Nervous System: Structure and Functions & Central Nervous System

Structure and Functions of the Nervous System

  • The nervous system maintains communication between body parts and homeostasis.

  • It governs behaviors, movements, memories, and senses.

Functions of the Nervous System

  1. Sensory Function:

    • Sensory receptors detect stimuli inside and outside the body.

    • Nerves are part of the afferent pathway.

  2. Integrative Function:

    • The nervous system processes sensory information and makes decisions.

  3. Motor Function:

    • Responses to integrative decisions.

    • Nerves are part of the efferent pathway.

Organization of the Nervous System

  • Central Nervous System (CNS)

    • Brain and spinal cord

  • Peripheral Nervous System (PNS)

    • Sensory Input

      • From sensory receptors responding to external stimuli.

      • From sensory receptors in skin, muscles, and joints.

      • From sensory receptors in internal organs.

    • Motor Output

      • Somatic Nervous System

        • Directs voluntary movements.

      • Autonomic Nervous System

        • Regulates involuntary bodily activities like heart and breathing rate.

        • Parasympathetic Nervous System

          • Governs bodily activities during restful conditions.

        • Sympathetic Nervous System

          • Prepares the body for stressful or emergency situations.

CNS vs. PNS

  • The nervous system consists of neural tissue i.e., specialized cells facilitating communication.

  • CNS: Brain and spinal cord.

  • PNS: Nerve cells and ganglia.

  • A ganglion is a cluster of nerve cell bodies outside the CNS.

  • The CNS integrates voluntary and involuntary functions.

  • The PNS connects the body to the CNS.

Central Nervous System

The Brain

  • The brain houses memories and feelings, aids decision-making, and maintains bodily functions.

  • Weighs less than 1.6 kg (less than 3% of body weight).

  • Divided into six structures:

    1. Cerebrum

    2. Corpus callosum

    3. Thalamus

    4. Hypothalamus

    5. Cerebellum

    6. Brainstem

Cerebrum

  • The bulk of the brain.

  • Surface features:

    • Gyri: elevated ridges of tissue.

    • Fissures and Sulci: grooves between gyri.

    • Gyri and fissures serve as anatomical landmarks (e.g., the longitudinal fissure).

  • The longitudinal fissure divides the brain into left and right hemispheres.

  • Each hemisphere receives sensory information from and directs the movement of the opposite side of the body.

  • The cerebrum is divided into three basic regions:

    • A superficial cortex (cerebral cortex) of grey matter, with some pockets deep inside.

    • An internal area of white matter.

    • The corpus callosum, made of white matter, connects the left and right hemispheres.

  • Gray Matter:

    • Nonmyelinated axons on neurons.

    • Important for integration and decision-making.

  • White Matter:

    • Myelinated axons on neurons.

    • Involved in communication (relaying signals).

Cerebral Cortex

  • The thin outermost layer of each hemisphere, made of grey matter.

  • Divided by sulci into lobes:

    • Frontal lobes

    • Parietal lobes

    • Temporal lobes

    • Occipital lobes

  • Different regions have different tasks:

    • Motor tasks

    • Sensory regions

    • Association regions

Functions of the Cerebral Cortex

  • Includes primary motor area, primary sensory area, taste (gustatory area), somatic association area, premotor cortex, frontal association area, speech area, hearing area, olfaction area, visual association area, and language understanding area.

Cerebrum – Sensory Areas

  • Sensory areas integrate information about the five senses.

  • Primary somatosensory area:

    • Receives touch, temperature, and pain input.

    • Receives information from sensory receptors in the skin, joints, and muscles.

    • Located in the parietal lobe.

  • Other sensory input processing regions:

    • Visual area

    • Auditory area

    • Gustatory area

    • Olfactory area

Cerebrum – Motor Areas

  • Motor areas control movement.

  • Primary motor cortex:

    • Conscious control of voluntary skeletal muscle movement.

    • Located in the frontal lobe.

  • Premotor cortex:

    • Coordinates movement of many muscle groups for repetitive motor skills.

    • Repeated movement patterns are stored in the premotor cortex, which stimulates the primary motor cortex when the pattern is repeated.

Cerebrum – Association Areas

  • Analyze signals picked up by sensory areas and communicate with motor areas to act on sensory input.

    • Example: Somatosensory association area communicates with other brain regions with stored memories to assign meaning to what sensory receptors from the primary somatosensory area are receiving.

Cerebrum – Association Areas: Prefrontal Cortex

  • Located at the most anterior part of the frontal lobe.

  • Predicts consequences of responses and determines the best response.

  • Aids in long-term planning, reasoning, and abstract thought.

  • Helps determine personality.

Thalamus

  • Located right under the cerebrum.

  • Important for sensory experience, motor function, and memory.

  • Gateway to the cortex: Information to the cerebral cortex must pass through the thalamus.

  • Sorts sensory information and sends it to the appropriate region of the cortex.

  • Provides a rough feeling of whether a sensation is good or bad.

Hypothalamus

  • Located under the thalamus.

  • Important for the autonomic nervous system and maintaining homeostasis.

    • Body temperature regulation

    • Regulates thirst and hunger

    • Metabolism

    • Breathing

    • Heart rate

    • Role in sleep/wake cycles

  • Coordinates nervous and endocrine function by influencing the pituitary gland.

  • Part of the limbic system, coordinates emotions.

Cerebellum

  • Located under the occipital lobe.

  • Like the cerebrum, has two hemispheres and a wrinkled surface, with an outer cortex of gray matter and an inner region of white matter.

  • Combines sensory and motor inputs for smooth movement and posture.

    • Functions: Timing and patterns for skeletal muscle activity, along with balance and equilibrium.

Brain Stem

  • About 3 inches long and consists of:

    1. The midbrain

    2. The pons

    3. The medulla oblongata

Brain Stem: Structure and Functions

  • Midbrain:

    • Processes information about vision and hearing.

    • Controls reflexes related to these stimuli (e.g., turning your head when you hear a loud noise).

  • Pons:

    • "Bridge" connecting the lower portions of the CNS to the higher brain structures.

    • Assists the medulla in controlling breathing.

  • Medulla:

    • Contains reflex centers (breathing, heart contraction, blood pressure, etc.).

    • Connects the spinal cord to the rest of the brain.

Reticular Activating System (RAS)

  • A group of neurons running from the medulla to the cerebral cortex.

  • Keeps the cortex alert.

  • Inhibited by sleep centers.

  • Filters out weak, repetitive signals but lets strong signals through.

Limbic System

  • Structures involved in emotions and memory.

  • Amygdala: Links events and sensations to feelings and memories.

  • Hippocampus: Plays a role in forming short-term and long-term memories.

Central Nervous System: The Spinal Cord

  • Long continuation of the brain stem located within the vertebral column.

  • Disks of cartilage separate the vertebrae and act as cushions.

Functions of the Spinal Cord

  1. Grey matter serves as a reflex center.

  2. White matter transmits messages to and from the brain.

    • Grey matter does not have myelinated axons.

    • White matter consists of tracts of myelinated axons:

      • Ascending tracts: Carry sensory information to the brain.

      • Descending tracts: Carry motor signals to the body.

Central Nervous System: Protective Structures

  • Components include:

    • Cerebrospinal fluid

    • Skull

    • Blood-Brain Barrier

    • Cerebellum

    • Brain Stem

    • Cerebrum

    • Spinal cord

CNS Protection

  • The brain and spinal cord both comprise tightly packed, fragile neurons that cannot be replaced if damaged.

  • Main Function: Integration of sensory information and issuing directive responses.

  • Protected by:

    • Bones

    • Meninges

    • Cerebrospinal fluid.

Bones

  • The brain is encased in the skull.

  • The spinal cord is encased in the vertebral column (spinal column), composed of the vertebrae, sacrum, and coccyx.

Meninges

  • Three layers that protect the brain and spinal cord:

    • Dura mater (outermost layer): Very tough.

    • Arachnoid (middle layer): Cobweb-like.

    • Pia mater (innermost layer): Thin and molded around the brain.

Cerebrospinal Fluid (CSF)

  • A fluid cushion that protects the brain and spinal cord.

  • Fills the space between meningeal layers and the cavities (ventricles) in the brain and the central canal of the spinal cord.

  • Functions:

    • Shock absorption

    • Support

    • Nourishment and waste removal

Blood-Brain Barrier

  • Acts as a selective barrier, controlling what enters the cerebrospinal fluid from the blood.

  • Separates neurons from other substances circulating in the blood.

  • Cells in the capillaries of the brain and spinal cord are relatively impermeable due to tight junctions.

  • Lipid-soluble substances pass through easily (e.g., caffeine and alcohol).