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Central and Peripheral Nervous Systems

• The nervous system is divided into two major regions: the central and peripheral nervous systems.
• Central nervous system (CNS) is the brain and spinal cord.
• Peripheral nervous system (PNS) is everything else.
• The brain is contained within the cranial cavity of the skull, and the spinal cord is contained within the vertebral cavity of the vertebral column.

Neurons

• Neurons are the primary type of cell that most anyone associates with the nervous system.
  • Responsible for the computation and communication that the nervous system provides.
  • Electrically active and release chemical signals to target cells.
  • Have a soma, or cell body, but they also have extensions of the cell; each extension is generally referred to as a process.
  • One important process that every neuron has called an axon.
  • Axon – the fiber that connects a neuron with its target.
  • Dendrites are responsible for receiving most of the input from other neurons.
• Gial cells (glia) play a supporting role for nervous tissue.

Types of Neurons

• Unipolar cells have only one process emerging from the cell. True unipolar cells are only found in invertebrate animals, so the unipolar cells in humans are more appropriately called “pseudo-unipolar” cells. Invertebrate unipolar cells do not have dendrites.
• Bipolar cells have two processes, which extend from each end of the cell body, opposite to each other. One is the axon and one the dendrite. Bipolar cells are not very common. They are found mainly in the olfactory epithelium (where smell stimuli are sensed), and as part of the retina.
• Multipolar neurons are all of the neurons that are not unipolar or bipolar. They have one axon and two or more dendrites (usually many more). With the exception of the unipolar sensory ganglion cells, and the two specific bipolar cells mentioned above, all other neurons are multipolar.

Glial Cells

• Astrocytes (CNS) and Satellite cells (PNS) function for support.
• Oligodendrocytes (CSN) and Schwann cells (PNS) function for insulation and myyelination.
• Microglia (CNS) function for immune surveillance and phagocytosis.
• Ependymal cells (CNS) function for creating CSF.

Myelin

• The appearance of the myelin sheath can be thought of as similar to the pastry wrapped around a hot dog.
• The glial cell is wrapped around the axon several times with little to no cytoplasm between the glial cell layers.
• For oligodendrocytes, the rest of the cell is separate from the myelin sheath as a cell process extends back toward the cell body.
• A few other processes provide the same insulation for other axon segments in the area.
• For Schwann cells, the outermost layer of the cell membrane contains cytoplasm and the nucleus of the cell as a bulge on one side of the myelin sheath.

Gray and White Matter

• The two regions within nervous system structures are often referred to as gray matter (the regions with many cell bodies and dendrites) or white matter (the regions with many axons).
• Gray matter is not necessarily gray. It can be pinkish because of blood content, or even slightly tan, depending on how long the tissue has been preserved.
• White matter is white because axons are insulated by a lipid-rich substance called myelin.

The Brain

• The adult brain is separated into four major regions: the cerebrum, the diencephalon, the brain stem, and the cerebellum.
• The cerebrum is the largest portion and contains the cerebral cortex and subcortical nuclei.
• It is divided into two halves by the longitudinal fissure.

Cerebrum

• Makes up most of the mass of the brain.
• Deep within the cerebrum, the white matter of the corpus callosum provides the major pathway for communication between the two hemispheres of the cerebral cortex.
• Many of the higher neurological functions, such as memory, emotion, and consciousness, are the result of cerebral function.
• Basal nuclei are responsible for cognitive processing, the most important function being that associated with planning movements.
• Basal forebrain contains nuclei that are important in learning and memory.
• The limbic cortex is the region of the cerebral cortex that part of the limbic system, a collection of structures involved in emotion, memory, and behavior.

Cerebral Cortex

• The cerebrum is covered by a continuous layer of gray matter that wraps around either side of the forebrain—the cerebral cortex.
• Responsible for the higher functions of the nervous system.
• Extensive folding in the cerebral cortex enables more gray matter to fit into this limited space.
• Lobes: frontal, parietal, temporal, occipital
• Gyri: precentral gyrus, postcentral gyrus
• Sulci: central sulcus, parieto-occipital sulcus, lateral sulcus

Diencephalon

• Deep beneath the cerebrum and constitutes the walls of the third ventricle.
• The two major regions of the diencephalon are the thalamus itself and the hypothalamus.

Thalamus

• A collection of nuclei that relay information between the cerebral cortex and the periphery, spinal cord, or brain stem.
• All sensory information, except for the sense of smell, passes through the thalamus before processing by the cortex.
• The cerebrum also sends information down to the thalamus, which usually communicates motor commands.

Hypothalamus

• A collection of nuclei that are largely involved in regulating homeostasis.
• The executive region in charge of the autonomic nervous system and the endocrine system through its regulation of the anterior pituitary gland.
• Involved in memory and emotion as part of the limbic system.

Brain Stem

• The midbrain and hindbrain (composed of the pons and the medulla) are collectively referred to as the brain stem.
• The midbrain coordinates sensory representations of the visual, auditory, and somatosensory perceptual spaces.
• The pons is the main connection with the cerebellum.
• The pons and the medulla regulate several crucial functions, including the cardiovascular and respiratory systems and rates.
• The cranial nerves connect through the brain stem and provide the brain with the sensory input and motor output associated with the head and neck, including most of the special senses.
• The major ascending and descending pathways between the spinal cord and brain, specifically the cerebrum, pass through the brain stem.
• The medulla is responsible for processing cranial nerve information and is related to sleep and wakefulness, such as general brain activity and attention.

Cerebellum

“Little brain”

• Responsible for comparing information from the cerebrum with sensory feedback from the periphery through the spinal cord.
• Primarily involved in coordinating movement and balance.

Spinal Cord

• Posterior regions are responsible for sensory functions
• Anterior regions are associated with motor functions.
• The name of a spinal cord region corresponds to the level at which spinal nerves pass through the intervertebral foramina:
  • Cervical region
  • Thoracic region
  • Lumbar region
  • Sacral region
• The nerves that emerge from the spinal cord pass through the intervertebral formina at the respective levels.
• See Spinal Cord Injuries topic for more information.

Function of the Nervous System

• Receiving information about the environment around us (sensation).
• Generating responses to that information (motor responses).
• Some regions of the nervous system are termed integration or association areas. The process of integration combines sensory perceptions and higher cognitive functions such as memories, learning, and emotion to produce a response.

Function of the Cerebral Cortex

• Orientation and memory
• Language and speech
• Sensorium (praxis, gnosis/stereognosis, graphesthesia)
• Judgment and abstract reasoning – planning and making decisions

Additional Concepts

Neuroplasticity

• Neuroplasticity: The ability of the nervous system to change its activity in response to intrinsic or extrinsic stimuli by reorganizing its structure, functions, or connections.
  • Synaptic Plasticity: Changes in the strength of synapses, which can result from increases or decreases in their activity.
  • Structural Plasticity: Changes in the organization and number of connections between neurons.

Neurotransmitters and Synaptic Transmission

• Neurotransmitters: Chemical messengers that transmit signals across a chemical synapse from one neuron to another ‘target’ neuron, muscle cell, or gland cell.
  • Acetylcholine (ACh): Involved in muscle activation, attention, and memory.
  • Dopamine: Plays a role in reward, motivation, and motor control.
  • Serotonin: Regulates mood, appetite, and sleep.
  • Gamma-Aminobutyric Acid (GABA): The primary inhibitory neurotransmitter in the brain.
  • Glutamate: The primary excitatory neurotransmitter involved in learning and memory.
• Synaptic Transmission: The process by which neurotransmitters are released by one neuron, cross the synaptic cleft, and bind to receptors on another neuron.

Neural Pathways and Circuits

• Neural Pathways: Bundles of neurons that connect different parts of the nervous system.
  • Corticospinal Tract: Conducts motor impulses from the brain to the spinal cord, involved in voluntary motor control.
  • Spinothalamic Tract: Transmits pain, temperature, and crude touch to the thalamus.
  • Dorsal Column-Medial Lemniscal Pathway: Carries fine touch, vibration, and proprioception information to the brain.
• Neural Circuits: Networks of neurons that work together to control specific functions.
  • Reflex Arc: The simplest neural circuit, consisting of a sensory neuron, interneuron, and motor neuron, responsible for reflex actions.
  • Central Pattern Generators (CPGs): Neural networks that produce rhythmic patterned outputs without sensory feedback, crucial for locomotion.

Higher Cognitive Functions

• Memory Systems:
  • Short-Term Memory (STM): Temporary storage of information.
  • Long-Term Memory (LTM): More permanent storage of information.
    • Declarative Memory: Memory of facts and events.
      • Episodic Memory: Memory of personal experiences and specific events.
      • Semantic Memory: Memory of general world knowledge.
    • Procedural Memory: Memory for the performance of particular types of action.
• Language and Speech:
  • Broca’s Area: Involved in speech production and located in the frontal lobe.
  • Wernicke’s Area: Involved in language comprehension and located in the temporal lobe.
• Executive Functions:
  • Prefrontal Cortex: Responsible for complex behaviors such as decision-making, social behavior, and personality expression.
  • Working Memory: The part of short-term memory that is concerned with immediate conscious perceptual and linguistic processing.

Neuroanatomical Structures and Their Functions

• Limbic System: A complex system of nerves and networks in the brain, involving several areas near the edge of the cortex concerned with instinct and mood.
  • Hippocampus: Essential for the formation of new memories and is also associated with learning and emotions.
  • Amygdala: Involved in emotion regulation and processing of fear and pleasure responses.
• Basal Ganglia: Group of nuclei in the brain associated with a variety of functions including control of voluntary motor movements, procedural learning, routine behaviors or “habits”, eye movements, cognition, and emotion.
  • Caudate Nucleus: Involved in learning and memory.
  • Putamen: Regulates movements and influences various types of learning.
  • Globus Pallidus: Regulates voluntary movement.
• Thalamic Nuclei:
  • Ventral Posterior Nucleus: Relays sensory information to the somatosensory cortex.
  • Lateral Geniculate Nucleus (LGN): Relays visual information from the retina to the visual cortex.
  • Medial Geniculate Nucleus (MGN): Relays auditory information to the auditory cortex.

Clinical Applications

• Neurological Assessments:
  • Mini-Mental State Examination (MMSE): Commonly used to screen for cognitive impairment.
  • Glasgow Coma Scale (GCS): Assesses consciousness level in patients with acute brain injury.
  • Functional MRI (fMRI): Measures brain activity by detecting changes associated with blood flow.
• Neurorehabilitation Techniques:
  • Constraint-Induced Movement Therapy (CIMT): Encourages the use of a hemiparetic limb by restraining the unaffected limb.
  • Neurofeedback: A type of biofeedback that uses real-time monitoring of brain activity to teach self-regulation of brain function.
  • Transcranial Magnetic Stimulation (TMS): Uses magnetic fields to stimulate nerve cells in the brain to improve symptoms of depression.

Bookmark

1. Betts, J.G., Young, K.A., Wise, J.A., Johson, E., Poe, B., Kruse, D. H., Korol, O., Johnson, J.E., Womble, M. & DeSaix, P. (2013). Anatomy and Physiology. OpenStax. http://cnx.org/content/col11496/latest/[↩]