found between neurons and blood vessels, provide structural support, join parts by their abundant cellular processes, and help regulate the concentrations of nutrients and ions within the tissue. Also form scar tissue.

4. Ependymal cells: form an ephithelia-like membrane that covers specialized brain parts (choroid plexuses) and forms the inner linings that enclose spaces within the brain (ventricles) and spinal cord (central canal).

Schwann cells: neuroglial cells that form a myelin sheath around axons.

9.4 NEURONS• Neurofibrils: fine threads, extend into the axons (in

the cell body).• Neuron cell body: granular cytoplasm, cell

membrane, organelles.• Chromatophilic substance: membranous sacs,

similar to rough endoplasmic reticulum.• Axon: conducts nerve impulses away from the cell

body.• Schwann cells: contain myelin, neurilemma sheath

that surrounds the myelin sheath.• Nodes of Ranvier: gaps between Schwann cells

• Axons with myelin sheaths are called myelinated.

• Unmyelinated: lack sheaths.• Peripheral nerve Axons can regenerate• Schwann cells help do this.• CNS axons usually cannot regenerate

themselves.

CLASSIFICATION OF NEURONS• Neurons differ in structure, size, and shape of

their cell bodies.• Trigger zone: sensitive region of the axon,

send a nerve impulse.

THREE MAJOR GROUPS OF NEURONS1. Multipolar neurons: many processes arising from

their cell bodies. Only one process of each neuron is an axon; the rest are dendrites.

2. Bipolar neurons: only two proceses, one an axon and the other a dendrite. Ex: eyes, nose, and ears

3. Unipolar neurons: single process, divides into two branches, single axon. One branch is associated w/ dendrites near a peripheral body part. The other branch enters the brain or spinal cord.

• Ganglia: masses of nervous tissue.• Located outside the brain and spinal cord

• Different functions of neurons:• Carry impulses into the brain or spinal cord• Transmit impulses out of the brain or spinal

cord• Conduct impulses from neuron to neuron

within the brain or spinal cord

GROUPS OF DIFFERENT NEURONS1. Sensory neurons: (afferent neurons) carry

nerve impulses from peripheral body parts into the brain or spinal cord. Either have specialized receptor ends at the tips of their dendrites, or they have dendrites that are closely associated with receptor cells in the skin or in sensory organs. (most unipolar, some bipolar)

2. Interneurons: (association or internuncial neurons) lie within the brain or spinal cord. Multipolar, link other neurons.

3. Motor neurons: (efferent neurons) are multipolar and carry nerve impulses out of the brain or spinal cord to effectors. Stimulate muscles to contract and glands to release secretions.

9.5 CELL MEMBRANE POTENTIAL• Polarized: surface of the cell membrane,

electrically charged, unequal distribution of positive and negative ions

• Action potential: change in charge of the membrane, forms a nerve impulse

DISTRIBUTION OF IONS• Active transport of sodium and potassium

ions.• More sodium ions are outside and more

potassium ions inside.• Cytoplasm: PO4-3, SO4-2, protein, K+• Potassium pass through cell membrane easier

than sodium ions.• K+ a major contributor to membrane

polarization.

RESTING POTENTIAL• Resting cell membrane is more permeable to K+

that to Na+, K+ diffuse out of the cell more rapidly.• Result, outside of cell gains a slight surplus of +

charge.• Inside – charge• Potential difference: difference in electrical charge

between two regions.• Resting potential: potential difference between

the region inside the membrane and the region outside the membrane.

• As long as a nerve cell membrane is undisturbed, the membrane remains in this polarized state.

POTENTIAL CHANGES• Nerve cells are excitable, they can respond to

changes in their surroundings.• Some detect changes in: temp, light, pressure• Many neurons respond to other neurons.• depolarizing: inside membrane becomes less

negative compared to outside.• Change in potential is proportional to the

intensity of stimulation.• Summation: change in potential is increases as

more stimulation comes to cell

Threshold potential: action potential occursACTION POTENTIAL• When threshold potential occurs: permeability

changes at the trigger zone of the neuron being stimulated.

• Channels highly selective for Na+ open and allow Na+ to diffuse freely inward.

• Aided by the negative electrical condition on the inside of the membrane, which attracts the positively charged sodium ions.

• As sodium ions diffuse inward, the membrane loses its negative electrical charge and becomes depolarized.

• Membrane channels open that allow potasssium ions to pass through, inside of membrane becomes negatively charged once more.

• Membrane returns to the resting potential.

• Rapid sequence of depolarization and repolarization, 1/1000 of a second, called action potential.

• http://www.youtube.com/watch?v=U0NpTdge3aw

9.6 NERVE IMPULSES• Wave of action potentials along a nerve axon –

nerve impulse.• Pg. 213IMPULSE CONDUCTION