A&P module 3 nervous system

Sensory input

Gathering info about the internal and external environment through sensory receptors

Integration

Interpreting sensory input to make decisions and form perceptions

Motor output

Activating effector organs (muscles and glands) to produce a response

Receptors

Specialized neurons that detect changes (stimuli)

Exteroreceptors

Detects stimuli from the external environment (touch, pain, temp, vision, hearing)

Proprioceptors

Monitors the position and state of skeletal muscles and joints

interoceptors

Detects stimuli from internal organs (blood pressure, blood pH, blood glucose levels)

Effectors

Organs that receive signals from the nervous system to produce a response. These include skeletal muscles, smooth muscles, cardiac muscle, and glands

CNS

Consists of the brain and spinal cord. The integration and command center

PNS

Consists of cranial and spinal nerves that carry info to and from the CNS

Sensory (afferent) division

Transmits sensory information from receptors to the CNS

Somatic sensory

Conveys impulses from the skin, skeletal muscles, and joints

Visceral sensory

Conveys impulses from visceral organs

Motor (efferent) division

Transmits motor commands from the CNS to effector organs

Somatic motor (voluntary)

Controls skeletal muscles

Autonomic nervous system (ANS)

Involuntary nervous system which regulates smooth muscle, cardiac muscle, and glands

Sympathetic division

Mobilizes body systems during activity

Parasympathetic division

Conserves energy and promotes housekeeping functions during rest

Neurons

Excitable cells that transmit electrical signals

Neuroglia (glial cells)

Supporting cells that provide physical support, insulation and nourishment to neurons

Astrocytes

Found in the CNS, the most abundant glial cells that support neurons,

Guide young neuron migration

Forms blood brain barrier

Controls chemical environment

Microglia

Neuroglia found in the CNS. Works as the immune cells that phagocytize microbes

Ependymal cells

Neuroglia in the CNS that line the central cavities in the brain and spinal cord, secreting cerebrospinal fluid (CSF)

Oligodendrocytes

Neuroglia of the DNS that form myelin sheaths around axons

Schwann cells

Neuroglia of the PNS that form myelin sheaths around axons in the PNS. Vital for the regeneration of damaged peripheral nerve fibers

Satellite cells

Neuroglia of the PNS that surround neuron cell bodies in PNS ganglia, delivering nutrients and removing waste

Cell body (Soma)

Biosynthetic center of the neuron, containing the nucleus and organelles. A receptive region

Dendrites

Short, tapered, branched processes that are the primary receptive (input) regions, receiving stimuli and initiating graded potentials

Axon

A single long process that conducts nerve impulses(action potentials) away from the cell body. It is the impulse-generating and conducting region

Axon hillock

The region where the axon arises from the cell body, it is where action potentials are initiated

Axon terminals (telodendria)

The branched endings of the axon that secrete neurotransmitters

Myelin sheath

A segmented, protein-lipid sheath that surrounds most long or large-diameter axons. it protects, electrically insulates and increases the speed of nerve impulse transmission

CNS myelination

Achieved by oligodendrocytes

PNS myelination

Achieved by schwann cells

Nodes of ranvier

Gaps in the myelin sheath where voltage-gated ion channels are concentrated, allowing for faster impulse conduction.

White matter

Composed primarily of dense collections of myelinated axons (nerve fibers). Found in tracts in the CNS and nerves in the PNS.

Gray matter

Composed mainly of neuron cell bodies, dendrites, unmyelinated axons, and glial cells. Found in the cortex and nuclei in the CNS, and in ganglia in the PNS.

Multipolar

One axon and several dendrites (most abundant, include motor neurons and interneurons).

Bipolar

One axon and one dendrite (rare, found in special sense organs like the retina).

Unipolar

A single, short process that splits into two branches (most sensory neurons).

Sensory (afferent)

Transmit impulses from receptors toward the CNS (often unipolar or bipolar).

Motor (efferent)

Carry impulses from the CNS to effectors (multipolar).

Interneurons (association neurons)

Shuttle signals between CNS neurons (multipolar, entirely within the CNS).

Neurons respond to adequate stimuli by generating what

An action potential (nerve impulse)

Chemically Gated (Ligand-Gated) Channels:

Open in response to binding of a specific chemical (neurotransmitter). Found on dendrites and the soma.

Voltage-Gated Channels:

Open and close in response to changes in membrane potential. Found in the axon hillock and axon.

Mechanically Gated Channels:

Open and close in response to physical deformation of the membrane.

What happens when gated channels open?

ions diffuse across the membrane along their electrochemical gradients, creating electrical currents and voltage changes.

Resting Membrane Potential (RMP)

The potential difference across the membrane of a resting cell. In neurons, it is approximately −70 mV (the inside is negative relative to the outside).

Differences in ionic makeup in the RMP

The intracellular fluid (ICF) has high K++ and negatively charged proteins, while the extracellular fluid (ECF) has high Na++ and Cl−− concentrations.

Differential permeability of the plasma membrane in the RMP

The membrane is more permeable to K++ than to Na++ at rest.

Sodium-Potassium Pump in the RMP

Actively transports 3 Na++ out of the cell for every 2 K++ pumped in, maintaining the concentration gradients.

Graded Potentials:

Short-distance signals that occur in dendrites and the soma.

The magnitude varies (is "graded") with the strength of the stimulus.

They decay with distance from the stimulus.

Can be depolarizations (EPSPs) or hyperpolarizations (IPSPs).

Initiated by stimuli opening chemically or mechanically gated ion channels.

Action Potentials (APs) or Nerve Impulses:

Long-distance signals of axons.

Always the same magnitude ("all-or-none").

Generated at the axon hillock when a graded potential depolarizes the membrane to threshold.

Depolarization

Occurs when the membrane potential becomes less negative (moves toward zero and positive).

Depolarization is caused by what

• Caused by the influx of positive ions, typically Na++.

• Increases the probability of generating an AP.

• Known as an Excitatory Postsynaptic Potential (EPSP).

Hyperpolarization

Occurs when the membrane potential becomes more negative (moves further away from zero).

Hyperpolarization is caused by what

• Caused by the efflux of positive ions (like K++) or influx of negative ions (like Cl−−).

• Decreases the probability of generating an AP.

• Known as an Inhibitory Postsynaptic Potential (IPSP).

Action Potentials (APs)

brief, all-or-none reversals of membrane potential that occur in muscle cells and axons. They are the primary means of long-distance neural communication.

Generation of an Action Potential

initiated at the axon hillock when depolarization reaches threshold (typically around −55 mV).

Resting state of Action potential

All voltage-gated Na++ and K++ channels are closed. RMP is −70 mV.

Depolarization of Action potential

• Stimuli cause graded potentials that depolarize the axon hillock.

• If threshold is reached, voltage-gated Na++ channels open rapidly.

• Na++ rushes into the cell, causing rapid depolarization to about +30+30 mV. This is a positive feedback loop.

Repolarization of action potential

• At +30+30 mV, voltage-gated Na++ channels close (inactivate).

• Voltage-gated K++ channels open slowly.

• K++ rushes out of the cell, restoring negative membrane potential.

Hyperpolarization (undershoot) of action potentials

• Some K++ channels remain open longer than necessary, causing the membrane potential to become more negative than RMP (e.g., −90−90 mV).
  

Return to resting state of action potentials

• Voltage-gated K++ channels close.

• The Na++/K++ pump restores the original ionic concentrations and RMP.

All-or-None Event

Once threshold is reached, an AP occurs with a consistent magnitude. Stimulus intensity is coded by the frequency of APs, not their amplitude.

Continuous Propagation:

Occurs in unmyelinated axons. Each segment of the axon membrane depolarizes and repolarizes sequentially. This is slower.

Saltatory Conduction

Occurs in myelinated axons. The AP "jumps" from one Node of Ranvier to the next. This is much faster (about 30 times faster than continuous conduction).

Absolute Refractory Period:

The time from the opening of Na++ channels until they reset. During this period, the neuron cannot generate another AP, ensuring unidirectional impulse transmission.

Relative Refractory Period:

Occurs during repolarization and hyperpolarization. A stronger-than-usual stimulus is required to generate an AP because the membrane is hyperpolarized and/or voltage-gated Na++ channels are still resetting

Axon Diameter:

Larger diameter axons have less resistance and conduct impulses faster.

Myelination

Myelinated axons conduct impulses much faster than unmyelinated axons due to saltatory conduction.

Group A Fibers:

Large diameter, heavily myelinated; fastest conduction. Found in somatic sensory and motor neurons.

Group B Fibers

Intermediate diameter, lightly myelinated; intermediate speed. Found in some ANS neurons.

Group C Fibers:

Smallest diameter, unmyelinated; slowest conduction. Found in ANS neurons.

The Synapse

the junction where information is transferred from one neuron to another or from a neuron to an effector cell.

Presynaptic Neuron:

Transmits the signal toward the synapse.

Postsynaptic Neuron

Receives the signal from the synapse.

Chemical Synapse

• Uses chemical neurotransmitters to transmit signals.

Axon Terminal:

The end of the presynaptic neuron, containing synaptic vesicles filled with neurotransmitters.

Synaptic Cleft:

The fluid-filled space between the presynaptic and postsynaptic membranes.

Postsynaptic Membrane:

Contains receptors for neurotransmitters.

Excitatory Postsynaptic Potential (EPSP)

A depolarization of the postsynaptic membrane caused by the influx of positive ions (e.g., Na++). Brings the neuron closer to threshold.

Inhibitory Postsynaptic Potential (IPSP):

A hyperpolarization of the postsynaptic membrane caused by the efflux of K++ or influx of Cl−− . Drives the neuron further away from threshold.

Temporal Summation

Occurs when multiple stimuli arrive from the same synapse in rapid succession, causing EPSPs to add up.

Spatial Summation:

Occurs when multiple stimuli arrive simultaneously from different synapses on the same postsynaptic neuron, and their effects are added together.

Neurotransmitters

chemical messengers released at synapses.

Neuromodulators

chemicals that affect neuronal function more broadly.

Acetylcholine (ACh)

Excitatory at neuromuscular junctions; can be inhibitory in cardiac muscle. Degraded by acetylcholinesterase (AChE).

Catecholamines (Biogenic amine)

Dopamine, Norepinephrine (NE), Epinephrine. Involved in mood, reward, alertness.

Indolamines (Biogenic amine)

Serotonin. Associated with mood and well-being.