Neuron Types: Functional and Structural

Neuron Classification

• Neurons are classified structurally and functionally

Functional Neuron Types

• Sensory neurons transmit impulses from sensory receptors to the CNS
• Motor neurons transmit impulses from the CNS to target organs like muscles or glands
• Somatic motor neurons are responsible for reflexes and voluntary control of skeletal muscles
• Autonomic motor neurons innervate involuntary targets, including smooth muscle, cardiac muscle, and glands
• Association/interneurons are located within the CNS and integrate nervous system functions

Structural Neuron Types

• Pseudounipolar neurons have a single short process that branches like a T into two longer processes, such as sensory neurons
• Bipolar neurons have two processes, one on either end, such as in the retina of the eye
• Multipolar neurons have several dendrites and one axon, and are the most common type

Action Potential Transmission Rate

• PNS: Schwann cells affect the rate of action potential transmission
• CNS: oligodendrocytes and myelinated axons affect the rate of action potential transmission

Blood-Brain Barrier

• Tight junctions in the blood-brain barrier prevent hydrophobic molecules from diffusing into the brain

Threshold Stimulus

• Only a supramaximal strong stimulus can bring a cell to threshold during the relative refractory period

Myelin Sheath

• The myelin sheath provides insulation and allows action impulses to conduct more rapidly

Synapse Types

• Chemical synapses have a slightly delayed impulse transmission

Neurotransmitter Inhibition

• Neurotransmitter inhibition examples: damage to the SNARE complex and blockage of calcium channels

Nicotinic ACh Receptor

• A nicotinic ACh receptor opens a channel for sodium and potassium

Stimulated Nicotinic-Gated Channels

• More sodium flows in than potassium out when nicotinic-gated channels are stimulated
• This depolarizes the cell, creating an EPSP (influx of sodium)

IPSP Production

• An IPSP on the postsynaptic membrane is produced by the opening of potassium or chloride channels, binding to some muscarinic receptors (potassium channels)

Receptor Agonists

• Nicotinic receptors: Nicotine is an agonist
• Muscarinic receptors: Muscarine is an agonist

ACh Receptor Binding

• Nicotinic ACh receptors induce EPSPs
• Muscarinic ACh receptors induce IPSPs

Hyperpolarization by ACh

• ACh can induce hyperpolarization by opening potassium channels (muscarinic receptors), creating IPSPs

Catecholamine Activation

• Catecholamines activate post-synaptic cells by increased production of cAMP

Brain EPSPs

• Glutamate produces EPSPs in the brain

GABA

• GABA is involved in motor control, inhibitory by opening chloride channels, and is the most common neurotransmitter

Spatial Summation

• Spatial summation occurs due to convergence of signals onto a single postsynaptic neuron
• All EPSPs and IPSPs are added together at the axon hillock

Rapid Action Potential Conduction

• Two characteristics of axons facilitate rapid action potential conduction: myelination (due to saltatory conduction) and increased neuron diameter (reduces resistance to charge spread)

Neuron Types for Muscles/Glands

• Motor neurons innervate muscles and glands

Brain and Spinal Cord Arrangement

• Spinal cord: white matter surrounds a gray matter core
• Brain: outer region of 2-4 mm of gray matter with underlying deep white matter

CSF Circulation

• The neural tube remains as ventricles in the brain and the central canal of the spinal cord, where CSF develops

Cerebrum Lobes

• The lobes of the cerebrum are the parietal, occipital, frontal, temporal, and insula

Central Sulcus

• The central sulcus separates the frontal and parietal lobes

Gyri

• Gyri are raised folds in the brain

Precentral Gyrus

• The precentral gyrus is responsible for motor control

Non-REM Sleep and Memory

• Non-REM sleep allows time for neuroplasticity mechanisms to consolidate memories

Cerebral Dominance

• Left hemisphere: language, speech, writing, analytical activity
• Right hemisphere: music, creativity, artistic expression (97%)

Limbic System

• The limbic system is responsible for emotional drives: aggression, fear, hunger/satiety, sex drive, and goal-directed behaviors

Semantic Memory

• Semantic memory is declarative memory based on facts

Major Afferent Relay

• The thalamus is the major afferent relay structure to the cerebrum, where most sensory information passes

Hypothalamus Function

• The hypothalamus maintains homeostasis and controls the autonomic nervous system
• The Hypothalamus contains centers for hunger, thirst, body temperature regulation, sleep and wakefulness regulation, sexual arousal and performance, emotions (fear, anger, pain, pleasure), and control of the endocrine system

Brain Stem Vital Centers

• The medulla oblongata contains vital centers that regulate breathing and cardiovascular response
• Vasomotor center controls blood vessel diameter
• Cardiac center controls heart rate
• Rhythmicity center helps the area in the pons controlling breathing

Cranial Nerve X

• Cranial nerve X (vagus nerve) carries sensory impulses from taste buds or the rear of the tongue, sensations from the auricle of the ear, and general visceral sensations

Nerve Pairs

• There are 12 pairs of cranial nerves and 31 pairs of spinal nerves in the human body

Spinal Nerve Count

• Spinal nerves emerging from each region of the spinal column: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal

ANS Effectors

• The effectors of the ANS are cardiac muscle, smooth muscle of visceral organs and blood vessels, and glands

ANS Control

• The ANS is involuntary

Autonomic Neuron Origin

• Preganglionic autonomic neurons originate in the midbrain or hindbrain, or from the thoracic, lumbar, or sacral spinal cord

ANS Divisions and Ganglia

• The sympathetic division of the ANS is associated with paravertebral and collateral ganglia

Parasympathetic Fibers

• Parasympathetic preganglionic fibers originate in the brainstem and sacral region of the spinal cord (craniosacral division)

Sympathetic Innervation

• Effectors which have only sympathetic innervation: adrenal medulla, arrector pili muscles in skin, sweat glands in skin, and most blood vessels

ANS Motor Division Dominance

• Fight or flight: sympathetic nervous system
• Normal body functions: parasympathetic nervous system

Postganglionic Sympathetic Nerves

• Most postganglionic sympathetic nerves release norepinephrine

Acetylcholine (ACh) Release

• All preganglionic neurons and most parasympathetic postganglionic neurons release ACh

Varicosities

• Varicosities are swellings on the axons of postganglionic neurons that release neurotransmitter along the axon

Adrenergic Receptor Actions

• Norepinephrine, G-proteins, and second messenger systems assist in the actions of adrenergic receptors

Postganglionic Transmitters

• Other neurotransmitters that may be released from postganglionic neurons: ATP, vasoactive intestinal peptide, nitric oxide

Medulla Control

• The medulla oblongata controls many cardiovascular, pulmonary, urinary, reproductive, and digestive functions through the vagus (cranial X) nerve

Medulla Regulator

• The hypothalamus is the major regulatory center in the ANS
• The limbic system is responsible for autonomic responses during emotional states