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Introduction to the Nervous System

• The nervous system helps maintain homeostasis and responds rapidly to external stimuli
• Two main systems: endocrine (hormones, slow response) and nervous (neurotransmitters, rapid response)

Organization of the Nervous System

• Central nervous system (CNS): brain and spinal cord
• Peripheral nervous system (PNS): nerves outside CNS
• Somatic nervous system: voluntary muscle control
• Autonomic nervous system: involuntary functions (heart rate, digestion)

Functions of the Nervous System

• Sensory: receives and interprets stimuli
• Motor: responds to stimuli with muscle or gland action
• Integrative: processes and integrates information

Histology of the Nervous System

• Two main cell types: neurons and neuroglia
• Neurons: electrically excitable, responsible for sensing, thinking, and controlling muscle activity
• Neuroglia: support, nourish, and protect neurons

Neurons

• Electrically excitable, with axons that transmit signals
• Classified structurally (multipolar, bipolar, unipolar) or functionally (sensory, motor, inter)

Neuroglia

• Not electrically excitable
• Five types: astrocytes, oligodendrocytes, microglia, ependymal cells, and satellite cells
• Functions: support, nourish, and protect neurons, as well as maintain the BBB (blood-brain barrier)

Myelination

• Insulates and increases nerve impulse speed
• In CNS: oligodendrocytes myelinate multiple axons
• In PNS: Schwann cells myelinate single axons

Electrical Signals in Neurons

• Ion channels allow movement of specific ions across the membrane
• Ion flow creates an electrical current that alters membrane potential
• Types of ion channels: leakage, ligand-gated, mechanogated, and voltage-gated

Resting Membrane Potential

• Result of unequal ion distribution and selective membrane permeability
• Na+/K+-ATPase pumps maintain the resting potential

Graded Potentials

• Small, localized changes in membrane potential
• Can be added together (summation) to create a larger response

Action Potentials

• Rapid sequence of events: depolarization, repolarization, and refractoriness
• All-or-none principle: the neuron either fires or doesn't

Refractory Periods

• Absolute refractory period: cannot fire again immediately
• Relative refractory period: can fire again, but with less intensity### Neurotoxins and Local Anesthetics
• Neurotoxins act on the nervous system, while local anesthetics block pain and somatic sensations.
• Tetrodotoxin (TTX) is a type of neurotoxin that prevents the opening of voltage-gated sodium channels.
• When TTX inserts into VG-Na+ channels, it prevents the opening of the channels, and pain signals do not reach the CNS.

Conduction of Nerve Impulses

• Continuous conduction occurs in unmyelinated fibers and muscle fibers, involving step-by-step depolarization and repolarization.
• Impulses propagate over short distances in continuous conduction.
• Saltatory conduction occurs in myelinated axons, where the impulse "leaps" from node to node.
• In saltatory conduction, the uneven distribution of voltage-gated channels leads to faster conduction.

Factors Affecting Propagation Speed

• Axon diameter, amount of myelination, and temperature affect the propagation speed of nerve impulses.

Encoding of Stimulus Intensity

• All nerve impulses are the same size, but the frequency of impulses and the number of sensory neurons activated by the stimulus encode stimulus intensity.

Signal Transmission at Synapses

• Synapses are junctions between neurons or between a neuron and an effector.
• There are two types of synapses: electrical and chemical synapses.

Electrical Synapses

• Electrical synapses involve the direct transmission of ions through gap junctions, allowing for faster communication and synchronization.
• Advantages of electrical synapses include faster communication, direct connection, and synchronization.

Chemical Synapses

• Chemical synapses involve the release of neurotransmitters (NTs) from the presynaptic neuron into the synaptic cleft, which then bind to receptors on the postsynaptic neuron.
• The binding of NTs to receptors leads to a postsynaptic potential, which can be either excitatory or inhibitory.

Postsynaptic Potentials

• Excitatory postsynaptic potentials (EPSPs) are depolarizing, while inhibitory postsynaptic potentials (IPSPs) are hyperpolarizing.
• A postsynaptic neuron can receive many signals at once, and the sum of these signals determines the effect on the neuron.

Neurotransmitters

• Neurotransmitters are classified into small molecule neurotransmitters, neuropeptides, and gas neurotransmitters.
• Examples of small molecule neurotransmitters include acetylcholine, amino acids, and biogenic amines.
• Examples of neuropeptides include substance P, enkephalins, and endorphins.

Acetylcholine

• Acetylcholine is an excitatory neurotransmitter at neuromuscular junctions and opens ligand-gated cation channels.
• It is inactivated by acetylcholinesterases.

Amino Acids

• Glutamate and aspartate are excitatory neurotransmitters, while GABA and glycine are inhibitory neurotransmitters.
• Glutamate is inactivated via reuptake.

Excitotoxicity

• Excitotoxicity occurs when there is an excess of glutamate in the interstitial fluid of the CNS, leading to the destruction of neurons.

Biogenic Amines

• Biogenic amines include dopamine, serotonin, and norepinephrine, which are involved in various physiological processes such as emotional response, addictive behavior, and regulation of mood.

Neural Circuits

• Neural circuits are complex networks of interconnected neurons.
• Types of circuits include simple series, diverging, converging, reverberating, and parallel after-discharge circuits.

Regeneration and Repair of Nervous Tissue

• The nervous system exhibits plasticity, but neurons have a limited ability to regenerate themselves.
• Regeneration is possible in the PNS, but not in the CNS.

Neurological Disorders

• Guillain-Barré syndrome is an acute, demyelinating disorder of the PNS, characterized by demyelination and acute inflammatory demyelinating polyneuropathy.
• Multiple sclerosis is an immune-mediated inflammatory disease of the CNS, characterized by demyelination and sclerosis of the white matter.Here are the study notes in bullet points:

The Spinal Reflexes

• A spinal reflex is a fast, automatic, unplanned sequence of actions that occurs in response to a particular stimulus.
• There are different types of reflexes:
  • Somatic reflex: contraction of skeletal muscle
  • Autonomic reflex: via autonomic nervous system (ANS)
  • Cranial reflex: integration in brain stem
  • Spinal reflex: integration in spinal cord grey matter

Components of a Reflex Arc

• A reflex arc consists of:
  • Receptor (detects stimulus)
  • Afferent neuron (transmits signal to CNS)
  • Integration center (spinal cord or brain stem)
  • Efferent neuron (transmits signal to effector)
  • Effector (responds to stimulus)

Types of Reflexes

• Stretch reflex:
  • Controls muscle length by causing muscle contraction in response to stretching of the muscle
  • Examples: tapping on tendons attached to knee, wrist, elbow, and ankle joints
  • Monosynaptic, ipsilateral reflex arc
• Tendon reflex:
  • Feedback mechanism to control muscle tension by causing muscle relaxation
  • Ipsilateral, polysynaptic reflex arc
  • Receptors: tendon (Golgi tendon) organs detect and respond to changes in muscle tension
• Flexor (withdrawal) reflex:
  • Polysynaptic, ipsilateral, intersegmental reflex arc
  • Withdrawal of limb from stimulus
• Crossed extensor reflex:
  • Acts with the flexor reflex
  • Helps maintain balance
  • Contralateral, polysynaptic reflex arc
  • Reciprocal innervation prevents muscle immobilization

Reflexes and Diagnosis

• Reflexes can be used to diagnose neurological disorders
• Examples: patellar reflex, Achilles reflex, Babinski reflex

The Brain

• The brain is protected by:
  • Cranial bones
  • Cranial meninges (pia, arachnoid, and dura mater)
  • Cerebrospinal fluid (CSF)
• The brain is supplied with blood via internal carotid and vertebral arteries
• The blood-brain barrier (BBB) protects the brain from harmful substances
• The brain is composed of:
  • Cerebrum
  • Cerebellum
  • Brain stem (midbrain, pons, and medulla oblongata)
  • Diencephalon (thalamus, hypothalamus, and epithalamus)

Blood Flow to the Brain

• The brain utilizes about 20% of the body's oxygen supply
• Any interruption of oxygen supply can result in weakening, permanent damage, or death of brain cells

Blood-Brain Barrier (BBB)

• The BBB is a barrier between the blood and the brain
• It is composed of tight junctions between endothelial cells of capillaries and a thick basement membrane
• Astrocytes secrete chemicals that maintain the permeability characteristics of the BBB
• The BBB can prevent the entry of therapeutic drugs

Cerebrospinal Fluid (CSF)

• CSF is a clear, colorless liquid that:
  • Carries oxygen, glucose, and chemicals
  • Circulates through ventricles and subarachnoid space
  • Total volume: 80-150 mL in adults
• Formation of CSF:
  • Produced by choroid plexuses in ventricles
  • Reabsorbed into blood via arachnoid villi at a rate of 20 mL/hr

Hydrocephalus

• Excess CSF accumulates in ventricles, causing CSF pressure to rise
• Can occur after head trauma, meningitis, or subarachnoid hemorrhage
• Can be relieved by draining excess fluid

Brain Stem

• The brain stem is composed of:
  • Midbrain
  • Pons
  • Medulla oblongata
• The medulla oblongata is the most inferior part of the brain stem
• It contains:
  • Respiratory rhythmicity center
  • Cardiovascular center
  • Reticular formation

The Midbrain

• The midbrain lies between the pons and the diencephalon
• It contains:
  • Cerebral peduncles
  • Superior and inferior colliculi
  • Red nucleus
  • Substantia nigra

The Cerebellum

• The cerebellum is attached to the brain stem via three pairs of cerebellar peduncles
• It regulates posture, balance, and movement
• Damage to the cerebellum can result in ataxia (loss of muscle coordination)

The Diencephalon

• The diencephalon is composed of:
  • Thalamus
  • Hypothalamus
  • Epithalamus
• The thalamus is a major relay station for sensory impulses
• The hypothalamus regulates autonomic functions, such as hunger, thirst, and body temperature