Nervous System: CNS and PNS

• Peripheral Nervous System (PNS) sends cranial and spinal nerves to Central Nervous System (CNS)
• The CNS includes the brain and spinal cord
• The PNS consists of sensory and motor divisions

Sensory Division

• Carries signals from receptors to the CNS
• Subdivided into somatic and visceral sensory
• Somatic sensory carries information from muscles, skin, and joints (e.g., pain, pressure, temperature)
• Visceral sensory carries information from organs

Motor Division

• Carries signals from the CNS to effectors
• Subdivided into somatic and autonomic motor
• Somatic motor sends signals to skeletal muscles (voluntary)
• Autonomic motor sends signals to smooth and cardiac muscle, and glands

Nervous Tissue: Neurons

• Specialized cells that send electrical signals
• Exhibit longevity and are largely amitotic (limited cell division)

Nervous Tissue: Neuroglia

• Support cells for neurons, outnumbering them 50:1
• Do not send electrical signals, but are mitotic

Neuroglia of the PNS: Satellite Cells

• Wrap around the cell body of a neuron
• Regulate movement of gases and ions
• Electrically insulate the cell body

Neuroglia of the PNS: Schwann cells

• Forms part of the myelin sheath around a single axon of cranial and spinal nerves

The Schwann Cell

• Wraps around a segment of an axon, forming two parts:
• Neurilemma: the outer layer containing cytoplasm, nucleus, and organelles, responsible for nerve regeneration
• Myelin Sheath: the inner layer composed of cell membrane, speeding up electrical signals

Neuroglia of the CNS: Astrocytes

• Largest and most numerous neuroglia
• Star-shaped cells that anchor neurons to blood vessels, regulate nutrients and ions, form the blood-brain barrier
• Secrete chemicals, making capillaries impermeable to ions or substances interfering with brain tissue

Neuroglia of the CNS: Microglia

• Smallest and rarest neuroglia
• Monocytes (macrophages) that wander the ECF of the CNS
• Use thorn-like extensions to probe for cell debris and pathogens

Neuroglia of the CNS: Ependymal Cells

• Ciliated cells that line the ventricles of the brain and spinal cord
• Make and circulate cerebrospinal fluid in association with the choroid plexus in the diencephalon

Neuroglia of the CNS: Oligodendrocytes

• Wrap extensions around multiple axons
• Form the myelin sheath in the CNS

Functional Classification of Neurons: Sensory/Afferent Neurons

• Carry impulses towards the CNS
• Cell bodies are located in the PNS, forming ganglia

Functional Classification of Neurons: Interneurons/Association Neurons

• Entire cell lies within the CNS
• Link neurons through impulses; cell bodies may form nuclei

Functional Classification of Neurons: Motor/Efferent Neurons

• Carry impulses away from the CNS to either a muscle or a gland
• Cell bodies are located in the ventral gray matter of the spinal cord

Reflexes

• Reflexes are rapid, predictable, and involuntary responses to stimuli
• The reflex arc, the neural pathway for reflexes, consists of five components: stimulus, sensory neuron, integration center, motor neuron, and effector

Types of Reflexes: Somatic Reflex

• Signal to skeletal muscle
• Voluntary muscle

Types of Reflexes: Autonomic Reflex

• Signal to cardiac/smooth muscle or glands
• Involuntary muscle

Structural Classification of Neurons

• Multipolar Neuron: 99% of neurons; motor, interneurons
• Bipolar Neuron: Special sense neurons (vision, hearing)
• Unipolar Neuron: General sense neurons (pressure, pain, temperature)

Organization of Neurons – Grey Matter

• Primarily dendrites and cell bodies of neurons
• Areas of interpretation

Organization of Neurons – White Matter

• Primarily myelinated axons
• Signal sending areas

Organization of Neurons in the PNS: Ganglia

• Gray matter; sensory neuron cell bodies on the dorsal side of the spinal nerve (dorsal root)

Organization of Neurons in the PNS: Nerves

• White matter; bundles of axons of sensory or motor neurons making up cranial and spinal nerves

Organization of Neurons in the CNS: Cerebral Cortex

• Grey matter; outer area of the cerebrum

Organization of Neurons in the CNS: Nuclei Basal Cortex

• Grey matter; clusters of neural cell bodies in the interior of the cerebrum that work with the cerebellum and motor cortex to prevent unwanted movement

Organization of Neurons in the CNS: Horns

• Grey matter; axon terminals or cell bodies of neurons in the spinal cord

Organization of Neurons in the CNS: Tracts

• White matter; bundles of axons with similar function and destination
• Ascending tracts are sensory tracts, and descending tracts are motor tracts

Organization of Neurons in the CNS: Column

• White matter; groups of tracts

Functional Properties of Neurons: Irritability

• Ability to respond to stimuli

Functional Properties of Neurons: Conductivity

• Ability to transmit an impulse

Synaptic Neurons: Pre-synaptic/pre-ganglionic Neuron

• Neuron sending signal (releasing the neurotransmitter)

Synaptic Neurons: Post-synaptic/post-ganglionic Neuron

• Neuron receiving signal (receptor for neurotransmitter)

Types of Neurotransmitters

• Excitatory: Increase post-synaptic membrane permeability to Na+ ions, increasing likelihood of reaching threshold (EPSP's)
• Inhibitory: Decrease likelihood of reaching threshold (IPSP's); may be due to opening of Cl- or K+ channels

Impulse Processing

• Neurons in the CNS are organized into neuronal pools
• Neuronal pools are groups of neurons connecting with each other and working together for a single function

CNS Regions of the Brain: Diencephalon

• Thalamus, hypothalamus, pineal gland

Layers of the Cerebrum

• Grey Matter: Cerebral cortex
• White Matter
• Basal Nuclei

Specialized Areas of the Cerebral Cortex: Primary Motor Cortex

• Voluntary movement of skeletal muscle

Specialized Areas of the Cerebral Cortex: Premotor Cortex

• Learned patterned motor skills (e.g., throwing a ball, walking, writing)

Specialized Areas of the Cerebral Cortex: Broca's Area

• Motor speech; movement of tongue

Specialized Areas of the Cerebral Cortex: Frontal Eye Field

• Eye movement

Specialized Areas of the Cerebral Cortex: Prefrontal Cortex

• Personality, judgement, reasoning

Specialized Areas of the Cerebral Cortex: Somatosensory area

• General sensory information from skin, muscles, joints (e.g., pain, pressure, touch)

Specialized Areas of the Cerebral Cortex: Somatosensory Association Area

• Integrates senses to understand objects felt (e.g., feeling change in pocket), knowing coins without looking

Specialized Areas of the Cerebral Cortex: Primary Visual Cortex

• Interprets and processes visual stimuli