Nervous System Divisions

• The nervous system is responsible for receiving sensory input, integrating information, controlling muscles and glands, maintaining homeostasis, and establishing and maintaining mental activity.

Divisions of the Nervous System

• The central nervous system (CNS) consists of the brain and spinal cord, connected at the foramen magnum of the skull.
• The peripheral nervous system (PNS) includes all nervous tissue outside the CNS, such as nerves and ganglia.

PNS Divisions

• Sensory division (afferent) conducts action potentials from sensory receptors to the CNS.
• Sensory neurons transmit action potentials from the periphery to the CNS.
• Motor division (efferent) conducts action potentials from the CNS to effector organs like muscles and glands.
• Motor neurons transmit action potentials from the CNS to the periphery.
• Somatic nervous system transmits action potentials from the CNS to skeletal muscles.
• Autonomic nervous system (ANS) transmits action potentials from the CNS to cardiac muscle, smooth muscle, and glands.
• The ANS includes the sympathetic and parasympathetic divisions.
• The enteric nervous system (ENS) is a unique PNS subdivision with sensory and motor neurons within the digestive tract to function independently of the CNS.

Cells of the Nervous System

• The two main types are neurons and glial cells.

Neurons

• Neurons, or nerve cells, receive stimuli, conduct action potentials, transmit signals, and require oxygen and glucose.
• The main parts include the cell body (soma), dendrites, and axon.
• The cell body contains a single nucleus and processes stimuli.
• Dendrites are short, branching extensions that receive stimuli.
• Axons are single, long processes that transmit stimuli, originating from the axon hillock.
  • Collateral axons are branches of the main axon.
  • Axoplasm is the cytoplasm of the axon, and the axolemma is its plasma membrane.

Types of Neurons

• Sensory neurons (afferent) carry action potentials toward the CNS.
• Motor neurons (efferent) carry action potentials away from the CNS.
• Interneurons conduct action potentials within the CNS from one neuron to another.
• Multipolar neurons have many dendrites and one axon, common as motor neurons and in the CNS.
• Bipolar neurons have one dendrite and one axon, found in special sense organs.
• Pseudo-unipolar neurons have a single process extending from the cell body, acting as an axon with no dendrites, typical of sensory neurons.

Glial Cells

• Glial cells (neuroglia) are supportive cells in the CNS and PNS and do not conduct action potentials.
• Glial cells enhance neuron function and maintain normal conditions in nervous tissue, and are more numerous than neurons with the ability to divide.

Glial Cells of the CNS

• Astrocytes are star-shaped and support CNS structure, regulate signaling, contribute to the blood-brain barrier, and aid tissue repair.
  • The blood-brain barrier is a permeability barrier formed by astrocytes and blood vessel endothelium.
• Ependymal cells are epithelial-like, line CNS fluid-filled cavities (ventricles and canals), and form choroid plexuses that produce CSF.
• Microglia are immune cells that protect the CNS from infection and become phagocytic in response to inflammation.
• Oligodendrocytes provide an insulating material by forming myelin sheaths around axons in the CNS.

Glial Cells of the PNS

• Schwann cells are single cells that form myelin sheaths around axons or enclose unmyelinated axons.
• Satellite cells surround neuron cell bodies in sensory and autonomic ganglia, providing insulation, support, nutrition, and protection.

Myelin Sheaths

• Myelin sheaths are specialized layers around axons formed by oligodendrocytes in the CNS and Schwann cells in the PNS.
• Myelinated axons conduct action potentials faster (3-15 m/s) due to Nodes of Ranvier.
• Nodes of Ranvier are gaps in the myelin sheath that allow action potential development, supporting saltatory conduction.
• Saltatory conduction involves action potentials jumping from one node to another.
• Multiple sclerosis involves myelin sheath deterioration, leading to loss of muscle function.

Organization of Nervous Tissue

• Gray matter consists of neuron cell bodies and dendrites with little myelin, found in the cortex (brain surface), nuclei (deeper clusters), and ganglia (PNS clusters).
• White matter consists of parallel axons with myelin sheaths, forming nerve tracts (CNS) and nerves (PNS) for action potential propagation.

Electrical Signals and Neural Pathways

• A resting membrane potential occurs when the outside of the cell is more positive (Na+) and the inside is more negative (K+).
• The resting membrane potential is maintained through a higher concentration of K+ inside the cell, a higher concentration of Na+ outside the cell, and greater permeability of the cell membrane to K+ than to Na+.
• Leak channels are always open and K+ channels are an example of a leak channel.
• Gated channels are closed until opened by specific signals.
  • Types of gated channels include chemically gated channels (opened by neurotransmitters) and voltage-gated channels (opened by a change in membrane potential).
• The sodium-potassium pump is required to maintain the greater concentration of Na+ outside the cell membrane and K+ inside.
• Active potentials are electricity that causes depolarization and repolarization.
• Local current is the movement of Na+ that causes the inside of the cell membrane to become positive
• Depolarization occurs when the inside of the cell membrane becomes positive, resulting in local potential.
• If the depolarization is large enough, Na+ enters the cell until it reaches a threshold value, which causes a voltage-gated Na+ channels to open.
• Hyperpolarization is the condition that the charge on the cell membrane briefly becomes more negative than the resting membrane potential.
• Continuous conduction occurs when an action potential is conducted along the entire axon cell membrane.
• Saltatory conduction occurs when action potentials "jump" from one node of Ranvier to the next along the length of the axon.

The Synapse

• The synapse is the junction where the axon of one neuron interacts with another neuron or with cells of an effector organ.
• Three major components include the presynaptic terminal (end of the axon), the postsynaptic terminal (membrane of the dendrite of effector cell), and the synaptic cleft (space separating the presynaptic and postsynaptic membranes).
• Neurotransmitters are chemical substances stored in synaptic vesicles in the presynaptic terminal.
  • Examples include acetylcholine (ACh) and norepinephrine.
  • Acetylcholinesterase breaks down acetylcholine.

Neurotransmitters and Their Effects

• Acetylcholine (ACh) is released at CNS synapses, ANS synapses, and neuromuscular junctions, and has excitatory or inhibitory effects.
• Norepinephrine (NE) is released at selected CNS synapses and some ANS synapses, and has excitatory effects.
• Serotonin is released at CNS synapses and generally has inhibitory effects.
• Dopamine is released at selected CNS synapses and some ANS synapses, and has excitatory or inhibitory effects.
• Gamma-aminobutyric acid (GABA) is released at CNS synapses and has inhibitory effects.
• Glycine is released at CNS synapses and has inhibitory effects.
• Endorphins are released at descending pain pathways and have inhibitory effects.

Reflexes

• A reflex is an involuntary reaction to a stimulus applied to the periphery and transmitted to the CNS.
• Reflexes allow for quick reaction to stimuli, more so than conscious thought.
• A reflex arc is a neuronal pathway by which a reflex occurs, it is the basic functional unit of the nervous system.
• The five basic components of a reflex arc are stimulus to a sensory receptor, sensory/afferent neuron, Interneurons within the central nervous system(CNS)(spinal cord), motor/efferent neuron and an Effector muscle or gland.
• A converging pathway has two or more neurons synapse with the same postsynaptic neuron; allows multiple information streams into single pathway.
• A diverging pathway has the axon from one neuron divides and synapses with more than one other postsynaptic neuron; allowing information streams into two or multiple pathways.
• Summation allows integration of multiple subthreshold local potentials, including spatial summation which happens when local potentials originate from different locations on postsynaptic neuron and temporal summation which happens when local potentials overlap in time from single input that rapidly fires, overlapping local potentials.

Spinal Cord

• The spinal cord extends from the foramen magnum to the 2nd lumbar vertebra
• Is protected by the vertebral column, allows movement, and ends at the conus medullaris.
• Cauda equina is the inferior end and spinal nerves exiting, resembling a horse's tail.
• White matter is composed of myelinated axons and is outside the spinal cord.
• Gray matter is composed of unmyelinated axons and is shaped as letter H or butterfly in center of the spinal cord
• The central canal is the fluid filled space in center
• Ventral root is where the spinal nerve rootlets on the ventral aspect combines
• Dorsal root is where spinal nerve rootlets on he dorsal aspect combines
• Ganglion are ovoid structures containing cell bodies of neurons and glial cells supported by connective tissue
• Arranged into 3 columns, these are dorsal, ventral and lateral.
• Ascending tracts consist of axons that conduct action potential towards brain.
• Descending tracts consist of axons the conduct action potential away from brain
• Posterior horns contain axons which synapse with interneurons
• Anterior horns contain somatic neurons
• Lateral horns contain autonomic neurons

Spinal Cord Reflexes

• Knee-jerk Reflex = patellar reflex and an example of the simplest stretch reflex
• Stretch reflex happens when muscle contract in response to a stretching force. This is used by clinicians to determine if higher CNS centers are functional
• Withdrawal(flexor) Reflex is to remove a limb and body part from a painful stimuli.

Spinal Nerves

• Spinal nerves arise along the spinal cord from unions of dorsal and ventral roots.
• Consisting of axons of both sensory and somatic motor neurons, these mixed nerves communicate between the spinal cord and body
• They are broken down into regions of the vertebral column
  • 31 pairs
    • cervical(C1-C8)
    • thoracic(T1-T12)
    • lumbar(L1-L5)
    • sacral(S1-S5)
    • coccygeal(Co)
• Dermatome are areas of skin supplied with sensory intervention by a pair of spinal nerves
• Plexuses are where neurons of several spinal nerves come together and intermingle
• Coccygeal Plexus originates from S5 and Co; Supplies motor innervation to the pelvic floor and sensory innervation to over the coccyx
• The major plexuses include cervical plexus. brachial plexus and Lumbosacral Plexus
• Cervical Plexus originates from spinal nerves C1 - C4 which has branches and innervates multiple muscles in the hyoid bone
  • contains the phrenic nerve which innervates in the diaphragm
• Brachial plexus originates from spinal nerve C5 to T1 with 5 being five major nerves to supple the upper limb and shoulder -Axillary nerves innervates shoulder and skins -Radial nerves innervates all the muscles in forearms and arms and has "crutch paralysis(dysfunction)".
  • Musculocutaneous nerve innervates anterior muscle of arms -Median nerve innervates forearms and hand
• Lumbosacral Plexus originates from L1 to S4 with four major nerves, supplying the lower limb -Obturator nerve innervates muscles the medial arm -Femoral nerve innervates anterior thigh muscles -Tibial nerve innervates posterior muscle ,anterior and posterior muscles also its in the feet and the skin over the soul -Common fibular nerve innervates lateral thigh and leg

Brain

• The brain occupies most of of the cranial cavity in the skull
• Its structure is broken down into the brainstem, cerebellum , diencephalon and cerebrum and covered by meninges and CSF
• Brainstem connects the spinal cord to the of the brain and composed of the medulla oblongata, pons and midbrain
• MedullaOblongata :The most inferior portion and continuous with the spinal cord which regulates heart rate, blood vessel diameter, swallowing , vomiting, hiccupping, coughing, sneezing and balance
• The pons are above the medulla and a functional bridge between the cerebrum and cerebellum to control breathing control swallowing ,salivation
• The midbrain is above pons and is the smallest region of the brain, coordinating pupil diameter, eye movement, turning head towards noise with the 4 mounds inside called culliculi which are in the midbrain inside substantia niga
• Reticular formation is the group of nuclei that is scattered within the brainstem and involved in regulating cyclical motor function like respiration, walking and chewing
• Reticular Activating System RAS plays a vital role in regulating and maintaining consciousness and sleeping cycles
• Cerebellum: Translates to "small brain '' and controls balance and muscle. Situated on the posteror portion of the cerebrum
  • Cerebellar peduncles are large connections attached to the brainstein

Diencephalon

• Diencephalon is between the brainstem and cerebrum
• Consisting of thalamus hypothalamus and epithalamus
  • The thalamus is the largest portion of the forebrain, effecting sleep and pain -Hypothalamus is below the thalamus, controlling homeostasis, body temp and hunger and contains mammillary bodies which initiate emotional responses.
  • Epithalamus Above the thalamus and stimulates visceral responses to odors and has Pineal gland

Cerebrum

• Largest and most prominent part
• Divided to left and right hemisphere and controls the body on the opposite sides
• Left hemisphere controls the right and controls ,science and logic
• Right hemisphere controls the left and controls art and music.
• Components of Cerebrum:
• Cerebral Medulla is white matter in the middle the context and the nuclides
  • Longitude Fissure divides the left and the right.
  • Corpus Callosum connects the left and right
  • Gyri are folds increasing surface areas
  • Sulci are shallow indentions

Lobes of the Cerebrum

• Frontal lobe-voluntary movements, motivation, mood, and smell
• Parietal lobe principal receiving sensory information; touch, pain, and balance
• Temporal lobe-hearing, smell. memory
• Occipital lobe-visual input
• Isla-fifth lobe, desires, cravings, an addiction
• Pathways in brain and spinal cord and indicates the origin and termination
• Transmit information peripheral
• Pathway: Spinothalamic=Pain, pressure, temp,tickle
• Pathway: Dorsal column= deep pressure, touch vibration
• Pathway: Spinocerebellar-Propriocepton

Somatic Motor Functions

• In motor areas of the cerebral context
• Primary context- Voluntarily ,frontal lobe
• Premotor-frontal lobe, motor organized before initation
• Prefrontal : Motation and foresight
• Upper motor neurons in context to lower motor to spinal and brainstem
• Direct Pathway= Lateral corticospin-skilled movement
• Indirect Pathway= Rubropsinal , reticulospinal

Somatic Motor Functions and Basal Nuclei

• Vestibulospinal- posture and balance
• Tectospinal- visuals
• Basal Nucli:
  • Group Function
  • Plan ,organize, Coordinate -two kinds= Corpus Striatum and Sustantia Negra

Other Brain Functions: Communication Between the Right and Left Hemispheres

• Right=input and the muscular activity on the left.
• Left= Input and the muscular activity on the right.
• Com=Connections between to hemi
• Corpus= brand of nerve tracks
• Speech; - left hemi and 2 areas is sensory and motor sensory speech and where and and comprended Motor-words and from and controls movements needed

Brainwaves and Memory

• electroencephalogram is to monitor brain waves
• types: alpha=normal, but quiet state with the eyes closed
• beta =higher freq occur with mental activity
• delta=occur during sleep on in certain sick patients
• Memory
• Encoding=lasts one second evaluate -Cons=short term memory last
• Storage- long term storage depending and how much -retrieve info and retrieval= often retrieved Types of
• 1. working-task. minute. info
• short-term -sus drugs
• long-term-depended
• declarative-relation under tone
• procudere- skills

Other Body Systems

• libic system is the emotional area responds to smell taste
• Meninges protect brain with 3 connecting layers.
• dura mater- outer
• subdural-dura and aachnoid
• ach-under layer

CSF

• csh protect spinal cord
• choroid plexus-producing the fluid
• hydro-accunulation .
• 12 nerves pair- named with names

Functions

• senor -tough ,pain motor=skeleton ,nect parasympatheti inner
• epidureal vertebrea .spinal taps take fluids