Human Nervous System Notes

Need for the Nervous System

  • Detecting and responding to environmental changes.
  • Coordinating body activities.

Responding to Environmental Changes

  • The nervous system detects changes (stimuli) in both the internal and external environments.
  • External environment: the surroundings outside the body.
  • Internal environment: conditions within the body (e.g., body temperature, blood sugar level, salt content, blood pressure).
  • Responses maintain a constant internal environment, a process known as homeostasis.

Coordinating Body Activities

  • The nervous system coordinates various activities and functions of the body.
  • Involuntary actions: not under conscious control (e.g., heartbeat, breathing rate, digestion, sweating).
  • Voluntary actions: under conscious control (e.g., walking, talking, writing).

Main Divisions of the Human Nervous System

  • Central nervous system (CNS).
  • Peripheral nervous system (PNS).

Central Nervous System (CNS)

  • Composed of the brain and the spinal cord.
  • Main function: Receive nerve impulses from receptors, interpret information, and transmit nerve impulses to effectors.

Protection of the CNS

  • Nervous tissue is soft and delicate and generally cannot repair itself.
  • Protection:
    • Bones: skull (cranium) protects the brain, vertebrae protect the spinal cord.
    • Cartilaginous discs and the s-shaped curve of the spine act as shock absorbers.
    • Meninges: protective membranes (pia mater, dura mater, arachnoid membrane).
      • Pia mater: innermost layer, rich in blood vessels, supplies oxygen and nutrients.
      • Dura mater: tough, outermost layer, lines the skull cavity and spinal canal.
      • Arachnoid membrane: thin, web-like membrane between the pia mater and dura mater.
    • Cerebrospinal fluid (CSF): found in the subarachnoid space.
      • Functions: shock absorber, supplies nutrients and oxygen, removes waste, prevents dehydration, maintains constant pressure.

Brain Structure and Functions

  • The brain is the upper, enlarged part of the spinal cord.
  • Hollow cavities called ventricles are filled with cerebrospinal fluid.
  • The brain gives rise to 12 pairs of cranial nerves.
  • Overall functions:
    • Receives and interprets sensations.
    • Controls higher thought processes (e.g., memory, judgment, reasoning).
    • Initiates and controls voluntary and involuntary actions.
    • Coordinates voluntary actions.
    • Maintains muscle tone, balance, and equilibrium.

Main Parts of the Brain

  • Cerebrum.
  • Hypothalamus.
  • Cerebellum.
  • Medulla oblongata.
Cerebrum
  • Largest part of the brain.
  • Structure:
    • Grey matter (outside) and white matter (inside).
    • Left and right hemispheres.
    • Right hemisphere controls the left side of the body.
    • Left hemisphere controls the right side of the body.
    • Corpus callosum connects the two hemispheres.
  • Functions:
    • Controls voluntary actions.
    • Receives and interprets sensory input (e.g., sight, taste, hearing).
    • Manages higher thought processes, including memory, judgment, and reasoning.
Hypothalamus
  • Located below the thalamus.
  • Functions:
    • Maintains homeostasis, regulating body temperature, blood pressure, sleep, emotions, appetite and thirst, and heartbeat.
    • Responsible for 'driving forces' such as defence and reproduction.
    • Controls emotions like fear and anger.
    • Controls the pituitary gland, which secretes hormones.
Cerebellum
  • Situated behind and below the cerebrum.
  • Structure: grey matter (outside) and white matter (inside).
  • Functions:
    • Coordinates voluntary movements, ensuring they are smooth and precise.
    • Maintains muscle tone, balance, and equilibrium.
Medulla Oblongata
  • Lowest part of the brain, continuous with the spinal cord.
  • Structure: white matter (outside) and grey matter (inside), contains cerebrospinal fluid.
  • Functions:
    • Transmits nerve impulses between the spinal cord and the rest of the brain.
    • Controls essential involuntary actions (breathing, heartbeat, salivation, swallowing).

Brain Imaging Techniques

  • MRI (Magnetic Resonance Imaging):
    • Uses magnets and radio waves to create detailed images.
    • Requires the patient to remain still; noisy; lasts about 30 minutes.
  • CAT scan (CT scan - Computerised Axial Tomography):
    • Uses specialized X-rays taken from multiple angles.
    • Contrast agents may be used.
  • T1 MRI (T1-weighted Magnetic Resonance Imaging):
    • Shows detailed brain anatomy.
    • Fatty tissues (e.g., white matter) appear bright; water-filled areas (e.g., cerebrospinal fluid) appear dark.
  • FLAIR MRI (Fluid-Attenuated Inversion Recovery MRI):
    • Highlights abnormalities or disease areas (e.g., inflammation, lesions like in MS) which appear bright.
    • Useful for detecting lesions near fluid-filled spaces.
  • CDT (Colour Doppler Transcranial Imaging):
    • An ultrasound technique showing blood flow in the brain's arteries.
  • [18F] FDG PET (Fluorodeoxyglucose Positron Emission Tomography):
    • A functional scan showing brain activity by tracking radioactive glucose (FDG).
    • Highly active areas appear brighter; low activity areas appear darker.

Spinal Cord

  • Downward continuation of the brain from the medulla oblongata.
  • Located within the vertebral column.
  • Protected by meninges and cerebrospinal fluid.
  • Spinal nerves enter and leave through intervertebral foramina.

Internal Structure

  • Cross-section:
    • H-shaped area of grey matter surrounded by white matter.
    • Central canal runs through the grey matter, contains cerebrospinal fluid.

Connection with Spinal Nerves

  • Spinal nerves connect via two roots:
    • Dorsal root: sensory neurons enter the spinal cord. Cell bodies are in the dorsal root ganglion.
    • Ventral root: motor neurons leave the spinal cord.
    • Dorsal and ventral roots join to form a mixed spinal nerve.

Functions of the Spinal Cord

  • Transmission of nerve impulses: pathway for impulses from receptors to the brain and from the brain to effectors.
  • Reflex centre: centre for many reflex actions (e.g., blinking, sneezing, coughing, knee-jerk reflex).

Peripheral Nervous System (PNS)

  • Motor neurons divided into:
    • Somatic nervous system.
    • Autonomic nervous system (ANS).
      • Sympathetic division.
      • Parasympathetic division.

Somatic Nervous System

  • Controls voluntary actions by sending nerve impulses to skeletal muscles.

Autonomic Nervous System (ANS)

  • Controls involuntary actions by sending nerve impulses to cardiac muscle, smooth muscle, and glands.
Divisions of the Autonomic Nervous System
  • Sympathetic division: prepares the body for emergency situations (fight-or-flight response). Increases heart rate, dilates pupils, diverts blood to muscles.
  • Parasympathetic division: returns the body to its normal, resting state (rest-and-digest response). Slows heart rate, constricts pupils, stimulates digestion.
  • Antagonistic action: the sympathetic and parasympathetic divisions have opposing effects on the same organs.

Autonomic Nervous System - Sympathetic vs. Parasympathetic

  • Sympathetic:
    • Increases heart rate.
    • Constricts blood vessels.
    • Increases blood pressure.
    • Decreases peristalsis.
    • Dilates pupils.
    • Widens bronchioles.
    • Stimulates sweat production.
    • Stimulates secretion of adrenaline.
    • Makes hair stand on end.
  • Parasympathetic:
    • Decreases heart rate.
    • Dilates blood vessels.
    • Decreases blood pressure.
    • Increases peristalsis.
    • Constricts pupils.
    • Constricts bronchioles.
    • No effect on sweat production.
    • No effect on adrenaline secretion.
    • No effect on hair standing on end.

Nervous Tissue and Neurones

  • Composed of millions of nerve cells called neurones.
  • Neurones form a continuous network.
  • Neurones are the structural and functional units of the nervous system.

General Structure of a Neurone

  • Cell body (soma).
  • Nerve fibres (outgrowths from cell body).
    • Dendrites.
    • Axon.
  • Myelin sheath.
Cell Body (Soma)
  • Contains cytoplasm and a nucleus.
  • Cytoplasm contains Nissl granules (rich in RNA, involved in protein synthesis).
Nerve Fibres
  • Dendrites:
    • Short, branched fibres.
    • Conduct nerve impulses to the cell body.
  • Axon:
    • Long, single fibre.
    • Conducts nerve impulses away from the cell body.
    • Forms terminal branches with synaptic knobs (axon terminals).
Myelin Sheath
  • Fatty insulating layer surrounding many axons.
  • Functions: insulates the axon and speeds up the transmission of nerve impulses.
  • Formed by Schwann cells (in the PNS).
  • Nodes of Ranvier: gaps between Schwann cells where impulses