Nervous System Function: Autonomic Control & Neural Signals

Posted on Apr 3, 2025 in Biology

Nervous System Organization

The Nervous System comprises two main parts:

  • Central Nervous System (CNS): Consists of the brain and spinal cord. It builds and maintains models of the environment, makes decisions, and processes sensory information. Grey matter contains dendrites and cell bodies, while white matter consists of myelinated axons. The blood-brain barrier tightly regulates substances entering the brain.
  • Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS, connecting it to the rest of the body. It is further divided into:
    • Somatic Nervous System: Controls voluntary movements via skeletal muscles.
    • Autonomic Nervous System (ANS): Controls involuntary functions via smooth muscle, cardiac muscle, and glands.

Autonomic Nervous System (ANS)

The ANS regulates involuntary bodily functions and has two opposing divisions:

Parasympathetic Division (Rest & Digest)

  • Origin: Brainstem and sacral spine.
  • Effects: Lowers heart rate (HR) and blood pressure (BP), constricts airways and pupils, increases digestion and salivation.
  • Activation: Targets specific organs individually. Extreme activation can cause fainting (vasovagal response).
  • Pathway: Uses two neurons: a preganglionic neuron (CNS to ganglion) and a postganglionic neuron (ganglion to target). Uses acetylcholine (ACh) at both synapses (nicotinic receptors in ganglia, muscarinic receptors at the target organ).

Sympathetic Division (Fight or Flight)

  • Origin: Thoracic and lumbar spine.
  • Effects: Increases HR and BP, dilates airways and pupils, decreases digestion and salivation.
  • Activation: Causes a full-body response. Extreme activation triggers an adrenaline rush via the adrenal medulla.
  • Pathway: Also uses preganglionic and postganglionic neurons. Uses ACh in ganglia (nicotinic receptors) but primarily norepinephrine (NE) at target organs acting on adrenergic receptors (exception: sweat glands use ACh).

Adrenal Medulla

Acts like a modified sympathetic ganglion, releasing epinephrine (adrenaline) and some norepinephrine directly into the bloodstream for a widespread, prolonged sympathetic response.

ANS Balance

Most organs receive dual innervation from both divisions, allowing for fine-tuned control. Their effects are typically antagonistic.

Peripheral Nervous System (PNS) Pathways

Somatic Motor Pathway

Involves upper motor neurons (in CNS) synapsing onto lower motor neurons (in spinal cord/brainstem). Lower motor neurons project from the PNS to skeletal muscles, using acetylcholine (ACh) and nicotinic receptors at the neuromuscular junction to trigger muscle contraction.

Cranial Nerves

There are 12 pairs of cranial nerves, primarily serving sensory and motor functions of the head and neck. The Vagus nerve (CN X) is a major exception, extending to innervate thoracic and abdominal organs as part of the parasympathetic system.

Spinal Nerves

Emerge from the spinal cord, containing both sensory (afferent) and motor (efferent) fibers. These fibers are sorted into:

  • Dorsal Root: Carries sensory input into the spinal cord. Contains the dorsal root ganglion (cell bodies of primary sensory neurons).
  • Ventral Root: Carries motor output away from the spinal cord.

Sensory Pathways

Sensory signals enter the spinal cord via the dorsal root. Primary sensory neurons synapse onto secondary neurons.

  • Signals for fine touch and proprioception ascend directly in the spinal cord to the brainstem.
  • Signals for pain and temperature synapse in the spinal cord, cross over, and then ascend to the brain.
  • Secondary neurons may ascend to the brain (thalamus and then cortex) for conscious perception or synapse within the spinal cord for reflex actions.

Reflex Pathways

  • Somatic Reflex Arc: A rapid, involuntary response to a stimulus (e.g., pulling back from pain). Involves a sensory neuron synapsing directly or indirectly (via interneuron) onto a motor neuron in the spinal cord, bypassing conscious brain processing for speed.
  • Autonomic Reflexes: Control involuntary functions like HR, BP, and digestion, involving sensory input and ANS motor output.

Neural Signaling Mechanisms

Neurons and Membrane Potential

Neurons transmit signals using electrical changes across their membranes.

  • Resting Membrane Potential (~ -70mV): The voltage difference across the membrane when the neuron is inactive. Maintained by the Na+/K+ pump (3 Na+ out, 2 K+ in) and leak channels (more permeable to K+ than Na+).
  • Equilibrium Potential: The membrane potential at which there is no net movement of a specific ion. Calculated by the Nernst Equation (e.g., Na+ ≈ +61mV, K+ ≈ -90mV).
  • Goldman Equation: Predicts the overall membrane potential considering the concentrations and relative permeabilities of multiple ions (primarily K+, Na+, and Cl-).

Action Potentials (APs)

Rapid, all-or-none electrical signals used for long-distance communication along axons.

  1. Threshold (~ -55mV): Sufficient stimulus causes depolarization to this level.
  2. Depolarization: Voltage-gated Na+ channels open rapidly; Na+ rushes in, making the membrane potential positive (≈ +30mV).
  3. Repolarization: Voltage-gated Na+ channels inactivate; voltage-gated K+ channels open slowly; K+ rushes out, restoring negative potential.
  4. Hyperpolarization: K+ channels close slowly, causing a brief overshoot beyond resting potential.
  5. Return to Rest: Na+/K+ pump and leak channels restore resting potential.

The Refractory Period (absolute and relative) follows an AP, preventing backward propagation and limiting firing frequency.

Graded Potentials vs. Action Potentials

  • Graded Potentials: Local, small changes in membrane potential (depolarizing/excitatory – EPSP, or hyperpolarizing/inhibitory – IPSP) that diminish with distance. Occur at dendrites and cell bodies. Can summate.
  • Action Potentials: Large, all-or-none signals that regenerate without decrement along the axon. Triggered if summed graded potentials reach threshold at the axon hillock.

Synaptic Transmission

Communication between neurons occurs at synapses.

  1. AP arrives at the presynaptic terminal.
  2. Voltage-gated Ca2+ channels open; Ca2+ enters.
  3. Ca2+ influx triggers the fusion of synaptic vesicles with the presynaptic membrane.
  4. Neurotransmitters are released into the synaptic cleft.
  5. Neurotransmitters bind to receptors on the postsynaptic membrane.
  6. Binding causes ion channels to open/close, generating a postsynaptic potential (EPSP or IPSP).

Key Neurotransmitters

  • Acetylcholine (ACh): Excitatory at neuromuscular junctions (somatic); used in ANS ganglia and by parasympathetic postganglionic neurons.
  • Norepinephrine (NE): Primary transmitter for sympathetic postganglionic neurons.
  • Epinephrine (E): Hormone released by adrenal medulla; also acts as neurotransmitter.
  • Glutamate: Major excitatory neurotransmitter in the CNS.
  • GABA & Glycine: Major inhibitory neurotransmitters in the CNS.
  • Dopamine: Involved in reward, motivation, and motor control.
  • Serotonin: Involved in mood, sleep, and appetite.

Key Receptors in the ANS

Muscarinic ACh Receptors (GPCRs)

Found on target organs of the parasympathetic nervous system.

  • M1: Involved in CNS, gastric acid secretion, pupil constriction, airway constriction, some digestive glands.
  • M2: Primarily in the heart; slows heart rate.
  • M3: Found in smooth muscle (contraction, e.g., bladder, GI tract), glands (secretion, e.g., salivary), pupils (constriction), and airways (constriction).

Adrenergic Receptors (GPCRs)

Found on target organs of the sympathetic nervous system; respond to norepinephrine (NE) and epinephrine (E).

  • Alpha-1 (α1): Primarily responds to NE. Causes constriction of blood vessels (↑BP), contraction of pupillary dilator muscle.
  • Alpha-2 (α2): Primarily responds to NE. Located presynaptically (inhibits NE release) and postsynaptically (e.g., reduces digestion, platelet aggregation).
  • Beta-1 (β1): Responds to NE and E equally. Increases heart rate and contractility. Stimulates renin release in kidneys.
  • Beta-2 (β2): Primarily responds to E. Causes dilation of bronchioles, dilation of blood vessels in liver, heart, and skeletal muscle; relaxes uterine smooth muscle; increases glycogenolysis.

Central Nervous System (CNS) Components & Function

Spinal Cord Structure

  • Dorsal Horns: Receive sensory information (grey matter).
  • Ventral Horns: Contain motor neuron cell bodies (grey matter).
  • White Matter Tracts: Ascending (sensory) and descending (motor) myelinated axons.

Key Brain Regions

  • Thalamus: Major sensory relay center; directs sensory information (except smell) to the appropriate cortical areas.
  • Hypothalamus: Controls homeostasis (temperature, hunger, thirst), autonomic functions, endocrine regulation, behavior, and motivation.
  • Midbrain: Involved in motor coordination, visual and auditory reflexes.
  • Cerebellum: Regulates movement, coordination, posture, and balance.
  • Parietal Lobe: Processes somatosensory information (touch, temperature, pain), spatial awareness, and navigation.
  • Temporal Lobe: Responsible for hearing, language comprehension, memory formation, and object recognition.

Vision and Sensory Processing

Retina and Visual Pathway

  1. Light enters the eye and passes through the lens to the retina.
  2. Light reaches photoreceptor cells (rods and cones) at the back of the retina.
  3. Photoreceptors: Convert light energy into neural signals.
  • Rods: Highly sensitive to dim light; provide black and white vision.
  • Cones: Require brighter light; responsible for color vision (detect red, green, blue wavelengths) and visual acuity.
Phototransduction:
  • In the dark: Photoreceptors are depolarized due to open cGMP-gated Na+ channels and continuously release the neurotransmitter glutamate.
  • In the light: Retinal pigment absorbs photons, activating opsin. This triggers a G-protein (transducin) cascade that activates phosphodiesterase (PDE). PDE breaks down cGMP, causing Na+ channels to close. The cell hyperpolarizes, reducing glutamate release.
Photoreceptors synapse onto bipolar cells. Bipolar cells process signals and synapse onto ganglion cells. Axons of ganglion cells bundle together to form the optic nerve, which exits the eye at the blind spot (optic disc). The optic nerve transmits visual information to the brain (via thalamus to visual cortex).

The retina’s layered structure allows for significant initial processing of visual information before signals leave the eye.

Perception and Sensory Processing

The brain actively interprets sensory input:

  • It compensates for missing visual information (e.g., filling in the blind spot).
  • It can perceive motion where none exists (e.g., motion aftereffect).
  • It adapts to continuous sensory stimuli (e.g., sensory adaptation leading to color afterimages, contrast illusions).
  • Visual illusions demonstrate how the brain’s interpretation can differ from physical reality.

Key Exam Tip

Focus on understanding ANS anatomy (origins, ganglia), neurotransmitters (ACh, NE, E), receptor types (nicotinic, muscarinic, adrenergic subtypes), and the functional effects of sympathetic vs. parasympathetic activation on major organs (heart, lungs, GI tract, blood vessels, pupils, glands).