BIOL 251 Nervous System

Sensory input

Information gathered via sensory receptors

Motor output

Process where the nervous system produces a response in the body through activation of the effector organ.

Effector organ

Typically a muscle or gland

Integration

Process information

What are the two main divisions of the nervous system?

The central nervous system and the peripheral nervous system

Central Nervous System (CNS)

Includes the brain and spinal cord

Peripheral Nervous System (PNS)

Includes all nerves not in the brain or spinal cord. Includes the cranial nerves and spinal nerves, specifically.

Somatic sensory receptors

Detect positioning of joins/muscles, touch, pressure, pain, and temperature.

Visceral receptors

Detect changes in internal organs like BP, presence of chemicals, or mechanical changes like stretch

Special receptors

Detect sensations of vision, hearing, taste, smell, and balance.

Sensory receptors are part of which division of the nervous system?

PNS

Afferent (sensory) signals

Sent to the CNS for integration and interpretation

Motor (efferent) division

Creates a response on an effector (target organ), which will respond to the nervous system.

What are the two divisions of the motor branch of the PNS?

Somatic nervous system, and autonomic nervous system

Somatic nervous system

Innervates skeletal muscle, sometimes called voluntary division

Autonomic nervous system

involuntary system, innervates glands, smooth muscle, cardiac muscle, adipose tissue

What are the two divisions of the autonomic system?

Sympathetic nervous system, parasympathetic nervous system

Sympathetic nervous system

Activates and prepare the body for vigorous muscular activity, stress, and emergencies.

Parasympathetic nervous system

Generally operates during normal situations, permits digestion, and conserves energy.

Receptors

Sensory structures that detect changes in internal or external environment

Effectors

Target organs who's activities change in response to neural commands

What protects the CNS?

Bone (skull and vertebrae), Meninges

What protects the PNS?

Connective tissue

Cell bodies of CNS?

Nuclei

Cell bodies of PNS?

Ganglia

Group of axons in CNS?

Tracts

Group of axons in PNS?

Nerve

What is the effect of damage to the CNS vs PNS?

CNS: Can cause global effects on body or affect large areas

PNS: Local effects

Regenerative capabilities of axons - CNS vs PNS

CNS: Do not repair

PNS: May repair depending on injury

Nervous tissue

Made up of neurons and support cells called neuroglia.

Neurons

Nerve cells that conduct electrical impulses and relay information throughout the body

Three basic parts of a neuron:

Dendrites, cell body, axon

Do neurons undergo cell division/mitosis?

No

Neuron cell body

Synthesizes all nerve cell products. Consists of large nucleus with surrounding cytoplasm containing organelles.

How many mitochondria does an individual neuron have?

Over 1000

Dendrites

Receiving end of neuron

Axon

Conducts nerve impulses away from the cell body to its axon terminals.

Axon terminals

Contain vesicles filled with neurotransmitters

Axon hillock

Where single axon arises

Neurotransmitters

Chemicals stored inside synaptic vesicles (axon terminal vesicles) at the end of the axon terminals. Carry transmission of nerve impulse from one neuron to another when released by axon terminal vesicles.

Synaptic vesicles (axon terminal vesicles)

Store neurotransmitters. At the end of the axon terminals.

Synaptic cleft

The gap between two neurons

Presynaptic neuron

The neuron before the synaptic cleft

Postsynaptic neuron

The neuron found after the synaptic cleft

Neurotransmitters are sent from a _____ neuron across the synapse to the _____ neuron.

Presynaptic, postsynaptic

How are neurons classified structurally?

According to the number of extensions from their cell body - multipolar, bipolar, or pseudounipolar

Multipolar neurons

Have 3 or more extensions from the cell body. One axon, many dendrites. Can be called motor neurons.

Bipolar neurons

Have a central body with two extensions. Found within the body as special receptor cells in the visual and olfactory systems.

Pseudounipolar (unipolar) neurons

Have one extension off the cell body that branches into two: one central process running to the CNS and another peripheral process running to the sensory receptor. These neurons are sensory neurons in the peripheral nervous system.

Sensory neurons (afferent)

Pseudounipolar, carry information from PNS to CNS (usually from skin or internal organs to CNS). Located outside the spinal cord in the ganglion. Info travels into CNS via dorsal root on posterior side of the spinal cord.

Interneurons (Association neurons)

Found only in CNS. Typically multipolar, transmit impulses within the CNS. Connect sensory and motor neurons.

Motor neurons (efferent)

Multipolar, send messages from CNS to organs, muscles, or glands. Moto neuron cell bodies are in the CNS and their axons extend out through the ventral root, on the anterior side of the spinal cord.

Neuroglial cells

Support cells for neurons. Make up the majority of nervous tissue with 5-10 neuroglial cells per neuron. Include ependymal cells, oligodendrocytes, astrocytes, microglial cells.

Ependymal cells

Line the ventricles and central canal of spinal cord. Simple cuboidal cells, line the fluid-filled passageways in the brain and spinal cord. Help to produce and regulate cerebrospinal fluid (CSF).

Cerebrospinal fluid

Clear fluid that only circulates in brain and spinal cord.

Oligodendrocytes

Act as the insulation for central nervous system axons by wrapping around axons and providing myelination. Can wrap around axons of multiple neurons

Astrocytes

Control the chemical environment of neurons by wrapping around the blood capillaries. This creates a physical barrier called the blood-brain barrier. Astrocytes also recycle neurotransmitters and form scar tissue when there is an injury.

Blood-brain barrier

Physical barrier that allows the passage of only certain substances into the central nervous system.

Microglial cells

Phagocytic cells that protect the CNS by scavenging dead cells and infectious microorganisms.

Schwann cells

Surround all peripheral nerve axons in short sections. Each Schwann cell covers a small section of an axon by wrapping around it. Responsible for making a white fatty layer called the myelin sheath

Myelin sheath

White fatty layer rolled around the axon, insulating the nerve fiber and increasing the speed of nerve impulses.

Nodes of ranvier

Gaps between Schwann cell-insulating sections

Neurilemma

Schwann cell wrapped around the myelin sheath and the axon at the nodes of Ranvier with its cytoplasm, nucleus, and outer cell membrane.

Crucial for helping nerve fibers grow back.

Satellite cells

Surround cell bodies of peripheral neurons in ganglia. Helps regulate the cell body environment.

Axonal regeneration

Process of peripheral nerves regenerating the axon length - Schwann cells make this possible. No axonal regeneration in the CNS.

Action potentials

Electrical impulses. Resting membrane potential becomes an action potential if the membrane becomes depolarized.

Polarized

Plasma membrane at rest

Resting membrane potential

Difference in electrical charge between the two sides of the axon. When the axon is not conducting an impulse, this is equal to about -70 millivolts in a neuron.

Leak channels

Used by passive chemical gradients to move down their concentration gradients across a cell membrane. Always open.

Sodium-potassium pumps (Na+/K+ ATPase)

Active pumps that use active transport (ATP) to carry ions across the plasma membrane against their concentration gradients.

Active channels

Gated. Only open in response to specific stimuli.

Three types: Ligand-gated channels, Voltage-gated channels, Mechanically gated ion channels

Ligand-gated channels (chemically gated channels)

Need a specific ligand (like a neurotransmitter) to bind and open the gate.

Voltage-gated channels

Open when they detect a change in the membrane potential in the cell. Characteristic of excitable membranes, such as axon membranes and the sarcolemma of skeletal muscle fibers.

Mechanically gated ion channels

Open in response to a physical change in shape of the membrane. Typically located only on dendrites of sensory neurons.

Graded potentials

Small, local changes in the membrane potential of the dendrites and cell body of the neuron

Threshold

Voltage that must be reached for the action potential to be triggered

Depolarization

Shift in the membrane potential that causes it to become less negative

Hyperpolarization

Increases the negative value inside the cell.

Repolarization

Changing from a depolarized state back to the resting membrane potential

Absolute refractory period

Axon membrane does not change based on additional stimulation because the sodium channels are already open during depolarization, and then become inactivated.

Relative refractory period

Axon can respond to additional stimulation, only if it is much larger than a normal stimulus

Phases of an action potential

1. Resting potential

2. Depolarization

3. Repolarization

4. Hyperpolarization

Why are action potentials self-propagating?

Ion channels are prompted to open whenever the membrane potential decreases (depolarizes) in an adjacent area.

All-or-nothing response

Either occurring at full strength or not

Continuous propagation

Segments of the unmyelinated axon are depolarized, eventually moving along the entire length of the axon to the axon terminals.

Saltatory conduction

The nerve impulse is forced to jump to the next node at exposed nodes of Ranvier. Greatly increases the speed of nerve impulse transmission along the axon.

Summation

Can be spatial or temporal

Temporal summation

The relationship of multiple action potentials coming from a single cell in quick succession.

Spatial summation

Occurs when the activity of two or more cells arrive at the same time to a neuron and combine effects.

The transmission of nerve impulses is _____ in nature.

Electrochemical

EPSP - Excitatory postsynaptic potential

Depolarization in a postsynaptic neuron. Makes an action potential in the postsynaptic neuron more likely to happen.

IPSP - Inhibitory postsynaptic potential

Make the interior of the postsynaptic cell more negative, moving the cell away from the threshold. This makes an action potential less likely to happen in the postsynaptic cell.

Acetylcholinesterase

Breaks down acetylcholine

Reuptake

Resorption of the neurotransmitter into the presynaptic neuron.

Inhibition

Prevention of continuous stimulation

Examples of neurotransmitters

Norepinephrine, Dopamine, Acetylcholine, Serotonin, GABA, Glutamate

Neuromodulators

Substances that change how fast the presynaptic neuron releases its neurotransmitter. Can also act on the postsynaptic neuron to change its response to the neurotransmitter

Ionotropic receptors

Ligand-gated ion channels, such as the receptor for acetylcholine or the glutamate receptor.

Metabotropic receptor

Complex of proteins that result in metabolic changes within the cell. Includes the transmembrane receptor protein, a G protein, and an effector protein

Effector protein

Enzyme that catalyzes the generation of a new molecule, which acts as a second messenger