Neuroscience Quiz: Brain Anatomy and Functions

The Nervous System: Nervous Tissue

• The nervous system is composed of nervous tissue.
• The nervous system has three main functions: sensory input, integration, and motor output.
• Sensory receptors monitor changes in internal and external environments.
• Information is processed in the brain and spinal cord.
• The nervous system is responsible for motor activity.

Learning Outcomes

• The learning outcomes included listing and describing the structures of the nervous system, its organization, and the basic functions of the system.
• Contrast of neurons and neuroglia, gray matter versus white matter, nerve impulse generation and conduction, and events of synaptic transmission with types of neurotransmitters were also included.

Structures of the Nervous System

• Brain: neurons enclosed within the skull.
• Spinal cord: connects to the brain and enclosed within the spinal cavity.
• Nerves: bundles of many neurons' axons
  • Cranial nerves (12 pairs) emerge from the brain.
  • Spinal nerves (31 pairs) emerge from the spinal cord.
• Ganglia: groups of neuron cell bodies located outside the brain and spinal cord.
• Enteric plexuses: networks in the digestive tract.
• Sensory receptors: monitor changes in internal or external environments.

Organization of the Nervous System

• Central nervous system (CNS) structures:
  • Brain
  • Spinal cord
• Peripheral nervous system (PNS) structures:
  • Cranial nerves and branches
  • Spinal nerves and branches
  • Ganglia
  • Sensory receptors

Organization of the Nervous System (PNS divisions)

• Somatic Nervous System (SNS):
  • Sensory neurons from head, body wall, limbs, special sense organs
  • Motor neurons to skeletal muscle: voluntary
• Autonomic Nervous System (ANS):
  • Sensory neurons from viscera
  • Motor neurons to viscera (cardiac muscle, smooth muscle, glands); involuntary
  • Sympathetic: "fight-or-flight"
  • Parasympathetic: “rest-and-digest"
• Enteric nervous system (ENS): "brain of the gut"
• PNS further divides into two main branches, somatic (voluntary) and autonomic (involuntary).
• This further divides into sympathetic and parasympathetic systems

Organization of the Nervous System (Enteric Nervous System)

• The enteric nervous system (ENS) is embedded in the wall of the gastrointestinal tract.
• It is responsible for regulating motility, secretion, and blood flow in the GI tract.
• Sensory neurons in the ENS monitor chemical changes and stretching of the GI wall.
• Motor neurons control contractions, secretions, and endocrine secretions (involuntary).

Histology of the Nervous System

• Neurons: can respond to stimuli and convert stimuli to electrical signals (nerve impulses) that travel along neurons.
• Neuroglia cells: support, nourish, and protect neurons; vital for homeostasis of interstitial fluid around neurons.
• Neuroglia help form the blood-brain barrier.
• Examples of neuroglia include astrocytes, oligodendrocytes, microglia, ependymal cells, Schwann cells, and satellite cells.

Neuronal Structure

• Cell body: nucleus, cytoplasm with typical organelles
• Dendrites: highly branched structures that carry impulses to the cell body.
• Axon: conducts away from the cell body toward another neuron, muscle, or gland.
• Emerges at a cone-shaped axon hillock.
• Axon terminals: contain synaptic vesicles that can release neurotransmitters.

Structural Classes of Neurons

• Multipolar: have several or many dendrites and one axon; most common type in brain and spinal cord.
• Bipolar: have one dendrite and one axon; found in retina of eye and inner ear.
• Unipolar: have fused dendrite and axon; sensory neurons of spinal nerves

Functional Classes of Neurons

• Sensory (afferent): convey impulses into CNS (brain or spinal cord).
• Motor (efferent): convey impulses from brain or spinal cord out through PNS to effectors (muscles or glands).
• Interneurons (association neurons): most within CNS; transmit impulses between neurons, such as between sensory and motor neurons.

Action Potentials

• Action potentials: nerve impulses (electrical signals).
• Require a membrane potential (charge difference across the cell membrane; polarization).
• Ion channels: allow ions to move by diffusion from high to low concentration.
• Leakage channels: allow ions to leak through membrane; more for K+ than for Na+.
• Gated channels: open and close on command; respond to changes in membrane so can generate and conduct action potentials.

Resting Membrane Potential

• The average resting membrane potential is -70 mV.
• Inside of the membrane is relatively more negative than the outside of the membrane because the cytosol contains many negative ions (amino acids and phosphates) and K+ that can easily leak out through many K+ membrane channels.
• Interstitial fluid has few negative ions and Na+, that does not leak out of the membrane easily, but the membrane pumps are responsible for quickly pumping out the Na+ that does leak into the cell.

Action Potential Generation

• Series of events that activate the cell membrane in a neuron or muscle fiber.
• An initial event (stimulus) is required.
• Triggers resting membrane to become more permeable to N+.
• Causes enough Na+ to enter the cell so that the cell membrane reaches threshold (~ -55 mv).
• If so, the following events occur: action potential that spreads along the neuron or muscle fiber.

Action Potential Phases

• Depolarizing phase: Na+ channels open → Na+ enters the cell, membrane potential rises and becomes positive (+30 mV).
• Repolarizing phase: K+ channels open → K+ leaves the cell, membrane potential returns to resting value (-70 mV).
• May overshoot: hyperpolarizing phase, membrane potential may become more negative than resting, before returning to normal.

Conduction of Nerve Impulses

• Nerve impulse conduction (propagation): each section triggers the next; Na+ channels open (domino effect).
• Types of conduction:
  • Continuous conduction: in unmyelinated fibers, slower (1 msec).
  • Saltatory conduction: in myelinated fibers, faster as impulses "leap" between nodes of Ranvier (100 msec).
• Factors that increase rate of conduction: myelin, large diameter, and warm nerve fibers.

Synaptic Transmission

• Synapse: site of signal transmission from one neuron to the next (neuronal junction, neuromuscular junction, neuroglandular junction).
• Triggered by action potential (nerve impulse).
• Components: presynaptic neuron, synaptic cleft, postsynaptic cell.

Synaptic Transmission-Electrical Synapse

• Cytoplasm of adjacent cells are connected; bidirectional transmission.
• Found in cardiac muscle, smooth muscle of the GI tract, and blood vessels.

Chemical Synaptic Transmission

• Action potential arrives at presynaptic neuron's end bulb.
• Opens voltage gated Ca2+ channels → Ca2+ flows into presynaptic cytosol.
• Increased Ca2+ concentration → exocytosis of synaptic vesicles.
• Neurotransmitter (NT) released into cleft.
• NT diffuses across cleft and binds to receptors in postsynaptic cell membrane, serving as chemical trigger for ion channels.
• Postsynaptic cell membrane may be depolarized or hyperpolarized, depending on the type of NT and type of postsynaptic cell.
• The sum of all NTs in neurons converging on a synapse determines the effect.
• If threshold reached, then postsynaptic cell action potential results (one-way).
• NT must be removed from the cleft, via diffusion out of the cleft or destruction by enzymes (such as ACh-ase) in the cleft.

Neurotransmitters

• Acetylcholine (ACh): common in PNS; excitatory (on skeletal muscles), inhibitory (on cardiac muscle).
• Amino acids: glutamate, aspartate, GABA, glycine
• Modified amino acids: norepinephrine (NE), dopamine (DA), serotonin
• Neuropeptides: endorphins
• Nitric oxide (NO)
• Neurotransmitters are crucial for communication between neurons and between neurons and other target cells.

Specific Types of Cranial Nerves

• Olfactory (I): special sensory (smell)
• Optic (II): special sensory (vision)
• Oculomotor (III), Trochlear (IV), Abducens (VI): motor (eye movement)
• Trigeminal (V): mixed (sensory and motor; face sensation, chewing).
• Facial (VII): mixed (taste, facial expression, tear glands, salivary glands)
• Vestibulocochlear (VIII): special sensory (hearing, balance)
• Glossopharyngeal (IX): mixed (taste, swallowing, salivary gland/parotid)
• Vagus (X): mixed (sensory from pharynx, ear, diaphragm; motor to palatal and pharyngeal muscles, viscera)
• Accessory (XI): primarily motor (sternocleidomastoid, trapezius)
• Hypoglossal (XII): primarily motor (tongue)

Spinal Cord

• Extends from medulla oblongata of brain to L2 vertebra.
• It is protected by the vertebral column.
• Contains ascending and descending tracts; consists of white matter.
• Has left and right halves partially separated by a fissure and sulcus.
• Contains a central canal filled with CSF.
• Regions of enlargement: cervical (upper limbs), lumbar (lower limbs)
• Cauda equine: collection of spinal nerve roots located inferior to the end of the spinal cord.

Spinal Nerve

• Part of the peripheral nervous system composed of 31 pairs, emerges from the spinal cord via intervertebral foramen.
• Formed from 2 roots: dorsal root (sensory), ventral root (motor)
• Divided into 4 main plexuses: cervical, brachial, lumbar, and sacral.

Spinal Plexuses

• Cervical: supplies posterior head, neck, shoulders, and diaphragm; Important nerves: phrenic nerve to diaphragm.
• Brachial: supplies upper limbs + some neck and shoulder muscles; Important nerves: radial, ulnar, axial, median.
• Lumbar: supplies abdominal wall, external genitalia, and parts of lower limbs; Important nerves: femoral and obturator.
• Sacral: supplies buttocks, perineum, and most of lower limbs; Important nerves: gluteal, sciatic, and pudendal.

Dermatomes

• Areas of skin innervated by a specific spinal nerve.

Somatic Nervous System: Sensory and Motor Pathways

• Sensory pathways: transmit sensory information; 3 neuron chain (1st in dorsal root ganglion, 2nd in spinal cord/brainstem, 3rd in thalamus, projecting to the cerebral cortex).
• Two major pathways:
  • Posterior column- medial lemniscus pathway (fine touch, pressure, vibration, proprioception)
  • Spinothalamic pathway (pain and temperature)
• Motor pathways: transmit motor commands; upper motor neurons (cortex, basal ganglia, cerebellum), lower motor neurons (anterior gray of spinal cord).

Spinal Cord Protection and Coverings

• Spinal cord is protected by three layers of meninges: dura mater, arachnoid mater, and pia mater.
• Epidural space: between vertebrae and dura mater, contains fat and blood vessels.

Somatic Senses

• Include tactile, thermal, pain, and proprioceptive sensations.
• Tactile sensations rely on receptors in the skin.
• Receptors adapt to stimuli (decrease their response to prolonged stimulation).

Definition of Sensation

• Conscious or subconscious awareness of change in external or internal environment.
• Requires: stimulus, sensory receptor, neural pathway, brain region for integration.

Reflex Arc

• Simple, automatic response to a specific stimulus.
• Components: receptor, sensory neuron, interneuron, motor neuron, effector.
• Several types, including knee-jerk reflex (patellar).

Cerebrospinal Fluid (CSF)

• Formed in choroid plexuses within ventricles of the brain, and circulates through ventricles, and subarachnoid space.
• Functions: cushions and supports the brain and spinal cord; provides nutrients.

Aging and Brain Function

• Rapid brain growth occurs in early life, from increased neuron/glia size, dendritic branches and synaptic contacts.
• Brain mass declines somewhat in adulthood and old age, leading to fewer synaptic contacts.

Brain Blood Supply and Blood-Brain Barrier

• Brain requires 20% of the body's O₂ supply.
• Protected by blood-brain barrier, which limits entry of harmful materials while allowing passage of lipid-soluble materials (O2, CO2, alcohol, and anesthetic agents).
• Blood-brain barrier created by tight capillaries and astrocytes.

Limbic System

• Ring of structures on the inner border of the cerebrum and floor of the diencephalon.
• Important in emotional responses, memory (including memory development) and certain types of behavior.

Reticular Formation

• Netlike arrangement of gray and white matter.
• Ascending part = reticular activating system (RAS), carries sensory pathways to cerebral cortex; helps maintain consciousness.
• Helps induce sleep.

Lateralization

• Left side of brain, controls right side of the body.
• Right side of brain, controls left side of the body.
• Left hemisphere important for language, right hemisphere for spatial/pattern recognition and emotional content.

Test your knowledge on key aspects of neuroscience, including neurotransmitter removal mechanisms, brain structures, and cranial nerves. Explore the functions of various brain parts and their specific roles in movement coordination, sensation, and autonomic control.