Behavioural Biology B11: The Nervous System - CNS & PNS

General Organization of the Nervous System

  • The nervous system has two major divisions:

    • Central Nervous System (CNS)
      • Consists of the brain and spinal cord.
      • Integrates incoming information and coordinates all voluntary and involuntary nervous functions.
    • Peripheral Nervous System (PNS)
      • Consists of ganglia and the nerves that branch from the CNS.
      • Keeps the CNS in contact with the rest of the body.

  • Overall Flow:

    • Sensory Input: From sensory receptors responding to external stimuli, skin, muscles, joints, and internal organs.
    • Central Nervous System: Processes input from the brain and spinal cord.
    • Motor Output: Directed via the Peripheral Nervous System.
      • Somatic Nervous System: Directs voluntary movements.
      • Autonomic Nervous System: Regulates involuntary bodily activities, such as heart rate and breathing rate.
        • Parasympathetic Nervous System: Governs bodily activities during restful conditions.
        • Sympathetic Nervous System: Prepares the body for stressful or emergency situations.

Cells of the Nervous System

  • Neurons (nerve cells):
    • Excitable cells that generate and transmit messages.
  • Neuroglial Cells (glial cells):
    • Outnumber neurons by 10 to 1.
    • Several types, each with a specific function (e.g., Schwann cells).
    • Provide structural support, growth factors, and insulating sheaths around axons.
  • Nerves:
    • Bundles of myelinated axons.
    • The predominant structures in the PNS.

The Central Nervous System (CNS)

Protection of the CNS

  • Bones:
    • Skull (cranial bones) protects the brain.
    • Vertebral column (vertebrae) protects the spinal cord.
  • Meninges:
    • Three connective tissue membranes that protect the brain and spinal cord.
    • Dura mater: Outermost layer.
    • Arachnoid: Middle layer.
    • Pia mater: Innermost layer.
    • Meningitis: Inflammation of the meninges, caused by bacteria and viruses, can lead to encephalitis (inflammation of the brain).
  • Cerebrospinal Fluid (CSF):
    • Location: Fills the space between the meninges, ventricles (internal cavities of the brain), and the central canal (cavity within the spinal cord).
    • Functions:
      • Shock absorption.
      • Support.
      • Nourishment and waste removal.
  • Blood-Brain Barrier:
    • Structure: Formed by tight junctions between cells in the walls of capillaries supplying the CNS.
    • Function:
      • Protects the CNS by selectively choosing which substances can enter the cerebrospinal fluid from the blood.
      • Inhibits many potentially life-saving, infection-fighting, or tumor-suppressing drugs that are not lipid-soluble from reaching brain tissue.

Brain: Command Center

Cerebrum

  • The thinking, conscious part of the brain, accounting for 83\% of total brain weight.
  • Separated into two hemispheres by the longitudinal fissure.
  • Each hemisphere receives sensory information from and directs movements of the opposite side of the body.
  • Each hemisphere has a thin outer layer called the cerebral cortex.
    • Outer layer: Gray matter (cell bodies of neurons).
    • Beneath the cortex: White matter (nerves, axons/dendrites).
  • Gray matter: Includes neuroglial cells, nerve cell bodies, and unmyelinated axons.
  • White matter: Consists mostly of myelinated axons, allowing for communication between various areas of the brain and between the brain and spinal cord.
  • Corpus Callosum: A band of white matter that connects the two cerebral hemispheres, allowing them to communicate.
  • Cerebral Cortex Lobes: Grooves on the surface of the brain mark the boundaries of four lobes on each hemisphere:
    • Frontal lobe
    • Parietal lobe
    • Temporal lobe
    • Occipital lobe
  • Cerebral Cortex Areas:
    • Sensory areas: Primary somatosensory area receives sensory information from the body.
    • Motor areas: Primary motor area controls skeletal muscles; premotor cortex coordinates learned motor skills.
    • Association areas: Communicate with sensory and motor areas and other brain parts to analyze and act on sensory input. The prefrontal cortex enables reasoning and thinking.

Basal Ganglia (Basal Nuclei)

  • Millions of cell bodies organized into groups located deep in the cerebrum.
  • Functions:
    • Helps execute desired voluntary motor behaviors (emotional and cognitive).
    • Helps suppress unwanted voluntary motor behaviors (emotional and cognitive).
  • Includes: Caudate (part of striatum), Putamen (part of striatum), Globus pallidus (GP), Subthalamic nucleus (STN), Substantia nigra (SN).

Limbic System

  • Defined on the basis of function rather than anatomy.
  • Includes several brain structures that produce emotions and memory.
  • Memory storage and retrieval occur in two stages:
    • Short-term memory: Holds a small amount of information for a few seconds or minutes.
    • Long-term memory: Stores limitless amounts of information for hours, days, or years.
  • The Hippocampus and Amygdala are involved in memory and emotion.

Thalamus

  • Serves as the relay station of the brain for all sensory information except smell.
  • Also directs motor activity, cortical arousal, and memory.

Hypothalamus

  • Maintains homeostasis by regulating blood pressure, heart rate, breathing rate, digestion, and body temperature.
  • Coordinates the nervous and endocrine systems by influencing the pituitary gland.
  • Regulates emotions (part of the limbic system).
  • Serves as the "master biological clock."

Reticular Activating System (RAS)

  • An extensive network of neurons that runs through the medulla and projects to the cerebral cortex.
  • Filters sensory input and keeps the cerebral cortex in an alert state.

Cerebellum

  • Integrates information from the motor cortex and sensory pathways to produce smooth, well-timed voluntary movements.
    • Comparison: Compares the actual position of a body part to where it ought to be.
    • Prediction: Calculates future positions of a body part during a movement.
  • Controls equilibrium and posture.

Brain Stem

  • Includes the medulla oblongata, midbrain, and pons.
  • Medulla Oblongata:
    • Contains reflex centers to regulate the rhythm of breathing, force and rate of the heartbeat, and blood pressure.
    • Serves as the pathway for all sensory messages going to the higher brain centers and all motor messages leaving the brain.
  • Midbrain:
    • Processes information about sights and sounds.
    • Controls simple reflex responses to these stimuli, such as turning your head toward a loud sound.
  • Pons (meaning "bridge"):
    • Connects the spinal cord and cerebellum with the cerebrum, thalamus, and hypothalamus.
    • Has a region that assists the medulla in regulating respiration.

Spinal Cord

  • Extends from the foramen magnum of the skull to between the 1^{st} and 2^{nd} lumbar vertebrae.
  • Protection: Protected by the bones of the vertebral column and the meninges (dura mater, arachnoid mater, pia mater, with associated epidural, subdural, and subarachnoid spaces).
  • Gray Matter:
    • Mostly cell bodies of neurons.
    • Dorsal horns: Contain cell bodies of interneurons; they receive information from sensory neurons in the dorsal root. Cell bodies of sensory neurons are housed in the dorsal root ganglion outside of the spinal cord.
    • Ventral horns: Contain cell bodies of motor neurons of the somatic (voluntary) nervous system; they send information out the ventral root.
    • Surrounds the central canal, which is filled with cerebrospinal fluid.
  • White Matter:
    • Composed of myelinated fiber tracts (myelinated axons).
    • Sensory (afferent) tracts: Conduct impulses toward the brain.
    • Motor (efferent) tracts: Carry impulses from the brain to skeletal muscles.

The Peripheral Nervous System (PNS)

Anatomy of the PNS

  • Connects the brain and spinal cord (CNS) to the rest of the body, mostly via nerves.
  • Cranial nerves: Take electrical impulses to and from the brain.
    • 12 pairs. Primarily serve the head and neck.
    • Only one pair, the vagus nerves, extends to the thoracic and abdominal cavities.
    • Some carry only sensory fibers, others only motor fibers, and some carry both.
  • Spinal nerves: Take impulses to and away from the spinal cord and brain.
    • 31 pairs of mixed nerves, each servicing a specific region of the body.
    • Spinal nerves are named according to where they emerge from (e.g., Cervical, Thoracic, Lumbar, Sacral).
    • The collection of spinal nerves at the inferior end is called the cauda equina.
    • All spinal nerves carry both sensory and motor fibers.
      • Sensory fibers enter the dorsal side of the spinal cord in a bundle called the dorsal root.
      • Axons of motor neurons leave the ventral side of the spinal cord in a bundle called the ventral root.
  • Ganglia: Collections of nerve cell bodies (e.g., dorsal root ganglion).

Sensory Receptors

  • Specialized to respond to changes in the environment (stimuli).
  • Classification by body location:
    • Cutaneous receptors: Located in the skin (respond to touch, pain, temperature, pressure).
    • Proprioreceptors: Located in muscles and tendons (respond to stretch/tension).
  • Classification by type of stimulus:
    • Mechanoreceptors: Respond to touch, pressure, vibration, and stretch.
    • Thermoreceptors: Sensitive to changes in temperature.
    • Photoreceptors: Respond to light energy (e.g., retina).
    • Chemoreceptors: Respond to chemicals (e.g., smell, taste, changes in blood chemistry).
    • Nociceptors: Sensitive to pain-causing stimuli (e.g., extreme heat or cold, excessive pressure, inflammatory chemicals).

Motor Division of the PNS

  • Consists only of motor nerves and is divided into the Somatic Nervous System and the Autonomic Nervous System.

Somatic Nervous System (SNS)

  • Stimulates skeletal (voluntary) muscle.
  • Motor neuron cell bodies originate inside the CNS, and their axons extend directly to the skeletal muscles they serve.
    • Neurotransmitter: Acetylcholine (ACh).
    • Effect: Skeletal muscle contraction.

Autonomic Nervous System (ANS)

  • Controls muscles in the heart, smooth muscle in internal organs (e.g., intestine, bladder, uterus), and regulates blood pressure, heart rate, and vascular salt concentrations.
  • Involves a chain of two motor neurons:
    • The first (preganglionic) motor neuron is in the brain or spinal cord (myelinated).
    • The second (postganglionic) motor neuron extends to the target organ (unmyelinated).
    • Neurotransmitters: Acetylcholine (ACh) or Norepinephrine (NE).
    • Effect: Smooth muscle contraction or relaxation, cardiac muscle increased or decreased rate and force of contraction, glands increased or decreased secretions.

Autonomic vs. Somatic Divisions - Key Differences

  • Both systems involve reflexes and connections to the CNS utilizing unipolar and multipolar sensory neurons, interneurons, and motor neurons.
  • Differences lie in:
    1. Their effectors (skeletal muscle vs. cardiac muscle, smooth muscle, glands).
    2. Their efferent pathways (single neuron vs. two-neuron chain).
    3. Target organ responses to their neurotransmitters.

Autonomic Subdivisions

  • Each autonomic subdivision dominates the other depending on the stimulus presented. Both operate continuously, responding in varying degrees to sensory information.
    • Sympathetic Nervous System:
      • "Fight or Flight" response.
      • E-response: Dominates during Exercise, Excitement, Emergency, Embarrassment.
      • Originates in the thoracic and lumbar regions of the spinal cord.
      • Effects: Dilates pupil, decreases salivation, increases breathing rate, increases heart rate, narrows blood vessels, slows digestive activity, stimulates secretion of epinephrine and norepinephrine, causes salt and water retention, relaxes bladder muscles, inhibits defecation.
    • Parasympathetic Nervous System:
      • "Rest and Digest" response.
      • D-response: Dominates during Digest, Defecate, Diuresis (urination).
      • Originates in the cranial and sacral regions of the spinal cord.
      • Effects: Constricts pupil, increases salivation, decreases breathing rate, slows heart rate, widens blood vessels, increases digestive activity, contracts bladder muscles, stimulates defecation.

Think Fast!

  • Pounding heart, rapid, deep breathing, cold sweaty skin, a prickly scalp, and dilated pupils belong to the Sympathetic Nervous System, as these are classic "fight or flight" responses.