Nervous System: Cranial Nerves & Function

• NEUR 1203 MIDTERM I

The Nervous System (NS)

• Coordinates actions by transmitting signals to and from different parts of the body
• Detects environmental changes (i.e., eyes detect changes in light, color, etc.)
• Responds to certain events (i.e., reflexes, moving out of the way)
• Divided into central NS and peripheral NS

Cranial Nerves

• Part of the somatic nervous system
• Consists of 12 pairs of nerves that control sensory information to the CNS
• Connects the brain and the internal organs, influencing several autonomic responses

Afferent Nerves

• Brings sensory information in from the periphery to the CNS
• Functions include sensation to the eyes, ears, mouth, and nose

Efferent Nerves

• Brings sensory information out
• Motor control over facial muscles, tongues, and eyes

The 12 Cranial Nerves:

• Olfactory (smell)
• Optic (vision)
• Oculomotor (eye movement)
• Trochlear (eye movement)
• Trigeminal (masticatory movements, facial sensations)
• Abducens (eye movement)
• Facial (facial movements, sensations)
• Auditory vestibular (hearing and balance)
• Glossopharyngeal (tongue/pharynx movement & sensation)
• Vagus (heart, blood vessels, viscera, movement of larynx and pharynx)
• Spinal accessory (neck muscles)
• Hypoglossal (Tongue muscles)

Spinal Nerves

• Functionally equivalent to cranial nerves of the head, extending from the spinal cord
• Controls and carries information about the body, trunk, and limbs
• Each spinal nerve integrates sensory information throughout the body

Spinal Cord

• Bilaterally symmetrical; each vertebra possesses a dorsal and ventral root
• A collection of fibers entering and exiting the spinal cord segment is called a root
• Dorsal root/fiber: afferent, carrying sensory information from fingertips to the spinal cord (in)
• Ventral root/fiber: efferent, carrying sensory information out

Steps of Spinal Fibers

• Fibers entering the dorsal root bring sensory information from sensory receptors
• Fibers leaving the ventral root carry motor information to the muscles
• Collateral branches of sensory neurons may cross to the other side and influence motor neurons there
• White-matter fiber tracts carry information to and from the brain

Law of Bell & Magendie

• Dorsal spinal cord handles sensory information.
• Ventral side handles motor function.
• Both send info to the CNS
• Allows inferences about location of spinal cord damage based on changes in sensation or movement experienced by a patient.

Autonomic NS

• Divided into sympathetic and parasympathetic divisions

Sympathetic Division

• The activating system
• Associated with fight or flight response
• Connected to thoracic & lumbar regions
• Spinal cord connects to autonomic control center, made up of ganglia with spinal connections to many ganglionic centers
• Causes increased heart rate, breathing, blood pressure, etc.

Parasympathetic Division

• Calming system
• Important for rest & digest pathway
• Connects through cranial nerves 3, 7, and 10
• Connected to the sacral region of spinal cord, allowing for all other processes to occur

Central Nervous System

• The spinal cord is the control center of the entire body
• Segments of the spinal cord are divided into 5 anatomical regions (from top to bottom)
• Dermatomes are segments of the body; each dermatome contains sensory nerves & motor nerves, controlling most body movement, divided into sections
• Regions:
  • Cervical (C1 - C8): Very top of the spinal cord
  • Thoracic (T1 - T12)
  • Lumbar (L1 - L5): Lower back
  • Sacral (S1 - S5)
• Spinal cord can act independently of the brain, enabling spinal reflexes and autonomic movements. Reflexes are hard for the brain to inhibit

Protecting The Brain

• Dura mater: tough double-layered fibrous tissue that encloses the brain & spinal cord
• Arachnoid layer: thin sheet of delicate connective tissue that follows the brain's contour and creates space for CSF
• Pia Mater: moderately tough membrane of connective tissue that clings to the brain surface and is directly attached like glue
• All of these layers are called meninges

Meningitis

• Inflammation of the meninges
• Bacterial infection of the meninges, particularly the pia mater and arachnoid space
• CSF is implicated
• Subarachnoid space is filled with CSF between pia mater and arachnoid layer

Intra Cranial Pressure (ICP)

• Inflammation puts pressure on the brain, leading to drowsiness, delirium, and coma

4 Lobes of the Brain

• Frontal lobe: executive function, decision-making, planning, impulse control; works with the parietal lobe for goal-directed movement
• Parietal Lobe: Tactile function, sensory & motor information processing - movement
• Occipital Lobe: visual function, visual cortices
• Temporal Lobe: auditory, visual, gustatory, emotion and memory

Dorsal and Ventral Views of the Brain

• Cerebrum: forebrain structure, two identical hemispheres, responsible for most conscious behavior (outer part of the brain)
• Cerebellum: controls and coordinates fine motor skills; coordinates the timing, precision, and accuracy of movements
  • Animals that are faster or move a lot have bigger cerebellums (i.e., cheetah vs. sloth)
• Brainstem: responsible for unconscious behaviors, structurally continuous with the spinal cord (sits under the cerebellum)
• Gyri: bumps & ridges of the cerebral cortex
• Sulci: cracks & valleys of the cerebral cortex, fissures are known as deep sulci
• Together, gyri and sulci create a larger surface area for the human brain

Lateral & Medial View of the Brain

• Larger cortical surface area = greater cognitive functioning
• Lateral fissure: goes very deep and is the longest sulci in the brain, separating the frontal and parietal lobes from the temporal lobe (aka central sulcus)

Cerebral Arteries

• Anterior cerebral artery
• Middle cerebral artery
• Posterior cerebral artery
• The 3 major arteries supply the cerebrum
• Blockage of any of these arteries leads to regional death = stroke

Inside The Brain

• Gray matter: composed of cell bodies and capillary blood vessels; processes information and supports behavior (outer portion of the brain)
• White matter: nerve fibers with fatty coverings; forms connections between cells and sends information to the outer layer
• Ventricles: Four cavities filled with cerebral spinal fluid, derived from blood plasma, NaCl, and other salts; three main functions are buoyancy, cushioning, and immune support
• Ependymal cells line the walls of the ventricles and produce CSF

Corpus Callosum

• Largest white matter tract that connects the right and left hemispheres
• Over 200 million nerve fibers connect the 2 hemispheres; this divides the brain into cortical (above corpus callosum) and subcortical (below corpus callosum) regions
• Allows interaction between both sides of the brain simultaneously, acting as a divisor

Split Brain

• Corpus callosum prevents cross-talk between hemispheres as the language center of the brain is usually on the opposite side of the dominant hemisphere
• Most of the brain is symmetrical
• Some functions are localized to one side
• Patients with a cut corpus callosum cannot name objects in the left visual field

The Brainstem

• All information travels through it
• Receives afferent nerves from all the body's senses and sends efferent nerves to the spinal cord
• Divided into 3 regions:
  • Hindbrain
  • Midbrain
  • Diencephalon

The Hindbrain

• Consists of:
  • Cerebellum: controls fine motor movement
  • Pons: connects the cerebellum to the rest of the brain
  • Reticular formation: located at the core of the brainstem; a netlike mixture of grey and white matter, helps send signals between the spinal cord and the brain
  • Medulla: controls breathing and cardiovascular system

The Midbrain

• Tectum: dorsal side of midbrain, receives sensory information from the eyes and ears, allows for production of oriented movements (reflexes)
• Tegmentum: contains superior colliculus (receives visual input) and inferior colliculus (receives auditory information)
• Red nuclei: motor coordination of the limbs
• Substantia nigra: initiates voluntary movements and is a dopamine system
• Periaqueductal grey matter - sexual behavior and pain

Forebrain

• Largest and most recently evolved, controls perception, movement; mostly found in mammals
• Divided into 4 parts:
  • The neocortex
  • Basal ganglia
  • Allocortex - limbic system
  • Olfactory system

Basal Ganglia

• Controls certain aspects of voluntary movements, procedural learning, and habit formation
• Consists of caudate nucleus, putamen, globus pallidus, and substantia nigra
• Above the brainstem

Allocortex

• Deeper in the brain but still considered cortical
• Includes hippocampus, amygdala and cingulate cortex.
  • Hippocampus: spatial memories; neurogenesis (production of new neurons)
  • Amygdala: emotional regulation, fear acquisition, memory enhancement and activation - information feeds into hippocampus to create memory.
  • Cingulate cortex: Helps aspects of memory formation and recollection which helps respond to future events.

Olfactory System

• Part of limbic system
• Contains olfactory bulbs, permits sense of smell, sends sensory information directly to pyriform cortex for processing
• Relatively small in humans compared to other animals (dogs, rats, cats)
• Plays a very significant role in memory formation;
• It is part of the limbic system; any other system goes directly to hypothalamus but this one goes through the allocortex then hypothalamus

Diencephalon

• Hypothalamus controls hormone production
• Thalamus, relay station that sends all information where it needs to go

Parts of A Neuron:

• Soma - core region, processes information
• Dendrites - branching extensions, collects information and sends it to the axon; the number of dendrites = amount of incoming information
• Dendritic spines: small synapses on a dendrite that serve as a point of contact with other axons
• Axon hillock: point at which the axon leaves the soma (cell body)
• Axon: carries information to other neurons through white matter tracts
• Myelin sheath: insulates axons, signals travel faster and further, electrical transmission
• Axon collaterals: point at which axon branches out, allows messages to be sent in multiple directions simultaneously
• Terminal button: stops extremely close to dendritic spine of another neuron, does not touch other neurons at the end of axon collaterals
• Synapse: junction between one neuron and the other; space between the terminal button and dendritic spine, where NTs are released

Sensory Neurons

• Carry out the brain's major functions
• Bring information to the brain (afferent).
• Structurally, they are the simplest type of neuron with one single dendrite on one side, cell body, and a single axon on the other side
• Subtypes:
  • Bipolar neurons (retinal bipolar cell)
  • Somatosensory neurons (multipolar cell)

Interneurons

• Links sensory and motor neurons, branch extensively to collect more information
• Subtypes:
  • Stellate cell (star-shaped): very small, many dendrites, extending around entire cell body – in the thalamus
  • Pyramidal cell (pyramid-shaped): long axon with multiple sets of dendrites – in the cortex
  • Purkinje cell: output cell, extremely branched dendrites – in the cerebellum

Motor Neurons

• Carry information (motor instructions) from the brain into the spinal cord and muscles (efferent).
• Contains extensive dendritic networks to collect information from multiple sources, large cell bodies to process information
• All outgoing information must pass through motor neurons to reach target muscles
• Includes upper and lower motor neurons

Glial Cells

• Cells that provide insulation and support to all neurons
• Types of glial cells include:
• Ependymal cell: located on walls of ventricles, produces CSF and are very small.
• Astrocyte: provides structural support, holds neurons in place, regulates the blood-brain barrier, star shaped and allows for more blood flow, glucose and oxygen
• Oligodendrocytes: insulates axons in the CNS, forms myelin around axons in the brain and spinal cord, can wrap around multiple axons at once through white matter tracts.
• Schwann cell: insulates axons in the PNS, wraps around peripheral nerves to form myelin.

Wallerian Degeneration

• Occurs when a nerve is cut or severely damaged, leads to nerve breakdown as it loses connection and nutrient access.
• Special immune cells (Schwann cells in the PNS or microglia in the CNS) come in to remove the debris
• In the peripheral nervous system (like in your arms and legs), Schwann cells help guide the axon to regrow, regrowth is limited in the central nervous system (like the brain and spinal cord)

Neuronal Repair Glial Cells

• Proximal axon regresses and the distal decomposes
• Schwann cells grow and form myelin
• Neurons send out axon sprouts
• Schwann cells shrink and form of path, though sometimes axons get lost, repair is much less common in CNS

How do neurons communicate?

• Axons carry information that connects neurons to each other
• Nerves are located when outside the CNS, and tracts are located within the CNS
• Neurotransmission occurs in two steps:

1. Electrical
2. Chemical

• A neuron must use both electrical and chemical signals to communicate with another neuron

Membrane Potential

• Part of electrical communication
• Each neuron has a resting membrane potential; at rest, the cell has no stimulus
• Occurs because the cell is negatively charged inside and positively charged outside
• Cell membranes are permeable, it is difficult to pass through which is why cells travel through channels
• Resting potential is -70mV

Maintaining Membrane Potential

• Large protein anions are made inside the cell and can't leave (negatively charged)
• Ungated potassium and sodium channels; free-moving positive ions
• Travel through a potassium-sodium pump

Electrochemical Gradient

• Helps neurons send signals by controlling ion movement.
• Ions move due to two forces:

1. Electrical force (opposites attract, like charges repel)
2. Chemical force (ions spread from high to low concentration)

• At rest, the neuron is more negative inside
• When activated, ions move, changing the charge and creating a nerve signal

Potential Changes in Electrical Communication

• Without stimulus a cell will remain at -70mV
• A stimulation is required to elicit a change in membrane potential
• Hyperpolarization: membrane potential is exaggerated, the difference between inside and outside is greater
• Depolarization: membrane potential is diminished, the difference between inside and outside is lessened
• Stimulus opens channels

Action Potential

• Brief but very large, reverses the polarity in the axon's membrane
• The inside of the cell becomes positive, relative to the outside, which becomes negative
• Change is abruptly reversed due to an influx of potassium, then it goes back to -70mV

Reaching Threshold

• Neurons receive both excitatory and inhibitory inputs: excitatory post-synaptic potentials (EPSP) and inhibitory post-synaptic potentials (IPSP)
• Spatial summation: presynaptic neurons release NT at different locations, combined signals trigger an action potential
• Temporal summation: single presynaptic neuron releases NT repeatedly over a short period of time, overlapping signals add up to trigger an action potential

Initiation of Action Potential

• When EPSP reaches -50mV, which is the threshold to trigger a response
• Large influx of sodium opens channels, and potassium leaves the cell
• All or nothing, continues until inside the cell reaches +30mV

What Happens at -50mV?

• Sodium and potassium channels are gated until -50mV is reached
• Sodium channels are faster and open first, then a second gate closes once +30mV is reached, no more sodium at peak action potential
• Potassium channels are slower and take longer to close (repolarization)

Action Potential Steps:

1. Stimulus - Signal or change that triggers cell to respond
2. Threshold (-50mV) - minimum charge needed for neuron to activate and start sending a signal
3. Depolarization (influx of sodium) - sodium rushes into the neuron making it positively charged
4. Peak (+30mV) - charge inside the neuron reaches its highest point during activation
5. Repolarization - potassium moves out of the cell, bringing the charge back down
6. Refractory period - neuron briefly recovers, can fire again but would need an even stronger stimulus
7. Returning to resting state - neuron goes back to -70mV ready for next signal

Action Potential Propagation

• In myelinated neurons, the signal jumps between gaps (nodes of Ranvier) for faster transmission (saltatory conduction)
• Myelin doesn't cover the whole axon

Chemical Transmission

• How information is passed to the next cell through the release of neurotransmitters
• NTs are released in the synaptic cleft (space between two terminals)
• NT = chemicals that can be excitatory or inhibitory
• Vesicle stores NT
• Synaptic cleft: space between button and spine
• Post-synaptic receptor: binding side of neurotransmitter

4 Main Criteria for a Molecule to be Classified as a Neurotransmitter

1. Must be synthesized in the neuron or otherwise be present in it
2. When the neuron is active, the transmitter must be released and produce a response in some target
3. The same response must be obtained when transmitter is experimentally placed on the target
4. A mechanism must exist for removing the transmitter from its site of action after work is done

• All NT are chemicals but not all chemicals are NT

Types of NTs

1. Monoamine: dopamine, norepinephrine, epinephrine, serotonin and histamine
2. Amino acid: GABA, glutamate, glycine, D-serine
3. Peptide: somatostatin, substance P
4. Transmitter gases: nitric oxide, carbon monoxide

4 Steps of Chemical Transmission:

1. Synthesis: synthesized from DNA and stored in vesicles
2. Release: transported to pre-synaptic membrane and released in response to an action potential
3. Receptor action: activates target receptors on the post-synaptic membrane
4. Inactivation: 4 different ways that the NT is returned to the terminal or stops working

Removal of NTs:

• Can be removed or inactivated in four main ways:

1. Reuptake – The NT is sucked back into the neuron that released it (like recycling)
2. Enzyme Breakdown – Special enzymes break down the NT (like cutting it into pieces)
3. Diffusion – The NT drifts away from the synapse (spreads out naturally)
4. Glial Cell Uptake – Nearby support cells (glia) absorb and remove the NT

Test your knowledge of the nervous system with questions covering cranial nerves and their functions. Review afferent and efferent nerve functions, and the roles of the brainstem and other nervous system divisions. Identify the primary functions associated with the nervous system.