Nervous System: Reception and Processing

• Living beings perform nutrition, reproduction, and interaction.
• Interaction involves receiving information (internal and external) and creating responses, coordinated by the nervous and endocrine systems.

Nervous System Function

• The nervous system rapidly transmits information within the body.
• The endocrine system, also known as the hormonal system, responds slower because it needs to synthesize and transport hormones.
• There are 3 phases in the nervous system that include, information reception, information processing, and response.

Information Reception

• The body continuously receives external or internal stimuli.
• External stimuli are related to senses like pain.
• Internal stimuli come from signal receptors in organs, like hunger.
• Specific neurons called sensory neurons, capture and translate stimuli into nerve impulses.
• Impulses are transmitted to sensory nerves, which carry signals to the brain and spinal cord.

Information Processing

• Impulses reach the brain.
• The brain correlates this with stored memory data to form a response (babies cry when hungry as an example).

Response Emission

• Once the brain creates a response is sent to motor neurons through motor nerves to effector organs (glands or muscles)
• Saliva production is related to glands, where we salivate when we eat something because saliva helps digest food.

Types of Cells in the Nervous System

• Neurons: Highly differentiated star-shaped cells that don't divide.
• As we age, neuron count decreases (80-year-olds have 30% fewer neurons).
• Neurons generate nerve impulses and transfer them from one another in a process called synapses.
• Neuroglia or Glia Cells are cells with the function of protecting neurons that provide isolation, support and nutrition.

Neuron Parts

• Dendrites: Thin, branched cytoplasmic extensions receive stimuli (signals).
• Neuronal Body or Soma: Contains the nucleus (DNA) and cytoplasmic organelles.
• Axon: A long extension from the soma to axon terminals; it's the pathway for nerve stimuli.
• Myelin Sheaths: Insulating layer on the axon improves nerve impulse transmission.
• Node of Ranvier: Spaces between myelin sheaths.
• Axon Terminations: Branches at the end of the axon produce and send neurotransmitters to the next neuron.
• Mielin acts as electrical current insulation during nerve impulses. Mielin is produced by Shwann cells. Neuron with mielin conducts saltatory nerve impulses.

Nerve Impulses

• Nerve impulses are electrochemical currents.
• Impulses originate from the potential difference as a consequence of a stimulus.
• Producing a stimulus, the neuron is at rest, and it is necessary to implement certain steps.
• Resting Potential: Neurons are inactive; their interior is negatively charged, and the exterior is positively charged (membrane potential is -70mV).
• Potential of Action: A neuron is stimulated by a sensory receptor or another neuron which rapidly increases sodium permeability.
• As a result, the membrane potential changes from -70mV to +40mV.
• Impulse Propagation: Depolarization in unmyelinated fibers triggers sodium channel opening in adjacent areas, causing continuous impulse propagation.
• Depolarization only occurs at the Ranvier nodes (saltatory conduction) in myelinated neurons
• Repolarization: After the impulse passes, sodium channels close, and potassium channels open, allowing potassium to flow out and restore the original polarization from the membrane potential.
• Excitability Threshold: A stimulus must have a minimum intensity (>55mV) to be perceived.
• Refractory Period: A neuron must return to its resting potential before it can be stimulated again.

Synapsis

• Neurons transmit nerve impulses from a presynaptic neuron to a postsynaptic neuron.

• This communication is called synapsis, which does not occur through physical contact.

• A synaptic cleft separates pre/postsynaptic neurons

• Once sensory neurons receives a stimulus they are charged of transmitting the nerve impulses to other neurons.

• Nervous current circulates through neurons thanks to forming connect circuits.

• There are two types of synapses:

  • Electric Synapsis: They are in physical contact through channels and present in invertebrates.
  • Chemical Synapsis: There is a synaptic space, so the impulse is transmitted by neurotransmitters (chemical molecules released from the axon).

• When a neuron receives a stimulus, there is a series of electrical charges in the neuron membrane.

• Impulses always propagate in one direction, starting from dendrites, and ending in axonic terminations

  • In axonic terminations there are vesicles, which produce chemical substances called neurotransmitters that cross to the next neuron.
  • When a neuron receives a nerve impulse (stimulus), it gets exited and electrical changes occur in its membrane to transmit the impulse to the next neuron.
  • Steps to trigger nerve impulses include:
    • Nerve currents cause calcium to enter the axonic terminations, and the vesicles release neurotransmitters into the synaptic cleft.
    • Neurotransmitters bind to specific receptors in the postsynaptic membrane (the impulse).
    • This binding causes depolarization and creates a new nerve current.

Types of Nervous Systems

• Anatomical parts division of the nervous system includes:
  • Central Nervous System (CNS): Includes the brain and spinal cord.
  • Peripheral Nervous System: Consist of nerves that depart from the CNS and are distributed across the body.
  • Functional parts division of the nervous system includes:
    • Somatic Nervous System: controls the voluntary actions from our body.
    • Autonomic/Vegetative Nervous System: controls involuntary actions.

Central Nervous System

• The CNS includes the spinal cord and the brain and those are protected by bone structures.
• The brain is protected by the cranium, and the spinal cord is protected by the vertebral column composed of the Meninges.
• The three membranes, from the inside out, are: pia mater, arachnoid, and dura mater, with a fluid between the inner two, called the cerebrospinal fluid, that protects and nourishes the brain and spinal cord.
• In vertebrates, the nervous system combines organs to become the central nervous system.

Central Nervous System

• The CNS is dorsal and includes the brain and spinal cord.
• The organs are protected by the meninges and bone structures in particular vertebral column and cranium.
• CNS has two zones that include gray and white matter
  • Gray matter contains neuronal bodies (somas), dendrites, synapses, and glial cells.
  • White matter contains axons covered with myelin.
  • Gray matter is in the cortexes and in the center of the spinal cord.
  • White matter lies beneath gray matter in the spinal cord.
  • The fuction of gray matter is processing/transmitting information.
• White matter connects different zones of the nervous system, allowing communication between the brain and spinal.
  • The function of white matter it to connect different zones of the nervous system and to quickly comunicate between the brain and spinal cord.

Brain

• The brain is divided in three parts, called anterior encephalon which include:
  • Telencephalon: Found in birds and mammals, is a nervous fiber accumulation and its centralized information and control.
  • Diencephalon: It is divided in three parts, thalamus, hypothalamus and the hypophysis.
• Mesencephalon is the brain in the middle.
• The posterior Encephalon includes:
  • Metencephalon: This part has the function of controlling equilibrium.
• Mielencephalon: Is the Rachidian Bulb (medulla oblongata); it controls automatic functions. This part unifies the Encephalon and the medullar column.

Medullar Column

• The medullar column begins with the base of the skull, and goes until the second lumbar vertebra.
• Is a nervous cylinder that is through to the interior from the vertebral column.
• It acts as intermediary from the central nervous system.
• Functions of the medullar column:
  • Conducter: Nerve fibers send the received information.
  • Controls reflex acts: Involuntary movements generated as a response of a stimulus.

Peripheral Nervous System

• The peripheral division (PNS) is the part of the nervous system outside of the central division (CNS).
• The PNS main function is to connect the brain and spinal cord with the rest of the body, allowing communication between organs, muscles and tissues The Peripheral Nervous System is formed by sensitive neurons (send the information from sensory organs into the SNC), and for motory neurons.
• Nerves: are associations from myelinated fibers axons. Nerves can be in the encephalon or in the Medulla zone.
• Nervous Ganglions: are associations of neurons, that act sending the information.

Peripheral Nervous System Types

• Sensory (afferent) nerves transport impulses from sensory organs to the CNS to be processed.
• The motor (efferent) nerves transmit orders from the CNS to skeletal muscles to allow for voluntary movement.

Autonomic Nervous System

• Regulates certain involuntary bodily functions, including heart rate, digestion, and respiration.
• Sympathetic increases heart rate and suppresses functions like digestion.
• Parasympathetic slows heart rate and increases digestive and excretory functions allowing to conserve energy.

Importance

• PNS enables communication between the body and external environment, and the correct functioning is crucial.
• It allows the perception of the world and realization of movements, while autonomous nervous regulate vital functions.