6.1 Organization and Function of the Nervous System

Organization and Function of the Nervous System
Overview

  • The nervous system is like the body's command center.

  • It takes in information and decides how the body should react, whether you're thinking about it or not.

  • It has two main parts:

    • Central Nervous System (CNS): This includes the brain and spinal cord.

    • Peripheral Nervous System (PNS): This includes all the nerves outside the brain and spinal cord.

Central Nervous System (CNS)

  • Made up of the brain and spinal cord.

Brain Subdivisions

  1. Hindbrain:

    • Includes the cerebellum, pons, and medulla oblongata.

    • Can be affected by sleep problems and strokes (CVAs).

    • Controls:

      • Coordinating movements.

      • Keeping your posture.

      • Breathing and blood flow.

  2. Midbrain:

    • Contains cerebral peduncles.

    • Has corticospinal tracts (the main pathway for motor nerves).

    • Controls eye movement.

    • Processes what you see and hear.

  3. Forebrain:

    • The front part of the brain.

    • Includes the diencephalon and the cerebral hemispheres.

    • Diencephalon:

      • Thalamus and subthalamus (dorsal horn).

      • Hypothalamus (ventral horn).

    • Cerebral Hemispheres:

      • Surround the diencephalon.

      • Connected by the corpus callosum (nerve fibers).

      • Divided into lobes:

        • Frontal: Has the primary motor cortex (for precise movements and learned activities).

        • Parietal: Has the primary somatosensory cortex (receives sensory info like touch).

        • Temporal: Has the primary auditory cortex (receives sound) and auditory association area (interprets sounds).

        • Occipital: Has the primary visual cortex (receives visual input) and visual association cortex (interprets what you see).

Basal Ganglia

  • These are masses of white matter deep inside the brain.

  • They help with natural, unlearned movements, making them smooth.

  • Examples: swinging your arms when you walk, or following through when you swing a bat.

  • Diseases like Parkinson's can affect these, leading to abnormal movements.

Limbic System

  • Found in the middle of the cerebrum.

  • Controls emotions and related behaviors.

  • Stimulating it can cause feelings like pleasure, anxiety, or rage.

Spinal Cord

  • Runs from the base of the brain to about the L1/L2 vertebrae.

  • Cauda equina: nerve roots that extend beyond the end of the spinal cord.

  • Oval-shaped.

  • Made of gray matter and white matter:

    • Gray matter: Butterfly-shaped, contains neuron cell bodies.

    • White matter: Surrounds the gray matter, contains nerve fibers that transmit information up and down the cord.

  • Horns:

    • Dorsal horns: receive sensory information from the body.

    • Ventral horns: send motor information to the body.

Protection of the Spinal Cord and Nerves

  • Vertebral column (the bony spine).

  • Vertebral body: the front part of each vertebra, supports weight.

  • Vertebral arch: the back part, forms the walls of the vertebral foramen.

  • Vertebral/spinal canal: the space through which the spinal cord runs.

  • Fibrocartilaginous discs: act like shock absorbers between vertebrae.

  • These discs create space for nerves and blood vessels to exit.

Meninges

  • Protective layers around the brain and spinal cord:

    1. Pia mater: the innermost, thin layer with blood vessels.

    2. Arachnoid layer: surrounds the CNS, nonvascular and waterproof.

      • Subarachnoid space: where cerebrospinal fluid (CSF) circulates.

    3. Dura mater: the strong, outermost layer.

Ventricular System

  • Cavities filled with cerebrospinal fluid (CSF).

  • CSF cushions the brain and spinal cord.

  • Keeps a stable environment for nutrients, electrolytes, and waste.

  • Ependyma: cells lining the ventricles and central canal.

  • Choroid plexus: makes CSF.

  • CSF Flow: Lateral ventricle → intraventricular foramen → third ventricle → cerebral aqueduct → fourth ventricle → foramina → subarachnoid space (where it bathes the brain and spinal cord).

Peripheral Nervous System (PNS)

  • Nerves and ganglia outside the CNS.

  • Includes 12 pairs of cranial nerves (for sensory and motor functions).

Cranial Nerves and Their Functions

  • CNI – Olfactory Nerve: Smell.

  • CNII – Optic Nerve: Vision.

  • CNIII – Oculomotor Nerve: Pupil control, eye movement, eyelid movement.

  • CNIV – Trochlear Nerve: Eye movement (down and inward).

  • CNV – Trigeminal Nerve: Chewing and facial sensation.

  • CNVI – Abducens Nerve: Eye movement (side to side).

  • CNVII – Facial Nerve: Facial expression and taste (front of the tongue).

  • CNVIII – Vestibulocochlear Nerve: Hearing and balance.

  • CNIX - Glossopharyngeal Nerve: Swallowing and taste (back of the tongue).

  • CNX – Vagus Nerve: Digestion, heart rate, and sensation in the digestive tract.

  • CNXI – Accessory Nerve: Neck and shoulder muscle control.

  • CNXII – Hypoglossal Nerve: Tongue movement.

Spinal Nerves

  • 31 pairs of spinal nerves from the spinal cord:

    • 8 cervical.

    • 12 thoracic.

    • 5 lumbar.

    • 5 sacral.

    • 1 coccygeal.

  • Each spinal nerve comes from the spinal cord via the dorsal root or the ventral root:

    • Dorsal root: carries sensory information to the cord.

    • Ventral root: carries motor information away from the cord.

  • The two roots join before the nerve leaves the spine.

  • All spinal nerves are mixed (carry signals to and from the cord).

  • Sensory information goes to the spinal cord, then to the brain.

  • Motor commands come from the brain, go down the spinal cord, and exit through the ventral root.

  • The signal travels to the muscle to cause a voluntary contraction or other response.

Efferent Division of PNS

  • Divided into the somatic nervous system and the autonomic nervous system.

  • Somatic System:

    • Usually under your control.

    • Controls skeletal muscles, skin, and joints.

  • Autonomic System:

    • Controls glands and smooth muscles.

    • Usually involuntary.

    • Divided into the sympathetic and parasympathetic nervous systems:

      • Sympathetic Nervous System: prepares the body for activity and stress.

      • Parasympathetic Nervous System: operates during rest, helps with digestion and conserving energy.