BIO 223 Exam 3

Organs of the nervous system

Brain, spinal cord, receptors of sense organs, nerves that connect to other systems

2 kinds of nervous tissue

Neurons and Neuroglia (glial cells)

This nervous tissue cell is used for intercellular communication

Neurons

This nervous tissue cell is essential for survival and functions of neurons. Preserves structure of nervous tissue 

Neuroglia (glial cells)

Anatomical divisions of the nervous system

Central nervous system, Peripheral nervous system, Enteric nervous system

Functional divisions of the PNS 

Afferent and Efferent divisions 

Central nervous system structure 

Brain and spinal cord. Consists of nervous tissue, connective tissue, and blood vessels 

Function of the nervous system

To process and coordinate sensory data from inside and outside body (integration) 

Includes all nervous tissue outside the CNS and ENS, delivers sensory information to the CNS, and carries motor commands to peripheral tissues 

PNS

Bundles of axons with connective tissue and blood vessels 

Nerves 

Nerves function

Carry sensory information and motor commands

Enteric nervous system

100 million neurons in walls of digestive tract. Uses the same neurotransmitters as the brain.

Initiates and coordinates visceral reflexes locally without instruction from the CNS and can be influenced by ANS 

ENS

Afferent division of PNS

Carries sensory information from receptors in peripheral nervous tissue and organs to CNS.

Receptors

Detects changes or responds to stimuli. May be neuron or specialized cells.

Somatic sensory receptors

Monitors skeletal muscles, joints, and skin surface

Visceral sensory receptors

Monitors internal organs

Special sensory receptors 

Monitors smell, taste, vision, balance, and hearing

Efferent division of PNS 

Carries motor commands from CNS to effectors 

Effectors

Target organs that respond to motor commands such as muscles, glands, and adipose tissue 

Somatic nervous system

Controls skeletal muscle contractions. Both voluntary and involuntary

Autonomic nervous system

Controls subconscious actions, contractions of smooth and cardiac muscle, and glandular secretions 

Neurons

Basic functional units of the nervous system. Sends and receives signal. Function in communication, information processing, and control 

Anaxonic neurons

Small. All cell processes look similar. Found in brain and special sense organs

Unipolar neurons (pseudounipolar neurons)

Axons and dendrites are fused. Cell body to one side. Most sensory neurons of PNS 

Bipolar neurons

Small and rare. One dendrite and one axon. Found in special sense organs

Multipolar neurons

Have one long axon and two or more dendrites. Most common in the CNS. All motor neurons that control skeletal muscles 

Three functional categories of neurons

Sensory, Motor, and Interneurons

Sensory neurons (afferent neurons)

Unipolar. Cell bodies grouped in sensory ganglia. Processes extend from sensory receptors to CNS

Monitors external environment

Somatic sensory neurons

Monitors internal environment 

Visceral sensory neurons 

Motor neurons (efferent neurons)

Mostly unipolar. Carry instructions from CNS to peripheral effectors (axons). 

Somatic motor neurons of SNS

Innervates skeletal muscles

Visceral motor neurons on ANS

Innervates all other peripheral effectors. Smooth and cardiac muscle, glands, adipose tissue

Interneurons

Multipolar. Most are in brain and spinal cord. Located between sensory and motor neurons.

Interneurons function

Responsible for distribution of sensory information, and coordination of motor activity. Involved in higher functions such as memory, planning, and learning. 

Neuroglia

Support and protect neurons. Make up half the volume of the nervous system

Neuroglia in the CNS

Astrocytes, Ependymal cells, Oligodendrocytes, Microglia

Neuroglia in the PNS

Satellite cells, Schwann cells

Astrocytes

Have large cell bodies with many processes. Maintain BBB. Create 3-D framework for CNS. Repair damaged nervous tissue. Guide neuron development. Control interstitial environment 

Ependymal Cells 

Form epithelium that lines central canal of spinal cord and ventricles., Produce and monitor cerebrospinal fluid. Have cilia that help circulate CSF. 

Oligodendrocytes

have small cell bodies with few processes. Many cooperate to form a myelin sheath. 

Increases speed of action potentials and make nerves appear white 

Myelin sheath 

Microglia

Smallest and least numerous neuroglia. Have many fine-branched processes. Migrate through nervous tissue. Clean up cellular debris, wastes, and pathogens. 

Satellite cells 

Surround ganglia. Regulate interstitial fluid around neurons

Schwann cells (neurolemmocytes)

Forms myelin sheath or indented folds of plasma membrane around axons. 

Neurolemma

Outer surface of Schwann cell. A myelinating Scwann cell sheaths only one axon.

Myelin Sheath

Composed of repeating layers of plasma membrane of schwann cells or oligodendrocytes.

Myelination

Process that forms myelin sheath from plasma membranes of neuroglial cells. Wrap themselves around axons forming multiple layers of membrane

Functions of myelin sheath

Neuron transmit an electrical current generate by movement of ions in body fluids. Lipid content of myelin sheath insulates which prevent electrical current from leaking into surround fluid. Increases speed of AP. 

Internodes

Segements of axon that are covered by neuroglia 

Node of Ranvier

Gap between adjacent neuroglia where myelin sheath is absent

White matter of myelin sheath

Composed of myelinated axons that appear white

Gray matter of myelin sheath

Composed of neuron cell bodies, unmyelinated dendrites and axons that appear gray

Forms path for new growth and wrap around new axon. In PNS, neurons can regenerate as long as the cell body remain intact 

Schwann cells 

Nerve regeneration in CNS

Limited by astrocytes which produce scar tissue and release chemicals that block growth

Generation of Action potential: Step 1

Depolarization to threshold

Generation of Action potential: Step 2

Activation of voltage gated Na channels

Generation of Action potential: Step 3

Inactivation of Na channels and activation of K channels

Generation of Action potential: Step 4

Return to resting membrane potential

Refractory period 

From beginning of AP until return to resting state. During which the membrane will not respond normally to additional stimuli 

Absolute refractory period

All voltage gated Na channels are already open or inactivated, Membrane cannot respond to further stimulation

Relative refractory period

Begin when Na channels regain resting condition but K channels are still open. Membrane is repolarizing/hyperpolarizing. Continues until membrane potential stabilizes 

Propagation

Moves an action potential along an axon in a series of steps

Continuous propagation of action potentials

Occurs in unmyelinated axons. Affects one segment of an axon at a time

Continuous propagation: Step 1

Action potential develops at initial segment

Continuous propagation: Step 2

Local current develops. Depolarizes second segment to threshold

Continuous propagation: Step 3

Action potential occurs in second segment. Initial segment begins repolarization

Continuous propagation: Step 4

Local current depolarizes next segment

Saltatory propagation of action potentials

Occurs in myelinated axons. Faster than continuous propagation and requires less energy, Myelin prevent continuous propagation, Local current jumps from node to node. Depolarization occurs only at nodes.

Synapse

Specialized site where a neuron communicates with another cell

Electrical Synapse

Direct physical contact between cells.

Chemical Synapse

Signal transmitted across a gap by a neurotransmitter

Chemical Synapse 

Presynaptic and postsynaptic membranes are locked together at gap junctions. 

Chemical synapse

Found in areas of brain responsible for programmed, automatic behaviors

Chemical synapse

Outside brain, found in cardiac and visceral smooth muscle to allow for coordinated muscle activity

Electrical synapse: Bidirectional transmission

Either neuron can be pre or postsynaptic depending on which direction current is flowing between them 

Electrical synapse: Instantaneous transmission

Only delay is time it takes presynaptic neuron to depolarize

Neurotransmitters

Chemical messengers contain within synaptic vesicles in axon terminal of presynaptic cell. Released into synaptic

Neurotransmitters

Affect receptors of postsynaptic membrane. Broken down by enzymes. Reabsorbed and reassembled by axon terminal 

Events at a chemical synapse: 1

An action potential in presynaptic neuron triggers voltage gated calcium ion channels in axon terminal to open

Events at a chemical synapse: 2

Influx of calcium ions causes synaptic vesicles to release neurotransmitter into synaptic cleft

Events at a chemical synapse: 3

Neurotransmitters bind to receptors on postsynaptic neuron

Events at a chemical synapse: 4

Ion channels open, leading to a local potential and possibly an action potential if threshold is reached

Terminating synaptic transmission: Method 1 

Some neurotransmitters diffuse away from synaptic cleft in ECF. Can be reabsorbed into a neuron or an astrocyte

Terminating synaptic transmission: Method 2

Neurotransmitter can be broken down in synaptic cleft by enzymes. By-products of reaction can be reabsorbed by presynaptic membrane for reassembly of original neurotransmitter

Terminating synaptic transmission: Method 3

Some neurotransmitters are reabsorbed into presynaptic neuron by a process called reuptake

Excitatory neurotransmitters

Cause depolarization of postsynaptic membranes. Promote action potentials

Inhibitory neurotransmitters

Cause hyperpolarization of postsynaptic membranes. Suppress action potentials

Effect of a neurotransmitter on postsynaptic membrane

Depends on the properties of the receptor. Not on the nature of the neurotransmitter

Major classes of neurotransmitters

ACH, Biogenic amines, Amino acids, Neuropeptides, Dissolved gases

ACH

Small molecule neurotransmitter widely used by nervous system. Cholinergic synapses use ACH. Found in neuromuscular junction, within brain and spinal cord and within autonomic nervous system. Largely excitatory but it does exhibit some inhibitory effects in PNS

Biogenic amines

Class of five neurotransmitters synthesized from amino acids. Used throughout CNS and PNS for many functions such as regulation of homeostasis and cognition

Norepinephrine

Found mainly in ANS where it influences HR, BP, and digestion. In CNS it regulates sleep/wake cycle, attention, and feeding behaviors

Epinephrine

Has similar functions as norepinephrine. More widely used as a hormone by endocrine system

Dopamine

Used extensively by CNS. Helps to coordinate movement. Involved in emotion and motivation. Drugs that bind to dopamine receptors are highly addictive

Serotonin

Synthesized from amino acid tryptophan. Most serotonin-secreting neurons are found in brainstem. Axons project into multiple areas of brain. Functions include mood regulation, emotions, attention, feeding behaviors, and daily rhythms

Histamine

Synthesized from amino acid histidine. Involved in regulation of arousal and attention 

Glutamate

Most important excitatory neurotransmitter in CNS. Binds receptors and opens channels that allow for flow of both sodium and calcium ions

Glycine and GABA

Both major inhibitory neurotransmitters. Open chloride ion channels. Hyperpolarize axolemma