Nervous System: Central and Peripheral Divisions

• Information on the nervous system includes gathering and responding to stimuli.
• Specialised nerve cells (neurones) are organised into the central and peripheral nervous systems.
• Peripheral nerves link sense organs, muscles, and glands with the brain or spinal cord.
• External and internal stimuli are received by receptors (sense organs).
• Impulses transmit through neurones to the nervous system which will then reach effector organs, such as muscles and glands.
• Effector organs will then produce responses, and feedback information.

Central Nervous System

• The central nervous system consists of the brain and spinal cord.
• It serves as an integrative control center.

Peripheral Nervous System

• Includes peripheral nerves (cranial and spinal).
• Facilitates communication between the CNS and the body.

Sensory (Afferent) Division

• Composed of sensory neurons.
• Conducts signals from receptors to the CNS.

Motor (Efferent) Division

• Composed of more neurons.
• Conducts signals from the CNS to the effectors.

Autonomic Nervous System

• Controls involuntary responses.
• Includes sympathetic and parasympathetic divisions.

Somatic Nervous System

• Controls voluntary movement.

Sympathetic Division

• Mobilises body systems.
• Deals with "flight or fight" responses.

Parasympathetic Division

• Conserves energy.
• Deals with "rest and digest" responses.
• A nerve bundle contains sensory and/or motor neurones consisting of bundles of axons surrounded by perineurium.

Types of neurones

• Motor neurones, relay neurones and sensory neurones are the three types of neurones featured.
• Axons can be myelinated, where the myelin sheath is formed by a Schwann cell wrapping itself around the axon.
• Between two sheath cells where an axon is myelinated is a node of Ranvier.

Generalised neurone structure

• Nucleus
• Dendrite
• Cell body
• Axon
• Myelin sheath
• Node of Ranvier
• Schwann Cell
• Axon terminal

Sensory neurones

• Conduct action potentials (impulses) from receptors (sensory cells) to the CNS.
• These neurones have a long axon and a cell body often centrally located.
• The cell body is located within the DRG (dorsal root ganglion), which causes swelling outside the spinal cord.
• The sensory neurones will terminate on a relay or motor neurone within the CNS.

Intermediate or Relay Neurones

• Conduct impulses from sensory to motor neurones.
• Integrate information from many sources.
• They originate entirely within the CNS.
• These neurones bear numerous short processes and are multipolar.

Motor Neurones

• Transmit impulses from the CNS to effectors.
• Adapted for receiving, integrating, and conducting impulses over long distances.
• The cell body is located within the CNS and has many cytoplasmic processes (dendrites and dendrons) that conduct impulses toward the cell body.
• The axon in a motor neurone is a long process that conducts impulses away from the cell body.

Nerve Impulses

• Nerve impulses aren't signals or messages.
• Electrical change in distribution of electrical charge across the plasma membrane caused by ion movement in and out of the axon.
• It is a self-propagating wave of electrical disturbance that travels along the surface of a neurone plasma membrane.
• It involves a momentary reversal in electrical potential difference in the membrane (action potential) caused by rapid ion movement.
• Axons may wrap with Schwann cells, which spiral around the myelin sheath; acting as electrical insulator.
• Spaces every 1-3 mm, uncovered regions = nodes of Ranvier (2-3 µm long)
• Myelin is insulating material made by Schwann cells.
• Gaps in the myelin sheath are called Nodes of Ranvier.
• Multiple Sclerosis damages myelin causing nerve signals to slow down or be blocked.

Reflex Arcs

• A reflex arc is a neural pathway that controls a reflex composed of sensory and motor neurones.
• A reflex arc may or may not involve the brain.
• It's a pathway of neurones with action potentials carried from receptor to effector without any conscious thought.
• The Babinski reflex tests the integrity of the spinal cord and involves observing toe movement in response to stimulation.
• A normal response in adults is flexion of the toes, whereas an abnormal response is extension or dorsiflexion of the large toe and fanning of other toes.
• A knee jerk is a useful diagnostic tool that tests the reflex arc involving sensory neurones, motor neurones and muscles.
• Reflexes are quick, stereotyped, and automatic responses, they are protective and involuntary.
• Immediate responses to harmful stimuli without time to think can range from simple to complex and are conditional.

Resting Potential

• Resting potential (electrical charge) is present across the membrane when the neurone isn't carrying an impulse.
• The inside of the membrane is negative compared to the outside.
• The Hodgkin and Huxley studies of the 1950s investigated resting potential.
• There is an unequal distribution of ions, inside of the axon is relatively negative.
• Potential difference across the plasma membrane of a neurone at rest.
• The plasma membrane is polarised and charged to -70mV.
• Sodium or Na+ is in excess on the outside, while Potassium of K+is in excess inside.
• There's more cations outside than inside, and large organic ions are inside.
• Maintained by selective permeable nature of membrane.
• Requires energy to maintain difference in electrical charge through polarisation that allows sensitivity to react.

Mechanisms Contributing to Resting Potential (RP)

• Action of Na+/K+ pumps (active transport), moves 3 Na+ out for every 2 K+ taken into the axoplasm.
• Differential permeability of the membrane, it's more permeable to K+ than Na+ because of facilitated diffusion.
• Na + - K + pumps in PM use active transport by carrier proteins to remove 3 Nat out of the axoplasm for every 2 K+ Uses ATP!
• It's opposed by some 'leakage' via diffusion through other parts of Plasma Membrane or PM.
• Plasma Membrane equals selectively permeable.
• Facilitated diffusion via channels includes:
  • Some always open while others are 'voltage-gated'.
  • MORE K+ channels open than Nat channels - thus more permeable to K+ so K+ DIFFUSES OUT again - and so Na+ diffuses in, down concentration gradients.
• The net result is loss of K+ (resulting in XS cations outside).
• Unequal distribution of ion causes movement of cations through plasma membrane of axon, allowing net outward movement of K+.

Nervous/Action Impulse

• Depolarisation: membrane becomes more permeable to Na+, enters the cell and reduces charge and occurs.

• To occur impulse must exceed threshold so temporary inside of membrane will have reversal that causes potential.

• Resting membrane is -65mV and the action potential peaks at +40mV.

• At resting state, gated Nat channels are closed.

• Stimulation above threshold opens and leads to entry of Na+ down an electron-chemical gradient.

• As Nat enter the cell making more Na+ enter, building charge is at ~ +40mV.

• At action potential Na + channels close causing no further entry so K+ is diffused out removing +ve charge (repolarisation).

• An AP moves in only 1 way because of the refractory period (Na channels don't respond so the Ap will still go towards the synapse).

• There are two phases of Action Potential that involve recovery or recovery phrase with pumps.

• Absolute is the resistance phase that stops stimulation during, so it won't impact locked large ion so AP can be stable.

• Relative to the resistance phase by having triggers to a new action that makes it difficult.

• All channels are still open for K+.

• AP is discrete from beginning to end by a refractory period that stops another and allows coordination by the membrane so re-establish.

• Information passes one way along itself is nerve impulse is waves of propagation that triggers production is ionic movement.

• Considers 2 channels (sensitive voltage to Altering and to spread).

Learn about the nervous system, its central and peripheral divisions, sensory and motor functions, and nerve cell organization. Explore how the body gathers and responds to stimuli through specialized nerve cells. Understand the roles of receptors, neurones, and effector organs in transmitting impulses and producing responses.