Neuroscience: Brain, Nerves, and Spinal Cord Key Concepts

Central nervous system (CNS)

brain and spinal cord

Peripheral nervous system (PNS)

nerves and ganglia

Nerve

a bundle of nerve fibers wrapped in fibrous connective tissue

Ganglion

a knot-like swelling of cell bodies in PNS

Excitability

ability to respond to stimuli

Conductivity

produce electrical signals that are conducted to other cells

Secretion

when signal reaches end of axon, the neuron secretes a neurotransmitter that stimulates the next cell

Sensory (afferent) neurons

detect stimuli and transmit information about them toward the CNS

Interneurons

receive signals from other neurons, process this information, and make resulting 'decisions'

Motor (efferent) neurons

send signals out to muscles and gland cells (the effectors)

Cell Body

Neurosoma, soma, perikaryon; Organelles; No centrioles: mature neurons cannot undergo mitosis after adolescence

Dendrite

Most numerous neurites; Resemble branching of a tree; Primary sites for receiving signals from other neurons; Neurons can have one or thousands

Axon

Long, cylindrical extension; Unbranched; Specialized for rapid nerve signals; Originates at axon hillock: mound on one side of cell body; Contains axoplasm and axolemma; Terminal arborization: branches

Multipolar Neuron

One axon and multiple dendrites; Most common type in body, most neurons in CNS

Bipolar Neuron

One axon and one dendrite; Olfactory cells, some neurons of retina, sensory neurons of ear

Unipolar Neuron

Single process leading away from cell body, splits into peripheral process and central process; Both processes comprise the axon; Short receptive endings of peripheral process are dendrites

Anaxonic Neuron

Many dendrites but no axon; Brain, retina, and adrenal gland

Axonal Transport

Two-way passage of materials along an axon; Anterograde transport: away from cell body; Retrograde transport: toward cell body

Anterograde transport

away from cell body

Retrograde transport

toward cell body

Supportive Cells

Approximately 86 billion neurons in the adult brain, and a roughly equal number of neuroglia; Form supportive tissue framework

Oligodendrocytes

Form myelin sheaths in CNS; Extends several processes that wrap around portions of axons

Myelin

Helps conduct electrical signal down the axon; Myelin sheath gap between segments; Node of Ranvier

Multiple Sclerosis (MS)

Oligodendrocytes and myelin sheaths in CNS deteriorate; Replaced by hardened scar tissue; Nerve conduction disrupted

Tay-Sachs Disease

Hereditary disorder in infants of Eastern European Jewish ancestry; Abnormal accumulation of glycolipid (GM2) that disrupts nerve signals; Blindness, loss of coordination, dementia

Ependymal Cells

Line internal cavities of brain; Secrete and circulate cerebrospinal fluid (CSF)

Microglia

Macrophages; Engulf debris, provides defense against pathogens

Astrocytes

Most abundant; Framework of nervous tissue; Perivascular feet contact capillaries and stimulate formation of blood-brain barrier (BBB)

Gliomas

tumors of glial cells; grow rapidly and are highly malignant

Satellite Cells (PNS)

Surround nerve cell bodies in ganglia; Provide insulation around cell body and regulate chemical environment

RMP

charge difference across membrane

K+ Influence on RMP

Greatest influence on RMP; More concentrated in ICF than ECF; Membrane permeable to K+ via leaky channels; Inside becomes more negative

Na+ Influence on RMP

More concentrated in ECF than ICF; Membrane much less permeable to Na+; Flows down electrochemical concentration gradient

Na+/K+ Pump Regulates RMP

Compensates for continual leakage of Na+ and K+; Moves Na+ out of cell; K+ into cell; Works continuously and requires ATP

Action Potentials

Dramatic, rapid up-and-down change in voltage produced by the coordinated opening and closing of voltage-gated ion channels

All-or-nothing

APs not graded like local potentials

Refractory Period

Period of resistance to stimulation; has two phases; Absolute refractory period: no stimulus of any strength will trigger another AP; Relative refractory period: an unusually strong stimulus needed to trigger new AP

Absolute refractory period

no stimulus of any strength will trigger another AP

Relative refractory period

an unusually strong stimulus needed to trigger new AP

Signal Conduction: Unmyelinated Axons

Voltage-gated channels along entire length; AP causes Na+ to enter axon and diffuse into adjacent regions

Signal Conduction: Myelinated Axons

AP can only generate at nodes; Voltage-gated ion channels concentrated; AP 'jumps' from node to node

Synapse

Point where axon terminal meets next neuron; AP arrives at end of axon of presynaptic neuron; Presynaptic neuron releases NT; Postsynaptic neuron responds to it

Synaptic cleft

gap between presynaptic and postsynaptic neuron

Neurotransmitters (NTs)

More than 100 NTs have been identified, most falling into these major categories.

Synaptic Transmission

Some NTs are excitatory and other inhibitory; Receptors; Ligand-gated ion channels; Intracellular second messengers.

Excitatory Cholinergic Synapse

ACh is NT; AP opens voltage-gated Ca2+ channels in presynaptic neuron; ACh released by exocytosis; ACh binds to gated ion channels; Na+ and K+ across membrane.

Inhibitory GABA-geric Synapse

𝛾-aminobutyric acid is NT; AP triggers GABA into synaptic cleft; GABA receptors are Cl- channels; Cl- hyperpolarizes postsynaptic membrane; Neuron inhibited.

Excitatory Andrenergic Synapse

Norepinephrine (NE) is NT; Bind to G-protein coupled receptors on postsynaptic membrane → second messenger signaling cascade; Advantage of enzyme amplification.

Cessation of the Signal

Important to turn off stimulus; NT bound ~ 1 ms; To end the signal; Presynaptic cell stops releasing NT; NT already in cleft is cleared in various ways; Degradation of NT in cleft.

Postsynaptic Potentials

Two types of postsynaptic potentials produced by NTs: Excitatory Postsynaptic Potential (EPSP) and Inhibitory Postsynaptic Potential (IPSP).

EPSP

Voltage change from RMP toward threshold; Results from Na+ flowing into cell; Glutamate and aspartate.

IPSP

Voltage becomes more negative than it is at rest; Results from Cl- entry or K+ exit from cell; Glycine and GABA.

Hebbian Theory

Neurons that fire together, wire together.

Synaptic plasticity

Ability of synapses to change.

Synaptic potentiation

Making transmission easier.

Long-Term Memory Formation

First demonstrated in Aplysia then mammalian brain (hippocampus).

Conduction

Conduct sensory and motor information up and down the spinal cord.

Neural integration

Neurons receive input from multiple sources, integrate it, and execute appropriate output.

Locomotion

Contains central pattern generators: groups of neurons that coordinate repetitive sequences of contractions for walking.

Reflexes

Involuntary responses to stimuli that are vital to posture, coordination and protection.

Cervical enlargement

Nerves of upper limb.

Lumbosacral enlargement

Nerves of pelvic region, lower limbs.

Medullary cone

Inferior point of cord.

Cauda equina

Bundle of nerve roots that occupy the vertebral canal from L2 to S5; resembles a horse tail.

Dura Mater

Forms loose-fitting sleeve (dural sheath) around spinal cord; Tough, thick membrane composed of dense irregular connective tissue.

Epidural space

Space between dura and vertebral bones.

Arachnoid Mater

Arachnoid membrane adhering to dura separated from pia by fibers; Subarachnoid space that is filled with cerebrospinal fluid (CSF).

Pia Mater

Delicate, transparent membrane that follows contours of spinal cord and continues inferiorly as a fibrous terminal filum.

Gray matter

Dull in color (no myelin); Site of neurosomas, dendrites, and synapses.

White matter

Bright, pearly white color due to myelin; Composed of tracts connecting regions of the brain to each other and to the spinal cord.

Cuneate Fasciculus

Sends touch, movement, limb position signals up spinal cord to brain.

Spinothalamic Tract

Carries temperature, pain, and light touch signals up spinal cord to brain.

Corticospinal Tract

Carries planned movement to motor neurons down spinal cord.

Epineurium

External to neurolemma.

Perineurium

Squamous cells that wrap fascicles.

Endoneurium

Irregular CT wraps entire nerve.

Sensory

Composed of only afferent fibers; signals from sensory receptors to CNS.

Motor

Composed of only efferent fibers; signals from CNS to muscles and glands.

Mixed

Consist of both afferent and efferent fibers.

Posterior root

Sensory input to spinal cord.

Anterior root

Motor output from spinal cord.

Poliomyelitis

Disease causing paralysis and muscle atrophy caused by poliovirus.

Amyotrophic Lateral Sclerosis (ALS)

Degeneration of motor neurons, leading to scarring of lateral regions of spinal cord.

Chickenpox

Common childhood disease caused by varicella-zoster virus (VZV).

Shingles

Adult disease caused by VZV when immune system is compromised.

Nerve Plexus

Have both somatosensory and motor functions.

Somatosensory

Sensory signals from bones, joints, muscles, skin.

Proprioception

Info about body position.

Cervical Nerve Plexus

In the neck; C1-C5; Supplies neck and phrenic nerves to diaphragm.

Brachial Nerve Plexus

Near the shoulder; C5-T1; Supplies upper limb and some of shoulder and neck.

Radial Nerve Injury

Nerve passes through axilla, may be compressed by crutches.

Wrist drop

Fingers, hands, wrist chronically flexed because extensor muscles paralyzed.

Lumbar Nerve Plexus

In lower back; L1-L4; Supplies abdominal wall, anterior thigh, genitalia.

Sacral Nerve Plexus

In pelvis; L4, L5, and S1-S4; Supplies remainder of lower trunk and lower limb.

Sciatic Nerve Injury

Sciatica: sharp pain that travels from gluteal region along the posterior side of thigh and leg to ankle.

Sciatica

Sharp pain that travels from gluteal region along the posterior side of thigh and leg to ankle.

Dermatome

Specific area of skin conveys sensory input.

Dermatome map

Diagram of innervation by each spinal nerve.

Cerebrum

Largest region of brain (telencephalon); 83% of total volume.

Cerebellum

Second largest brain region; Contains > 50% of neurons.