Neurophysiology

This set of flashcards captures key vocabulary and definitions related to neurophysiology, helping to reinforce understanding of the nervous system's structure and function.

Nervous System

The communication and control system of the body, responsible for collecting information, processing it, and initiating responses.

CNS (Central Nervous System)

Composed of the brain and spinal cord.

PNS (Peripheral Nervous System)

Consists of nerves (fiber bundles) and ganglia (clusters of cell bodies) outside the CNS.

Sensory nervous system

Afferent nervous system- receives sensory information from receptors and transmits it to CNS; Somatic sensory system and Visceral sensory system

Visceral sensory system

Detects stimuli we typically do not perceive - ex. Signals from the heart or kidneys (sensory nervous system)

Somatic sensory system

Detects stimuli we consciously perceive (sensory nervous system)

Motor nervous system

Efferent nervous system- initiates motor output and transmits it from CNS to effectors; somatic motor system and Autonomic motor system

Somatic motor system

Sends voluntary signals to skeletal muscles (motor nervous system)

Autonomic motor system

(Visceral motor) sends involuntary commands to heart, smooth muscle, and glands; has sympathetic and parasympathetic divisions (motor nervous system)

Nerve

A bundle of parallel axons in the PNS; have connective tissue wrappings

Epineurium

Encloses entire nerve; thick layer of dense irregular connective tissue

Perineurium

Wraps fascicle (small bundle of axons in nerve); layer of dense irregular connective tissue

Endoneurium

Wraps an individual axon; delicate layer of areolar connective tissue; separates and electrically insulates each axon

Cranial nerves

Extend from brain

Spinal nerves

Extend from spinal cord

Structural classification of nerves

Cranial and spinal nerves

Functional classification of nerves

Sensory, motor, and mixed nerves

Sensory nerves

Contain sensory neurons sending signals to CNS

Motor nerves

Contain motor neurons sending signals from CNS

Mixed nerves

Contains both sensory and motor neurons; most named nerves are in this category; individual axons in these nerves transmit only one type of information

Excitability, conductivity, secretion, extreme longevity, amitotic

neurons (nerve cells) traits

Excitability

Responsiveness to a stimulus; stimulus causes change in cells membrane potential

Conductivity

Ability to propagate electrical signal; voltage-gated channels along membrane open sequentially

Secretion

Release of neurotransmitter in response to conductive activity; messenger is released from vesicle to influence target cells

Extreme longevity

Cell can live throughout person’s lifetime

Amitotic

After fetal development, mitotic activity is lost in most neurons

Cell body (soma)

Plasma membrane encloses cytoplasm (perikaryon); contains nucleus; initiates some graded potentials, receives others from dendrites and conducts these potentials to axon; contains chromatophilic substance (nissl bodies) made of ribosomes (free and bound)

Dendrites

Short, unmyelinated processes branching off cell body; receive input and transfer it to cell body

Axon

Long process emanating from cell body; makes contact with other neurons, muscle cells, or glands; attaches to cell body at axon hillock; cytoplasm called axoplasm, membrane called axolemma; splits into branches called axon collaterals; ends in several telodendria (axon terminals); tips of telodendria are synaptic knobs (terminal boutons); synaptic knobs house synaptic vesicles containing neurotransmitter

Function is to conduct action potentials and then release neurotransmitter at synaptic knobs

Cytoskeleton

Composed of microfilaments, intermediate filaments, microtubules

Neurofilaments

What are intermediate filaments termed? They provide tensile strength and aggregate to form bundles, neurofibrils

Anterograde transport

From cell body; moves newly synthesized material towards synaptic knobs; transports of vesicles, organelles, glycoproteins

Retrograde transport

To cell body; moves used materials from axon for breakdown and recycling in soma

Fast anoxal transport

Occurs at about 400mm per day; involves movement along microtubules; powered by motor proteins that split ATP; anterograde or retrograde motion possible

Slow axonal transport

Occurs at about 0.1 to 3mm per day; results from flow of axoplasm; substances only moved from cell body toward knob (enzymes, cytoskeleton components, new axoplasm)

Multipolar neurons

Many dendrites, one axon extend from soma; most common type

Bipolar neurons

One dendrite and one soma extend from soma; limited number; ex. Retina of the eye

Unipolar neurons (pseudounipolar)

One axon extends from soma; axon splits into two processes- peripheral process and central process

Anaxonic neurons

Have dendrites but no axons

Peripheral process

Splits into several receptive dendrites (unipolar neurons)

Central process

Leads to synaptic knobs in CNS (unipolar neurons)

Sensory neurons

Conduct input from somatic and visceral receptors to CNS; most are unipolar, few bipolar

Motor neurons

Conduct output from CNS to somatic and visceral effectors; All are multipolar

Interneurons (association neurons)

Association neurons; Receive, process, and integrate information from many other neurons; communicate between Ensor and motor neurons; locked within CNS, make up 99% of our neurons; generally are multipolar

Synapse

place where a neuron connects to another neuron or an effector; two types- chemical and electrical

Electrical synapse

Presynaptic and postsynaptic neurons bound together by gap junctions; fast: no synaptic delay in passing electrical signal

Chemical synapse

Presynaptic neuron’s axon terminal produces signal; postsynaptic neuron receives signal (most commonly with one of its dendrites) synaptic cleft

Synaptic cleft in chemical synapse

Small fluid-filled gap between the two neurons

Events of synaptic communication

Neurotransmitter molecules released from vesicles of synaptic knob into cleft; transmitter diffuses across cleft and binds to postsynaptic receptors; binding of transmitter to receptor initiates postsynaptic potential (a graded potential); synaptic delay: time it takes for all of these events

Glial cells (neuroglia)

Nonexcitable, support cells found in CNS and PNS; smaller but farther outnumber neurons, account for about half the volume of nervous system; capable of mitosis; protect and nourish neurons; provide physical scaffolding for nervous tissue; critical for normal function at neural synapses

Astrocytes (star shaped cells)

Have processes that end in perivascular feet; most abundant glial cell in CNS; help form blood-brain barrier by wrapping feet around brain capillaries; regulate tissue fluid composition; form structural framework, strong cytoskeleton helps support nearby neurons; assist development by secreting chemicals that regulate synapse formation; occupy the spac of neurons that have died

Blood brain barrier

Controls which substances enter brains nervous tissue from blood

Ependymal cells

Lined internal cavities of brain and spinal cord; ciliated simple cuboidal or simple columnar epithelial cells; form choroid plexus with nearby blood capillaries; have cilia to help circulate CSF

Choroid plexus

Helps produce cerebrospinal fluid; liquid that bathes CNS and fills its cavities

Microglia

Small, rare cell that wander CNS and replicate in infection; phagocytic cells of immune system that engulf infectious agents; remove debris from damaged CNS tissue

Oligodendrocytes

Large cells with slender extensions; extensions wrap around axons of neurons forming myelin sheath; myelin insulation allows for faster action potential propagation

Satellite cells

Arranged around neuronal cell bodies in a ganglion; electrically insulate and regulate the exchange of nutrients and wastes

Neurolemmocytes (Schwann cells)

Elongated, flat cells that ensheath PNS axons with myelin; allows for faster actin potential propagation

Neurolemmocyte (Schwann cell)

Glial cells in the PNS that myelinate and insulate axons, allowing for faster action potential propagation.

Tumors

Neoplasms from unregulated cell growth; sometimes occurs in CNS; originating from the brain; typically originate in supporting tissues (tissues with capacity to undergo mitosis; from meninges or glial cells)

Gliomas

Glial cell tumors; may be relatively benign, may be malignant (capable of metastasizing)

Myelination

The process of wrapping an axon with myelin, which increases the speed of action potential propagation.

Myelin

Several layers of membrane of glial cells; high liquid content gives glossy-white appearance and insulates axon; the glia are neurolemmocytes in PNS; oligodendrocytes in CND

Myelination in the PNS

Neurolemmocyte encircles neuron axon and wraps it in layers forming myelin sheath; neurolemmocytes’s cytoplasm and nucleus and pushed to periphery forming neurilemma; a neurolemmocyte can myelinate only 1 mm of axon, so several are needed for one axon

Neurofibril nodes (nodes of Ranvier)

Gaps between neurolemmocytes

Unmyelinated axons

Exist in PNS and CNS

In PNS: axon sits in depressed portion of neurolemmocyte; not full ensheathed by it

In CNS: unmyelinated axons not associated with oligodendrocytes

Multiple sclerosis

Progressive demyelination of neurons in CNS; autoimmune disorder: oligodendrocytes attacked immune cells; repeated inflammatory events causing scarring and permanent loss of function

Gullian-Barre syndrome

Loss of myelin from peripheral nerves due to inflammation; muscle weakness begins in distal limbs, advances to proximal muscles; most function recovered with little medical intervention

Neuron cell body is intact and enough neurilemma remains

Amount of damage is less intensive, distance between site of damage and structure it innervates is shorter

When is regeneration of PNS axons possible after traumatic injuries?

When is success more likely?

Steps of axon regeneration

1. Axon served by trauma

2. A. Proximal to the cut: the axon seals off and swells B. Distal to the cut, the axon and sheath degenerate (wallerian degeneration) but the neurilemma survives

3. Neurilemma and endoneurium form a regeneration tube

4. Axon regenerates guided by nerve growth factors released by neurolemmocytes

5. Axon reinnervates original effector or sensory receptor

Oligodendrocytes secrete growth-inhibiting molecules, not growth factors; large number of axons crowd the CNS; regrowth obstructed by scars from astrocytes and connective tissue

Why is CNS axon regeneration extremely limited?

Pumps

Membrane proteins that maintain a concentration gradient by moving substances against their concentration gradient; requires cellular energy; neurons have sodium-potassium pumps and calcium pumps in their membranes

Channels

Protein pores in the membrane that allow ions to move down their concentration gradients (into or out of the cell); when open they allow a specific type of ion to diffuse

Leak, chemically gated, voltage-gated

Leak (passive) channels

Always open for continuous diffusion

Chemically gated channels

Normally closed, but open when neurotransmitter binds

Voltage-gated channels

Normally closed, but open when membrane charge changes

Resting state, activation sate, inactivation state

What are the 3 stages of voltage gated sodium channels

Resting state of voltage gated Na+ channels

Activation gate closed; inactivation gate open

Activation state of voltage-gated Na+ channels

Activation gate open (due to voltage changes), inactivation gate open; Na+ moves through channel

Inactivation state of voltage-gated Na+ Channels

Activation gate open, inactivation gate closed; entry of Na+ prevented; this state lasts a short time- the channel quickly resets to resting state

Modality gated channels

Normally closed, but open in response to a stimulus other than a chemical or a voltage change; found in membranes of sensory neurons that respond to changes in their environment (ex. Some receptor neurons of the skin have mechanically gated channels that sense pressure)

Receptive segment (dendrite and cell body)- chemically gated channels (ex. Chemically gated Cl- channels)

initial segment (axon hillock)- voltage-gated Na+ channels and voltage-gated K+ channels

conductive segment (axon and its branches)- voltage-gated Na+ channels and voltage-gated K+ channels

transmissive segment (synaptic knobs)- Voltage-gated Ca2+ channels and Ca2+ pumps

Functional segments of neuron have additional channels and/or pumps which are

Electrical current

What is neuron activity dependent upon

Voltage (potential energy)

Amount of difference in electrical charge between two places; measured in volts or millivolts

Current

Movement of charged particles across barrier separating them; can be harnessed to do work

Resistance

Opposition to movement of charged particles (the barrier); an increase in resistance lowers the current

Ohm’s law

Current=voltage/resistance; currents increases with larger voltage and smaller resistance

How Ohm’s law is applied to neurons

Charged particles are ions, and current is generated when ions diffuse through channels; voltage exists across the membrane due to unequal distribution of ions; the membrane offers resistance to ion flow, and this resistance changes due to the actions of gated channels (resistance decreases when channels open)

Characteristics of resting neurons

Ions are unevenly distributed across the plasma membrane due to the actions of pumps; gated channels are closed in the functional segments of the cell; there is an electrical change difference across the membrane (this difference can be measured with microelectrodes- one inside cell, one outside- and a voltmeter)

Resting Membrane Potential (RMP)

The electrical charge difference across the membrane of a neuron at rest, typically around -70 mV.

K+ diffusion

What is the most important factor in setting RMP

Na+ because there are a few Na+ leak channels

What also influences RMP and why

The rule of Na+/K+ pumps

By pushing 3 positive charges out and pushing only 2, the pump contributes about -3mV (of the -70mV total); more importantly, it maintains the concentration gradients for these ions

Graded Potential

A localized change in membrane potential that can vary in size and does not travel far.

Depolarized, hyperpolarized

What is a cell when it is less negative than RMP? More negative?

Postsynaptic potential

Triggered by reception of neurotransmitters

Neurotransmitter binds to chemically gated ion channels and opens them; ions, diffuse across membrane changing its electrical potential; the voltage change is a graded potential; it is a local potential, it starts at dendrites or Soma and does not go far; the direction of the potential depends on what type of ion channel opens; it is a short-lived potential lasting only milliseconds

Action Potential

Conducted by an axon due to ion flow through voltage gated channels; involves depolarization and depolarization; propagated down axon to synaptic knob; are all or none once threshold is reached

Propagation

An impulse or nerve signal

Neurotransmitter

Chemical substances released from presynaptic neurons that signal target cells by binding to their receptors.

EPSP (Excitatory Postsynaptic Potential)

A depolarization of the postsynaptic membrane potential that makes the neuron more likely to fire an action potential; caused by cation entry