Chapter 7 - Nervous System and Neuronal Excitability

Central nervous system (CNS)

Brain and spinal cord. Processes many different kinds of incoming sensory information and is the source of thoughts, emotions, and memories.

Peripheral nervous system (PNS)

Consists of all nervous tissue outside the CNS.

Nerve

Bundle of axons that lies outside the brain and spinal cord in the PNS

Sensory receptor

A structure that monitors changes in the external or internal environment.

Afferent division

Conveys input into the CNS from sensory receptors in the body.

Somatic senses

Tactile, thermal, pain, and proprioceptive sensations

Special senses

Smell, taste, vision, hearing, and equilibrium

Efferent division

Conveys output from the CNS to effectors.

Somatic nervous system

Conveys output from the CNS to skeletal muscles only.

Autonomic nervous system

Convey output from the CNS to smooth muscle, cardiac muscle, and glands.

Parasympathetic nervous system

Rest and digest activities

Sympathetic nervous system

Helps support exercise or emergency actions (fight or flight responses)

Enteric nervous system (ENS)

Part of the autonomic nervous system. Network of neurons confined to the wall of the GI tract. Help regulate the activity of the smooth muscle and glands of the GI tract

Sensory function

Sensory receptors detect external or internal stumuli, and convey it through the cranial and spinal nerves of the PNS int the brain and spinal cord of the CNS

Integrative function

CNS processes sensory information by analyzing it and making decisions for appropriate responses

Motor function

Once sensory information is integrated, the CNS may elicit an appropriate motor response. Motor information is conveyed from the CNS through the cranial abdomen spinal nerves of the PNS to effectors.

Neurons (nerve cells)

Basic functional units of the nervous system

Dendrites

Short, highly branched processes that extend from the cell body. Main input portions of the neuron

Cell body (soma)

Contains most of the organelles and the nucleus. Functions as the control centre of the neuron. Can receive signals from other neurons.

Ganglion (ganglia)

Cluster of neuronal cell bodies in the PNS

Nucleus (in the CNS)

Cluster of neuronal cell bodies in the CNS

Axon

Single, long, thin process that extends from the cell body. Functions as the output portion of the neuron by generating action potentials.

Axon hillock (trigger zone)

Cone-shaped region where the axon connects to the cell and generates action potentials.

Axon collaterals

Side branches on the axon

Axon terminals

End of the axons and its collaterals.

Synaptic end bulbs

The tips of the axon terminals that swell to form synapses with other cells.

Synapse

Site of communication between a neuron and a target cell (another neuron, muscle fibre, or gland cell).

Synaptic vesicles

Found in the synaptic end bulbs that store chemical neurotransmitters

Axonal transport

Materials being moved between the cell body and axon terminals.

Anterograde

Forward direction

Retrograde

Backward direction

Tract (in the CNS)

Bundle of axons in the CNS

Somatic motor neurons

Convey action potentials to skeletal muscles

Autonomic motor neurons

Convey action potentials to cardiac muscle, smooth muscle, or glands.

Neuroglia (glia)

“glue” that holds the neurons together as well and nourishes and protects neurons, as well as maintain homeostasis in the interstitial fluid.

Astrocytes

Type of glia that is most numerous. It wraps around capillaries in the CNS. They secrete chemicals to help maintain the tightness in tight junctions.

Oligodendrocyte

Type of glia that is responsible for forming and maintaining the myelin sheath around axons of neurons in the CNS.

Microglia

Type of glia that functions as phagocytes by removing cellular debris formed during development.

Ependymal cells

Type of glia that lines the ventricles of the brain and central canal of the spinal cord.

Schwann cells

Type of glia only found in the PNS. They form the myelin sheath around axons of the PNS neurons.

Myelin sheath

Multilayered covering composed of lipids and proteins. It insulates the axon of a neuron and increases the speed of conduction of action potentials.

Nodes of Ranvier

Gaps in the myelin sheath that appear at intervals along the axon.

Myelinated

Axons that have myelin sheaths

Unmyelinated

Axons without myelin sheaths.

White matter

Composed primarily of myelinated axons

Gray matter

Composed of neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia.

Plasticity

Capability to change based on experience

Membrane potential

The difference in electrical charges that exists just across the plasma membrane

Electrical excitability

The ability to response to a stimulus and convert it into an action potential

Nerve action potential (nerve impulse)

An action potential that occurs in a neuron

Muscle action potential (muscle impulse)

An action potential that occurs in a muscle fibre

Upper motor neuron

Type of neuron that synapses with a lower motor neuron farther down in the CNS in order to contract a skeletal muscle

Lower motor neuron

Type of neuron that directly supplies skeletal muscle fibres.

Neuromuscular junction

Type of synapse formed between a neuron and a skeletal muscle fibre

Electrochemical gradient

A concentration difference plus an electrical difference

Leak channels

Randomly alternate between open and closed positions. They are important for establishing the resting membrane potential.

Ligand-gated channel

Opens or closes in response to a specific ligand (chemical) stimulus. Participate in the generation of graded potentials.

Mechanically-gated channel

Opens or closes in response to mechanical stimulation in the form of touch, pressure, tissue stretching, or vibration. Involved in the formation of graded potentials

Voltage-gated channel

Opens in response to a change in membrane potential. Responsible for the generation and conduction of action potentials.

Voltage

The electrical potential difference between opposite charges that are separated from each other. Measured in volts or millivolts

Membrane potential

The voltage that exists across the plasma membrane of a cell.

Resting membrane potential

Resting voltage. Unstimulated cell.

Current

The flow of charged particles

Resistance

The hindrance to the flow of charges

Conductors

Substances that permit fast current flow because they have low resistance

Insulators

Substances that decrease current flow because they have high resistance.

Polarized cell

A cell that exhibits a membrane potential

K+ equilibrium potential

K+ electrical gradient becomes equal to the magnitude to the opposing K+ concentration gradient and there is not net movement of K+ ions into or out of the neuron.

Equilibrium potential

The membrane potential at which the concentration gradient and electrical gradient for a particular ion are equal in magnitude but opposite in direction and there is no net movement of that ion across the plasma membrane. K+ initially moves out of the cytosol into the ECF

Na+ equilibrium potential

Na+ electrical gradient becomes equal in magnitude to the opposing Na+ concentration gradient and there is no net movement of Na+ ions into or out of the neuron. Na+ initially moves from the ECF into the cytosol.

Electrogenic

Contribution to the negativity of the resting membrane potential.

Graded potential

Small deviation from the membrane potential that makes the membrane either less polarized (inside less negative) or more polarized (inside more negative).

Depolarizing graded potential

When a response makes the membrane less polarized (inside less negative).

Hyperpolarizing graded potential

When a response makes the membrane more polarized (inside more negative).

Local current flow

The passive movement of charges from one region of membrane to adjacent regions of membrane due to differences in membrane potential in these areas.

Decremental conduction

Mode of travel where graded potentials die out as they spread along the membrane due to charges being lost through leak channels.

Summation

Process by which graded potentials add together and become stronger.

Postsynaptic potential

When a graded potential occurs in the dendrites or cell body of a neuron in response to a neurotransmitter

Receptor potential

When a graded potential occurs in sensory receptors

End plate potential

When a graded potential occurs in the plasma membrane of a skeletal muscle fibre at the neuromuscular junction

Depolarizing phase (rising phase)

The negative membrane potential becomes less negative, reaches zero, and then becomes positive.

Repolarizing phase (falling phase)

Membrane potential is restored to the resting state of -70mV.

After-hyperpolarizing phase (undershoot)

The membrane potential temporarily becomes more negative than the resting level.

Subthreshold stimulus

A stimulus that is a weak depolarization that cannot bring the membrane potential to threshold.

Threshold stimulus

A stimulus that is just strong enough to depolarize the membrane to threshold.

Suprathreshold stimulus

A stimulus that is strong enough to depolarize the membrane above threshold.

All-or-none principle

Characteristic of an action potential to either completely occur or to not occur.

Refractory period

The period of time after an action potential begins during which and excitable cell cannot generate another action potential in response to a normal threshold stimulus.

Relative refractory period

Period of time during which a second action potential can be initiated, but only by a larger than normal stimulus.

Propagation

Action potential has a continuous flow and does not die out, it maintains it strength as it travels along.

A fibers

Large-diameter, myelinated axons that conduct urgent information like sensory signals that cause muscle contraction.

C fibers

Small-diameter, unmyelinated axons that conduct less critical information like motor signals that cause contraction of smooth muscles in digestive organs.

Synapse

Site of communication between two neurons or between and neuron and an effector cell.

Presynaptic neuron

The neuron sending the signal

Postsynaptic neuron

The neuron receiving the message

Axodendritic

Synapse between an axon and a dendrite

Axosomatic

Synapse between and axon and cell body

Axoaxonic

Synapse between two axons

Electrical synapse

Action potentials conduct directly between adjacent cells though gap junction

Chemical synapse

In response to an action potential, neurotransmitters are released into the synaptic cleft from the presynaptic neuron and bind to receptors on the postsynaptic neuron.