Nervous System - Bio 12

Responsibilities of nervous system

Sensory input (PNS) - Gather information to monitor changes occuring both inside and outside body, e.g. senses, pressure, proprioreceptors

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Integration (CNS) - Process and interpret sensory input to decide action

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Motor output (PNS) - Response to integrated stimuli, response activates muscles or glands

Nervous system development

Formed during first month of embryonic development

Folate is important for development of nervous system during pregnancy

Maternal infections can have very harmful effects

Hypothalamus develops last

No more neurons formed after birth, but growth/development continues for years and can grow dendrites (neuroplasticity)

Brain reaches max weight as an adult

CNS components

Brain - Result of centralization of nervous system

Spinal cord - Protected by vertebrae, PNS nerves extend outside of vertebrae

Grey matter contains cell bodies, white matter contains myelinated fibers

Nerve cell components

Dendrite - Transmits signals to other cells

Axon terminal - Transmits signals to muscles, glands, other neurons

Cell body - Has nucleus

Myelin sheath

Nodes of Ranvier

Schwann cells

Part with dendrites + cell body + axon is part for integration of inputs and decision making

Neuroplasticity of teen brain

Brain goes through second critical period of heightened neuroplasticity between 12-25 years old

Significant remodeling of neural connections to improve efficiency, rapid synaptic pruning and white matter growth

Good time for learning, skill acquisition, environmental adaptation

Key aspects of adolescent neuroplasticity

“Use it or lose it” - Synaptic pruning eliminates underused neural connections to strengthen frequently used ones, optimizing brain function

Front-to-back development - Prefrontal cortex, which is responsible for decision-making, planning, impulse control - is last to mature

Enhanced learning and adaptability - High plasticity makes brain very adaptable, allows faster learning/skill development but also more likely to have long-term effects of environmental stress

Risk-taking and emotion - Reward system (limbic system) matures faster than prefrontal cortex, so heightened emotional sensitivity, increased sensation-seeking, and focus on social rewards

Impact of neuroplasticity of teen development

Environmental influence - High plasticity, so environment actively sculpts the brain’s final structure

Mental health vulnerability - Many mental illnesses like anxiety and depression emerge due to intense remodeling of brain circuitry

Social focus - Brain is uniquely primed for social connection and exploring new environments, essential for developing independence

PNS components

Nerves, neurons, sensory organs outside CNS

Consists of somatic and autonomic systems

PNS functions

Sends (sensory) information to CNS

Receives and transmits motor signals from CNS

Stimulates effectors

Somatic nervous system

Voluntary movement

Motor neurons control voluntary movements by activating skeletal muscles

Also involved with involuntary movements like reflexes that need skeletal muscles to contract

Soma = body

Autonomic nervous system

Involuntary movements

Motor neurons control involuntary responses involving organs, glands, smooth muscles

Some voluntary control can come from relaxation, meditation, which reduces perceptions of stress which reduce stress response

Involves CNS and neurons in ganglia

Two divisions: Sympathetic and Parasympathetic

Sympathetic nervous system

Increases activities

“Fight or flight” response - dilate pupils, increase heart rate, constrict blood vessels, inhibit digestion

“E” division - exercise, excitement, emergency, embarrassment

Parasympathetic nervous system

Conserves energy

“Rest and ruminate/digest” response - constrict pupils, dilate blood vessels, reduce heart/breathing rates, stimulate digestion

Maintains daily necessary body functions

“D” division - digestion, defecation, diuresis

Nuerons/Nerve cells

Receive stimuli and transmit action potentials

Organization: cell body, dendrites (input), axons (output)

Sensory neurons: afferent nerve fibers that carry information to CNS

Motor neurons: efferent nerve fibers that carry impulses away from CNS

Neuroglia/Glial cells

Support neurons

Glia - Non-neuronal cells maintaining homeostasis, from myelin, provide support and protection for neurons in brain and other parts of nervous system

Different cells in CNS and PNS

CNS: astrocytes, oligodendrocytes, ependymal cells, microglia

PNS: Schwann cells, satellite cells

Glial cells in CNS

Astrocytes, oligodendrocytes, ependymal cells, microglia

Glial cells in PNS

Schwann cells, satellite cells

Neuroglia vs Neurons

Neuroglia divide, neurons don’t but can be induced to repair, and astrocytes have stem cell like properties

Most brain tumors involve neuroglia cells and not neurons, so most brain tumors are “gliomas”

• Because glial cells can divide and cancer is uncontrolled cell division that disrupts homeostasis

Astrocytes

In CNS

Most abundant cells in brain, astro = star because star-shaped

Functions:

• Clean up brain “debris”

• Transport nutrients to neurons

• Hold neurons in place

• Digest parts of dead neurons

• Regulate content of extracellular space

• Promote synaptic connections

• Clear excess neurotransmitters

• Ensure continued function of neurons

Also help maintain permeability of blood-brain barrier where they sense glucose and ion levels inside brain and regulate their flow in and out

Also regulates extracellular fluid transport (glymphatic pathway, brain’s lymphatic system)

Oligodendrocytes

In CNS

Myelin-forming cells of CNS

Composed of layered phospholipid membranes and supports and insulates axons, allowing faster impulse transductions

Have same role as Schwann cells in PNS

One oligodendrocyte myelinates multiple axons

Incapable of replication upon injury

Ependymal cells

In CNS

Lines cavities of brain and spinal cord (ventricles of brain, central canal of spinal cord)

Creates cerebrospinal fluid (CSF)

• Clear body fluid in brain and spinal cord, acts as cushion/buffer for brain’s cortex, also important for autoregulation of cerebral blood flow

• Created in the choroid plexus

Cells have cilia on their surfaces to circulate CSF

Microglia

In CNS

Spider-like immune cells

Functions:

• Maintain brain homeostasis

• Phagocytose debris

• Regulate inflammation

Often described as brain’s “first responders, janitors, superheroes” but can also cause neurodegeneration when dysfunctional

Key functions and usage examples:

• Phagocytose dead cells, extracellular debris, protein aggregates (e.g. Alzheimer’s amyloid-beta plaques)

• Release cytokines to fight infections/respond to injuries like stroke or traumatic brain injury

• Role in brain development by removing excess/redundant synapses to refine neural networks

• Facilitate repair and regrowth of neural tissue after inflammation

Schwann cells

In PNS
Type of satellite cell that functions similarly to oligodendrocytes in CNS

Create myelin sheath around nerves, which helps insulate them for faster conduction

Nodes of Ranvier are gaps in myelin sheath, needed for nerve impulses to travel rapidly

Can also help repair and regenerate damaged axons. stimulating the regeneration may require release of growth factors

In multiple scleroses (autoimmune disease), the myelin sheath is destroyed and gets hardened into a tissue called scleroses

Satellite cells

In PNS
Maintain health and activity of neurons in PNS

Provide support and nutrition to peripheral nerves and regulate their electrolyte and neurotransmitter levels (important factors for proper nerve conduction)

Release neurotrophic factors that help protect neurons from degeneration or death

Essential for developing, maintaining, repairing peripheral nerves

Neuron/Nerve Cell vs Nerve

Neuron:

• Individual unit of nervous system

• Composed of cell body (soma), dendrites (receive signals), an axon (send signals)

• Generates and conducts electrochemical impulses

• In CNS and PNS

Nerve:

• Bundle of axons outside brain and spinal cord

• Many axons wrapped together

• Transmits signals to PNS

• In PNS

Types of neurons

Sensory/Afferent

• Carries impulses towards CNS from sensory receptor

• Cell body located in dorsal root ganglion adjacent to vertebrae

• Axon enters grey matter of spinal cord and either synapses with dendrite/cell body end of an interneuron or with dendrite/cell body end of motor neuron

Motor/Efferent

• Carries impulses away from CNS

• Axon synapses with effectors with structures called motor end plates

• Cell body is located in spinal cord (grey matter), characterized by long axon and short dendrite

Interneurons/Association neurons

• In spinal cord

• Connects sensory and motor neurons

• In grey matter of spinal cord

• Function in reflex arcs which are structural and functional components of reflexes

Structure of neurons

Cell body:

• Nucleus and metabolic center of cell

• Integrates and coordinates

• After response to stimulus decided, nerve impulse or action potential is initiated by cell body at axon hillock, then travels down axon

Processes:

• Processes - Fibers that extend from cell body (dendrites and axons)

• Cell body - Nucleus and metabolic center of cell

Neuron process

1. Synaptic terminals - Bring signals from other neurons

2. Dendrites - Receive signals from other neurons

3. Cell body - Integrate signals, coordinate metabolic activities

4. Axon hillock - Action potential (nerve impulse) starts

5. Axon - Transmits action potential

6. Myelin - Insulates axon, speeds conduction

7. Synaptic terminals - Transmit signals to other neurons

8. Dendrites of other neurons receive signals

Axons

Axon is typically a single, longer projection that carries signal away from body

Main functions:

• Conduct action potentials to effector (other neurons, muscle cells, gland cells)

• Structure makes them especially suited for long-distance communication in nervous system

• Many axons wrapped in myelin, which speeds up connection and reduces signal loss

Axon/Synaptic terminals at end of axon:

• Stores neurotransmitters in vesicles

• Neurotransmitters released by action potential into the synapse to pass message to next cell (neurotransmitter causes reaction by other cell

  • E.g. acetylcholine released into neuromuscular synapse causes muscle to contract

Dendrites

Short, branched extensions from cell body

Branched shape allows neuron to form many synaptic connections, more branches = more opportunities to receive information

Neural communication - Receives incoming signals from other neurons, information is received in form of neurotransmitters released from axon terminal

Integrates information - Computes and summates synaptic inputs, determines whether neuron generates action potential in response

• Single neuron can receive thousands of inputs at once, dendrites collect and transmit those towards cell body

• The inputs can be excitatory (increase chance neuron will fire) or inhibitory (decrease chance neuron will fire)

Dendrites and neuroplasticity

Many dendrites contain dendritic spines, which are small protrusions where synapses often form

Dendritic spines can remodel or change based on brain activity, which is crucial for learning and memory

Reflex vs Reaction

Reflexes are involuntary, used to protect body, faster than reactions

Also negative feedback loop usually, helps body return to homeostasis

Components of reflex arc

1. Sensory component/Afferent neuron - Takes in info and translates to electrical signal that gets sent to CNS

2. Integrating center/Interneuron - Sensory processing centers that determine magnitude of response to the incoming stimulus, located in CNS

3. Efferent portion/Motor neuron - Takes info from interneuron and sends it to effectors which activate a response, the effectors are usually in muscle fibers or a gland

Patellar reflex arc

Knee reflex arc, is a spinal reflex

Sends two signals

1. Motor neuron leads back to quadriceps. Quad springs lower leg up when receive info from quad' muscle’s motor neuron

2. Interneuron sends signal to motor neuron leading to hamstring that tells it to relax, so no negative force is acting on quads when it contracts

Both signals work together, all happens in spinal cord without brain (not needed)